2/8 ELEMENTS OF ELECTRICAL ENGINEERING. 



Therefore every detail of action of an induction motor running 

 at slip s would be reproduced at standstill by increasing the rotor 

 resistance in the ratio I /s ; except that while running, the power 

 delivered to the rotor consists of mechanical power and electrical 

 power in the ratio of I s to s, whereas at standstill, although 

 the same total power would be delivered to the rotor, it would 

 all appear as electrical power in the increased resistance of the 

 rotor windings. For example, the action of an induction motor 

 running at $ slip would be reproduced as pure transformer action 

 at standstill if the rotor resistance were increased twenty times, 

 the action of an induction motor running at ^ slip would be 

 reproduced at standstill if the rotor resistance were increased ten 

 times, and so on. That is, the larger the slip (the larger the 

 load on the motor) the less the rotor resistance would have to be 

 at standstill to reproduce the details of the induction motor action 

 as pure transformer action, or, in other words, the heavier the 

 motor load the heavier the load on the equivalent standstill trans- 

 former, the variations of load on the equivalent standstill trans- 

 former being due solely to changes of secondary (rotor) resistance. 



136. The application of the circle diagram to the induction 

 motor. Since the action of an induction motor with varying load 

 is equivalent to the action of a transformer (the equivalent stand- 

 still transformer) with varying secondary resistance, it is evident 

 that the circle diagram of the transformer, which represents the 

 action of a transformer with non-inductive secondary receiving 

 circuit of variable resistance (see Art. 121), may be used to rep- 

 resent the action of the induction motor. 



Determination of the circle diagram by experimental tests on a 

 given motor. The magnetizing current M in each phase of the 

 stator winding may be determined by driving the motor at zero 

 load, the effective value of M being measured by a properly 

 inserted ammeter, and the phase difference, m , between E' (of one 

 phase) and M being calculated from the measured values of E 1 ', 

 and M, together with the power E f M cos m as measured by a 

 wattmeter. Lay off the line OM, Fig. 242, representing to scale 



