CHAPTER XIV 



129. Generator action of induction motor at hypersynchronous speed. Phase relation 

 of stator and rotor currents 130. \\-ctor diagram of e.m.f.'s and its transforma- 

 tion 131. Generalized circle diagram 132. Characteristic features of induc- 

 tion generator 133. Speed control of induction motors. Rheostatic control 

 134. Speed control by change in number of poles 135. Tandem control. 

 Multiple motor method 136. Single-phase induction motors. Theory of motor 

 at rest 137. Torque exerted by single-phase induction motor when running 

 138. Effect of varying resistance of rotor circuits 139. Starting of single- 

 phase induction motors. 



129. Generator Action of Induction Motor at 

 Hypersynchronous Speed. Phase Relation of 

 Stator and Rotor Currents 



A POLYPHASE induction motor in many respects resembles an ordinary 

 shunt-wound, continuous-current motor. Each runs at a speed which 

 decreases but little with increase of load, so that practically we may 

 regard each type as a constant-speed motor. Each develops a torque 

 which increases, for ordinary load conditions, practically in propor- 

 tion to the decrease of speed (slip) from no-load speed. Further, 

 each, if driven mechanically at a sufficient speed above the no-load 

 speed, is capable of acting as a generator. We are thus led to con- 

 sider the generator action of an induction motor when its rotor is 

 mechanically driven above the speed of synchronism. An induction 

 motor when so used forms an induction generator. 



In order to bring out as clearly as possible the relation connecting 

 the phase differences between the primary and secondary currents in 

 the two cases when the speed is a certain amount below and above 

 that of synchronism, we may consider the four diagrams (a), (>), (c), 

 and (d) of Fig. 140. The stator field is supposed to move from left 

 to right in each case. The arrows drawn along the face of the stator 

 core and at right angles to it indicate the polarity of the stator field, 

 and are drawn at the points where the induction due to the stator 

 current alone reaches a maximum value. Diagrams (a) and (c) refer 

 to the case of a motor running with 2>ositive slip, i.e. below the speed 

 of synchronism. The sine wave of magnetic flux due to the stator 

 current in sweeping past the rotor conductors induces e.m.f.'s in them, 



