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448 EE Laboratory Experiment 4 3-Phase Induction (Squirrel-Cage) Machines EE 448 Fall 2006 Experiment #4 11/15/2006 EE448_LAB04A.doc 3-PHASE INDUCTION (SQUIRREL-CAGE) MACHINES -1- EE 448 Laboratory Experiment 4 3-Phase Induction (Squirrel-Cage) Machines I. INTRODUCTION OBJECTIVES: 1. To examine the construction of the three-phase squirrel cage motor. 2. To determine its starting, no load and full load characteristics. BACKGROUND SUMMARY Courtesy of: http://www.knoware-online.com/motors.html The simplest and most widely-used rotor for induction motors is the socalled squirrel cage rotor, from which the squirrel cage induction motor gets its name. The squirrel cage rotor consists of a laminated iron core -2- EE 448 Laboratory Experiment 4 3-Phase Induction (Squirrel-Cage) Machines which is slotted length-wise around its periphery. Solid bars of copper or aluminum are tightly pressed or embedded into the rotor slots. At both ends of the rotor, short-circuiting rings are welded or brazed to the bars to make a solid structure. The short-circuited bars, because their resistance is much less than the core, do not have to be specially insulated from the core. In some rotors the bars and end rings are cast as a single integral structure for placement on the core. The short-circuiting elements actually form shorted turns that have high currents induced in them by the stator field flux. Compared to the intricately wound and arranged wound rotor or the armature of the DC motor, the squirrel cage rotor is relatively simple. It is easy to manufacture and is essentially trouble-free in actual service. In an assembled squirrel cage induction motor, the periphery of the rotor is separated from the stator by a very small air gap. The width of this air gap, in fact, is as small as mechanical clearance needs will permit. This insures that the strongest possible electromagnetic induction action will take place. When power is applied to the stator of a practical induction motor, a rotating magnetic field is created by any one of the means you learned about. As the field begins to revolve, its flux lines cut the shorted turns embedded around the surface of the squirrel cage rotor and generate voltages in them by electromagnetic induction. Because the turns are short-circuits with very low resistance, the induced voltages cause high currents to circulate in the rotor bars. The circulating rotor currents then produce their own strong magnetic fields. These local rotor flux fields produce their own magnetic poles, which are attracted to the rotating field. Thus, the rotor revolves with the main field. If torque from an external source is applied to the shaft of the induction machine such that it rotates at a speed greater than its synchronous speed, then electrical power may be generated in the stator windings. The generation of electrical power relies on the existence of the stator rotating magnetic field. The magnetizing reactive power must be supplied to the stator windings and may come from a capacitor or from synchronous machines connected to the stator. Induction generators are not commonly used as sources of electrical power because of the need for a separate source of magnetizing reactive power. Synchronous alternators are the common means of generating electricity. However, small generators used to supply a farm or residence from wind energy can be induction generators. These generators draw their magnetizing reactive power from the utility system and generate electrical power for the local load and/or supply power to the utility system. -3- EE 448 Laboratory Experiment 4 3-Phase Induction (Squirrel-Cage) Machines INSTRUMENTS AND COMPONENTS Squirrel Cage Induction Motor Module Electro-dynamometer Module Three-Phase Wattmeter Module Power Supply Module (0-120/208V 3) AC Metering Module (250V) AC Metering Module (2.5/2.5/2.5/8A) Timing Belt Strobotac EMS 8221 EMS 8911 EMS 8441 EMS 8821 EMS 8426 EMS 8425 EMS 8942 General Radio 1531-AB Squirrel Cage Induction Motor and Electro-dynamometer Timing belt -4- EE 448 Laboratory Experiment 4 3-Phase Induction (Squirrel-Cage) Machines Strobotac II. PRE-LAB TEST QUESTIONS AND CALCULATIONS Fill in the blanks. 1. The relative difference in speed between the actual rotor speed and the synchronous speed of the stator field is called the _______________. 