Welcome to Linear Controls Quarterly Training
Introduction to Power Generation
Objectives Supply attendees with basic knowledge of power generators and voltage regulators and provide the fundamentals of generator and voltage regulator field diagnostics.
A Few Words About Safety PLEASE REMEMBER SAFETY FIRST. If you are not sure of the instructions or procedures, seek qualified help before continuing. Before any service work is done, disconnect all power sources and, where appropriate, lock out all controls to prevent an unexpected startup of the generator set. Proper grounding in compliance with local and national electrical codes must be provided. These safety precautions are necessary to prevent potential serious personal injury, or even death.
A Few Words About Safety
A Few Words About Safety Whenever the generator is running, always assume and proceed as if voltage is present. Residual voltage is present at the generator leads and at the regulator panel connections, even with the regulator fuse removed. Caution must be observed. Otherwise, serious personal injury or death can result. Whenever solvents, cleaners, or flammable liquids are present, adequate ventilation must be available to avoid fire, explosion, and health hazards. Always avoid breathing vapors and use suitable personal protective equipment to prevent personal injuries (such as eyes, face, and hand protection). Repairs should only be attempted by qualified, trained people. Each installation will create its own set of circumstances. No manual can cover every possible situation.
Generator Basics In 1831 a scientific discovery was made which is considered one of the most important findings in scientific history. Mr. Michael Faraday discovered that if a conductor is moved through a magnetic field, an electrical voltage is generated. The magnitude of the induced voltage is directly proportional to the rate of change and the strength of the magnetic field. This natural phenomenon is identified as generator action and the law by which it is governed, is called Faraday s Law of Electro Magnetic Induction. We make use of Faraday s law of generator action by building a rotating electrical mass whose purpose is to change mechanical energy into an electrical alternating current.
Generator Basics There are many different types of generators: DC, Induction, Synchronous, etc. Each type has its own unique application, but all utilize the same basic principles of generator action. In this discussion we are most concerned with synchronous machines, their properties, characteristics, and performance.
Components
Components A synchronous machine consists of a stationary armature winding with many coils connected together to obtain a desired generator terminal voltage. The armature winding is placed into a slotted laminated steel core, with good magnetic properties. The number of slots is generally based upon a symmetrical polyphase winding where the coils of the windings are displaced by 120 electrical degrees.
Components A synchronous machine also consists of a rotor, which is a revolving field. Its function is to produce magnetic lines of force whose flux lines cut and induce an EMF (electromagnetic field) into the coils of the stator. The revolving field is essentially an even set of laminated pole cores with coils of wires embedded around the poles to create an exciting field. A DC current is fed into the revolving field by means of slip rings from a brush type rotary or static exciter, or more commonly today, a directly coupled rotating brushless exciter. The revolving field is designed for standard DC voltages of 63, 125, 250, and on very large machines 375 volts.
Electrical Flow with PMG
Electrical Flow with PMG The flow begins with PMG. 1. When rotating, the magnetic fields from the PMG rotor induces an A/C voltage into the PMG stator which flows to the voltage regulator. 2. The voltage regulator converts the A/C voltage to a D/C voltage which flows to the exciter stator. 3. The magnetic fields of the exciter stator induces an A/C voltage into the exciter armature with flows to the rectifier. 4. The rectifier converts the A/C to a D/C voltage that flows to the main rotor. 5. The magnetic fields of the main rotor induces an A/C voltage into the main stator that flows out through the output leads. 6. The voltage regulator samples the generator output A/C voltage and adjusts the D/C voltage output to the exciter stator to control the generator output voltage.
Electrical Flow without PMG
Electrical Flow without PMG Without a PMG the generator relies on the residual magnetism of the exciter stator to start the flow. 1. The residual magnetism of the exciter stator induces an A/C voltage into the exciter armature that flows to rectifier. 2. The rectifier converts the A/C voltage to a D/C voltage that flows to the main rotor. 3. The magnetic fields of the main rotor induces an A/C voltage into the main stator that flows out through the output leads. 4. The voltage regulator uses the generator output A/C voltage for its power supply and samples it to adjust the D/C voltage output to the exciter stator to control the generator output voltage.
