INDUCTION MOTOR 343 



get the value of locked current at rated voltage, raise the values 

 of IL in the ratio EI : EL,. To get the values of locked watts and 

 locked torque at rated voltage, raise the values of WL and TL in 

 the ratio Ei 2 : EL*. To get accurate results for the circle diagram 

 it is better to reduce the values of watts and torque to terms 

 of power current per phase. This is done by dividing the 

 values of W L and T L by V% E L . 

 Plot on a base of impressed voltage, 



~W 1 



(1) I L , (2) -^-> (3) 



These three loci should be straight lines passing through the 

 origin and can be produced till they cut the ordinate at the rated 

 voltage of the motor. 



The following results are obtained. 



Value of IL at rated voltage = os on the diagram. 



W L 

 Value of /- at rated voltage = sw on the diagram. 



3. Measure the resistance of the stator per phase = r\. Then 

 the stator copper loss locked at rated voltage is 3 osVi watts 



- 2 



and 7= -= vt on the diagram. 



V3#! 



The rotor copper loss, locked at rated voltage, is \/3 E\ X st 

 and is known since st = sw wv vt. 



The rotor copper loss also represents the starting torque in 

 synchronous watts and therefore if 7\ is the value of TL at rated 

 voltage 



Ti X 27r X (sync, speed r.p.m.) , . A/3 EI X st 



33,000 -should equal --- 



The circle diagram can be drawn in from the values obtained 

 above. 



The motor has been assumed to be Y-connected and the volt- 

 age EI is the line voltage. 



218. Methods of Starting. Except in the case of small ma- 

 chines, induction motors should not be started by connecting 

 them directly to the mains, since the large starting current at low 

 power factor disturbs the voltage regulation of the system. 



