VFD 101 Lesson 1. Functions of an Variable Frequency Drive (VFD)

Transcription

1 VFD 101 Lesson 1 Functions of an Variable Frequency Drive (VFD) This lesson covers the basic functions of a Variable Frequency Drive (VFD) as it applies to fans. 10/2/2003 Here is the basics outline for this lesson. Outline: A. 3-phase AC Motor B. Functions of an VFD 1. Start/Stop 2. Change Speed 3. Constant Speed 4. Limits 5. Ramping 6. Forward/Reverse 7. Save Energy 1 of 98

2 VFD 101 Lesson 1 This lesson covers the basic functions of an Variable Frequency Drive (VFD) on a 3-phase AC (alternating current) motor. Pictured above is an AC motor. 10/2/2003 The 3-phase motor pictured above is for commercial use, but in your home, AC motors are used as well. A vacuum cleaner uses an AC motor to clean the carpet; a blender uses an AC motor to process food; and the clothes dryer uses an AC motor to dry clothes. In each of these examples, how is the AC motor controlled? When controlling motors in the home we control them by applying AC power, and removing it, usually through a switch. Obviously when power, 120 or 240 VAC, is applied to the motor it runs. With no power, the motor stops. With the use of a Variable Frequency Drive (VFD) not only can the AC motor be started and stopped as in the home, but more sophisticated controls are accomplished. A VFD can send a modulating signal to the motor, which allows a variety of speeds to be delivered not just an ON/OFF signal. This variety of speeds can be used to match the motor to a particular task. There are a number of functions that the VFD accomplishes with commercial 3-phase AC motors, which are covered in the pages that follow. Motors in the home are almost always single-phase motors which require additional electric parts to rotate the magnetic field. Because of these extra parts, single-phase motors do NOT operate correctly with a VFD. 2 of 98

3 To understand the functions of an VFD better, an example of cooling tower fans is used. What must the fans do? 10/2/2003 The cooling tower in the picture above must maintain a certain temperature perhaps 30 C (85 F) for the condenser water temperature. Looking at this example, see if you can identify some of the functions that must be performed by the VFD, AC motor and fans? In other words, what must the fans be able to do? Take a couple of minutes to jot down the functions. The fans must A few of the basic functions of an VFD in controlling the AC motor and fans are covered on the pages that follow. 3 of 98

4 Function #1 Start and Stop The VFD must be able to START and STOP the cooling tower fans. 10/2/2003 Function #1 Start and Stop START One function of the VFD is to start the fans. This could be done locally off the keypad of the drive or remotely from a switch. This remote switch could be a continuous singlepole-double-throw (SPDT) switch or a momentary (push button) switch. STOP In the picture above, the SPDT switch is used to stop the pump. If there are 2 separate push button switches, one to Start and one to Stop, this arrangement is known as a 3-wire Start/Stop. If only one switch, a continuous switch, is used, then it is referred to as a 2-wire Start/Stop. In the picture above, since there is one switch, so this is a 2-wire Start/Stop. 4 of 98

5 Function #2 Change Speed The VFD must be able to Change the Reference, Hz. The Reference could also be temperature or PSI if a transmitter were attached to the VFD. 10/2/2003 Function #2 Change Speed The speed of the cooling tower fans must be variable to allow for a slower speed when there is little demand for cooling and a higher speed when more cooling is needed. This allows the operator to match the speed of the fans to a particular demand. The setting of this speed is known as the Reference. In most examples, reference refers to speed in Hertz (Hz), maximum reference of 60Hz, and minimum reference of 6Hz for fans and 18Hz for pumps. It could also be used in regards to a pressure setting, maximum reference of 100psi (690kPa), minimum reference of 40psi (275kPa), if a transmitter were attached to the VFD. In the picture above, the display of an VFD, a Danfoss VLT 6000, is shown. Speed in Hz is the reference. The plus (+) key is used to increase the reference making the fans go faster and the minus (-) is used to decrease the reference point slowing the fans down. 5 of 98

6 Function #3 Maintain a Constant Speed 10/2/2003 Light load or heavy, the drive should maintain the same speed. Function #3 Maintain a Constant Speed Another function of the VFD is to maintain the speed of the fans regardless of the temperature and humidity in the air. The VFD automatically compensates the current and torque to accommodate changes in the load. 6 of 98

7 Function #4 Limits Limits on current, torque, speed, heat and voltage and others protect the VFD & motor. 10/2/2003 Function #4 Limits It is important that limits be placed on an VFD. Speed limits can be placed in the program of the VFD so an operator can not go beyond a maximum speed or less than a minimum speed. The maximum speed of the fans should not exceed 60Hz, due to excessive power consumption. Because of the possibility of overheating, fans should not be run less than 6Hz. For the same reason as the fans, pumps should not be run more than 60Hz. For lubrication purposes a pump should have a minimum speed of at least 18Hz. If the fans gets stuck, there are torque limits that the VFD monitors stopping the motor if they are exceeded. Current limits are also important for protection of the drive and motor. In the picture above the maximum reference is set to 60 Hz. Notice that in the diagram there is a minimum reference of 6Hz. 7 of 98

8 Function #5 Ramping To reduce mechanical wear, it is important to control the acceleration, ramp up and deceleration, ramp down. 10/2/2003 Function #5 Ramping The VFD also ramps the fans up and ramps them down. When the fans starts, acceleration, it is important that there is no sudden jump to the reference speed, or there can be stress on the gear boxes. In the example above, a ramp-up slowly increases the speed from stopped or 0Hz up to the reference, 34Hz, over a certain amount of seconds perhaps 10. If this ramp up is too short, the drive can trip on an over current alarm or torque limit. If the VFD is tripped, the fans stop and it might require an operator to manually reset the VFD. Many VFDs have an automatic reset setting of 1 time to infinite times. Ramping is very important for pumps, to avoid water hammer. 8 of 98

9 Function #5 Ramping All ramp times are based on motor speed, 60Hz in Western Hemisphere, 50Hz in the Eastern Hemisphere. 10/2/2003 A ramp is also present on the stop side. This is referred to as a ramp down or deceleration. It is important that the fans do NOT stopped abruptly. A ramp-down of 60 seconds might be entered into the program for this application. If the ramp is too short, the drive can trip on over voltage. All ramp times are based on the motor speed, 60Hz in the Western Hemisphere. This means if the the ramp time is set for 60 seconds as in the picture above, but the reference is set to 30Hz (1/2 of 60Hz), it takes 30/60 x 60seocnds (½ the time) or 30 seconds to ramp up. In the rest of the world 50Hz is used for the motor speed. Using the same ramp up time (10) and reference (30), the motor then takes 30/50 X 60seconds or 36 seconds to ramp up to 30Hz. Calculations for the ramp down time would be the same. A special feature of the Danfoss VLT 6000 is automatic ramping. The VFD automatically extends the ramp times, during ramp up and ramp down, to avoid tripping of the drive. 9 of 98

10 Function #6 Forward/Reverse 10/2/2003 Change of Direction Forward to cool water Function #6 Forward/Reverse Operation FORWARD One function of the VFD is to operate the motor in a forward direction, to move the air through the cooling tower and out the top. In its default (factory set) condition the VFD is only allowed to go forward. Some fans if driven backwards may have problems. 10 of 98

11 Function #6 Forward/Reverse Reverse to defrost cooling tower 10/2/2003 REVERSE In the cooling tower example the fans need to operate in Reverse in order to complete a defrost cycle, when the outside temperature is very cool. Power going to the motor must be changed to move the fans backwards (Reverse). If there were no VFD, 2 of the 3 leads of the 3-phase motor would be switched in order for the motor to change its direction and go backwards. This switching of the motor leads is done inside the VFD. 11 of 98

12 Function #7 Saving Energy The most important function for the VFD with this fan application is to save energy. 10/2/2003 Function #7 Saving Energy In many applications, particularly involving fans and pumps, the major function of the VFD is to save energy. Before VFDs, cooling tower fans might have been cycled On at full power, when a temperature setting in condenser water of 32 C (90 F), was reached. When the water cooled to 28 C (82 F) the fans were turned OFF coming back ON when the temperature rose again to 32 C (90 F). This arrangement uses a great deal of energy and the frequent cycling causes a great deal of wear on equipment. A drive is placed on the fans, which slows the fans down to perhaps 30Hz to constantly maintain the required condenser water temperature. The fans speed up or slow down following demands. On the chart above, if the fan is running at 30Hz, half of the full speed, assuming no friction losses, the energy level is 1/8 th the HP at full speed. This same energy savings is seen on pump applications. This concludes Lesson 1. There is a Post Test to review this information. 12 of 98

13 VFD 101 Lesson 2 Control Arrangements for a VFD This training covers the major control arrangements for a VFD, starting with the simplest arrangement then moving to the more complex. Many of the control arrangements shown in the previous lesson (Lesson 1: Functions of an VFD) were Closed Loop Control. There can be other arrangements which are covered in detail in this lesson. 1) Local or Hand Control 2) Remote or Auto Control 3) Multi-motor 4) Master/Slave 5) Closed Loop 6) Cascade Control Fixed Stages 7) Cascade Control Variable Stages 8) Build Automation System (BAS) - Enable 9) BAS Enable and Reference 10) BAS Serial Communications 13 of 98

