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Unformatted text preview: I NDUCTION M OTOR T ESTS (NoLoad Test, Blocked Rotor Test) The equivalent circuit parameters for an induction motor can be determined using specific tests on the motor, just as was done for the transformer. NoLoad Test Balanced voltages are applied to the stator terminals at the rated frequency with the rotor uncoupled from any mechanical load. Current, voltage and power are measured at the motor input. The losses in the noload test are those due to core losses, winding losses, windage and friction. Blocked Rotor Test The rotor is blocked to prevent rotation and balanced voltages are applied to the stator terminals at a frequency of 25 percent of the rated frequency at a voltage where the rated current is achieved. Current, voltage and power are measured at the motor input. In addition to these tests, the DC resistance of the stator winding should be measured in order to determine the complete equivalent circuit. NoLoad Test The slip of the induction motor at noload is very low. Thus, the value of the equivalent resistance in the rotor branch of the equivalent circuit is very high. The noload rotor current is then negligible and the rotor branch of the equivalent circuit can be neglected. The approximate equivalent circuit for the noload test becomes Induction machine equivalent circuit for noload test Note that the series resistance in the noload test equivalent circuit is not simply the stator winding resistance. The noload rotational losses (windage, friction, and core losses) will also be seen in the noload measurement. This is why the additional measurement of the DC resistance of the stator windings is required. Given that the rotor current is negligible under noload conditions, the rotor copper losses are also negligible. Thus, the input power measured in the noload test is equal to the stator copper losses plus the rotational losses. where the stator copper losses are given by From the noload measurement data ( V NL , I NL , P NL ) and the noload equivalent circuit, the value of R NL is determined from the noload dissipated power. The ratio of the noload voltage to current represents the noload impedance which, from the noload equivalent circuit, is and the blocked rotor reactance sum X l 1 + X m 1 is Note that the values of X l 1 and X m 1 are not uniquely determined by the no load test data alone (unlike the transformer noload test). The value of the stator leakage reactance can be determined from the blocked rotor test. The value of the magnetizing reactance can then be determined. Blocked Rotor Test The slip for the blocked rotor test is unity since the rotor is stationary. The resulting speeddependent equivalent resistance goes to zero and the resistance of the rotor branch of the equivalent circuit becomes very small. Thus, the rotor current is much larger than current in the excitation branch of the circuit such that the excitation branch can be neglected. The resulting equivalent circuit for the blocked rotor test is shown in the...
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 Fall '08
 Staff
 Volt

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