8-A] OPERATION AND LOAD TEST. 277 



ance loss. Reducing the value of R a makes the speed more nearly 

 constant. There is no generally satisfactory way for varying the 

 speed; the following methods,* however, may be used. 



60. Rheostatic Control. A variable speed may be obtained by 

 varying the secondary resistance. As already stated, this method is 

 inefficient, the operation being the same as that of a shunt motor with 

 a resistance in series with the armature. When there is much reduc- 

 tion in speed, the speed will vary greatly with load. 



61. Changing Number of Poles. A multispeed motor with a 

 limited number of definite speeds can be obtained by changing the 

 number of poles and this can be accomplished by re-grouping the coils 

 of the primary winding or by using a separate winding for each speed. 



62. Tandem or Cascade Operation. This involves the use of two 

 motors, the rotors of which are mounted on the same shaft or other- 

 wise mechanically connected, and arranged so that the secondary of 

 the first motor supplies current to the primary of the second motor. 

 The two motors may have the same number of poles, but usually the 

 number of poles is different. The effect produced by the second motor 

 is that of adding its poles to or subtracting them from the first motor 

 so that four synchronous speeds are produced, two for the motors 

 independently and two for the combinations. 



63. By Varying Frequency. The synchronous speed of an induc- 

 tion motor may be varied by changing the supply frequency by means 

 of apparatus external to the motor or constructed as a part of the 

 motor itself. Such variation may also be obtained by supplying cur- 

 rents of constant frequency to the primary and of variable frequency 

 to the secondary. 



*For full discussion, see Multispeed Induction Motors, by Reist and 

 Maxwell, A. I. E. R, Vol. XXVIII., p. 601 ; also articles by H. C. Specht, 

 A. I. E. E., Vol. XX VII, p. 1177, and Electric Journal, Vol. 6, pp. 421, 

 492, 577, 611, 73i. 





