260 INDUCTION MOTORS. [\-^v. 



constructed with separate poles, as in Fig. i. The primary wind- 

 ings are usually distributed in slots and are so arranged in groups 

 that a series of poles are produced which correspond to those 

 shown in Fig. I. There are a number of slots for each pole. A 

 lap or a wave winding (see 3b, Exp. i-A) can be used, accord- 

 ing to various winding schemes, as described in text-books. 



8. Squirrel Cage Secondary. The secondary winding usually 

 consists of a squirrel cage, made up of parallel copper bars set 

 in slots with ends connected by two short-circuiting rings. Such 

 a construction is strong and simple; it makes possible a low 

 secondary resistance which gives high efficiency and good speed 

 regulation but low starting torque. The squirrel cage secondary 

 is self-contained and has no outside connection. 



9. Phase-wound Secondary. To obtain a higher secondary 

 resistance and hence a greater starting torque,* the secondary 

 is sometimes wound, or phase-wound as it is called. Extra sec- 

 ondary resistance, either internal or external to the motor, may 

 be included in the circuit on starting to increase the starting 

 torque, this resistance being cut out as the motor speeds up so as 

 to give higher efficiency and better speed regulation while run- 



* (pa). Maximum Torque. Torque is proportional to secondary 

 electrical input, EJ Z cos # 2 , see 54. Starting torque is proportional to 

 secondary input at standstill and is a maximum when Rj=X 2 , for h 

 then lags 45 behind E z and has a maximum power component in phase 

 with E 2 . (In any constant potential circuit with constant reactance, cur- 

 rent has a maximum power component when the lag angle is 45, as can be 

 seen in Fig. 2, Exp. 4-B.) Increasing R 2 beyond a certain amount will 

 decrease the starting torque. 



When running with a slip s, the secondary reactance becomes sX^ and 

 the torque is a maximum when R 2 = sX 2 . The secondary resistance can 

 be given such a value as will give the motor its maximum torque at any 

 desired slip, as for example at standstill when s= i.oo. 



The maximum value which the torque can have, irrespective of speed, is 

 independent of R, being dependent solely upon the input E -s- 2X, except 

 for the small effect of primary losses ; see I2a, 22, Exp. 8-B. Changing 

 R 2 can not alter this maximum, but can cause it to occur at any desired 

 speed. 



