388 THE INDUCTION MOTOR. 



relations are seen to hold in the case of the experj- 

 mentally-obtained curves. 



As the rotor current is practically proportional to 

 the load within the usual working range, 'the curves 

 in Fig. 189 will be similar to curves of primary current 

 plotted on a load base. The similarity to these curves 

 of the curves of stator current shown in Figs. 185 and 186 

 confirms this statement. 



Power Absorbed and Efficiency. The losses occurring in the 

 motor are practically of three kinds. (a) The loss in the 

 stator and rotor windings ; (6) iron losses in stator and 

 rotor cores ; (c) frictional losses. 



(a) In a three-phase motor, the watts lost in the stator 

 winding are equal to ^c^r^ where c, is the primary current 

 per phase, and r l is the resistance of the winding per phase. 

 If the rotor winding is a short-circuited three-phase wind- 

 ing the watts lost in the rotor are 3c 2 2 f 2 where the symbols 

 ,, r, denote the current and resistance per phase of the 

 rotor winding. Both losses increase together as the load 

 on the motor is increased. 



(6) The iron losses in the stator will depend on the 

 saturation of the iron and the frequency of the magnetic 

 changes. It has already been shown that the strength of 

 the primary field is constant at all loads, depending only 



on the supply voltage. The rotating field makes revs 



per min. when n = frequency of supply and p = number of 

 pairs of poles. Thus the iron of the stator passes through 

 a complete magnetic cycle n times per second. This source 

 of loss is consequently independent of the load. 



The iron losses in the rotor will be proportional 

 to the slip, since the polarity of the rotor core will 

 change p times for each rotation of the rotating field 

 relative to the rotor. The slip under ordinary working 

 conditions is so small that the maximum rate of magnetic 

 reversal is only about 5 per cent, of the speed of the 

 rotating field. The iron losses in the rotor may conse- 

 quently be safely neglected in comparison with the other 

 losses. 



(c) The frictional losses consist of bearing friction 

 and windage. Since these depend almost entirely on the 

 speed, which is practically constant, and not appreciably 



