466 ELEMENTS OF ELECTRICAL ENGINEERING, 



NOTE. Imagine the machine to be running at its ideal zero-load speed with no 

 volts applied to its brushes which are in the neutral axis. Under these conditions the 

 induced electromotive force (counter electromotive force) between the brushes is no 

 volts, and the induced electromotive force between two auxiliary brushes 15 ahead 

 of the neutral axis is (no 2.e). If the main brushes are now shifted 15 forwards, 

 taking the place of the two auxiliary brushes, then the speed must increase sufficiently 

 to make the induced etectromotive force equal to the applied voltage. In this problem 

 and the one that follows the armature current is assumed to be negligibly small. It is 

 to be noted that the speed equation (23^), page 98, is true only when the brushes are 

 in the neutral axis. 



87. The ideal zero-load speed of the motor specified in prob- 

 lem 78 increases to 1,700 revolutions per minute when the brushes 

 are shifted 25 from the neutral axis. Find the electromotive 

 force between : (a) an auxiliary pair of brushes in the neutral 

 axis ; (b) one of the main brushes and an auxiliary brush in the 

 neutral axis. Ans. (a) 187 volts; (b) 38.5 volts or 148.5 volts. 



88. Ignoring the demagnetizing action of the current in the 

 armature, calculate : (a) the resistance that must be connected in 

 series with the armature of the motor specified in problem 78 to 

 reduce its speed to 800 revolutions per minute when its load is 

 such as to make the armature intake 50 amperes ; (b~) the watts 

 lost in this resistance expressed in per cent, of the total power 

 intake of the motor. Ans. (a) 0.33 ohm; (b) 833 watts or 14.4 

 per cent. 



89. The field current of the motor described in problem 78 is 

 increased to 4.8 amperes by cutting out a portion of the field 

 rheostat. This reduces the full-load speed of the motor to 800 

 revolutions per minute. Calculate the increase of power deliv- 

 ered to the field circuit due to the increase of field current from 

 2.6 to 4.8 amperes, and express it in per cent, of the power lost 

 in the armature rheostat in problem 88. Ans. 242 watts or 29.1 

 per cent. 



90. The motor of problem 78 has its field permanently con- 

 nected to I lo-volt mains and its speed is reduced to 200 revolu- 

 tions per minute at full-load (50 amperes in the armature) by 

 having its armature connected to low voltage supply mains. 

 Calculate : (a) the electromotive force of the low voltage mains, 



