122 ALTERNATING CURRENTS 



with the high starting torque and moderate starting current charac- 

 teristic of the wound rotor designed for use with an external starting 

 resistance. One of the most ingenious solutions of this problem is 

 to be found in the motor recently patented by Mr. F. Lewis, and 

 manufactured by the Electric Construction Co., of Wolverhampton. 

 The construction of this motor is based on the fact that whereas a 

 squirrel-cage rotor may be used in connection with any rotating field, 

 quite independently of the number and sequence of the magnetic 

 poles, a wave-wound rotor will only develop a torque in a field having 

 the correct number and sequence of magnetic poles. In the Lewis 

 motor, the rotor is provided with a double winding a low-resistance 

 wave winding, which is permanently short-circuited on itself instead 

 of being connected to slip-rings, and a high-resistance squirrel-cage 

 winding. At starting, the low-resistance winding is rendered in- 

 operative by reversing the normal polarity of one half of the field. 

 Thus, considering the case of an eight-pole motor, in which the 

 normal sequence of poles is NSNSNSNS, at starting the sequence is 

 arranged to be NSNSSNSN. The rotor, therefore, behaves as if it 

 were provided with the high-resistance squirrel-cage winding alone, 

 and starts with a powerful torque. If it were allowed to run on this 

 winding, not only would an excessive temperature rise take place, 

 but the efficiency would also be poor. The normal polarity is there- 

 fore restored as soon as the speed has reached a certain limit, and 

 the low-resistance wave winding now comes into play. The speed 

 rises, the slip decreasing, and with it also the currents in the squirrel- 

 cage. Although under normal running conditions the squirrel-cage 

 conductors are still traversed by feeble currents and do part of the 

 driving, yet the bulk of the torque is due to the wave winding, the 

 presence of the squirrel-cage being simply equivalent to a slight 

 increase of cross-section in the conductors of the wave winding. The 

 squirrel-cage consists of conductors of small cross-section arranged in 

 the upper parts of the slots containing the large conductors which 

 form the wave winding. 



65. Example of Induction Motor. Variation of 

 Torque with Position of Rotor 



In Fig. 95 is shown a 5-h.p. three-phase induction motor, having 

 a short-circuited rotor.* The stator is provided with a three-phase 

 coil winding embedded in open slots, the coils being wound on 

 formers and then fitted into position in the slots. The stator 

 stampings or laminations are supported by a hollow cylindrical 



* The author is indebted to the Electric Construction Co., Ltd., of Wolverhampton, 

 for drawings from which Fig. 95 has been prepared. 



