SPEED CONTROL 263 



that in this case E r is practically balanced by V, so that there is 

 no current in the rotor except the very small magnetizing current 

 (V beiiiL, r slightly greater than Er). The rotor will remain at rest, 

 as there is no torque. 



Lastly, consider the case in which C r > C,, and which is of more 

 practical interest than the others. We now have the resultant rotor 

 e.m.f. positive, according to equation (1), so that a current flows 

 through the rotor windings, as in an ordinary induction motor, in 

 the direction of the induced rotor e.m.f. Hence the rotor experiences 

 a driving torque in the direction of the rotating field, and it will 

 run up to a speed at which the induced e.m.f. is nearly balanced by 

 the p.d. If a load be applied to the rotor, the speed will decrease 

 slightly, the current and torque increasing to the amounts required 

 to deal with the resisting torque corresponding to the load. The 

 increase in the rotor ampere-turns due to the load current must be 

 balanced by an equivalent number of opposing ampere-turns in the 

 stator. Now, since the rotor load current flows in the direction of 

 the induced e.m.f., the stator load current must flow in the direction 

 of the p.d., i.e. the stator will absorb power from the mains, and the 

 rotor return power to them. 



As in the case of an ordinary induction motor, there will be a 

 very small percentage change of speed from no load to full load.* 

 This is due to the low resistance of the rotor windings. The motor 

 will, therefore, run at nearly constant speed. The slip s of the rotor 

 may easily be found approximately, since, if m denotes synchronous 

 speed, we must, neglecting resistance drops and magnetic leakage, 

 have the relations 



E, = KmC, = V = E r = KsmCr 



K being a constant. If we denote the rotor speed (I s)m by m r , 

 the above equation gives 



mC 8 = smCr = (m w r )C r ...... (2) 



The speed of the motor may, therefore, be arranged to have any 

 desired value between zero and near synchronism by suitably 

 choosing C r ,t and may even be made negative by making C r < C*. 



This suggests a simple means of varying the speed of such a 

 motor without any sacrifice of efficiency such as is involved in the 

 rheostatic method of control sometimes employed with induction 



* By introducing non-inductive resistances between the mains and the rotor brushes, 

 the speed could be controlled as in an ordinary induction motor. 



t In this respect, a commutator induction motor closely resembles nn ordinary 

 shunt-wound continuous-current motor, which is also a nearly constant-speed motor, and 

 whose speed, for a given p.d. of supply, may be made to have any desired value by 

 suitably choosing the number of armature conductors. 



