398 THE INDUCTION MOTOR. 



constant at all loads. Their value is determined from 

 the no-load running test (see page 370). The power taken 

 by the motor when running light is nearly all spent in 

 iron losses, and running friction. A small part will be 

 due to copper losses in the windings, and this should be 

 subtracted from the wattmeter readings to give the true 

 iron and friction losses. The value of the copper losses 

 is easily derived from the results of the determination of 

 the total oopper losses. 



From the results of these tests the efficiency of the 

 motor at any load can be approximately calculated, since 

 the iron and friction losses remain practically constant 

 at all loads, and the losses due to resistance are at once 

 seen from the curves obtained in the first test (see Fig. 

 183, page 375). 



The power factor may also be calculated from the 

 results of the two loss tests just described. 



The idle magnetising current is determined from the 

 readings of the wattmeter, ammeter, and voltmeter 

 taken in the no-load running test. 



From these readings cos <' = rr - , where cos <t> 1 



volts x amps. 



is the no-load power factor. The magnetising current 



is then (no-load amps) x sin <f> 1 ). This idle current 



will remain approximately constant at all loads. Thus 



the total current at any load will be given by 



C* = (energy] load current + energy loss current) 2 -)- (idle 



current) 2 . 



In a 3-phase motor the total output = 3 C 1 E when 

 C l and E are current and volts per phase. Thus total 

 current at any load 



- c - w 

 = 



When W = total watts output. 



w = total calculated losses. 

 C = magnetising current. 



W -f w 



mi energy current W + w 



The power factor is - jff- r = 3 E = - 7rir?T - 



total current 



a 



The percentage slip at any load can be also approxi- 

 mately calculated from the results already obtained, since 



