FIGURE 6-3-1: Squirrel Cage Power and Torque CharacteristicsLbf-In / N-mI1AmpsWattsRPMFIGURE 6-3-2: SQUIRREL CAGE FULL-VOLTAGE STARTECE 4501 Power Systems Laboratory Manual Rev 1.16.0 INDUCTION MOTORS6.1 WOUND ROTOR INDUCTION MOTOR 6.1.1 OBJECTIVETo examine the construction of a three-phase wound rotor induction motor and understand the concepts of exciting current, synchronous speed, slip and induced voltages.6.1.2 DISCUSSIONWhen the three windings in the stator of an induction motor are connected to a three-phase voltagesource, currents flow in the windings and a rotating magnetic field is established in the stator. If there is no load connected to the motor shaft, the three-phase current drawn by the stator windings is called the exciting current. This current, at line voltage, provides the reactive power necessary to establish the rotating magnetic field in the stator and the real power dissipated in the copper windings and core.The speed of the rotating stator field is determined by the frequency of the three-phase waveforms supplied by the source, 60 Hz in North America, and by the number of magnetic poles with which the stator is built. This speed is known as the synchronous speed and is measured in revolutions per minute, RPM. Poles always come in pairs ( a north pole and a south pole) and for a two-pole motor, the field will complete 60 revolutions every second and thus synchronous speed is 3600 RPM. This is as fast as any induction motor can ever turn when excited by 60 cycle waveforms. Electric utilities maintain system frequency with great precision (in order to make electric clocks run accurately, among other things). Therefore, synchronous speed may be considered a constant value for a given motor.The rotor of an induction motor consists of a laminated steel core with slots and some type of winding. The two most common types of winding are the squirrel cage rotor and the wound rotor (using copper windings). The squirrel cage rotor will be discussed in a later section of the experiment. In the wound rotor, three sets of windings are set in the slots of the core material. Each winding is brought out to a slip ring on the shaft of the rotor. Terminating the windings on slip rings allows flexibility in the manner in which the windings are configured by allowing resistors to be placed in WYE or DELTA across them. The resistors are sized to accurately control the magnitude of currents in the rotor windings.The rotating three-phase magnetic field produced by the stator induces an alternating voltage on each of the rotor windings. If the rotor is not turning, the rate at which each rotor winding cuts thelines of flux produced by the magnetic field will be equal to the synchronous speed and the induced voltages in the rotor will be at the same frequency as the source voltage. This condition iscalled 100% slip. As the rotor is turned in the same direction as the rotating magnetic stator field, the rate at which the rotor windings cut lines of flux will decrease and the induced voltages in the rotor windings will decrease in frequency and magnitude. If the rotor is turned at a rate equal to the synchronous speed, its windings will not cut any lines of flux and the induced voltages will be zero in magnitude and frequency. This condition is called 0% slip. The torque produced by the motor drops to zero at 0% slip and thus, for all practical purposes, an induction motor cannot actually achieve synchronous speed. Conversely, if the rotor is turned in the opposite direction Page 1ECE 4501 Power Systems Laboratory Manual Rev 1.1with respect to the stator field, but at synchronous frequency, the induced voltages will have twice the magnitude and frequency as compared to the 100% slip condition.6.1.3 INSTRUMENTS AND COMPONENTSPower Supply Module EMS 8821AC Metering Module (2.5 A) EMS 8425AC Metering Module (250V) EMS 8426DC Motor/Generator Module EMS 8211Wound Rotor IM Module EMS 8231Three-Phase Wattmeter Module EMS 8441Hand Tachometer EMS 89206.1.4 PROCEDURECAUTION! – High voltages are present in this Experiment. DO NOT make any connections with the power supply ON. Get in the habit of turning OFF the power supply after every measurement.1) Examine the construction of the Wound Rotor Induction Motor, EMS 8231, paying close attention to the slip rings, the rotor windings, the stator windings, and the connection schematic.2) What is the rated current of the stator windings? ___________ Rated voltage? ___________3) What is the rated current of the rotor windings? ___________ Rated voltage? ___________4) Are the rotor windings configured WYE or DELTA? ___________5) What is the rated speed of the induction motor? ___________ Rated Horsepower? ___________6) Connect the following circuit, coupling the two motors with a timing belt:Page 2ECE 4501 Power Systems Laboratory Manual Rev 1.1FIGURE 6-1-1: INDUCTION MOTOR TURNED BY A DC MOTOR7) Note that the DC Shunt Motor* will be used to turn the rotor of the Induction Motor. Also note that the DC motor is connected with fixed (120 Vdc) shunt field excitation and variable (0 – 120 Vdc) excitation for the armature.* Alternatively the Prime Mover/ Dynamometer can substitute for the DC shunt motor, in which case supply power to it using 1-N terminals at power supply to 1-2 terminals of Dynamometer. 8) Turn the field rheostat on the DC motor to its full clockwise position (for minimum field resistance). If Dynamometer is used instead of DC motor adjust the load control to minimum9) Note that the stator of the Induction motor is WYE connected and that voltmeter, V1 will measure input voltage and V2 will measure the induced voltage on the open circuited rotor windings.10) Make sure that both motors are coupled by a timing belt.11) Turn on the 24 Vac power supply and the Main Power Supply, but DO NOT turn the voltage control (the DC motor should not turn).12) Measure and record the following (it is OK if W1 and W2 have different signs):V1 = Volts W1 = Watts I1 = AmpsV2 = Volts W2 = Watts I2 = AmpsPage 3ECE 4501 Power Systems Laboratory Manual Rev 1.1I3 = Amps13) Turn OFF the main power supply.Page 4ECE 4501 Power Systems Laboratory Manual Rev 1.114) Calculate the following (by the 2-wattmeter method, three phase real power will be W1 + W2):Apparent Power, S3Reactive Power, Q3 VA VARReal Power,