THE INDUCTION MOTOR 347 



scheme is only applicable where two equal motors are con- 

 nected to the same load, as in electric cars. 



There are other methods for controlling the speed of an 

 induction motor but they are all either inefficient or com- 

 plicated and high in first cost. We must conclude as before 

 that the induction motor is essentially a constant speed 

 motor. 



99. Single-phase Induction Motor. All of the previous 

 analyses have been carried out with a polyphase motor 

 in mind. In some respects the single-phase induction 

 motor differs from the polyphase motor. 



Single-phase Motor has no Starting Torque. Unless sup- 

 plied with some additional starting device, such as a com- 

 mutator, or extra stator winding with inductances or con- 

 densers, etc., the single-phase induction motor has no starting 

 torque. But when the motor is operating near synchronous 

 speed (with not more than about 20% slip) its characteristics 

 are exactly like those of a polyphase induction motor. 

 There are some slight differences, such as a lower power 

 factor and less slip in the single-phase motor than in the 

 polyphase, but these differences are slight. 



The absence of starting torque is due to the fact that the 

 stator has only one set of coils and is supplied with single- 

 phase current only; it can, therefore, produce only an oscillating 

 magnetic field and not a rotating magnetic field. To develop 

 torque it is necessary to have a rotating magnetic field as 

 was shown for the polyphase motor. When the motor is 

 running at nearly synchronous speed the rotor currents, 

 acting in conjunction with the single-phase stator winding, 

 actually do produce a rotating magnetic field and for this 

 reason the single-phase and polyphase motors act alike near 

 synchronous speed. 



Split-phase Method for Starling a Single-phase Motor. 

 One way of designing a single-phase motor to give it a 

 starting torque is known as the split-phase method. 

 Although there is only one winding on the stator which can 



