226 ALTERNATING CURRENTS 



time the motor is running at its normal speed. It is, however, even 

 in other cases used in combination with the methods described 

 below. 



134. Speed Control by Change in Number of 



Poles 



The speed of synchronism of an induction motor being given, in 



ft 



revs, per sec., by p, n being the frequency of the stator currents and 



P the number of pairs of poles, it is evident that doubling the 

 number of poles will halve the speed. By suitably arranging the 

 stator winding,* and providing a pole-changing switch, it is possible 

 to run at two different speeds, the lower of which is about half the 

 higher. This method of speed control has been used by the Oerlikon 

 Co. since 1893. In some motors, two independent windings, 

 corresponding normally to different numbers of poles, each of which 

 may be halved by a pole-changing switch, are provided, so that four 

 different speeds are available. Thus, a motor wound for 12 and 

 6 poles, and 8 and 4 poles, will give (synchronous) speeds, at a 

 frequency of 50, of 500, 1000, 750, and 375 revs, per min. 



135. Tandem Control. Multiple Motor Method 



Another method which has been used is that known as the 

 cascade or tandem or concatenation method of coupling induction 

 motors. In its simplest form, it consists in using two motors 

 mechanically coupled (the coupling may be direct, as when the 

 motors are mounted on the same shaft ; or indirect, as in the case of 

 the two motors of a tramcar or railway carriage; or some form of 

 mechanical gearing having any desired speed ratio may be used) so 

 as to run at the same speed, the stator of the first motor, which we 

 may call motor I, being connected to the mains, and the rotor of this 

 motor being connected to the stator of the second motor (motor II), 

 whose rotor circuits are closed, either by being short-circuited or 

 through suitable resistances, as at starting. Let us suppose that 

 rotor II has been short-circuited, and that stator I is switched on. 

 The motors start, their torques at first increasing as they gain speed. 

 But as the speed increases, the frequency of the currents in stator II 



* With a squirrel-cage rotor, no special device is required in the rotor when the 

 number of stator poles is changed. But wound rotors must be of special construction 

 such as that illustrated in Fig. 139, in order to enable them to run when the number 

 of poles is altered. 



