INDUCTION MOTOR TESTS

1 INDUCTION MOTOR TESTS (No-Load Test, Blocked Rotor Test)The equivalent circuit parameters for an INDUCTION MOTOR can bedetermined using specific TESTS on the MOTOR , just as was done for TestBalanced voltages are applied to the stator terminalsat the rated frequency with the rotor uncoupled fromany mechanical load. Current, voltage and powerare measured at the MOTOR input. The losses in theno-load test are those due to core losses, windinglosses, windage and Rotor TestThe rotor is blocked to prevent rotation andbalanced voltages are applied to the statorterminals at a frequency of 25 percent of therated frequency at a voltage where the ratedcurrent is achieved.

2 Current, voltage andpower are measured at the MOTOR addition to these TESTS , the DC resistance of the stator winding should bemeasured in order to determine the complete equivalent TestThe slip of the INDUCTION MOTOR at no-load is very low. Thus, thevalue of the equivalent resistancein the rotor branch of the equivalent circuit is very high. The no-load rotorcurrent is then negligible and the rotor branch of the equivalent circuit canbe neglected. The approximate equivalent circuit for the no-load testbecomesInduction machineequivalent circuit for no-load testNote that the series resistance in the no-load test equivalent circuit is notsimply the stator winding resistance.

3 The no-load rotational losses(windage, friction, and core losses) will also be seen in the no-loadmeasurement. This is why the additional measurement of the DCresistance of the stator windings is required. Given that the rotor currentis negligible under no-load conditions, the rotor copper losses are alsonegligible. Thus, the input power measured in the no-load test is equal tothe stator copper losses plus the rotational the stator copper losses are given byFrom the no-load measurement data (VNL, INL, PNL) and the no-loadequivalent circuit, the value of RNL is determined from the no-loaddissipated ratio of the no-load voltage to current represents the no-loadimpedance which, from the no-load equivalent circuit, isand the blocked rotor reactance sum Xl1 + Xm1 isNote that the values of Xl1 and Xm1 are not uniquely determined by the no-load test data alone (unlike the transformer no-load test).

4 The value of thestator leakage reactance can be determined from the blocked rotor test. Thevalue of the magnetizing reactance can then be Rotor TestThe slip for the blocked rotor test is unity since the rotor is resulting speed-dependent equivalent resistancegoes to zero and the resistance of the rotor branch of the equivalent circuitbecomes very small. Thus, the rotor current is much larger than current inthe excitation branch of the circuit such that the excitation branch can beneglected. The resulting equivalent circuit for the blocked rotor test is shown in thefigure machineequivalent circuit for blocked rotor testThe reflected rotor winding resistance is determined from the dissipatedpower in the blocked rotor ratio of the blocked rotor voltage and current equals the blocked reactance sum isNote that this reactance is that for which the blocked rotor test isperformed.

5 All reactances in the INDUCTION machine equivalent circuit arethose at the stator (line) frequency. Thus, all reactances computed basedon the blocked rotor test frequency must be scaled according to relativefrequencies (usually, a factor of 4 since TBR is usually ). The actualdistribution of the total leakage reactance between the stator and the rotoris typically unknown but empirical equations for different classes of motors(squirrel-cage motors) can be used to determine the values of Xl1 and following is a description of the four different classes of A Squirrel-Cage INDUCTION MOTOR - characterized by normalstarting torque, high starting current, low operating slip, low rotorimpedance, good operating characteristics at the expense of high startingcurrent, common applications include fans, blowers, and B Squirrel-Cage INDUCTION MOTOR - characterized by normalstarting torque, low starting current, low operating slip, higher rotorimpedance than Class A.

6 Good general purpose MOTOR with commonapplications being the same as Class C Squirrel-Cage INDUCTION MOTOR - characterized by highstarting torque, low starting current, higher operating slip than Classes Aand B, common applications include compressors and D Squirrel-Cage INDUCTION MOTOR - characterized by highstarting torque, high starting current, high operating slip, inefficientoperation efficiency for continuous loads, common applications arecharacterized by an intermittent load such as a punch press. Blocked Rotor Leakage MOTOR Reactance DistributionSquirrel-cage Class AXl1 = = Class BXl1 = = Class CXl1 = = Class DXl1 = = rotorXl1 = = these empirical formulas, the values of Xl1 and Xl2N can be determinedfrom the calculation of XBR from the blocked rotor test data.

7 Given thevalue of Xl1, the magnetization reactance can be determined according toExample (No-Load/Blocked Rotor TESTS )The results of the no-load and blocked rotor TESTS on a three -phase, 60hp, 2200 V, six-pole, 60 Hz, Class A squirrel-cage INDUCTION MOTOR areshown below. The three -phase stator windings are test Frequency = 60 HzLine-to-line voltage = 2200 VLine current = AInput power = 1600 W Blocked-rotor test Frequency = 15 HzLine-to-line voltage = 270 VLine current = 25 AInput power = 9000 WStator S per phaseDetermine (a.) the no-load rotational loss (b.) the parameters of theapproximate equivalent circuit.

8 (a.)(b.) The voltage at the input terminals of the per-phase equivalent circuit,given the wye connected stator windings, isThe equivalent circuit for the INDUCTION MOTOR is shown MACHINE TORQUE AND POWER(MACHINE PERFORMANCE CHARACTERISTICS)In order to simplify the determination of torque and power equationsfrom the INDUCTION machine equivalent circuit, we may replace the networkto the left of the reflected components by a Thevenin equivalent source. The Thevenin voltage (open-circuit voltage) for the stator portion of theequivalent circuit (to the left of the air gap) isThe Thevenin impedance (impedance seen after shorting V1) isInserting the Thevenin equivalent source into the INDUCTION machineequivalent circuit yields the following equivalent the equivalent circuit, the total real power per phase that crosses theair gap (the air gap power = Pair gap) and is delivered to the rotor is The portion of the air gap power that is dissipated in the form of ohmic loss(copper loss) in the rotor conductors isThe total mechanical power (Pmech)

9 Developed internal to the MOTOR is equalto the air gap power minus the ohmic losses in the rotor which givesorAccording to the previous equations, of the total power crossing the air gap,the portion s goes to ohmic losses while the portion (1!s) goes tomechanical power. Thus, the INDUCTION machine is an efficient machinewhen operating at a low value of slip. Conversely, the INDUCTION machineis a very inefficient machine when operating at a high value of slip. Theoverall mechanical power is equal to the power delivered to the shaft of themachine plus losses (windage, friction).The mechanical power (W) is equal to torque (N-m) times angularvelocity (rad/s).

10 Thus, we may writewhere T is the torque and T is the angular velocity of the MOTOR in radiansper second given bywhere Ts is the angular velocity at synchronous speed. Using the previousequation, we may writeInserting this result into the equation relating torque and power givesSolving this equation for the torque yieldsReturning to the Thevenin transformed equivalent circuit, we findNote that the previous equation is a phasor while the term in the torqueexpression contains the magnitude of this phasor. The complex numbersin the numerator and denominator may be written in terms of magnitudeand phase to extract the overall magnitude term magnitude of the previous expression isInserting this result into the torque per phase equation givesThis equation can be plotted as a function of slip for a particular inductionmachine yielding the general shape curve shown in Figure ( ).