INDUCTION MOTORS 131 



(Always see that the wattmeter switches are closed when- 

 ever a change in the field current is made.) The exciter 

 field switch is then closed and the voltage brought up 

 slowly until the motor starts and reaches normal speed. 

 The machine should then be inspected to see that it is 

 operating normally and the amperes and volts in the 

 different phases read and any unbalancing corrected 

 or its cause discovered. 



The end play of the motor should be tested next, 

 since the rotor must always run centrally in the frame. 

 A slight pressure against one side will change the friction 

 watts and give an incorrect value to the core loss. Small 

 motors should be run about one hour and a half and large 

 ones tw T o hours and a half or more, to obtain constant 

 friction before starting tests. If the wattmeter needle 

 goes off the scale in a negative direction when connected 

 in circuit, the current leads on the current terminals 

 should be interchanged. On a two-phase circuit, with a 

 machine underload, both wattmeters should read positive. 



For running light readings on a three-phase machine 

 the sign of the meter must be determined, since one 

 reads negative on the upper part of the curve. With 

 both meters reading positive, one of the phases contain- 

 ing the current coil of the wattmeter should be opened 

 and the other meter observed. If the needle drops off 

 the scale below zero the meter reads negatively. If the 

 needle drops to some value above zero the reading is 

 positive. This process must be repeated for determining 

 the readings of the other wattmeter. 



The alternator speed must be held constant during 

 the test and about 130 per cent normal volts used for 

 the first reading; volts amperes, watts and speed of gen- 

 erator and motor being read and recorded. The volts 

 should then be decreased in steps so as to obtain about 

 20-25 points on the curve, down to 10 or 15 per cent of 

 normal volts. Here the conditions are no longer stable. 

 The meter with the negative sign will read less than the 

 other, and its readings will fall off more rapidly, becom- 

 ing less and less until zero is reached and its sign changes. 

 When it becomes positive, the current leads must be 

 interchanged. 