14 1) Local (Hand) Control Local or Hand control means controlling the system through the keypad on the VFD. 1. Local (Hand) Control In the picture above, a VFD, motor and fan are operated from the keypad on the front of the drive. Local (Hand) control of the VFD means that operation of the VFD is completed strictly through the keypad on the front of the drive, or Local Control Panel (LCP). An operator monitors the readings and controls the VFD by using this keypad. Even if the keypad, LCP is remotely mounted away from the drive, maximum of 3m (10 ), the control arrangement inside the program of the VFD is still considered as LOCAL. If any line is labeled as LOCAL in the program, think KEYPAD. 14 of 98

15 1) Local (Hand) Control HAND START key starts the drive in Hand (local control) speed set by + and - keys OFF STOP key stops the drive locally AUTO START key ends manual control RESET clears the drives from an alarm condition. In the picture above there are 2 VFDs, each being controlled by its own LCP or keypad. The one on the right uses a remote keypad kit to place the keypad in a convenient location. The operational site on both VFDs is considered as LOCAL or HAND. The Hand Start key starts the VFD, assuming safeties have been enabled. The display on the keypad changes as seen on the last page. This allows the operator to increase (+ key) or decrease (- key) the speed of the motor. Other start commands are ignored. The OFF Stop key stops the drive. The display starts to flash to indicate that this key has been pressed. Other start commands are ignored. To remove this stop command, the Hand Start or Auto Start must be pressed. The Auto Start key ends Local or Hand control. This means that remote controls, which are described in the pages that follow are in control. The Reset key clears an alarm from the VFD, assuming that the alarm has been corrected and is set for manual reset. Some alarms require that power be removed (Disc.Mains) before they can be reset. 15 of 98

16 2) Remote Control Remote signals are wired into the control section of the VFD. They come in 4 types: Digital Inputs (DI), Analog Inputs (AI), Analog Outputs (AO), Digital/Relay Outputs (DO). 2. Remote (Auto) Control Other arrangements are possible including remote signals. If there is a problem with the fan and it must be stopped immediately, it might be time consuming to run back to the VFD to stop it. Stop switches can be placed at key positions to stop the VFD, AC motor and fan. It is important that the VFD accept these stop signals as well as other remote signals. These remote control signals come in four types: 1) Digital Inputs (DI) are 2-position (ON/OFF) signals sent into the VFD. These commands check safeties, then tell the VFD to Start, to Stop, etc. A DI requires 24Vdc which is supplied by a terminal on the drive. 2) Analog Inputs (AI) are proportional or modulating signals sent into the VFD. These commands tell the VFD what the reference speed should be or tell the VFD what a feedback signal is doing such as static pressure. These signals are usually from 0-10Vdc or 4-20mA. 3) Analog Outputs (AO) are modulating signals sent by the VFD to a device such as a meter which could display feedback, speed or current. 4) Digital/Relay Outputs (DO/RO) are 2-position (ON/OFF) signals sent by the VFD to a device such as a light to indicate an Alarm, or when the feedback signal has reached a certain limit. Digital Outputs have power 24Vdc attached and Relay Outputs do not have power, which are known as dry contacts. 16 of 98

17 2) Remote Control Above, connectors are shown for remote signals. These signals are divided into 4 types Digital Inputs, Analog Inputs, Analog Outputs and Digital/Relay Outputs. The terminal numbers are listed on the cover plate shown above. Besides stop switches, other signals can be sent into the VFD. These could be a reference pot to change the speed, increase and decrease buttons which would also change the speed, remote Start and Stop switches, or other signals. All the different options for remote signals are considered as control wiring. In the program of the drive, if the Hand keys are used, remote signal except for safeties are ignored. On the VFD shown above, all the control wiring terminals are shown on the black plastic cover just under the LCP keypad. The amount of connections for the control wiring is a means of comparison between manufacturers of VFDs. As an example, the Danfoss drive shown above has the ability to accept the following signals: 8 Digital Inputs 3 Analog Inputs (2 are setup for 0-10Vdc; 1 for 0-20mA) 2 Analog/Digital Outputs (0-20mA or 4-20mA signals or these can be programmed as Digital Outputs with 24Vdc attached) 2 Relay Outputs (dry contacts) 17 of 98

18 3) Multi-Motor Operation One VFD is used to operate 4 separate fans. The VFD must be able to handle the maximum current for the 4 motors. Some features of the VFD in this arrangement are restricted. 3. Multi-Motor Operation The multi-motor arrangement is usually done because of strict cost considerations. Not only must the VFD have the current capacity for all the motors, but each individual motor must have overload protection. In the picture above, one VFD operates 4 AC motors, which in turn operates the 4 cooling tower fans together. They operate at the same speed or close to the same speed. When this arrangement is used some VFD features are restricted. First the motors and fans must all run at the same speed. Another restriction is that the VFD can not be tuned to a individual motor. Motor Tuning is where the VFD is tuned or matched to an individual motor for better performance and energy savings. The last restriction given here is that the slip compensation (calculating the difference between the field speed and rotor speed) should be set to OFF. 18 of 98

19 3) Multi-Motor Operation In the example above each of the 4 AC motors is the same size and has a maximum current rating, Full Load Amps (FLA) of 15 amps. The VFD must be sized for 60 amps. Note individual overload protection on each motor. If the cooling tower fans were operating with a very light load but motor 3 got jammed. The amp draw on motors 1,2 and 4, might be low, say 5 amps each, but the amp draw on motor 3 would need to surpass 45 amps before the drive saw any problem. This would cause damage to motor 3. As shown in this example, individual overload protection, such as thermal overloads are needed to protect each motor. Motors in this arrangement operate close to the same speed. 19 of 98

20 4) Master/Slave (Leader/Follower) In the Master/Slave arrangement a Master or Lead VFD monitors the pressure sensor and operates a single fan. It sends a corresponding signal to another following supply fan. Features of the VFD which were restricted in the multi-motor can be used in this arrangement. 4. Master/Slave (Leader/Follower) This arrangement allows motors to operate closer together than in the multi-motor application. Each VFD also provides safeties for its own motor. One VFD is selected as a Master or Leader drive. It is setup to send a reference signal and ON/OFF commands to the Slave or Follower drive. In the example above, 2 fans are used for Supply Air on a Variable Air Volume (VAV) system. The VFD operating the top fan is considered the Master or Leader. It varies its speed to match the static pressure needs in the supply duct. The bottom fan and VFD, follows the top supply fan and is known as the slave or follower. The slave can match the speed to within 0.3Hz of the master, over the operating range from about 6Hz to 60Hz. Rather than always matching the speed of the master VFD, the slave VFD can operate at a percentage of the reference. If a positive pressure needs to be maintained in a zone, the slave VFD on a return fan can be slightly behind (-10% of reference) of the master VFD on the supply fan. The supply fan always runs faster than the return fan causing a positive pressure in the zone. In this last application using a volumetric sensor comparing the CFM (L/s) from supply and return to control the return fan would give a much greater accuracy than the Master/Slave arrangement. 20 of 98

21 4) Master/Slave (Leader/Follower) A Static Pressure sensor sends a signal to the master/ leader. Using a 4-20mA signal from its AO the Master sends a signal to the AI of the Slave. In the picture above, the VFD on the first supply fan is the Master/Leader and it generates a reference signal for one of its analog outputs (AO). The VFD on the second supply fan is the Slave/Follower, and it monitors this 4-20mA reference signal from the master using the slave s analog input (AI). It is possible with Danfoss drives to use a Digital Output (DO) as a pulsed reference. The follower uses a Digital Input (DI) to follow this pulsed reference signal. These 2 fans must run closely together, the greatest variance between drives using the pulsed signal is around 0.2Hz along the entire range of frequencies (6 to 60Hz). There is a time delay in this arrangement. The master drive starts going to its reference before it sends the signal to the slave drive. Usually this delay is very small, in milliseconds, and of no consequence in this application. 21 of 98

22 5) Closed Loop/PID Control In this arrangement a 4-20mA static pressure transmitter is wired directly into the VFD. This is a Feedback signal and is always referred to as Closed Loop. The VFD monitors its own signal and result. 5. Closed Loop/PID Control Up to this point most of the previous control arrangements have been closed loop, which means that there is feedback signal monitoring the controlled variable, going directly to the VFD. Closed Loop is used for stand alone control. In the example above, the VFD monitors the signal coming from the 4-20mA static pressure sensor in the supply duct. In a variable air volume (VAV) system it is important to maintain Static Pressure in the duct for proper operation of the VAV boxes. In all closed loop applications, additional parameters must be programmed. These include a setpoint, and PID settings. In this application, the VFD is constantly comparing the static pressure setpoint, 2.5 wc, (625 pascals) with the actual feedback value coming from the pressure transmitter. The VFD modulates the speed of the supply fan to maintain that pressure. Controller action is one of the parameters that must be checked in the VFD. There are 2 selections which are as follows: Normal Control (Reverse Acting) which increases the speed of the fan when the signal decreases from the pressure sensor, as in the example above. Inverse Control (Direct Acting) increases the speed of the fan when the signal increases from the sensor. 22 of 98

