INDUCTION MOTORS 



143 



This ' ' no load ' ' scale reading must be recorded and sub- 

 tracted from all subsequent readings taken. 



A small weight is now placed on A and the spring 

 balance G pulled up until the pointer on lever L reaches 

 K ; when the motor volts and speed of the generator 

 are normal and all meters are steady, the volts, amperes, 

 watts, weights on A, spring balance deflection, and 

 speed given by the tachometer should be read. A read- 

 ing should also be taken with the slip machine. Continue 

 to add weight to A and take readings until the breakdown 

 load of the motor is reached. The readings should be 

 recorded in the following manner : 



A rope of small diameter gives better results than one 

 of larger diameter, even though it may require more time 

 to make the tests on account of having to renew the 

 rope more frequently. For motors up to 20 h.p., a ^ i n - 

 oiled hemp rope is best, and for motors from 20 h.p. 

 to 50 h.p., a J/2 m - r P e can be used. The rope will 

 usually last longer if doubled, and two strands used in 

 parallel. The rope should be wrapped around the pulley 

 one and a half times, care being taken to have no 

 strands twisted or crossing each other ; all -strands should 

 lie closely and evenly together on the face of the pulley. 

 The tension read on the balance G will vary with the 

 temperature of the rope and may differ widely with 

 different loads. 



The additional weight added to A each time should be 

 such as to allow of from fifteen to twenty readings be- 

 tween no-load and breakdown. 



When the breakdown point has been reached and 

 complete readings taken and recorded, the diameter 

 of the pulley should be carefully measured. 



Weight on A (tension on balance) ("no Load" 

 reading on balance) = actual load in pounds = P. 



Normal speed slip = actual speed of motor. 



R= radius of pulley in inches + H diameter of rope. 



