D.C. dynamic braking for direct determination of rotor impedance and power factor of induction machines

The paper presents a new method of determining from tests the rotor leakage reactance and resistance of a 3-phase induction motor treated as a d.c. dynamic brake. Sinusoidal stationary flux distribution in the air gap is produced by applying direct currents, corresponding to 3-phase alternating currents, to the stator. The air-gap spatial flux distribution along the periphery is determined with distributed search coils pasted over the slot wedges and enclosing the stator core. The measurement of flux distribution is made when the rotor is stationary and when it is run as a d.c. dynamic brake. The method of computing the rotor leakage reactance and resistance therefrom is developed. The method is then verified with the computed and the actual braking power as a d.c. brake. Further comparison is also made between the performance as an induction motor, predicted with the test results obtained by this method, and the actual load test. It is concluded that the experimental method is universal and can be applied for the determination of rotor impedance and power factor of all types of induction machine including solid-ferromagnetic-rotor machines and deep-bar and double-squirrel-cage machines, in which, in addition to rotor reactance, the resistance varies as a function of rotor frequency.