EFFICIENCY 197 



temporarily introducing resistances into the rotor windings, and that 

 int Mentally we thereby gain the advantage of a smaller starting 

 current. 



In the case of rotors having permanently short-circuited windings, 

 of which the best-known and most widely used type is the squirrel- 

 cage winding ( 61), the introduction of starting resistances into the 

 rotor windings is, of course, impossible. Such motors are, therefore, 

 incapable of starting against a heavy load. Further, if the output 

 of such a motor is large, the starting current would reach a dangerous 

 value if the full p.d. were applied to the stator windings. Hence 

 it becomes necessary to introduce resistances into the stator windings, 

 or to use an auto-transformer ( 58) in order to limit the current ; 

 but this, as we have seen,*reduces the torque still further. The 

 squirrel-cage type of winding is, therefore, totally unsuitable for 

 motors which are required to start under load. 



114. Effect of Stator Core Loss. Efficiency 



of Motor 



We have hitherto neglected the stator core loss. In the diagram 

 (Fig. 126) of the equivalent transformer, this is represented by the 

 power absorbed by the shunting non-inductive resistance h. The 

 presence of this resistance has the effect of adding to the primary 

 current a component IA which is in phase with the p.d. The addition 

 of this to the primary current vector in Fig. 129 would displace all 

 points on the primary current locus a distance representing IA to the 

 right. It is simpler, however, to leave the diagram unaltered, and 

 to measure the primary current not from 0, but from a point 0' 

 to the left of 0, such that O'O represents, on the scale of primary 

 current, I h . The primary angle of phase difference will be PO'A 

 instead of POA. 



The total mechanical power is easily calculated from the known 

 value of the torque and the speed. This latter is easily determined 

 from the slip, as it is equal to (1 s) X speed of synchronism. If 

 we express the total mechanical power in watts, then its ratio to the 

 total watts supplied to the primary gives us the electrical efficiency 

 of the motor. The usc.ful power is obtained by subtracting the rotor 

 friction loss from the total mechanical power, and the ratio of the 

 useful power to the total power supplied to the primary gives us 

 the commercial efficiency. 



