GENERAL BEHAVIOUR OF INDUCTION MOTOR 125 



coil will thus be somewhat irregular, and will differ from that of the 

 ends of the remaining two sets of coils. This asymmetry may, 

 however, be entirely done away with by adopting the arrangement 



FIG. 96. Type of Stator Winding. 



shown in Fig. 96. Instead of being made alternately straight and 

 bent, the coil ends are here all of the same type, being bent obliquely 

 so as to clear each other. 



67. General Characteristics of Induction Motor. 

 Air-gap Length. Effect of Rotor Eccentricity 



In its general characteristics the polyphase induction motor 

 resembles the ordinary shunt-wound continuous-current motor. It 

 is essentially a constant-speed motor, the drop of speed from no load 

 to full load being very slight. One of the disadvantages of this type 

 of motor is its somewhat low average power factor, on account of 

 which the current drawn from the mains is larger than it need be for 

 the amount of power developed by the motor. The power factor of 

 an induction motor which is running light is very low, being generally 

 less than 0'2. It increases, however, very rapidly with increase of 

 load. The slip at full load, when expressed as a percentage of the 

 speed of the stator field, varies from about 10 per cent, for very small 

 motors to below 2 per cent, for very large ones. The following 

 table may be taken as representing the average performance of 

 modern induction motors : 



Horse-power 1 5 20 300 1000 



Efficiency at full load 80 85 88 93 95 



Full-load power factor 0'80 0'84 0'87 0'91 0'94 



Slip as percentage of field speed 10 6 4 2 T2 



