274 INDUCTION MOTORS. [Exp. 



is E--X, as in a transformer, where X is the total leakage react- 

 ance of both primary and secondary ; as X becomes smaller, the 

 diameter becomes larger, the curve shown in Fig. 5 becoming more 

 nearly a straight line. 



APPENDIX I. 



MISCELLANEOUS NOTES. 



50. Operation of a Polyphase Motor. If a cylinder of glass, wood 

 or other material in which there are no electric or magnetic losses is 

 rotated in a stationary magnetic field, no torque is required friction 

 or mechanical load being neglected. If, however, the cylinder is con- 

 structed of a material as copper, in which there are losses due to the 

 currents induced by the cutting of magnetic flux, it will require a 

 torque to turn the cylinder depending upon the losses. 



51. Similarly, in a rotating magnetic field, the cylinder in which 

 there are no losses will remain at rest; there will be no tendency for it 

 to turn and no torque will be required to hold it stationary. On the 

 other hand, the cylinder of copper in which there are losses will tend 

 to rotate with the field; to hold it stationary would require a torque 

 depending upon the losses. 



52. In a like manner, it may be seen that an induction motor would 

 have no torque and would not start if there were no losses in the rotor. 

 Starting torque is dependent upon rotor losses. 



53. Torque in an induction motor, as in any motor, is due to the 

 force exerted by the magnetic field upon conductors carrying current 

 in that field, 3, Exp. 2-A. 



After an induction motor starts, it continues to acquire speed until, 

 when there is no load, nearly synchronous speed is reached. If the 

 rotor did revolve at synchronous speed the same speed as that of 

 the rotating magnetic field there would be no cutting of flux and 

 hence no induced current in the secondary and" no torque. The motor 

 would then slow down and the cutting of flux would increase until the 

 secondary current is sufficient to produce enough torque to maintain 

 rotation. 



54. Torque, Slip and Secondary Losses. When an induction 

 motor is loaded, it slows down according to the load, in order to have 