23 5) Closed Loop/PID Control In the picture above, Static Pressure in the supply duct is maintained by modulating the speed of the supply fan. Proportional gain, and integral settings must be setup in the VFD, along with a setpoint and proper controller action. There is always a difference between the setpoint and the actual feedback pressure. This is referred to as Offset, off the setpoint or error. PID settings attempt to reduce this error. P stands for Proportional Gain which can be considered as a multiplier of the error. The higher the gain the more accurate, but if it is set too high, the control can become unstable and jittery. With too high of a gain setting, the VFD oscillates between maximum speed and minimum speed, hunting for the correct speed. The gain must be high enough to be sensitive but not too high to cause hunting. Each application is different, but a proper starting setting for pumps is 4.0 and 2.0 for fans. I stands for Integral which looks at the error over a certain amount of time. The lower the number the more frequently it checks the error. If the I setting is too low, the motor again appears to be hunting. Based on most applications, a pump has its I setting for 20 seconds and 30 seconds for fans. D stands for derivative which, if used, compensates for momentary changes in the load. In most HVAC applications, this parameter is not used, keeping it OFF. 23 of 98

24 6) Cascade Control Card Fixed Stages There are occasions where a group of pumps or fans must work together. There is an optional Cascade Card that may be placed inside the VFD. This card operates up to a maximum of 5 motors together. 6. Cascade Control Card Fixed Stages There are some applications where multiple pumps or fans must operate together. It is desired to only operate the number of pumps necessary to achieve the load requirements, keeping other pumps off to save energy. A separate Cascade Controller card which can be mounted inside the VFD, can be used to coordinate the operation of up to a maximum of 5 pumps. One way to operate these pumps is to have the first pump be varied by the VFD and the fixed stages using soft starts. When it reaches 100% output, a fixed stage is started. Each time the 1 st pump reaches 100%, another stage is enabled. In the example above, the variable pump is the first pump to start. When this variable pump goes to 100% or 60Hz, a relay on the Cascade Card is enabled which starts a soft-starter on Pump #1. Since soft-starters do not modulate, pump #1 goes to 60Hz. The variable pump then drops its output. When the load requires more output, and the variable pump again reaches 100% then the soft-starter on Pump #2 is enabled. 24 of 98

25 6) Cascade Control Card Fixed Stages The diagram above shows the operation of the fixed stages. Notice that every time the VFD signal reaches 100%, a fixed stage comes ON and every time the VFD drops to minimum, a fixed stage goes OFF. The diagram above shows the operation of the variable pump, VFD at the bottom of the chart and how it relates to the fixed stages, which are shown at the top. Notice that every time the VFD reaches 100% or 60Hz, a fixed stage on the Cascade Controller card is enabled. When the fixed pump is running, this causes the VFD to drop its signal. With an increase in demand the VFD goes to 100% again causing the second fixed stage to come ON. When the demand drops and the VFD goes to 0% or minimum speed, 20Hz, the fixed stages go OFF. With a loss of a fixed pump, the VFD then increases its signal. Every time the VFD drops to minimum speed, a fixed stage is turned OFF. It is possible to use 4 stages, but only 3 stages are shown here. 25 of 98

26 7) Cascade Control Card Variable Stages Rather than having pumps come ON as fixed stages, it is desired to modulate all the pumps. In the example shown, pump #1 has the VFD with the Cascade Controller and pumps 2-4 each have a VFD. 6. Cascade Control Card Fixed Stages There are some applications where multiple pumps or fans must operate together, but the operator wants each pump or fan to have a VFD. The same Cascade Controller card discussed in the last arrangement can be used to coordinate the proportional operation of up to a maximum of 5 pumps. When the lead pump reaches a certain output, say 75%, the second VFD is enabled and now they both modulate using the same reference. As the first VFD has its output go progressively higher say to 80%, an additional VFD is enabled. A further explanation of this operation is shown on the chart on the next page. 26 of 98

27 7) Cascade Control Card Variable Stages The diagram above shows the operation of the variable stages. The lead VFD which has the Cascade Controller, on pump #1, enables the other stages as the demand is increased from 75% up to 85%. The diagram above shows the operation of the variable pump, VFD at the bottom of the chart and how it relates to pumps 2-4. When the 1 st pump reaches 75%, pump 2 is enabled and quickly follows the same signal that goes to pump 1. If the demand for both pumps continues to increase to 80%, pump 3 is enabled. Pump 4 is enabled when the demand goes to 85%. When the demand drops all the pumps start to slow down. Although this is NOT shown above, when the demand drops to 30%, pump 4 is disabled. When the demand drops to 20%, pump 3 is disabled; and pump 2 is disabled at 10%. Pump 1 then continues to monitor the demand. Remember that 0% is still a minimum speed of 20Hz. 27 of 98

28 8) Building Automation Enable Building Automation Systems (BAS) coordinate the use of the VFD with numerous other schedules and commands. In the picture above, a local Direct Digital Controller (DDC) enables/disables the VFD by use of a Digital Output. 8) Building Automation - Enable On numerous occasions a VFD works with a Building Automation System or BAS. The BAS coordinates the VFD with more information and commands such as occupancy schedules, holidays, energy optimization, electric demand limiting to name a few. There are a few ways to wire the VFD with the BAS; three arrangements are covered in this lesson. In the example above, a local area controller, also known as a Direct Digital Controller (DDC) which monitors and issues commands to an AHU, enables and disables the VFD based on BAS schedules. This is done with a DO coming from the DDC controller wired to the DI of the VFD which issues the Start and Stop commands. Any wire attachment to the DDC controller, other than communications, is known as a point. On some DDC controllers, points or certain types of points, can be restricted. Here only one point is used on the DDC which saves on wiring costs. In this arrangement, the pressure sensor is wired directly to the VFD. The closed loop inside the VFD modulates based on a setpoint to keep the proper pressure in the AHU. The BAS system only enables or disables the VFD. The DDC controller can not monitor or control the setpoint for the pressure, which makes this arrangement the least desirable for the BAS system. 28 of 98

29 8) Building Automation Enable A Start/Stop command from a DO of the DDC Controller goes to a DI of the VFD. The signal from the pressure sensor is wired into the VFD. Notice in the example above, a DO on the DDC controller, a relay output, which has no power of its own, completes the circuit between terminal 12, +24Vdc and terminal 18 of the VFD which is the start/stop input. The pressure transmitter is wired directly to the VFD, which saves a point on the DDC controller, but limits the operational use and adjustments by the BAS for this air handling unit. The operator must go to the VFD through its keypad, to monitor the pressure in the duct and to change the setpoint. Other information from the VFD, such as speed, temperature, alarms, etc. displayed through the keypad would also be done locally at the VFD. In this control arrangement, the BAS system can only indicate if the VFD had been enabled or disabled. 29 of 98

30 9) BAS Enable & Reference A local DDC of a BAS sends an enable/disable (Start/Stop) command to the VFD by use of a Digital Output (DO); and it sends a reference (speed) command to the VFD by use of an Analog Output (AO). 9) BAS Enable and Reference On the example above additional connections or points are used to gather more information and send more commands from the DDC controller. Notice that the DDC Controller sends a Start/Stop command and also sends a reference. The pressure sensor monitoring the static pressure in the AHU is sent to the DDC controller. A closed loop inside the DDC controller monitors the pressure and adjusts the reference to maintain the pressure setpoint. The VFD, programmed for open loop, receives the reference and Start/Stop commands from the DDC controller. In this arrangement, the static pressure and setpoint can be monitored and modified by the BAS system. Additional connections can be made from the VFD back to the DDC controller to confirm commands and monitor alarms. Each connection requires an additional hard-wire point from the DDC controller. 30 of 98

31 9) BAS Enable & Reference A Start/Stop command from a DO of the DDC Controller goes to a DI of the VFD. A Reference or Speed command from an AO of the DDC Controller goes to a AI of the VFD. In the example above, the DO of the DDC controller is wired to the VFD as in the previous example. Now, however, an AO from the DDC controller which sends a 4-20mA signal, is wired directly to the current input, AI of the VFD. The 4-20mA pressure transmitter is wired directly to the DDC controller, which is not shown above. More information and adjustments can be accomplished through the BAS system using this control arrangement. For each piece of new information such as alarm status, current going to the motor, and energy usage, wires are needed between the VFD and DDC controller. 31 of 98

