168 



TREATISE ON ALTERNATING CURRENTS. 



been done, to halve the speed by doubling the number of stator- 

 poles, or, generally, to reduce the speed by increasing the number 

 of stator-poles ; but such variation is not continuous, and involves 

 complicated windings. The only way of producing a continuous 

 variation of speed is by supplying the motor with current at a 



/YvS 1 

 /OO tfolfs dp Per facts /4OO 



Motor 

 m 



Output B.H P. 



FIG. 71. 



varying frequency, for which purpose a frequency trans- 

 former is required. As, however, no frequency transformer of 

 wide range has hitherto been constructed, a satisfactory solution of 

 the variation of speed of induction motors has not been arrived at. 

 The method of changing the speed by altering the P.D. applied 

 between the terminals of the stator windings is not satisfactory. 

 The P.D. is varied by means of variable resistances, placed in 

 series with the stator windings, entailing a serious waste of energy. 

 An additional objection is that the motor cannot give its full 

 rated output for any length of time when the P.D. is lowered, on 

 account of the heavy currents it requires to do so. 



114. The noticeable feature of induction motors is their 

 simplicity of structure combined with great mechanical strength. 

 The absence of commutators is an immense advantage, since there 

 is no sparking limit to the output as in direct-current motors ; 



