CHAP. VII] M.M.F. OF DISTRIBUTED WINDINGS 133 



Sometimes the secondary winding consists of coils individually 

 short circuited; this is an intermediate type of winding between 

 an ordinary squirrel-cage winding and a three-phase winding 

 such as is used with slip-rings. Let the foregoing motor be 

 provided with such a winding, of the two-layer type, and let the 

 rotor have 71 slots, 6 conductors per slot, the coils being placed 

 in slots 1 and 14. In formula (64) m stands for the number of 

 symmetrically distributed phases, the current in each phase being 

 displaced in time by 2n/m with respect to that in the next phase. 

 In the winding under consideration, each coil represents a phase, 

 and one has to go over a pair of poles until one finds the next coil 

 with the current in the same phase. Thus, in this case, the num- 

 ber of secondary phases is equal to the number of slots per pair of 

 poles, or m=35.5. Each coil has 3 turns, but there is only one coil 

 per pair of poles, so that n= 1.5. Substituting these values into 

 eq. (64), and also M =5580, fc 6 =0.912, we find i= 128 amp. As a 

 matter of fact, in this case it is not necessary to decide what the 

 values of n and m are, because eq. (64) contains only the product 

 mn, which is the total number of turns per pole. Thus, in our case 

 ron=(7lX3)/4. 



Formula (64) holds also for a squirrel-cage winding, the number 

 of secondary phases being equal to the number of bars per pair of 

 poles. Since there is but one bar per phase, each bar can be con- 

 sidered as one-half of a turn, and in formula (64) n=0.5 and k b = 1, 

 so that it becomes 



M=0.45iC 2 /p, (64a) 



where C 2 is the total number of rotor bars, and p is the number of 

 poles. Or else, one may say that the total number of turns per 

 pole is equal to one-half the number of bars per pole, so that 

 mn = JC2/p. This again gives eq. (64a). For a direct proof of 

 formula (64a) see problem 15 below. Applying this formula to 

 the same rotor with 71 slots we find that the current per bar is 

 700 amp. 



(6) The Equivalent Secondary Winding Reduced to the Primary 

 Circuit. When investigating the general theory of the induction 

 motor or calculating the characteristics of a given motor, it is cou- 

 nt to replace the actual rotor winding by an equivalent wind- 

 ing identical with the primary winding of the motor. In this case 

 the primary current transmitted into the secondary is equal to the 



