EXPERIMENT 5 CONTROLLING A SQUIRREL-CAGE INDUCTION MOTOR WITH SIMATIC S7-300 PLC

Transcription

1 YEDITEPE UNIVERSITY ENGINEERING & ARCHITECTURE FACULTY INDUSTRIAL ELECTRONICS LABORATORY EE 432 INDUSTRIAL ELECTRONICS EXPERIMENT 5 CONTROLLING A SQUIRREL-CAGE INDUCTION MOTOR WITH SIMATIC S7-300 PLC Introduction: The main objective in this experiment is to: Understanding the programmable logic controller and its peripherals. Programming the PLC with the STEP 7 software. Applying the PLC to drive a squirrel cage motor. Equipments: Table 1. List of Equipments Three-Phase Supply Unit with FCCB Measurement Indicator Magnetic Powder Brake Control Unit for the Magnetic Powder Brake Tachogenerator AC Current Transformer (2V/1A) PLC Basic Unit Squirrel-Cage Induction Motor, 1.6kW 3RT1016-1BB41 24V DC Relays (x2) 3SB A Normally Open (NO) Push-button switches (x3) Metra Hit 25S Multimeter (x 2) Siemens SIMATIC S7-300 General Information: THREE-PHASE INDUCTION MOTOR An induction motor is one in which alternating current is supplied to the stator directly and to the rotor by induction or transformer action from the stator. When excited from a balanced three-phase source, the stator winding will produce a magnetic field in the air gap rotating at synchronous speed as determined by the number of stator poles and the applied stator frequency f e. The rotor of a three-phase induction machine may be one of two types. A wound rotor is built with a three-phase winding similar to, any wound with the same number of poles as, the stator. The terminals of the rotor winding are connected to insulated slip rings mounted on the shaft as shown in Figure 1(a). Carbon brushes bearing on these rings make the rotor terminals available external to the motor. The second type is squirrel-cage rotor with a winding consisting of conducting bars embedded in slots in the rotor iron and shortcircuited at each end by conducting end rings. The three-phase induction motor with squirrel-cage rotor is shown in Figure 1(b). EE432 Industrial Electronics, Spring 2012 Experiment 5, page 1/9 Last updated March 29, :14 PM by D. Yildirim

2 brushes Three phase windings on rotor Slip rings (a) Short circuiting ring Aluminum rotor bars embedded in rotor iron (b) Figure 1. Cutaway view of a three-phase (a) wound-rotor and (b) squirrel-cage induction motor. EE432 Industrial Electronics, Spring 2012 Experiment 5, page 2/9 Last updated March 29, :14 PM by D. Yildirim

3 The quantity slip is the difference between the synchronous speed and the actual speed of motor, as measured in rpm. Slip is more usually expressed as a fraction of synchronous speed. The fractional slip s is ns n 120 f s, where ns (1) n p s Where f is the frequency of applied voltages, p is the pole number and n is the mechanical speed of rotor. The rotor terminals of an induction motor are short circuited; by construction in the case of a squirrel-cage motor and externally in the case of a wound-rotor motor. The rotating air-gap flux induces slip-frequency voltages in the rotor windings. The rotor currents are then determined by the magnitudes of the induced voltages and the rotor impedance at slip frequency. At starting, the rotor is stationary (n=0), the slip is unity (s=1), and the rotor frequency equals the stator frequency f e. The field produced by the rotor currents, therefore, revolves at the same speed as the stator field, and a starting torque results, tending to turn the rotor in the direction of rotation of the stator-inducing field. If this torque is sufficient to overcome the opposition to rotation created by the shaft load, the motor will come up to its operating speed. The operating speed can never equal the synchronous speed however, since the rotor conductors would then be stationary with respect to the stator field; no current would be induced in them, and hence no torque would be produced. MAGNETIC POWDER BREAK Magnetic powder break is mechanically connected to the three-phase induction motor for loading purposes. It applies breaking torque to the induction motor. The amount of breaking torque can be adjusted from the break control unit (732 55). Control unit also takes speed information from the tachogenerator (732 59) and displays the speed with an analog panel meter on the break control unit. SIMATIC S7-300 PLC SIMATIC S7-300 PLC will be used to drive the squirrel-cage induction motor. More information can be obtained from the Experiment 2 laboratory handout. TIMER TYPES Timer format: S5T# ah_bbm_ccs_ddms a. where a = hours, bb = minutes, cc = seconds, and dd = milliseconds b. The time base is selected automatically, and the value is rounded to the next lower number with that time base. For example: 500ms = S5T#500MS, 1min 10sec = S5T#1M_10S There are five different timer types in Simatic Ladder programming as depicted in Figure 2 where each timer starts with an input signal. EE432 Industrial Electronics, Spring 2012 Experiment 5, page 3/9 Last updated March 29, :14 PM by D. Yildirim

