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Basic Electrical Technology (DET 211/3 - Hardware Version) Laboratory Module EXPERIMENT 5 TITLE: AC INDUCTION MOTORS – SINGLE PHASE INDUCTION MOTORS OBJECTIVES To demonstrate the main operating characteristics of single-phase induction motors using the Capacitor-Start Motor module. EQUIPMENTS EMS Workstation Model 8110, Four Pole Squirrel-Cage Induction Motor Model 8221, Capacitor-Start Motor Model 8251, Capacitive Load Model 8331, Power Supply Model 8821 and Data Acquisition Interface Model 9062. INTRODUCTION It is possible to obtain a single-phase squirrel-cage induction motor using a simple electromagnet connected to a single-phase ac power source as shown in Figure 5.1. Figure 5.1 Simple Single-Phase Squirrel-Cage Induction Motor The operating principle of this type of motor is more complex than that of the three-phase squirrel-cage induction motor. The simple induction motor of Figure 5.1 can even be considered as an eddy-current brake that brakes in an intermittent manner since the sinusoidal current in the stator electromagnet continually passes from peaks to zeros. However, when the rotor of the simple induction motor of Figure 5.1 is turned manually, a torque which acts in the direction of rotation is produced and the motor continues to turn as long as ac power is supplied to the stator electromagnet. This torque is due to a rotating magnetic field that results from the UNIVERSITI MALAYSIA PERLIS (UniMAP) – Exp5 1 Basic Electrical Technology (DET 211/3 - Hardware Version) Laboratory Module interaction of the magnetic field produced by the stator electromagnet and the magnetic field produced by the currents induced in the rotor. A graph of speed versus torque for this type of motor is shown in Figure 5.2. The curve shows that the torque is very small at low speeds. It increases to a maximum value as the speed increases and finally decreases towards zero again when the speed approaches the synchronous speed ns. Figure 5.2 Speed versus Torque Characteristics of a Single-Phase Induction Motor The low torque values at low speeds are due to the fact that the currents induced in the rotor produce magnetic fields that create forces which act on the rotor in various directions. Most of these forces cancel each other and the resulting force acting on the rotor is weak. This explains why the single-phase induction motor shown in Figure 5.1 must be started manually. To obtain torque at low speeds (starting torque), a rotating magnetic field must be produced in the stator when the motor is starting. Figure 5.3 Adding a Second Electromagnet to the Simple Induction Motor of Figure 5.1 UNIVERSITI MALAYSIA PERLIS (UniMAP) – Exp5 2 Basic Electrical Technology (DET 211/3 - Hardware Version) Laboratory Module Figure 5.3 shows the simple induction motor of Figure 5.1 with the addition of a second electromagnet placed at right angle to the first electromagnet. The second electromagnet is identical to the first one and is connected to the same ac power source. The currents I1 and I2 in the electromagnets (winding currents) are in phase because the coils have the same impedance. However because of the inductance of the coils of the electromagnets, there is a phase shift between the currents and the ac source voltage as illustrated in the phasor diagram of Figure 5.3. Since currents I1 and I2 are in phase, there is no rotating magnetic field produced in the stator. However, it is possible to phase shift current I2 by connecting a capacitor in series with the winding of electromagnet 2. The capacitance of the capacitor can be selected so that current I2 leads current I1 by 90° when the motor is starting as shown in Figure 5.4. The capacitor creates the equivalent of a two-phase ac power source and allows the motor to develop starting torque. Figure 5.4 Adding a Capacitor Allows the Induction Motor to Develop Starting Torque UNIVERSITI MALAYSIA PERLIS (UniMAP) – Exp5 3 Basic Electrical Technology (DET 211/3 - Hardware Version) Laboratory Module PROCEDURE CAUTION High voltages are present in this laboratory exercise! Do not make or modify any banana jack connections with the power on unless otherwise specified! 1. Install the Power Supply, Data Acquisition Interface, Four-Pole Squirrel Cage Induction Motor, Capacitor-Start Motor and Capacitive Load modules in the EMS Workstation. 2. Make sure that the main switch of the Power Supply is set to the O (OFF) position and the voltage control knob is turned fully counter clockwise (ccw). 3. Ensure that the DAI LOW POWER INPUT is connected to the main Power Supply, set the 24V-AC power switch to the I (ON) position. 4. Display the Metering application. 