32 10) Serial Communications Here serial communications is used to enable the VFD and to give it a reference or speed. Verification of the VFD speed and status is sent back to the DDC Controller through the serial connection. This connection is usually an RS-485 connection. 10. Serial Communications Here is the last interface with a BAS system. A serial communications protocol is used inside the VFD to talk directly on the Local Area Network or Bus. With this arrangement numerous bits of information can be see at the DDC controller and the BAS. This information includes the start/stop command, static pressure, and reference, as in the previous example; but it also includes information about energy usage, alarming, running hours, input status, internal temperatures, and motor information. This information is sent over a 2-wire, RS-485 connection. In the example above, it can be seen that the VFD is wired directly to the same bus that connects the Local Area controllers. This bus connection could be a LonWorks protocol. Local Area controllers operate specific devices in a building, such as a cooling tower, chiller, boiler and air handling unit. The Local Controllers are in turn connected to a Global Area Controller. These Global Controllers operate individual buildings or large parts of a building. Global Controllers are wired together to form a Global Network. This connection could use an Ethernet protocol. Both LonWorks and Ethernet are popular within the HVAC market. Notice that a PC is wired to the Global Network. This PC has a Graphic User Interface program which is also known as a GUI. 32 of 98

33 10) Serial Communications The Start/Stop command and reference command along with numerous other bits of information travel through the 2-wire, RS-485 connection. In the example above, the 2-wire, RS-485 connection goes between a local controller and the VFD. Normally this 2-wire bus connection must be in a Daisy Chain arrangement. This arrangement only allows 2 ends. All the positive or + connections are wired together and all the negative or connections are wired together. Addressing of each device must be unique, and can be accomplished with hardware switches or programmed in software, which is the case with this VFD. This concludes Lesson 2. There is a Post Test to review this information. 33 of 98

34 VFD 101 Lesson 3 Parts of a Variable Frequency Drive (VFD) This lesson covers the parts that make up the Variable Frequency Drive (VFD) and describes the basic operation of each part. Here is the basics outline for this lesson. Outline: Parts and Operations of a Drive 1. VFD part of a Larger System 2. Rectifier 3. Soft Charge Circuit 4. Intermediate Circuit (DC Link) 5. Brake Circuit 6. Inverter 7. Pulse Width Modulation 8. Control & Regulation Section Note: Other names are used for this device, such as Adjustable Frequency Drive (AFD), Variable Speed Drive (ASD), Frequency Converter and Inverter, but Variable Frequency Drive (VFD) or just Drive is used throughout this lesson. 34 of 98

35 VFD 101 Lesson 3 This lesson covers the basic parts and operation of a Variable Frequency Drive (VFD). These parts are divided into 4 sections: Rectifier, Intermediate circuit (DC Link), Inverter and Control & Regulation. 1. VFD in a Larger System This section covers the parts and operation of the Variable Frequency Drive (VFD). It is important to keep in mind that the Drive is just one part of a system. In the diagram above, notice the disconnect switch, fuses, bypass switch, thermal overloads, BAS, etc. all play an important part in making an application work correctly. Inside the VFD there are 4 major sections: rectifier, intermediate circuit (DC Link), inverter and control/regulation. This fourth section, control and regulation, interfaces with the other 3 sections. In very general terms the operation of the drive is as follows. Power first goes into the rectifier, where the 3-phase AC is converted into a rippling DC voltage. The intermediate circuits then smoothes and holds the DC Voltage at a constant level or energy source for the inverter. The last section, the inverter, uses the DC voltage to pulse the motor with varying levels of voltage and current depending upon the control circuit. The pattern of the pulses going to the motor makes it appear similar to an AC sinusoidal waveform. Each one of these sections is reviewed in some detail in the pages that follow. 35 of 98

36 To understand the parts of an VFD better, an example of a 450kW (600Hp) drive is used. Fuses Disconnect In the picture notice the fuses and disconnect switch. As each part is explained pictures of these parts on a 450kW (600Hp) drive are displayed. This large drive is used in this lesson for the size of the parts are easy to identify. One of the options for these large drives kW ( Hp) is to have fuses and a disconnect switch mounted inside the drive. With smaller size drives fuses and a disconnect are separate but are still part of the overall system as described on the previous page. 36 of 98

37 2) Rectifier Section Its function is to change 3-phase AC into DC. 2. Rectifier The 3-phase AC voltage goes into the rectifier section which is made up of a group of gated diodes (silicon rectifiers or SCRs). In most VFDs, these diodes are in a group of 6 as diagramed above. One VFD manufacturer has stressed that there should be more sets of diodes, 12, 18, even 24. Reasons for/against this are covered in lesson 5. Diodes (D1 through D6) allow current to flow only in one direction when enabled by the gate signal. In this diagram, the AC power on L1 goes into Diodes D1 and D2. Because of the position of these diodes, current flow can only go up. The D1 diode conducts when the AC is positive and D2 conducts when the AC goes negative. This drives the top line (+) more positive and the bottom line (-) more negative. Diodes D3 and D4 convert L2 power to DC and Diodes D5 and D6 convert L3. A volt ohmmeter or VOM can be used to measure this DC voltage. In this type of circuit, the DC voltage is 1.35 times the AC line voltage. If 240 Vac is coming in, 324 Vdc is generated. If 380 Vac is coming in, 513 Vdc is generated. If 460 Vac is the line voltage, 621 Vdc is generated. If 575 Vac is the line voltage, 776 Vdc is generated. Because of line (power coming in) and load (power to the motor) changes, the DC Voltage level is constantly moving above and below this expected value. 37 of 98

38 2) Rectifier Section SCR Heatsinks Incoming Power The Rectifier section contains terminals for incoming power, silicon rectifiers (SCR) and heatsinks The picture above highlights the rectifier section of the drive. Six SCRs are used to change the incoming power from AC to DC. This rectification can generate a considerable amount of heat, so the SCRs are mounted onto a gold-colored heatsink. The fins of the heatsink are facing the other way inside a special ductwork where the air flow removes the heat. Four fans mounted across the top of this VFD pull the air across the heatsink. Remember that heat is the enemy both to the drive and to the motor. Any practice which makes either run cooler makes them last longer. Because of the high amperage (750A for this unit) there are bus bars connecting the rectifiers to the incoming power. Even the largest wire size is too small for this unit. 38 of 98

39 3) Soft Charge Circuit On large drives, 22kW (30Hp) and larger, a soft charge circuit is added in helping charge the cap banks before main power is applied to the rectifier. 3. Soft Charge Circuit On larger drives, kW (30 600Hp), a part of the rectifier section is known as the soft charge circuit, which is used to power up the drive. With this circuit, when power is applied, the inrush of current is restricted going to the large capacitors in the DC Link, so that they may charge up slowly (within a couple of seconds). If this circuit was absent, line fuses would be blown every time the VFD was started. The soft charge circuit on some of the VFDs has a resistor or two in line with the current to slowly allow charging of the capacitors. This current resistor even has its own safety, a thermal switch, which shorts out if the current rush is too high in the soft charge circuit. The shorted thermal switch blows fuses on the soft charge circuit preventing the drive from starting. Once main power is applied to the drive, the SCRs in the main rectifier section remain off. The much smaller rectifier section in the soft charge circuit starts, applying DC power through the current resistors charging up the capacitors in the DC Link. When these capacitors are charged to the DC voltage minimum value, the control section starts the firing of the SCRs in the main rectifier. Because of the amp draw through the current resistors in the soft charge circuit, time is needed to cool them off, so the kW ( Hp) drives are limited to 2 start per minute. 39 of 98

40 3) Soft Charge Circuit Soft Charge Circuit Soft Charge- Snubber Fuses Here is a picture of the soft charge circuit in the 450kW (600Hp) drive. In the picture above, the soft charge circuit card is shown. This circuit card on the 450kW (600Hp) drive is in the upper left corner, just above the rectifier section. Notice that the soft charge fuses are just to the right of the circuit card. The soft charge circuit card on the kW ( Hp) drives uses small IGBTs instead of resistors to limit power going to the capacitors. This is referred to as an active soft charge. 40 of 98

41 3) Soft Charge Circuit Current Resistors Soft Charge- Resistor Fuses Soft Charge Fuses Soft charge circuit in the 160kW (200Hp) drive. On the 160kW (200Hp) drive, shown above, the soft charge circuit is exactly like the schematic diagram shown on the previous page. Notice the 2 large black current limiting resistors used to limit power going to the capacitors in the DC Link section. 41 of 98

42 4) Intermediate Circuit (DC Link) Using a large bank of capacitors and DC reactors the rippling DC voltage becomes more stable. 4. Intermediate Circuit (DC Link) The Intermediate Circuit also known as a DC Link, can be seen as a power storage facility for the next section, the inverter section. There are 2 major components to the DC Link section, capacitors and coils. In the diagram above only one capacitor is shown but it is always a series of capacitors. With Danfoss VFDs, this intermediate section always uses DC coils also known as DC Line Reactors or DC chokes. For cost considerations, most other VFD manufacturers do not offer these DC Line Reactors as standard equipment. Danfoss regards these coils as essential for two main reasons; one is the ability to reduce harmonic noise (interference) by 40% and the other is the ability to ride through a temporary loss of power. This allows this drive to avoid numerous nuisance shut downs. In the diagram above, notice that the rippled DC voltage coming in has now been filtered to a relatively constant voltage. Remember that this DC Link Voltage is 1.35 times the input voltage. The value of the DC Link voltage can be read from the display on the front of the drive. When ever working around the drive always be careful and give it a healthy respect. The largest drive produces 620Vdc at 750 A. 42 of 98

