372 THE INDUCTION MOTOR. 



switches this may be accomplished without increasing 

 the number of instruments required. 



Instructions. (1) After starting the motor in the usual way, 

 light and with belt off, adjust the primary voltage to the 

 full working pressure of the motor. Take readings of the 

 stntor and rotor currents, watts supplied (by readings on 

 the wattmeter with the shunt coil connected first across 

 one pair of mains and then across the other). Observe 

 also the primary voltage and the slip (by any of the 

 methods described on p. 362). 



Repeat the readings for gradually decreased values of 

 the primary voltage, the periodicity of the supply being 

 maintained constant. 



Readings should be continued in this manner until a 

 point is reached when the motor speed falls rapidly. The 

 motor should then be stopped and the voltage reduced to 

 its minimum value. With the rotor winding short- 

 circuited, close the switch in the primary circuit and take 

 similar readings to those already taken for gradually 

 increased values of the applied volts up to the point where 

 the motor speed rapidly rises to approximate synchronism. 



(2) Tie or clamp the rotor shaft in such a way that it 

 cannot rotate. Apply first a low voltage, and then gradu- 

 ally increasing voltages to the stator windings with the 

 rotor short-circuited as before. Take the readings 

 as in the previous case, continuing the experiment until 

 the rotor current becomes excessive. 



From the readings of the wattmeter and primary 

 ammeter and voltmeter, the value of the power-factor 

 cos should be calculated. The power in a three-phase 

 circuit = y8~(7^ cos </> where C and$ are the line current 

 and voltage as measured on A l and F. Consequently the 

 power-factor is calculated from the formula 



true watts 



COS 9 = >o 



>o ~rt -- in 



A/3. C x E 



The following table shows a few sample readings from 

 a test on a 2|-h.p. Brown-Boveri motor, and illustrates 

 the manner of entering up the results. The readings are 

 plotted on the curves shown in Fig. 182, the armature 

 being free to rotate. la Fig. 183 are shown curves, 

 obtained with the armature clamped, 