PMG PMG stands for permanent magnet generator. The basic components of the PMG are the: 1. Rotor which contains the permanent magnets. 2. Stator which contains the field windings and the output leads.
PMG Depending on make and model of the generator you are servicing, the PMG may look very different from others.
Advantages of a permanent magnet generator: Provides an economical and simple means of reliable, responsive and stable input power to the voltage regulator. Enhances manual voltage control regulation as the PMG provides a more stable power source to the manual control. Provides full exciter power, regardless of alternator voltage, for motor starting and is a separate voltage source for use external to the generator set, such as a tachometer and relay options.
How the PMG Works The PMG exciter provides input power to the voltage regulator on a brushless revolving field generator to help maintain rated output voltage during sudden load changes. The PMG is a separate power source for the voltage regulator. In the event of a sudden load change due to a motor starting application or even a sudden short-circuit condition on the generator output, the PMG will supply rated voltage to the regulator. This will force the generator into saturation and supply the necessary output current to start the motor or clear the fault condition. The generator will produce up to 300% or more short-circuit current during a threephase fault condition. This is normally more than enough current to trip a properly sized circuit breaker.
How the PMG Works A revolving field brushless generator obtains excitation from a direct-connected brushless exciter. The voltage regulator regulates the generator output voltage by automatically regulating the DC current fed to the exciter field. The constant output voltage from the PMG pilot exciter is fed to the brushless exciter field. When the generator rotor begins to turn, the PMG rotating magnetic field produces voltage in the PMG stationary armature winding. The output voltage from this PMG armature winding is then used to power the voltage regulator.
How the PMG Works The rotating permanent magnet field assembly is mounted on the generator shaft. As shown in Figure 2, the PMG armature is mounted outboard for easy removal. Typically both the PMG and the exciter are mounted in the same frame. The PMG will provide constant power to the voltage regulator through a wide range of transient conditions. The only external input required is rotational energy from the prime mover.
Rectifiers There are many styles of rectifiers. They are typically made up of the same components, with the simple purpose of converting A/C voltage to D/C voltage.
Rectifiers Below is an exploded view of a rectifier
How the Rectifier works As the induced voltage flows from the exciter armature (rotor) to the main rotor it must pass through the rectifier diodes (Normally three standard and three reverse) causing the A/C voltage to be converted to pulsating D/C voltage.
How the Rectifier works This type of rectifier is referred to as a full-wave bridge rectifier. Full-wave rectification Bottom half of the wave is flipped and not clipped Yet still is pulsating DC Diode arrangement is called a bridge Diodes mounted on a heat sink
Diode Testing Some generators maybe equipped with an integrated rectifier instead of individual diodes and plates. The function and testing is electrically the same.
Diode Testing To check an ordinary silicon diode using a digital multimeter, put the multimeter selector switch in the diode check mode. Connect the positive lead of multimeter to the anode and negative lead to cathode of the diode. If multimeter displays a voltage between 0.5 to 0.8, we can assume that the diode is healthy. This is the test for checking the forward conduction mode of diode. The displayed value is actually the potential barrier of the silicon diode and its value ranges from 0.5 to 0.8 volts depending on the temperature. The voltage for a germanium diode test voltage is slightly lower, between 0.3 0.5 volt.
Diode Testing Now connect the positive lead of multimeter to the cathode and negative lead to the anode. If the multimeter shows an infinite reading (over range), we can assume that the diode is healthy. This is the test for checking the reverse blocking mode of the diode.
Surge Suppressors The surge suppressor is a type of varistor, metal oxide. There many shapes and sizes of varistors depending on voltage and current demands.
How the Surge Suppressor Works A surge suppressor is a voltage dependent variable resistor. When disconnected or operating at a normal voltage level the surge suppressor has a very high resistance, near infinity.
How the Surge Suppressor Works When a surge or spike causes voltage rises above a predetermined point. The resistance of the suppressor begins to fall. This is know as clamping voltage. As long as the voltage on the line stays above clamping voltage the suppressor allows current to flow through it shunting the excess voltage.
Testing Surge Suppressors To test a varistor by the book requires equipment that you will not likely find on a production platform. However there are ways to determine whether or not the surge suppressor is the cause of problems in your generator.