43 4) Intermediate Circuit (DC Link) Cap Banks The blue capacitors banks (black on newer units) are a major part of the DC Link and store a great deal of energy. When looking at a drive, some of the most striking components are the 2 devices that make up the DC link. The 3 banks of blue capacitors on the 450kW (600Hp) drive, shown on the left, are quite prominent. On newer units these blue capacitors have been replaced with black ones. They are in the center of the drive, just to the right of the rectifier section. There are 3 banks of 12 capacitors in each bank for a total of 36 capacitors. Capacitor numbers vary with each size of drive. This 3-bank arrangement is to allow for easier service. The plate on the right side of each capacitor bank has full voltage. 43 of 98

44 4) Intermediate Circuit (DC Link) DC Reactors There are 2 sets of coils shown above. The DC Link coils are always the ones with 2 terminals, shown here on the left. In the picture above there are 2 sets of coils. The coils on the left at the bottom center of the drive, the ones that have 2 connections (DC +, DC -) are the DC Coils, also know as DC Reactors or DC Chokes. The other set of coils to the right, with 3 terminal connections, are discussed in the pages that follow. 44 of 98

45 5) Brake Circuit When drives are ordered with dynamic braking, the drive comes with Brake IGBTs. When the voltage gets too high on the DC Bus, the brake IGBT activates sending power to the brake resistor. This is NOT an option with HVAC drives. 5) Brake Circuit This Brake Option, also known as Dynamic Braking, is used with devices that need to stop or change directions quickly, such as conveyors, hoists and centrifuges. On drives that have the brake option, an additional IGBT transistor is used to remove extra power coming back into the drive when the motor, which has a large inertia, is stopping or changing direction. The only HVAC related application that might use dynamic braking is for some fans for boiler combustion. This option is not required for the vast majority of HVAC applications. 45 of 98

46 6) Inverter Section The Inverters take the voltage from the DC Bus and using Pulse Width Modulation (PWM) sends a signal which appears to the motor as an AC signal. 6) Inverter The next part of the VFD is the Inverter section. This section takes the DC voltage from the intermediate section and, with the help of the control section, fires each set of IGBT (Insulated Gate Bipolar Transistors) to the U, V and W terminals of the motor. This firing of the IGBTs is known as Pulse Width Modulation (PWM) and is described in the next couple of slides. Notice in the diagram above that sensors monitor the current going to each terminal of the 3-phase motor. Unlike some other manufacturers, Danfoss monitors all 3 phases continuously. There are some manufacturers, who in an attempt to cut costs, only have 2 sensors and guess on the output to the 3 rd. There are others that only monitor the outputs when the first run command is given. Another component that Danfoss insists on including are the motor coils on any drive larger than 18.5kW (25Hp). These coils smooth the waveform going to the motor. The smoother the waveform the less heat is generated at the motor and the longer the motor lasts. The standard distance used by Danfoss between its drive and the motor is 300m (1000 feet) using unshielded cable. There are other manufacturers that are limited to shorter distances 100m (330 feet) or less. Some end users have used distance as a sign of quality. The longer the distance, the better the drive. 46 of 98

47 6) Inverter Section Current Sensors Terminals Motor Coils Motor coils, current sensors and motor terminals are located in the lower right corner of the 450kW (600Hp) drive. The current sensors monitor the current going to the 3-phases of the motor. These sensors detect and alarm when a short circuit or grounded circuit is discovered. Some manufacturers only check for short circuits or grounding on the first run command, but Danfoss monitors for these faults all the time. This allows Danfoss to place a motor disconnect between the drive and the motor. If the motor is disconnected from the drive during operation, the drive might trip, but because of this constant monitoring, it suffers no damage. A disconnect switch between the motor and drive is not allowed by many other manufacturer. If a disconnect switch is used on a few drives, it causes severe damage in other words the smoke is let out. 47 of 98

48 6) Inverter Section IGBTs IGBT snubber card The inverters, IGBTs and snubber card, are mounted on heatsinks under each of the 3 cap banks. The IGBTs are mounted on the heatsinks behind the capacitors in the middle of the 450kW (600Hp) drive. The picture on the right shows two IGBTs with the circuit card which is used to help control them, know as a snubber card. The picture on the left shows the IGBTs without the snubber card. The correct mounting pattern for the 6 screws (done in a rotating manner) on each is critical, so that there is proper contact between the IGBT and the heatsink. 48 of 98

49 7) Pulse Width Modulation All manufacturers of drives use PWM, but there are differences when it comes to the shape of the pulses going to the motor. 7) Pulse Width Modulation In the diagram above, a close up view of the waveform that goes to the motor shows the switching frequency of the IGBTs. The switching-pattern shown above is known as pulse width modulation or PWM. As the length of time is increased for the IGBT to be ON and then OFF, the motor responds to it as a sinusoidal waveform. The positive IGBT fires first in the diagram followed by its negative counterpart. Only one motor terminal (U) is shown but the same type of activity would appear on V and W. In the diagram above only 7 pulses are shown on each side, but actually 1750 pulses or more should be shown. This PWM frequency can vary from 3.5KHz to 15 khz, which means it is audible. It is also known as the Carrier Frequency, which is Variable by most VFD manufacturers. A low carrier frequency can have an annoying noise, but a higher carrier frequency generates more heat in the drive and motor. If the carrier frequency noise is too loud particularly with supply fans, LC filters can be placed between the VFD and motor and the noise stops at this filter. 49 of 98

50 7) Pulse Width Modulation Without a drive, the motor can go full speed or OFF; With a drive, the motor can go to a number of different speeds. At first glance the function of a drive might look rather confusing. Taking 50Hz or 60Hz power input then changing it to DC only to change it back to look like AC to the motor. Due to the electronics in the drive, the DC voltage can be manipulated in a much easier and adaptable fashion. In the example, without the drive, the only signal the motor sees is ON (50Hz/60Hz) or OFF (0Hz). With a drive the motor can operate with 20Hz, 40Hz, 60Hz, 90Hz or any frequency in between, making it much more adaptable to any application. 50 of 98

51 8) Control & Regulation The control and regulation section monitors the other 3 sections, making numerous calculations and adjustments to the outgoing signal. 8) Control & Regulation Section The control section coordinates and regulates signals inside the drive. This is where numerous calculations are completed to properly switch the IGBTs. This control section uses Vector technology, which separates the torque producing current from the magnetizing current. In the diagram above the current going to the AC motor is being monitored. The Danfoss VLT 6000 has a special program, algorithm, called Automatic Motor Adaptation (AMA), which determines the electrical parameters for the connected motor while the motor is at a standstill, a feature introduced by Danfoss in Many competitors must decouple and spin the motor for tuning. Because the AMA measures the resistance and reactance of the motor s stator establishing a motor model, the magnetizing current can be calculated. This motor model is used to calculate the slip and load compensation. The control section uses the frequency (f), voltage (V) and phase angle (theta) to control the inverter. This means that the torque producing current can be controlled more accurately. This robust sensor-less regulation scheme Voltage Vector Control (VVC+), which is patented by Danfoss, can compensate for rapid load changes. In most HVAC applications where there is a minimum speed greater than 5Hz, the AMA feature is only a minor benefit. It is very important if used on motors spinning at 5Hz or less. 51 of 98

52 8) Control & Regulation Power Card Control Card Gate Card Three cards make up the control and regulation section, the control card, the power card and the gate card. On smaller units the power and gate cards are combined. In the picture above, on the 450kW (600Hp) drive, 3 circuit cards make up the control and regulation section for the drive. These cards are mounted onto the frame used for the capacitor banks. The Control Card is the same card used on all drives from kW (1-600Hp). The Local Keypad (LCP) fits into this control section. The LCP is used to program and monitor the drives operation. The next card is known as the power card, which is specific for a particular size of drives. It relays signals to the gate card, monitors the current from the current sensors, coordinates the fan operation and a number of other functions. The third is known as the gate card, whose major function is to send signals to the IGBTs. On smaller drives these last 2 circuit cards are together on one card. 52 of 98

