Stray-Load Losses in Polyphase Induction Maclines

The total stray-load loss of a polyphase induction machine of the squirrel-cage type is given by the formula: WL = WC + WE + WSL1 + WSL2 + WZ + WB + WK (13) where the several items, except Wc, are given by equations 1, 7, 6, 9, 10, and 12. The sum of WE and Wc can be determined separately from the other losses by measuring the input to the stator when balanced polyphase currents are flowing, with the rotor removed, and subtracting from this the I2R losses calculated on the basis of the d-c resistance of the winding at the test temperature, as required in the Morgan method of load-loss measurement. However, the end loss will differ somewhat from that under load, because the rotor end winding currents are not present. The total loss can be measured by the dynamometer method, either as a motor alone, or by the differential dynamometer method as the average of a motor and a generator, or by the pump-back method or the Morgan method, as outlined in a companion paper. 1 From a theoretical point of veiw, all these methods should give equivalent results, except that the Morgan method would be expected to give a stray loss slightly lower than the true value when the rotor has closed slots.