2. The direction of rotation of an induction motor may be changed by _____________. 3. In order to operate an induction machine as a generator the rotor has to be driven at a speed _______________ than the synchronous speed. Answer the following questions. 4. The general formula relating power, torque and speed is P = T with P in watts, T in Newton meters, in radians/sec. Derive the formula with appropriate constants for P in hp, T in lbf-in, in r.p.m. 5. The 3 induction machine in your lab has the following rated values. current = 1.2 A voltages (L-L) = 208 V speed = 1670 r.p.m. power = HP -5- EE 448 Laboratory Experiment 4 3-Phase Induction (Squirrel-Cage) Machines From these values, calculate the rated torque in lbf-in. Trated = _______ lbf-in. 6. A 3, y connected induction motor is operating at rated load conditions. At this rated load, the following data is obtained. TLOAD = 9 lbf.in I = 1.0A VLL = 200V N = 1680 r.p.m. W1 + W2 = Total Power Input = 250W Calculate the following quantities. a. Apparent power S _______ b. Reactive power, Q _______ c. Input power factor, pf _______ d. Horse HP power, _______ e. Efficiency _______ 8. a. Draw a typical torque versus speed curve for an induction motor. Carefully show the location of the following points on the axes of the graph. i. ii. iii. Synchronous speed, ns, Starting torque, Ts Maximum torque, Tmax b. Identify the region of the curve in part 8a where the motor cannot be operated. IIIa. EXAMINING THE EXPERIMENTAL SETUP CAUTION: HIGH VOLTAGES ARE PRESENT IN THIS LABORATORY EXPERIMENT! DO NOT MAKE ANY CONNECTIONS WITH THE POWER ON! THE POWER SHOULD BE TURNED OFF AFTER COMPLETING EACH INDIVIDUAL MEASUREMENT! 1. Examine the construction of the Squirrel Cage Induction Motor Module EMS 8221, paying particular attention to the motor, connection terminals and the wiring. -6- EE 448 Laboratory Experiment 4 3-Phase Induction (Squirrel-Cage) Machines a. Identify the stator windings. Note that they consist of many turns of small diameter wire evenly spaced around the stator. (The stator windings are identical to that of the wound rotor induction motor). b. Identify the end rings of the squirrel cage rotor. c. Note the thickness of the air gap between the stator and the rotor. d. Is there any electrical connection between the rotor and any other part of the motor? ______________________________________ 2. Viewing the front face of the module: a. The three separate stator windings are connected to terminals ___________ and ___________, __________ and __________, __________ and ___________. b. What is the rated current of the stator windings? ___________ c. What is the rated voltage of the stator windings? ___________ d. What is the rated speed and horsepower of the motor? r/min = __________ hp = __________ IIIb. EXPERIMENTAL WORK 1. Using your EMS Squirrel Cage Motor, Electrodynamometer, ThreePhase Watt-meter, Power Supply and AC Metering Modules, connect the circuit shown in Fig. 1. DO NOT COUPLE THE MOTOR TO THE DYNAMOMETER AT THIS TIME! Note that the stator windings are wye connected through the wattmeter to the variable 3 output of the power supply, terminals 4, 5 and 6. To measure the speed of the motor two methods can be used. One method is to connect the tachometer to the shaft of the motor. The other method is to aim the strobotac at the pulley of the motor and adjust the strobotac till a constant line is visible on the pulley while the motor is running. -7- EE 448 Laboratory Experiment 4 3-Phase Induction (Squirrel-Cage) Machines Ammeter + + - I1 W1 + W2 + + Squirrel Cage Induction Motor - Power Meter 1 2 3 4 5 6 1 2 3 4 5 4 4 6 V1 Voltmeter Ammeter + 6 + + Ammeter 200 Volt Line-Line 5 3-Phase 6 Source N 8821 5 I2 1 2 3 4 5 6 - - I3 1 1 Electro-Dynamometer 120 V AC Source 8821 N - Figure 1: Motor Connection Diagram 2. a. Turn on the power supply and adjust V1 to 200Vac. The motor should be running. b. Measure and record in Table 1, the three line currents, the two wattmeter indications, and the motor speed. c. Return the voltage to zero and turn off the power supply. 3. a. Couple the motor to the electrodynamometer by using the timing belt. Make sure the belt rides in-between the belt tensioners as show in figure 2. Figure 2: A Properly Installed Timing Belt. -8- EE 448 Laboratory Experiment 4 3-Phase Induction (Squirrel-...