4 CONTACTOR Figure 2. Timer types A contactor is a simple electromechanical relay that is used to connect loads to the power supplies the contactor illustrated in Figure 3 will be used to connect a threephase induction motor to a three-phase voltage source in this experiment. This contactor has four normally open (NO) contacts and contacts are activated (closed) by applying 24V DC voltage to terminals A1+ and A2- (relay coils). A freewheeling diode must be connected across the relay coil terminals (cathode of diode is connected to A1+ and anode is connected to A2-) in order to prevent PLC output from destructive inductive-kick overvoltages. EE432 Industrial Electronics, Spring 2012 Experiment 5, page 4/9 Last updated March 29, :14 PM by D. Yildirim

5 Figure 3. Contactor for switching motor (see Appendix A for data sheet) Procedure of Experiment: Circuit Set-up: Assemble the circuit shown in Figure 4. First, connect the power circuit cables of the motor. Then, connect the control circuit cables for an easy set-up. Do not forget to connect the PE (ground) connections! Connect the 24 V DC Relay to the three-phase Supply Unit (726 75). Connect a multi-meter to measure the line current. Connect the Relay to the stator windings of the induction motor. The windings connections can be plugged to the connection box on the induction motor. The stator must be WYE connected. Connect the tachogenerator to the breaker control unit (732 55). Make sure that the torque knob in the brake unit is at minimum, i.e., rotate all the way to the CCW direction. Connect the NO and NC switches to the digital input module of the PLC via PLC Basic Unit ( ). For each switch, connect one end to 24 V socket on PLC Basic Unit, and connect other end to the corresponding input sockets on PLC Basic Unit. Connect the voltage output of current sensor (735 24) to the analog measurement indicator. Connect the PLC output from the PLC Basic Unit ( ) to the 24 V DC Relay. Connect A2- end of relay to zero volt on PLC Basic Unit and connect other end to the corresponding output socket. Open the STEP 7 software to program the PLC: Make an online connection between the PLC and the software by running the command from File/Connect Online. Create a new project. Then, in Program/OB1 write your code in FBD/LD or STL. Right Click on OB1 and choose Download to CPU command. When file transfer is finished, switch PLC to RUN mode. EE432 Industrial Electronics, Spring 2012 Experiment 5, page 5/9 Last updated March 29, :14 PM by D. Yildirim

6 Figure 4. PLC control of induction motor operation.. EE432 Industrial Electronics, Spring 2012 Experiment 5, page 6/9 Last updated March 29, :14 PM by D. Yildirim