53 Summary The function of all the parts of the drive, Rectifier, DC Link, Inverter and Control/Regulation is to make a clean waveform to the motor. The operation of the control and regulation section produces a very clean waveform going to the motor. The picture to the left shows an oscilloscope trace of a motor phase current provided by a conventional pulse width modulation system with harmonic elimination. To the right is the output from the Danfoss VVC+ system. The more sinusoidal the wave form going to the motor the easier it is on the motor and the less heat resulting in longer motor life. Here are some of the advantages of the control and regulation program that is in the Danfoss drives. Advantages of Voltage Vector Control (VVC +) Up to 110% motor torque for 1 minute. Automatic Energy Optimization Flying Start Sleep Function at minimum speed, Wake up speed on demand PID LOOP with 4-20mA input and/or 0-10Vdc inputs Fast system response to speed and load changes (3 ms updates) Disconnect switch allowed between the drive and motor Sensor calculation: Max of 2, Min of 2, Average of 2, etc. Long Cable lengths (1000 unshielded) between drive and motor. This completes this lesson Review Questions in Post-Test 53 of 98

54 VFD 101 Lesson 4 Application Terminology for a VFD This lesson covers the application terminology associated with a Variable Frequency Drive (VFD) and describes each term in detail. When applying a Variable Frequency Drive (VFD) to a particular application, a number of terms are involved. This lesson attempts to clarify most of those terms. Here is the basics outline for this lesson. Outline: Application Terminology 1. Application Curves 2. Starting Torque 3. Open or Closed Loop 4. Closed Loop and PID 5. Different Types of VFDs Note: Other names are used for this device, such as Adjustable Frequency Drive (AFD), Variable Speed Drive (ASD), Frequency Converter and Inverter, but Variable Frequency Drive (VFD) or just Drive is used throughout this lesson. 54 of 98

55 Outline Application Terminology 1. Application Curves CT, CP, VT 2. Starting Torque HO, Breakaway, NO 3. Open or Closed Loops 4. Closed Loop & PID Action, Setpoint, Offset, Proportional, Integral, Derivative 5. Different Types of VFDs This module covers some of the application terminology used with VFDs. This terminology becomes very important for selecting the correct VFD and programming the correct settings needed for an application. Applications include terms such as Constant Torque (CT), Constant Power (CP) and Variable Torque (VT). Starting Torque is also covered, which uses High Overload (HO), Breakaway Torque, and Normal Overload (NO). Terms such as open loop and closed loop are also covered. Terms in closed loop, also explore action, setpoint and PID terminology. The last part covers a summary of the different types of VFDs. It shows the Volts/Hz, Voltage Vector, Flux Vector as they compare to a DC Servo in terms of response, accuracy and speed range. A more detailed explanation of each type is scheduled for a later lesson. 55 of 98

56 1. Application Torque Curves This part of the lesson covers the application terminology associated with a VFD, starting with application torque curves. VFD Application Terminology 1. Application Torque Curves a) Constant Torque (CT) b) Constant Power (CP) c) Variable Torque (VT) 2. Starting Torque 3. Open or Closed Loop 4. Closed Loop & PID 5. Different Types of VFDs 56 of 98

57 1. a) Constant Torque (CT) CT used on Reciprocating Compressors Torque stays relatively constant from 5Hz to 60Hz. AC motors have low torque at slow speeds. The graph above shows Torque in relationship to Speed. This graph was made when the VFD and loaded motor, were running at Synchronous Speed, 60 Hz, 1800 rpm on a 4-pole 3- phase AC motor. The speed of the VFD was slowly turned down to 0 Hz. Notice that the torque drops down to its nominal rating on an AC induction motor at the lower speeds, 5Hz or less. Please be aware that this particular CT application, reciprocating compressor, should never run less than 30Hz. Constant Torque applications require a constant level of torque throughout a process regardless of different speeds. The vast majority of constant torque applications are for industry, which involve conveyors, mixers, elevators, feeders, hoists, and bottling machines. In HVAC reciprocating compressors, and positive displacement pumps might be encountered which require CT. Some VFD manufacturers attempt to apply a CT drive to fans and pumps used in HVAC, because they do not make a specific VFD for variable torque applications. More information is given in the pages that follow. On most CT applications, constant torque must be maintained from 1 to 60Hz. Notice that the torque curve drops when the speed is reduce below a minimum of 5Hz. To compensate for this drop, some VFDs can perform Motor Tuning, which matches the VFD settings to a particular motor. This tuning allows for more precise calculations by the VFD giving more torque throughout the CT curve, but particularly noticeable is the torque boost at those slow speeds from 1 to 10Hz. In HVAC, Motor tuning is only a slight advantage for fans and pumps, saving only a small portion of energy. Because of motor cooling, most fans and pumps have a minimum speed of 10Hz or greater. 57 of 98

58 Volts per Hertz Chart Voltage increases as speed increases CT linear relationship between volts and hertz 30Hz = 230V; 60Hz = 460V on 460VAC motor VFDs attempt to generate the correct level of torque needed for its application. Torque is directly related to current. In CT applications, voltage going from the VFD to the motor is increased in a linear manner as the speed increases. In the example above, on a 460Vac motor, 460 Volts is only sent to the motor when the speed reaches 60Hz and higher. The chart above displays the Volts/Hz relationship for a CT application. Constant Torque is achieved because the VFD is increasing the voltage to the motor as it increases the speed. In this chart, when the VFD sends a signal of 30Hz, it is also sending a 230V signal to the motor. When the VFD sends a 60Hz signal, the voltage is at 460V. This relationship keeps the current and in turn the torque to the motor relatively constant. The changes seen in the motor current are based on the load. Notice that when the speed reaches 60Hz, base speed, the voltage going to the motor from the VFD can not go higher, it has reached its limit, 460V. If the speed is increased above base speed, to say 90Hz, the voltage from the VFD stays at 460V, but the current and torque drop. This Volts/Hz chart is different for motors that use different voltages. 58 of 98

59 1. b) Constant Power (CP) CP applications are rare for the VFD, none in HVAC. CP applications are always above base speed. Torque drops significantly above base speed (60Hz). Constant Power applications are run at synchronous speed, 60Hz and higher. On a CT Curve, the area above synchronous speed is known as the Constant Power Curve. As speed continues above 60Hz, torque continues to drop. The few applications that require these high speeds are typically saws, and grinders, and no applications in HVAC. A point of clarification between synchronous speed and base speed is useful. Synchronous speed, in an induction motor, is the RPM of the magnetic field when the motor reaches its nameplate voltage and frequency. The formula for synchronous speed is as follows: Synchronous speed = (Motor frequency) * 120 / (# poles) Example: (60Hz * 120) / 4 poles = 1800RPM Base Speed is the speed in rpm that results when the motor is at its nameplate voltage, frequency and current. Most Asynchronous 4-pole induction motors using 60Hz have a base speed between 1725 to 1770rpm. 59 of 98

60 1. c) Variable Torque (VT) VT applications save a great deal of energy Used with centrifugal pumps and centrifugal fans. Variable Torque (VT) applications almost always involve centrifugal pumps and centrifugal fans. If a VFD manufacturer has an HVAC drive, it is most likely designed and programmed for VT applications only, such as the VLT 6000 from Danfoss. The torque required to operate a pump or fan is very low until it starts to approach its base speed, say 1750rpm at 60Hz. There are formulas that show the relationship of power to rpm. Ideally, assuming no friction losses, the pump at half the speed only requires 1/8 the power. VFDs on VT applications save a great deal of energy and money. Once above base speed, a fan or pump requires a considerable amount of power, so they are rarely run above 60Hz. 60 of 98

61 Automatic Energy Optimization (AEO) VT voltage lower at speeds than CT AEO - Reduces voltage until speed is effected 30Hz = 160V straight VT; 90V with AEO A couple of VFD manufacturers, such as Danfoss, offer Automatic Energy Optimization (AEO). This is used in all VT applications. This function automatically and continuously monitors and adjusts the output voltage to maximize energy savings. After the motor reaches the set speed, say 30Hz, the AEO function reduces the output voltage to the motor, if the load allows. When the load is light, the voltage is reduced perhaps from 160V to 90V. This reduced voltage not only saves energy, but reduces motor heating, motor noise, and increases efficiency. 61 of 98

62 2. Starting Torque Centrifuge This part of the lesson covers the application terminology associated with starting torque. Outline: VFD Application Terminology 1. Application Torque Curves 2. Starting Torque a) High Overload (HO) b) Breakaway Torque c) Normal Overload (NO) 3. Open or Closed Loop 4. Closed Loop & PID 5. Different Types of VFDs 62 of 98

63 2. a) High Overload (HO) High Overload allows 160% starting current for 1 min. Most CT applications require High Overload. If drive exceeds 160% or 1 minute, VFD trips. High Overload (160%) is used to apply the torque to start a load. The inertia to start a reciprocating compressor, a positive displacement pump or in the example above a hoist requires a great deal of starting torque. It can be as high as 160% of the current for 1 minute. Once started the torque drops back to 100% or less. If the load does not move and the drive exceeds 160% or exceeds 1 minute, the drive trips, which means that the VFD does not move the motor. This trip gives an Over-current or Torque Limit alarm and for protection sake a reset button must be pressed to clear the alarm and restart the VFD. Some VFDs have a High Overload limit of 150% for 1 minute. It is possible to use a High Overload or CT Start to get a fan or pump moving, but most HVAC applications do not require High Overload. 63 of 98

