Determination of G0 and B0
If the total leakage reactance X 01 of the motor, exciting conductance G0 and exciting susceptance B0 are found, then the position of the circle O¢BC¢ is determined uniquely.
One method of finding G0 and B0 consists in running the motor synchronously so that slip s = 0. In practice, it is impossible for an
induction motor to run at synchronous speed, due to the inevitable presence of friction and windage losses. However, the induction motor may be run at synchronous speed by
No-load Test
In practice, it is neither necessary nor feasible to run the induction motor synchronously for getting G0 and 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 are included in the circuit of Fig. 35.7. 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 W 1 and W 2. The readings of the total power input W 0 , I0 and voltage V are plotted as in Fig. 35.8. If we extend the curve for W 0, it cuts the vertical axis at point A . OA represents losses due to friction and windage. If we subtract loss corresponding to OA from W 0, then we get the no-load electrical and magnetic losses in the machine, because the no-load input W 0 to the motor consists of
(i) small stator Cu loss 3 I 2 R (ii) stator core loss W = 3G V 2
(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 W CL, G0 and B0 can be found, as discussed in Art. 35.4.