7 1. Unlatched Circuit Configuration with Indicator: Write a program in STEP 7 such that when S 1 button is pushed on momentarily, it will start the motor, and when S 2 button is pushed on momentarily, it will deenergize motor. When motor is operating, the indicator light L d must be turned on continuously, otherwise it must be in off position. Motor must be initially loaded with minimum torque, therefore, torque adjustment knob must be set to low value. Start the motor and write down the starting current (peak value), running current (using the analog measurement unit), torque, and speed in Table 1. Load the motor using the brake unit by turning the torque adjustment knob slowly in CW direction. Monitor the motor current and stop when the motor current reaches the rated value written on the nameplate. Stop the motor. Restart the motor with this loaded condition and write down the starting current (peak value), running current (using the analog measurement unit), torque, and speed in Table Delayed Circuit Configuration with Indicator: Write a program in STEP 7 such that when S 1 button is pushed on momentarily, it will start the motor, and when S 2 button is pushed on momentarily, it will deenergize motor. The motor will either start or stop (depending on the previous operating condition) after predetermined delay time when S 3 button is pushed on momentarily, i.e., if the motor is operating, pushing S 3 button will stop the motor after 10 seconds. If motor is at standstill, pushing S 3 button will start the motor after 6 seconds. When starting the motor using S 3 button, the indicator light L d must be flashing (0.3sec on, 0.8sec off) during the delay time. When motor is energized after this delay (in operating condition), the indicator light L d must be continuously turned on. Once S 3 button is pushed on for stopping the motor, the indicator light L d must be flashing (0.4sec on, 0.5sec off) during delay time and must be turned off when motor is denergized. Conclusion: Bidirectional operation of an induction motor with Star-Delta starting will be realized in your report. Use the induction motor nameplate values available in the Industrial Electronics laboratory. Selection of the contactors will be based on for this motor. There will be three push buttons (normally open type) for controlling: one stop button, one clockwise (CW) button and one counterclockwise (CCW) button. Pressing either CW or CCW button will start the motor in Star-Delta configuration, i.e., stator windings first will be connected in WYE (star) and then three-phase voltage will be applied to stator windings. 10 seconds later stator windings will be switched from WYE connection to DELTAconnection. You must make sure that all safety precautions are implemented in your program so that no awkward operations result in. One example of such an operation is short-circuiting of three-phase supply when both EE432 Industrial Electronics, Spring 2012 Experiment 5, page 7/9 Last updated March 29, :14 PM by D. Yildirim

8 CW and CCW are pressed on. Please note that there may be other unwanted situations. a) Describe the operation of the system and draw electrical connection diagram indicating three-phase supply, induction motor, and contactors. b) Draw single-line relay logic diagram for this operation. c) Write a PLC program in ladder diagram form to run this operation properly: Determine the I/O addresses of the PLC which you use for this device. Determine the number of networks and function of each network. d) Determine all the contactor types (specify model number) and select proper contactors from a manufacturer s data sheet. Compare the type of contactor (i.e., number of poles) requirements both for relay logic realization and for PLC implementation. Include data sheet of these contactors to your report (PLC realization only). References: [1] T. E. Kissell, Industrial Electronics: Applications for Programmable Controllers, Instrumentation and Process Control and Electrical Machines and Motor Controls, Prentice Hall, 3 rd edition, [2] T. J. Maloney, Modern Industrial Electronics, 5th Ed., Prentice Hall, [3] S. J. Chapman, Electric Machinery Fundamentals, 4th ed., McGraw-Hill, [4] A. E. Fitzgerald, C. Kingsley, S. D. Umans, Electric Machinery, 6th ed., McGraw-Hill, [5] T. Bartelt, Industrial Control Electronics, 3 rd ed., Thomson Delmar Learning, EE432 Industrial Electronics, Spring 2012 Experiment 5, page 8/9 Last updated March 29, :14 PM by D. Yildirim

9 E X P E R I M E N T R E S U L T S H E E T This form must be filled in using a PEN. Use of PENCIL IS NOT ALLOWED EXPERIMENT 5: CONTROLLING A SQUIRREL-CAGE INDUCTION MOTOR WITH SIMATIC S7-300 PLC STUDENT NO STUDENT NAME SIGNATURE DATE 1 2 INSTRUCTOR APPROVAL 3 4 starting current (A) Table 1: Motor operating with minimum load running current (A) torque (Nm) speed (rpm) Table 2: Motor operating at rated load starting current running current torque (A) (A) (Nm) speed (rpm) Please include flowchart of your program in here indicating inputs and outputs (ladder diagram code must be included in your report). EE432 Industrial Electronics, Spring 2012 Experiment 5, page 9/9 Last updated March 29, :14 PM by D. Yildirim