INDUCTION MOTORS 133 



other points in the curve should be carefully read and 

 recorded as a check on the balance of the motor. As the 

 lowest point of the curve, or friction reading is ap- 

 proached, many readings should be taken. This portion 

 is the most difficult part of the curve to locate, especially 

 in the case of large motors, as in many instances "hunt- 

 ing" begins at a low voltage. A reading taken when the 

 motor is accelerating is of greater value than the steady 

 reading. 



Hunting usually makes the meter needle swing with 

 a slow beat, the range of the beat varying with the 

 size of the motor and degree of hunting. Bad cases of 

 hunting are not numerous and reliable readings can gen- 

 erally be secured between beats. To test successfully, 

 the speed of the driving generator must be kept constant 

 and no reading taken until the speed is properly adjusted. 

 The tachometer used must be carefully checked. 



The excitation tests on all forms of induction motors 

 are the same. 



The form M motor is provided with collector rings 

 for the external resistance. These must be short cir- 

 cuited at the brush-holder terminal and the brushes 

 carefully sandpapered until they fit the rings accurately. 



Calculation of Excitation Test on Induction Motors 



All readings must be corrected for the instrument 

 constants and ratios used. Special care should be taken 

 to use the proper signs for the wattmeter readings. 

 Table XVI shows the form used in calculating an exci- 

 tation test, and Fig. 39 the method of plotting it. The 

 friction and windage watts are obtained from the exci- 

 tation curve by producing the watt curve to zero volts. 



Impedance 



In the motor having a symmetrical bar winding in 

 the rotor the impedance is the same for any position of 

 the stator relative to the rotor. 



On motors which have wound rotors, a position curve 

 is first taken. Two-thirds of the distance between two 

 consecutive poles on a three-phase motor and one-half 

 that distance on a quarter-phase motor are marked off 