5. Connect the equipment as shown in Figure 5.5. Figure 5.5 Three-Phase Squirrel-Cage Induction Motor Two-Phase and Single-Phase Operation of a Three-Phase Squirrel-cage Induction Motor 6. Turn on the Power Supply and set the voltage control knob so that the voltage applied to each of the motor windings (indicated by meter E1) is equal to the nominal voltage of these windings. Note: The nominal voltage and current of the windings of the Four-pole Squirrel-Cage Induction Motor are indicated on the module front panel. Does the squirrel-cage induction motor start readily and rotate normally? Why? UNIVERSITI MALAYSIA PERLIS (UniMAP) – Exp5 4 Basic Electrical Technology (DET 211/3 - Hardware Version) Laboratory Module 7. In the Phasor Analyzer window, select voltage phasor E1 as the reference phasor then select proper sensitivities to observe voltage phasor E1 and current phasors I1, I2 and I3. These phasors represent the ac source line-to-neutral voltage and the line currents in the three-phase squirrel-cage induction motor. Are phasors I1, I2 and I3 all equal in magnitude and separated by a phase angle of 120° thus showing they create a normal rotating magnetic field? Why? 8. Turn off the Power Supply. Open the circuit at point A shown in Figure 5-5. Make sure that VOLTAGE INPUT E1 of the DAI module remains connected to the ac power source. 9. Turn off the Power Supply. Does the squirrel-cage induction motor start readily and normally? Why? In the Phasor Analyzer window, observe current phasors I2 and I3. Is there a phase shift between current phasors I2 and I3 to create a rotating magnetic field? Why? 10. Turn off the Power Supply and turn the voltage control knob fully counterclockwise. Open the circuit a point B shown in Figure 5-5. 11. Turn on the Power Supply, set the voltage control knob to about 50%, wait approximately 5 seconds, then turn off the Power Supply and turn the voltage control knob fully counterclockwise. Does the squirrel-cage induction motor start readily and rotate normally? Why? 12. Use the Capacitive load module to connect a capacitor to the motor circuit as shown in Figure 5-6. Set the capacitance of the capacitor to the value indicated in figure. 13. Turn on the Power Supply and slowly set the voltage control knob to 100%. Whiel doing this, observe phasors I2 and I3 in the Phasor Analyzer window as the voltage increases. Does the squirrel-cage induction motor start to rotate? Briefly explain why. UNIVERSITI MALAYSIA PERLIS (UniMAP) – Exp5 5 Basic Electrical Technology (DET 211/3 - Hardware Version) Line Voltage (V ac) 240 Laboratory Module C1 (µF) 4.6 Figure 5-6 Adding a Capacitor to the Motor Circuit. 14. On the capacitive Load module, open the switches to disconnect the capacitor from the motor circuit and cut off the current in one of the two windings of the squirrel-cage induction motor. Does the squirrel-cage induction motor continue to rotate, thus showing that it can operate on single-phase ac power once it has started? Turn off the Power Supply and turn the voltage control knob fully counterclockwise. Operation of a Single-Phase Induction Motor (Capacitor-Start Type) 15. Remove all leads except the 24-V ac power cable then set up the capacitor-start motor circuit shown in Figure 5-7. Figure 5.7 Capacitor-Start Motor Circuit UNIVERSITI MALAYSIA PERLIS (UniMAP) – Exp5 6 Basic Electrical Technology (DET 211/3 - Hardware Version) Laboratory Module 16. Turn on the Power Supply and set the voltage control knob to about 10%. In the Phasor Analyzer window, select proper sensitivities to observe voltage phasor E1 and current phasor I1. Observe that current phasor I1 (main winding current) lags voltage phasor E1 (source voltage). On the Power Supply, set the voltage control knob to the 50% position. Does the capacitor-start motor start to rotate? Why? 17. Turn off the Power Supply and turn the voltage control knob fully counterclockwise. Set the 24-V Ac power switch to the O (Off) position and remove all leads and cables. UNIVERSITI MALAYSIA PERLIS (UniMAP) – Exp5 7 Basic Electrical Technology (DET 211/3 - Hardware Version) Laboratory Module Name: _________________________ Matrix No.: _____________ Date: __________ RESULTS 6. ________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ 7. ________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ 9. ________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ 11. ________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ 13. ________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ Instructor Approval: _____________________________________ Date: __________ UNIVERSITI MALAYSIA PERLIS (UniMAP) – Exp5 8 Basic Electrical Technology (DET 211/3 - Hardware Version) Laboratory Module Name: _________________________ Matrix No.: _____________ Date: __________ 14. ________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ 16. ________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ Instructor Approval: _____________________________________ Date: __________ UNIVERSITI MALAYSIA PERLIS (UniMAP) – Exp5 9 Basic Electrical Technology (DET 211/3 - Hardware Version) Laboratory Module Name: _________________________ Matrix No.: _____________ Date: __________ DISCUSSION ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ CONCLUSION ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ Instructor Approval: _____________________________________ Date: __________ UNIVERSITI MALAYSIA PERLIS (UniMAP) – Exp5 10