ELECTRICAL MACHINERY 489 



rection of the current and the turning effect is thrown to 

 the other side and the armature moves on. 



Counter electromotive force of a motor. The armature 

 wires of a motor rotating in its own magnetic field cut 

 the lines of force as if the motor were being driven as a 

 dynamo, consequently there is an induced E.M.F. in 

 them. The direction of this induced E.M.F. is op- 

 posite to that of the applied pressure. Such an induced 

 E.M.F. is known as counter electromotive force and/ is 

 an important property of the motor. A motor without 

 load will run with sufficient speed that its counter electro- 

 motive force will very nearly equal the applied pressure. 

 The counter E.M.F. will never be as great as the ap- 

 plied force. There will always be a difference between 

 these, equal to the loss due to resistance in the motor 

 armature. The power of a motor increases as the counter 

 E.M.F. decreases until the counter E.M.F. is one-half of 

 the applied E.M.F., then the power of the motor decreases. 

 The maximum power of a motor is reached when the counter 

 E.M.F. is one-half of the applied E.M.F. 



Losses of a motor. The losses of a motor, like those 

 of a dynamo, are due to resistance in the armature fric- 

 tion, eddy currents and hysteresis. 



660. Operating motors. The resistance in the arma- 

 ture of a motor is so low that if a motor were directly con- 

 nected to the supply mains, too great a current would 

 flow through it before a counter E.M.F. could be set up, 

 consequently the machine would be practically short-cir- 

 cuited and the windings damaged. For this reason a 

 rheostat known as a starting rheostat is inserted into the 

 armature circuit of a shunt motor. To start the motor, 

 switch A (Fig. 368) is closed, and this throws the cur- 

 rent into the fields and excites them; then the arm is 

 moved over the starting box to point one, and when 