Testing Surge Suppressors If you disconnect and check the resistance of a surge suppressor with a digital volt meter and the meter doesn t read out off limits. The suppressor is likely failed (internally shorted). If the generator has symptoms that are not spike or surge related and you suspect a failed suppressor. Remove the surge suppressor and retest the generator. If the problem is resolved with the suppressor removed, replace the suppressor with new replacement. If the problem still exists with the suppressor removed reinstall the suppressor continue with your diagnostics.
Main Rotor Field AC Impedance Test Theory: The main rotor resistance can be measured with a very accurate meter that is able to measure low (1 ohm) resistance, but it is difficult to determine if there are turn-to-turn shorts in the field pole windings. One shorted turn would only change a resistance reading on the order of one half of one percent. The AC impedance test measures the impedance (inductance and resistance) of the field pole coils. Shorted turns in the field pole windings change the coil inductance to a much greater degree than the resistance.
Main Rotor Field AC Impedance Test Step 1: The rotor must be supported on a nonmagnetic surface such as a wooden skid. Do not use a steel table that would create a magnetic short circuit between the poles. Step 2: Apply 120 volts AC to disconnected main rotor leads F1 and F2. Step 3: Measure and record voltages across each pole. Between points A and B, B and C, C and D, and D and E Step 4: The voltage readings should balance within one volt. Results: If the AC voltages are not balanced (30V ± 1V AC with 120V AC input) across each pole, the winding has shorted turns and should be rewound.
Megging Basics Insulation resistance is a measurement of the integrity of the insulating materials that separate the electrical windings from the generator s steel core. This resistance can degrade over time or due to contaminants (dust, dirt, oil, grease, and especially moisture). Most winding failures are due to a breakdown in the insulation system. In many cases, low insulation resistance is caused by moisture collected when the generator is shut down. The problem can be corrected simply by drying out the windings.
Megging Basics Normally the resistance of the insulation system is on the order of millions of ohms. It is measured with a device called a megger which is a megaohm meter (meg is for million) and a power supply. The power supply voltage varies, but the most common is 500 volts. A megger voltage over 500 is not recommended, except for measuring medium voltage (2400/4160) stators only.
Megging Basics The minimum acceptable value can be calulated using the following formula. Minimum Insulation Resistance = Gen. Voltage / 1000 + 1 Example: For a 480 volt generator 480 / 1000 + 1 = 1.48 Megohms If the reading is below the recommended value, the winding must be dried out or repaired.
Megger Types The two commonly use meggers used in the production field are the analog hand crank and the digital battery operated. This is an example of an analog hand crank type. The hand crank must be manually rotated to produce the output voltage for megging.
Megger Types This is an example of a digital megger. They typically resemble a digital multimeter and often have some the same basic functions. The output voltage for megging is produced by the internal batteries.
Megging Exciter Stator 1. Disconnect the F1 and F2 leads form the voltage regulator. Never subject the regulator to a megger. 2. Connect the F1 and F2 leads together to the positive megger lead. 3. Connect the negative megger lead to the generator frame. 4. Take the meg reading and record it. 5. If the reading is below the minimum spec the exciter stator must be dried or repaired.
Megging- Exciter Rotor 1. Disconnect the exciter rotor winding leads from the rectifier assembly. 2. Connect all the leads together to the positive megger lead. 3. Connect the negative megger lead to the generator frame. 4. Take the meg reading and record it. 5. If the reading is below the minimum spec the exciter stator must be dried or repaired.
Megging Main Stator Phase to Ground 1. Before megging, the ground T0 must be isolated. Make sure that L1,L2 and L3 are isolated from the bus and ground, also any external wiring must be disconnected from the stator leads. 2. Connect the positive megger lead to L1. 3. Connect the negative megger lead to the generator frame. 4. Take the meg reading and record it. 5. Repeat on L2 and L3. 6. If any of the meg reading are below the minimum spec the main stator must be dried or repaired.
Megging Main Stator Phase to Phase 1. Before megging, each phase ground must be separated and isolated. Make sure that L1,L2 and L3 are isolated from the bus and ground, also any external wiring must be disconnected from the stator leads. 2. Connect the positive megger lead to L1. 3. Connect the negative megger lead to L2. 4. Take the meg reading and record it. 5. Repeat for L1 to L3 and L2 to L3. 6. If any of the meg reading are below the minimum spec the main stator must be dried or repaired. Note: phase to phase megging is not possible on generators that the grounds are hard wired in generator windings. ie. 4 lead and 10 lead generators.
