SPEED CONTROL 225 



capacity equal to some 30 per cent, of that of the induction generators 

 for providing the necessary exciting or wattless current. It is mainly 

 for this reason that induction generators have not come into use, in 

 spite of the fact that a great deal of ingenuity has been devoted to 

 special types of this machine.* 



Since an induction generator does not run at synchronous speed, 

 it is frequently termed an asynchronous generator. 



The fact that an induction motor when driven above synchronous 

 speed is capable of acting as a generator is turned to useful account 

 in connection with railways employing such motors, the motors in 

 descending a steep gradient being made to return power to the line 

 by their generator action; the power is thereby saved, instead of 

 being dissipated by the use of ordinary brakes. 



'33 Speed Control of Induction Motors. 

 Rheostatic Control 



The induction motor is, as we have seen, practically a constant 

 speed motor, its slip at full load amounting to not more than 5 per 

 cent, even in the case of small motors, and to much less in larger 

 motors. Now, for many practical purposes such as traction or 

 crane work it is essential to have a motor whose speed may be 

 varied within much wider limits. We shall now consider some of 

 the methods of speed control which are used in practice. 



The method most commonly employed consists in introducing 

 resistances into the rotor windings. We have already seen ( 106) 

 that the introduction of non-inductive resistances into the rotor 

 circuits has the effect of reducing the speed while leaving the torque, 

 the currents, and their phase relations unaltered. The total power 

 drawn from the mains is obviously also unaltered, and the loss of 

 mechanical power due to speed reduction (at constant torque) is 

 accounted for by the power dissipated in the external resistance. 

 This method, although giving any desired range of speed variation, 

 is obviously very wasteful, especially at the lower speeds. Hence it 

 can only be tolerated when the periods of variable speed are 

 comparatively short, and when during the greater portion of the 



* Leblanc has devised an interesting form of exciter for use with an induction 

 generator (Eclairage Ekctrique, vol. xyiii. pp. 161, 376 (1899)). McAllister bus found 

 that the wattless current may be supplied by condensers (Electrical World and Engineer, 

 vol. xli. p. 109). An induction generator requires no synchronizing before being thrown 

 into parallel with other machines, and is entirely free from the trouble of " hunting " 

 or " phase-swinging " to which synchronous generators are occasionally liable. The 

 special forms of induction generators in which commutators arc used arc dealt with in 

 Chapter XVI. 