64 2. b) Breakaway Torque High Overload has a breakaway torque that allows 180% starting current for 0.5 seconds. If drive exceeds 180% or 0.5 seconds, VFD trips. Breakaway Torque (180%) also known as Starting Torque, usually refers to the first 0.5 seconds after start. In the example a mixer has a very thick substance, perhaps a batch of adhesive. When the VFD starts to turn the motor the resistance of the substance is very high. The VFD increases the current output up to 180% for half a second to get the mixer started. Once it starts, the VFD continues at the High Overload setting of 160% for 1 minute. If it does not move, the VFD trips for protection of the VFD, motor and mixer. This is rarely needed for HVAC applications. 64 of 98

65 2. c) Normal Overload (NO) Normal Overload has a starting torque that allows 110% starting current for 1 minute. Most VT applications require Normal Overload. VT VFDs are sized differently 200Hp for CT; 250Hp for VT Normal Overload (110%) is almost always used with HVAC applications basically with centrifugal fans and pumps. When a centrifugal fan is started it takes very little starting torque to turn the fan against air, so the 110% limit is adequate. If the 110% for 1 minute is exceeded, the VFD trips. Since the current output on starting is less for VT applications, smaller VFDs can operate slightly larger motors. If a 200Hp VFD which is normally Constant Torque (CT) has a VT setting, the VFD could be used to operate a 250Hp motor with little difficulty. 65 of 98

66 3. Open or Closed Loop All applications fall into 2 categories Open Loop or Closed Loop Closed loop has additional settings which include controller action, setpoint, and PID settings Outline: VFD Application Terminology 1. Application Torque Curves 2. Starting Torque 3. Open or Closed Loop a) Open Loop b) Closed Loop 4. Closed Loop & PID 5. Different Types of VFDs A quick definition for Open and Closed Loop is shown here. Open Loop control does NOT have a direct feedback signal. Closed Loop has a direct feedback signal coming into the VFD. 66 of 98

67 3. a) Open Loop The VFD is programmed for open loop, because there is NO feedback signal going to the VFD but the system is closed loop. Open Loop control does NOT have a direct feedback signal. In the example above, a static pressure sensor is wired to a Direct Digital Controller (DDC). Because the feedback signal, pressure, goes to the Controller, a closed loop must be setup in it to control the static pressure. This closed loop inside the Controller monitors the pressure comparing it against a setpoint and then calculates a reference signal. The Controller then sends this reference signal to the VFD for proper modulation of the fan. The reference signal is sent to the VFD, which in turn controls the motor and fan. In this example, the VFD is programmed for Open Loop and it receives a reference command from the Controller. If the pressure sensor were wired directly into the VFD, then the drive would need to be programmed for Closed Loop in order to have proper control of the fan. The system is a closed loop system because it senses its own action, its feedback, but because the feedback signal does not go directly to it, the VFD is programmed for open loop. 67 of 98

68 4) Closed Loop Sensor monitors the feedback signal. Controller, the VFD, gives the sensor a desired value, action and PID settings and calculates a response. Controlled Device, the fan, responds to the signal from the controller, increasing or decreasing in speed. Closed Loop is where the controller inside the VFD can be used to modulate the speed of a motor to maintain a process. It is the job of the sensor or transmitter to measure a variable, in the example above, this is static pressure. This sensor sends a signal (usually a 4-20mA) to the controller over a particular range, in this example from 0 to 5 wc. The sensor is wired to the current input of the VFD, the controller. This signal is programmed in the VFD as the feedback signal. This feedback signal must be given engineering units, from our example, inches of water column ( wc) or Pascals. Inside the VFD the feedback signal must also be given the range that matches the sensor. When the sensor sees 4mA it is 0 wc (0 Pascals) and when the sensor sees 20mA, the reading is 5 wc (1250 Pascals). The VFD, controller, has a setpoint, action and PID settings (explained later) to calculate the response necessary to maintain the desired value seen at the sensor. The VFD sends a signal to the fan to speed up or slow down to maintain a certain level. The fan, controlled device, responds to match the signal from the VFD, controller. It speeds up when the level of the static pressure decreases and slows down when the level of the static pressure is too high. 68 of 98

69 4. a) Control Action Normal Control incoming signal increases; VFD speed decreases. Inverse Control incoming signal increases; VFD speed increases. In closed loop applications, such as controlling pressure by monitoring the volume of return air, shown above, the VFD must always be programmed for the correct action. There are 2 choices: Normal Control (also know as Reverse Acting). In the top return fan example, when the CFM Transmitter sends a signal that indicates it is above the setpoint, the output from the VFD decreases. The action is correct if the sensor is down stream from the controlled device, the return fan. As the CFM sees an increase, the signal to the fan decreases, slowing the fan down. As the CFM transmitter sees a drop, the speed of the fan increases. Inverse Control (also know as Direct Acting). In the bottom return fan example, when the signal from the CFM Transmitter increases above the setpoint the output from the VFD increases. This action is correct if the CFM transmitter is upstream from the return fan. If the wrong action (Inverse) were programmed in the VFD on the top return fan, it would speed up as the CFM in the duct rose higher. Eventually the speed of the return fan would go to its maximum. This could put a major stress on the ductwork. If the incorrect action is programmed in an application the VFD ends up locked at minimum or at maximum speed. 69 of 98

70 4. b) Setpoint, Error & Gain Setpoint or reference is the desired value for a controlled variable; example: 2.00 wc (500pa). Error (offset) is the amount the feedback is from setpoint at any given time, example: 0.15 wc (38pa) Proportional Gain is a multiplier times the error (offset). Example above the Gain starts at 0.3 Setpoint is the desired value or reference of the controlled variable for a closed loop application. In controlling static pressure, shown above, 2.00 wc (500pa) is the desired setpoint or reference. Error is a difference between the setpoint, 2.00 wc (500pa) and the actual feedback pressure, 1.85 wc (462pa) for this system. This difference is also known as Offset (or off the setpoint). Proportional Gain is multiplied by the error to create an output. The larger the proportional gain, the larger the output change for a given error. If the proportional gain is too small (0.01) this multiplier times the error has no effect on the speed of the VFD. If the proportional gain is too large (3.00 or higher) the system can become unstable, modulating between minimum speed and maximum speed which is called hunting. A good starting place for the Proportional Gain is about 1.00 but changes depending on the application. If Proportional Gain is only used, without I or D (explained on the next slide), the output is relatively close to setpoint but there is always an error. 70 of 98

71 4. c) Integral & Derivative Integral checks the offset over a period of time and makes corrections. Derivative checks the offset and corrects for the speed of the integral correction. Integral Gain is based on the history of the error. The integral function maintains a running total of the error and creates an output based on this total. The lower the number used for the integral gain the more frequently the error is checked and the larger its influence. If the integral time is too low (less than 5 seconds), the system can become unstable and starts to hunt. A good place to start is 10 seconds, but again this can vary based on the application. Integral gain eliminates the steady error inherent to proportional control. As pictured above, integral gain adds overshoot every time the load changes, but eventually settles close to setpoint. Integral is used frequently in HVAC applications. Derivative or differential is based on the rate of change of the error. It is used to limit overshoot and dampen system oscillation. The larger the derivative time, the larger the influence. If the derivative time is too large, the system becomes unstable. Derivative is very sensitive to noise on the feedback signal and historically seldom used in HVAC closed loop systems. 71 of 98

72 4. b) PID Summary: There are many different PID algorithms. All contain these 3 features, Proportional Gain, Integral, and Derivative. The ways these terms are combined may be different. In some implementations the 3 values operate independently of each other and in others, changing one value effects the others. Proportional Gain contributes to the output in a direct relationship to the error between the feedback and setpoint. Integral contributes to the output based on the history of the error. Derivative contributes to the output based on the rate of change to the error. Setpoint is the desired value for the variable being controlled. Feedback is the actual value of the variable being controlled. Error (offset) is the difference between the setpoint and feedback Hunting is an unstable condition which causes the speed of the motor to continuously vary from maximum to minimum. 72 of 98

73 5) Different Types of VFDs There are different types of VFDs, which are briefly covered here. They include Volts/Hz, Voltage Vector, Voltage Vector +, Flux Vector and Servo. Volts/Hz drives also known as a Basic Scalar drives, are the least expensive drive with the least features. This drive is usually setup for CT and Open Loop only. Voltage Vector drives also known as Space Vector drives have more features and may have the ability to do both CT/VT and Open/Closed Loop. Voltage Vector Plus drives have more features than the others and therefore have more cost than previously mentioned drives. They can calculate motor characteristics without spinning the shaft of the motor. These last 3 types of VFD account for 95% of all applications. There are a few that need greater accuracy which follow. Flux Vector drives have more calculations which makes them more expensive and more accurate. Some Flux vector drives require special motors. Servo drives are DC and do not operate with AC induction motors. They are expensive in comparison to VFDs. They are the most responsive to dynamic changes. These drive types are explored in greater detail in a later lesson. 73 of 98