Megging PMG Stator 1. Disconnect both PMG leads form the voltage regulator. Never subject the regulator to a megger. 2. Connect both leads together to the positive megger lead. 3. Connect the negative megger lead to the generator frame. 4. Take the meg reading and record it. 5. If the reading is below the minimum spec the PMG stator must be dried or repaired.
Exciter Field Flashing Restoring residual magnetism/field flashing The direct current necessary to magnetize the revolving field is obtained from the exciter. Upon starting the generator, current and voltage is induced into the exciter by the magnetic lines of force set up by residual magnetism of the exciter field poles. Residual magnetism of the exciter Field poles may be lost or weakened by a momentary reversal of the field connection, a strong neutralizing magnetic field from any source, or nonoperational for a long time. If the generator fails to generate voltage after it has come up to rated speed, it may be necessary to restore residual magnetism.
Exciter Field Flashing 1. Open the output circuit breaker, and stop the engine. 2. Disconnect the exciter field coil wires F1 at the terminal F1 and F2 at the terminal F2, and connect the battery positive lead to the field coil lead F1. 3. Flash the field by momentarily touching the battery lead to the field coil circuit terminal F2. 4. Disconnect the battery leads. 5. Reconnect the field coil lead F1 to terminal F1, and reconnect the field coil lead F2 to terminal F2. 6. Start the generator, and check for voltage build up. Reflash if the generator output voltage does not build up, or flash with the generator running, the field coil wires connected to the regulator, and a 3-amp or larger diode off the positive terminal of the battery.
Exciter Field Flashing Do not flash the exciter with the generator running with out a diode between the battery positive and F1. Voltage back feed can cause the battery to explode. Do not firmly connect the negative battery lead to F2, only momentarily touch the negative lead to F2.
WYE Connections
WYE Connections
WYE Connections
WYE Connections
WYE Connections
WYE Connections
WYE Connections
Center Tap Connections The diagram shows the taps configured for high volt output. Typically 480 volts. The voltage reading phase to phase would read 480 vac. The voltage reading phase to ground would read 277 vac. By tapping onto T4&T7 or T8&T5 or T9&T6 you would read: Phase to phase 240 vac. Phase to ground 120 vac. By using center taps you can achieve 240 vac single phase, 120 vac single phase, and 120 vac three phase from a generator with a 480 vac three phase output.
Center Tap Connections The most common uses of centers taps are: 1. Voltage sensing for regulators, meters and end devices. 2. Voltage regulator supply power and panel control voltage supply. Warning: Since the center tap is connected before any current measuring devices only applications with very small current loads should be used with center tapping.
Voltage Regulators There are many voltage regulators available depending on the generator requirements. Most regulators offer the same basic functions and connections. For this discussion we will be referencing the Kato KCR 360 and Basler SR4.
Voltage Regulators The typical voltage regulator connections consist of : 1. Power supply 2. Sensing 3. Exciter output 4. Paralleling circuit ( if equipped)
Voltage Regulator Operation The voltage regulator uses an A/C voltage for it s power supply. The regulator converts this A/C voltage to a D/C voltage for the output to the generator exciter. The regulator automatically varies the D/C voltage output based on A/C voltage inputs to the sensing circuits from the generator and amperage inputs to the paralleling circuits( if equipped ). D/C voltage output can be manually varied by the internal voltage adjust rheostat and/or external voltage adjust rheostat ( if equipped ).
Voltage Regulator Bench Test The Voltage Regulator Bench Test can be used to test the basic function of a voltage regulator. Most voltage regulator operators manuals provide bench test instructions. Most operators manuals can be down loaded free of charge via the manufactures website. Bench testing can save time and money when dealing with a suspected failed regulator.
