276 ELEMENTS OF ELECTRICAL ENGINEERING. 



magnetism and rotor induces electromotive forces in the rotor 

 windings, these induced electromotive forces produce currents in 

 the rotor, and the magnetizing action of these rotor currents is 

 balanced by the equal and opposite magnetizing action of additional 

 currents in the stator windings, exactly as in a transformer of 

 which the secondary delivers current to a receiving circuit ; this 

 additional current in each phase of the stator winding is called 

 the load current and it is represented in the following discussion 

 by the letter /'. 



Slip. In considering the action of the induction motor under 

 load, it is convenient to express the relative speed n n f of 

 stator magnetism and rotor in terms of the speed n of stator 

 magnetism (synchronous speed as it is called) taken as unity. 

 When so expressed the relative speed is equal to (it n')/n, it 

 is called the slip of the motor, and it is represented in the fol- 

 lowing discussion by the letter s. It is well to think of slip 

 always as signifying merely the relative speed of stator mag- 

 netism and rotor, and much confusion of thought may be 

 avoided by remembering that the slip of an induction motor is 

 usually less than unity, being zero when the rotor runs at syn- 

 chronous speed and being equal to unity when the rotor stands 

 still. 



Energy transformations in the rotor of an induction motor. 

 To reduce the action of an induction motor to an entirely equiva- 

 lent transformer action, it is necessary to consider the energy 

 transformations in the rotor ; and in order to consider these 

 transformations in the simplest possible way, let us return to the 

 conception of the stator of an induction motor as a mechanically 

 rotated field magnet. From this point of view, to assume that 

 the rotating field magnet is without friction or other losses is to 

 assume that the total power, 2irnT, required to drive it against 

 the torque T with which the stator and rotor act on each other, 

 is the total power actually delivered to the rotor. Of this total 

 power a portion 27rn'T appears as mechanical power driving 

 the rotor, and the remainder 2irnT 27rn'T\_= 27r(;/ n f )T~\ 