74 5) Different Types of VFDs Summary: Type Response Accuracy Speed Range V/Hz ms 10% + 10:1 180rpm Voltage Vector ms 2% - 8% 50:1 36rpm Voltage Vector ms 0.2%-2% Open 100:1 18rpm Closed 900:1 2rpm Flux Vector 5 60ms 0.02% - 0.1% 1400:1 1.3rpm Servo 2 8ms 0.007% % 1800:1 1rpm As a comparison above, each type of motor control shows, Response to dynamic changes Accuracy and Speed Range. Speed Range indicates how slow the motor can turn but still have proper operation. Speed Range is always indicated as a ratio. The higher the ratio the better control at slow speeds. The minimum rpm for each type has been calculated, assuming 1800 base speed. Voltage Vector Plus has 2 values, one for open loop and the other for closed loop. This completes this lesson. There are Review Questions in the Post-Test section. 74 of 98

75 VFD 101 Lesson 5 Power Input Terminology for a VFD This lesson covers the terminology associated with the incoming power to a Variable Frequency Drive (VFD) and the efforts to protect both the VFD and incoming power. When connecting a VFD to a system, there are 3 separate connections that are made: incoming power, motor wiring and control wiring. This lesson deals with the terminology concerning the first of these three, incoming power. Here is the basics outline for this lesson. VFD Power Input Terminology 1. Protecting the VFD a. Switching on the Input b. Surge/Sag c. Transients Spikes d. Phase Balancing e. Single Phase f. Fuses, connectors and voltages 2. Protecting the Supply Line a. Radio Frequency Interference (RFI) b. Harmonics, IEEE 519 c. Managing Harmonic Distortion 75 of 98

76 Incoming AC Power Inputs labeled L1, L2, and L3. R, S and T are older designations Terminals 91, 92 and 93 for Danfoss This lesson covers the terminology involved in connecting a Variable Frequency Drive (VFD) to incoming AC power. The terminals used to connect incoming power to the VFD are labeled L1, L2 and L3. Older designations label these as R, S and T. In the drive shown above both labels are shown, along with the terminal numbers for Danfoss which are 91, 92 and 93. When connecting the drive to incoming power the current needed by the drive is always the primary consideration. It is important that the transformer, wires and fuses can handle the full load amps of the drive. It is also very important that local electric codes are followed. The National Electric Code (NEC) requires branch protection and UL certification requires fuses be used with the drive. There are 2 major concerns when wiring incoming power to the drive. The first is that the VFD must be protected from any variants from the incoming supply line. The second is that the supply line must also be protected from noise made by the drive. Anytime diodes or SCRs are used to change AC into DC, switching occurs creating a non-linear load. This non-linear load can have an adverse effect (noise) on the supply power going to the VFD. In the notes that follow this noise is explained in more detail. 76 of 98

77 1) Protecting the VFD Numerous Stray Voltages and power changes from incoming power can effect the VFD. The first section covers the protection of the VFD from the changes in incoming power. The first item covered is the switching of power coming into the drive. There can also be Surges and Sags in the incoming AC power. If these surges are too high, or the sags are too low, the VFD shuts down for its own protection. There must also be protection against transients or spikes in the incoming power. Another item to be covered is a Phase Imbalance between the 3 phases of power coming into the drive. The last item to be covered are Fuses, connectors and different voltages which might be encountered. 77 of 98

78 a) Switching on the Input Maximum switching on input is 2 times per minute Soft Charge circuit heats up with excessive starts, and charging and discharging of the capacitors needs to be limited. Rapidly and repeatedly switching power on the input to the VFD can have a very negative effect on the drive. The power going into the drive requires the capacitors to be charged, which requires power to travel through a soft charge circuit. If this is started repeatedly, the current going through the current limiting resistors can cause the resistors to overheat and blow fuses on the soft charge circuit, disabling the drive for its own protection. The stress on the capacitors powering up during the ON cycle, then discharging during the OFF cycle can also shorten the life of the capacitors and also the life of the drive. For these reasons it is important that the drive be limited to just 2 starts per minute. If the drive needs to start and stop its motor repeatedly, it is best to always use the drive s control circuitry. 78 of 98

79 b) Surge and Sag Surge is an incoming voltage above its expected level Sag is an incoming voltage below its expected level Frequency shifts between 45Hz to 65Hz, can also occur. It is important that a VFD be able to operate even though the voltage is changing coming into the drive. With an increase in voltage, a surge, the VFD continues to run its load until an upper limit is reached. To protect itself, once the voltage exceeds this limit, the drive trips into an alarm condition. In the example above, 7 manufacturers are shown in the tolerance range for changes in the incoming voltages. Danfoss has one of the widest ranges concerning incoming voltage shifts. Notice that a 480Vac drive has a surge trip point of 550Vac. A drop in voltage, a sag, is far more common, particularly in the summer. In the example above, using a 480Vac drive, the sag trip point is 342Vac. As the voltage drops from 480Vac down, the VFD starts to drop its maximum output down from 100%. It continues to operate but at a lower level during these Brown-Out conditions. Once the voltage drops below 342Vac, the VFD trips to protect itself. If there is a slight frequency shift away from 60Hz in North America, or 50Hz in the rest of the world, the VFD must continue to operate. In both cases, if the voltage swings or if the frequency shifts, the wider the range the drive can handle the fewer nuisance shut-downs. In both cases, no one exceeds Danfoss in the range of voltage or frequency as indicated in the example above. 79 of 98

80 c) Transients and Spikes Spikes or Transients Metal Oxide Varistors (MOV) Another problem that might be encountered with incoming power is spikes or transients of voltage, which occur for a very short amount of time. These spikes may be caused by heavy loads in the main supply being switched ON then OFF, or even by lightning strikes. A VFD must have the capacity to handle these spikes in the same way that personal computers need surge protection. Some manufacturers, such as Danfoss, use a fast acting Metal Oxide Varistor (MOV), zenner diodes and oversized DC bus capacitors to provide protection against high potential spikes. The MOV, which looks like a small capacitor, is pictured above. When the voltage exceeds 2.3 times the expected incoming voltage for 1.3 milliseconds, the MOV shorts, protecting the internal parts of the drive. Danfoss places 4 of these MOVs in its drive, one on each of the 3 inputs and one attached to the DC Link. Danfoss drives are designed and built to meet a tough German specification for surge suppression (VDE 0160). 80 of 98

81 d) Phase Imbalance Voltage Imbalance on one of the Phases causes excessive stress on filter capacitors, so the VFD shuts down and sends out an alarm. The closer to maximum load of the VFD the more sensitive it becomes to an incoming voltage imbalance. Another problem with incoming power can be a phase imbalance between the 3 phases. A voltage imbalance on one of the phases causes excessive stress on filter capacitors. When the VFD sees an amount beyond 2%, the VFD shuts down and alarms the operator letting him know that there is a problem with the incoming power. This 2% imbalance becomes more crucial as the drive approaches its maximum load. The closer to maximum load the more sensitive the drive becomes to an incoming voltage imbalance. There are some parts of the world that almost always have a phase imbalance of at least 2% or more. Some VFD manufactures, such as Danfoss, allow the turning off of the phase imbalance protection for those locations. It must be remembered with a large constant phase imbalance, the expected life of the VFD is reduced. 81 of 98

82 e) Single Phase Input Some small VFDs are designed for single phase It is NOT recommended to place single phase on a 3-phase only - VFD Reduces life of filtering capacitors Single Phase into the drive is possible with some smaller size drives, usually 3Hp and under. Notice in the picture above single phase terminals are labeled N and L1. It is possible to use single phase input on a 3-phase drive when properly engineered, it is NOT officially recommended or supported by Danfoss. Performance and long term reliability problems may result. One of the biggest concerns with single-phase input is the effect it has on the filtering capacitors. 82 of 98

83 f) Fuses, Connectors and Voltages Fuses No Power factor correction capacitors Fuses must be placed between the VFD and the transformer. Instruction manuals for each size of VFD identifies the correct type and size of fuses needed for the installation. According to UL, fuses must be used with a VFD because of the quickness of fuse protection, when compared with circuit breakers. It should be noted that fuses do NOT completely protect the drive but do reduce the damage during a short circuit or other problem. On a retrofit, if power factor correction capacitors are found with the motor, and the customer wants them with the new VFD installation, they must be placed between the transformer and the VFD and never between the VFD and the motor. 83 of 98

84 f) Fuses, Connectors and Voltages Quick Connections are needed for quick replacement. Cable clamps are very helpful, but must also have a quick release. A note in passing is that most drives need quick release connectors to remove power, motor and control wiring. In the diagram above plugs are used to remove the incoming power. This is very important particularly in the industrial market where lost time is measured in the thousands of dollars per minute. Cable clamps are another helpful addition to drives. These clamps relieve the tension from the wire connectors. Incidental pulling of wires on those drives that do not have clamps can have a very adverse effect on the drives. 84 of 98