CHAPTER XVI 



INDUCTION MOTORS 



The preliminary tests made on induction motors 

 include the measuring of the air gap, bearing and end 

 play, slip and resistance, as well as the tests for starting, 

 running light, excitation and static impedance. 



Special measuring scales are used for taking induction 

 motor air gap. 



Bearing play is taken by measuring the gap at the 

 top, bottom and on each side. With the rotor in the 

 same relative position to the stator, that is, without 

 turning the rotor, the motor is turned over in all four 

 positions of the quadrant and the same measurements 

 of air gap taken. Any defects in the bearings which will 

 affect the air gap of the machine are thus disclosed. 



A starting test on form K * motors is made by switching 

 the machine onto the line at a low voltage and then in- 

 creasing the voltage until the motor starts, the current 

 and voltage at this point being recorded. The starting 

 current should not exceed 200 per cent normal current. 

 This test is occasionally made with a compensator. 



With all the internal resistance in the rotor circuit, 

 full line voltage should be impressed on form L motors 

 and the starting current recorded. This current should 

 not exceed normal current. 



Form M motors are started at full line voltage with 

 all the external resistance in the rotor circuit, and the 

 starting current is recorded, which should not exceed 

 normal value. Sometimes the collector rings on form M 

 motors are short circuited and the starting test made at 

 reduced voltage, as in the case of form K motors. 



* K, M and L are trade symbols, denoting the three types of induction 

 motors. K is the squirrel-cage machine; M the slip-ring type; while L has a 

 wound rotor with an internal resistance which may be cut out in steps. 



[126] 



