MEASUREMENT OF PARAMETERS
The tests used to obtain the equivalent circuit parameters of a cage induction motor are essentially the same as those described for the transformer in Section 7.4.5.
To simulate the ‘open-circuit’ test we would need to run the motor with a slip of zero, so that the secondary (rotor) referred resistance (R 0 =s) became inWnite and the rotor current was absolutely zero. But because the motor torque is zero at synchronous speed, the only way we could achieve zero slip would be to drive the rotor from another source, e.g. a synchronous motor. This is hardly ever necessary because when the shaft is unloaded the no-load torque is usually very small and the slip is suYciently close to zero that the diVerence does not matter. From readings of input voltage, current and power (per phase) the parameters of the magnetising branch (see Figure 7.16) are derived as described earlier. The no-load friction and windage losses will be combined with the iron losses and represented in the parallel resistor – a satisfactory practice as long as the voltage and/or frequency remain constant.
The short-circuit test of the transformer becomes the locked-rotor test for the induction motor. By clamping the rotor so that the speed is zero and the slip is one, the equivalent circuit becomes the same as the short- circuited transformer. The total resistance and leakage reactance parameters are derived from voltage, current and power measurements as already described. With a cage motor the rotor resistance clearly cannot be measured directly, but the stator resistance can be obtained from a d.c. test and hence the referred rotor resistance can be obtained.