SPEED VARIATION OF INDUCTION MOTORS. 169 



the consequence is that the output per unit weight is much greater 

 in an induction motor than in a direct current machine. It is not 

 difficult to construct induction motors of a moderate size, which 

 only weigh 60 or 70 Ibs. per horse-power, and this without any 

 sacrifice of efficiency. 



Unless considerably overloaded, an induction motor will run 

 quite cool compared with direct-current motors of the same weight 

 and output. This is due to the large section which can be given 

 to the conductors and to the fact that the winding is so arranged 

 that the copper losses are not localized, whilst the laminated 

 character of the iron facilitates ventilation. 



Although polyphase induction motors are termed non- 

 synchronous, it should be borne in mind that there is always 

 a tendency towards synchronism. The speed of an induction 

 motor is, in fact, almost independent of the load ; the variation in 

 speed from no-load to/ull-load seldom exceeds 5 per cent. 



It is usual to make large machines multipolar in order to 

 reduce the peripheral velocity of the rotor. Small machines can 

 safely run at 2000 revolutions or more per minute, and so can be 

 made bi-polar. The angular velocity of the rotor, for a given 

 frequency in the stator currents, varies inversely as the number 

 of poles, so that large machines are necessarily of the multipolar 

 type. 



115. Before concluding this chapter, we draw the attention 

 of the reader to a few points which have to be carefully noted in 

 the design of monophase induction motors. 



There are two distinct stator- windings ; one the running coil 

 must be such as to provide a counter E.M.F. nearly equal to 

 the applied P.D. and capable of carrying the full-load current 

 continuously; the other the starting- coil which must likewise 

 provide a counter E.M.F. nearly equal to the applied P.D., but 

 since it is only in circuit for a very short time, it may run at a 

 much higher current density than the running-coil. 



Let us examine closely the conditions which the two coils 

 have to satisfy. 



When running on load, the power-factor of the motor must be 

 as high as possible. The running- coil, therefore, must be wound 

 so as to have as small an equivalent self-induction as possible ; 

 that is to say, the mutual induction between the running coil and 

 the rotor-windings should be as large as possible. The wattless 



