Fig:1 Fig: 2
In practice, it is neither necessary nor feasible to run the induction motor synchronously for getting conductance, G0 and susceptance, B0. Instead, the motor is run without any external mechanical load on it. The speed of the rotor would not be synchronous, but very much near to it ; so that, for all practical purposes, the speed may be assumed synchronous.
The no load test is carried out with different values of applied voltage, below and above the value of normal voltage. The power input is measured by two wattmeters, I0 by an ammeter and V by a voltmeter, which is included in the circuit of Fig. 1.
As motor is running on light load, the p.f. would be low i.e. less than 0.5, hence total power input will be the difference of the two wattmeter readings W1 and W2. The readings of the total power input W0 , I0 and voltage V are plotted as in Fig. 2. If we extend the curve for W0, it cuts the vertical axis at point A. OA represents losses due to friction and windage. If we subtract loss corresponding to OA from W0, then we get the no-load electrical and magnetic losses in the machine, because the no-load input W0 to the motor consists of
(i) small stator Cu loss 3 I02 R1
(ii) stator core loss WCL = 3G0 V2
(iii) loss due to friction and windage.
The losses (ii) and (iii) are collectively known as fixed losses, because they are independent of load. OB represents normal voltage. Hence, losses at normal voltage can be found by drawing a vertical line from B.
BD = loss due to friction and windage
DE = stator Cu loss
EF = core loss
Hence, knowing the core loss WCL, G0 and B0 can be found by
V = applied voltage/phase; I0 = motor current / phase
W = wattmeter reading i.e. input in watt;
Y0 = exciting admittance of the motor.
Additionally, φ0 can also be found from the relation