464 ELEMENTS OF ELECTRICAL ENGINEERING. 



CHAPTER IV. THE OPERATION OF THE DYNAMO AS A MOTOR. 



78. A shunt motor, connected to I lo-volt supply mains, takes 

 2.6 amperes through its shunt field winding, and, when the 

 motor is unloaded, it takes 3.0 amperes through its armature and 

 runs at a speed of 997 revolutions per minute. The rated full- 

 load armature current of this motor is 50 amperes, and the 

 resistance of its armature including brushes and brush contacts is 

 0. 1 1 ohm. At what speed would this motor run if the friction, 

 eddy current, and hysteresis losses in the armature were zero ? 

 Ans. 1,000 revolutions per minute. 



NOTE. This speed is called the ideal zero-load speed of the motor. It is the 

 speed at which the motor would run if no torque were required to supply the losses 

 due to friction, eddy currents, and hysteresis. If these losses were zero, the armature 

 current at zero-load would be zero, the ri drop in the armature would be zero, and 

 the induced electromotive force would be equal to E x . 



79. From the data given in problem 78 find : (a) the counter 

 electromotive force of the motor at full-load, and (b) the full-load 

 speed of the motor (assuming 4> to be constant). Ans. (a) 104.5 

 volts ; (B) 945.5 revolutions per minute. 



80. The actual full-load speed of the motor of problem 78 is 

 observed to be 980 revolutions per minute instead of the speed 

 as calculated in problem 79. Find the value of the ratio 4>/<J>' 

 where <I> is the armature flux due to the field winding alone, and 

 <f>' is the actual value of the armature flux at full-load due to the 

 combined action of field and armature currents. Ans. 1.036. 



81. (a) Calculate the resistance that must be connected in 

 series with the armature of the motor of problem 78 to give a 

 speed of. 500 revolutions per minute under full-load (50 amperes 

 in the armature) ; (b) find the speed of the motor when, with this 

 resistance in the armature circuit, the load is reduced to one half 

 (25 amperes in the armature) and to one quarter (12.5 amperes 

 in the armature) respectively. In this problem assume that <I> 

 remains the same as at zero-load. Ans. (a) 0.99 ohm ; () 750 

 and 875 revolutions per minute. 



82. (a) Find the value of the induction factor, 3>Z', of the 