Voltage Regulator Bench Test (SR4A) 1. Move the wire on the sensing transformer (T1) to the terminal listed below: SR4A: Move to 120 V tap. SR8A: Move to 240 V tap. 2. Adjust the voltage stability potentiometer (R4) fully counter-clockwise (CCW). 3. Connect the voltage regulator as shown on the next page. The bulb should be 120 V and not more than 300 W. See Note 1 of the drawing for the SR8A. 4. Adjust the voltage adjust potentiometer for maximum resistance. 5. Connect the regulator to the power source. The bulb should flash on momentarily and then extinguish. 6. Slowly adjust the voltage adjust rheostat toward minimum resistance. The light bulb should reach full brilliance before minimum resistance is attained. (If the light does not illuminate, adjust the centering adjustment (R3). 7. At the regulating point, a small change in the voltage adjust potentiometer should turn the light bulb on or off. Note: If the light stays on, the regulator is defective. 8. This test may not reveal a stability problem, however, rotating the stability adjustment (R4) should affect the light's turn on/turn off time. 9. Before reinstalling the voltage regulator into the system, reconnect the sensing transformer (T1) as it was before performing Step 1.
Voltage Regulator Bench Test (SR4A)
Voltage Regulator Bench Test (KCR 360) Voltage regulator operational test: Use the following test procedure to determine if the regulator is basically operational: 1. Connect regulator as shown on the next page. 2. Connect internal wire from terminal E-3 to the 120 volt tap on sensing transformer T1. 3. Connect jumper across terminals CT1 and CT2. 4. Adjust the external voltage adjust for maximum resistance (complete counterclockwise position). 5. Connect light bulb across terminals F+ and F- and wires to terminal El, E3, P1 and P2 as shown in Figure 9. 6. Connect to 120 Vac power source. 7. Turn the external voltage adjust clockwise. Before reaching the maximum clockwise position the bulb should come on to near full brilliance. 8. At the regulating point a small change in adjustment of the external voltage adjust rheostat should turn the light on or off. If the light does not come on or stays on at full brilliance, the regulator is probably defective. 9. Before re-installing in generating system, connect regulator as it was before steps 2, 3, and 4.
Voltage Regulator Bench Test (KCR 360)
Generator Drying Electrical components must be dried before placing in operation if tests indicate that the insulation resistance is below a safe value. Machines that have been idle for sometime in unheated and damp locations may have absorbed moisture. Sudden changes in temperature can cause condensation or the generator may have become wet by accident. Windings should be dried out thoroughly before being put into service.
Generator Drying Space Heaters Electric space heaters can be installed inside of the generator. When energized (from a power source other than the generator), they will heat and dry the inside of the generator. If an alternate source of electricity is not available, enclose the generator with a covering and insert heating units to raise the temperature 15 18 F (8 10 C) above the temperature outside of the enclosure. Leave a hole at the top of the enclosure to permit the escape of moisture.
Generator Drying Oven Place the machine in an oven and bake it at a temperature not to exceed 194 F (90 C). The voltage regulator and any electronic component accessories must be removed from the generator when using this method. Forced Air A portable forced air heater can be used by directing heat into the air intake (conduit box) and running the generator with no load and without excitation (this can be accomplished by removing the regulator fuse). Heat at point of entry should not exceed 150 F (66 C).
Generator Drying Short Circuit Method 1. The generator can be dried out quickly and thoroughly by using this method. 2. Disconnect exciter leads F1 and F2 from the regulator. 3. Connect a battery or other DC power source of approximately 20 35 volts to the exciter leads F1 and F2. An adjustable voltage source is desirable, however a rheostat (rated approximately 2 amps) in series with the DC power source will work. 4. Short circuit the generator output lead wires to each other (L1 to L2 to L3). If using jumpers, be sure they are large enough to carry full load amperage. 5. Start the generator and measure the current through the output leads with a clip-on ammeter. 6. Adjust the voltage source to produce approximately 80% of the rated AC nameplate amps, but in no case exceed nameplate amps. If an adjustable source is not available and current is excessive, use a lower DC source voltage or a larger resistor in series with the source. 7. Running time will be determined by the amount of moisture present in the machine. Insulation resistance checks should be taken every one to four hours until a fairly constant value is obtained. 8. After the generator is dry and the insulation resistance is brought up to specifications, remove the short circuit from the line leads, disconnect the DC source, and reconnect the F1 and F2 leads at the regulator. Be sure all connections are tight and correct before attempting to run the generator.
Formulas