172 ELECTRICAL MACHINERY 



flow in the armature circuit may be calculated from the 

 equation 



E = e-\-IR a , and as e = Q, 



= IR a or 1 = - ..... (35) 

 ria 



Necessity of Resistance for Starting a Motor. Now the 

 current calculated from this equation will be ten or twenty 

 times the full load current and will be disastrous in its 

 results. 



Suppose a 110- volt motor, the full-load current of which 

 is 40 amperes. The armature resistance of such a motor 

 would be about 0.2 ohm. If the stationary armature were 

 connected directly to the 110-volt line, the current through 

 the armature would be 110-7-0.2 = 550 amperes, whereas the 

 full-load current is only 40 amperes. The current of 550 

 amperes would burn the brushes, commutator, and wind- 

 ing, and also would probably blow the fuses in the supply 

 line. 



After the motor is running there is not an excessive 

 current flowing through the armature because the current 

 is limited by the counter e.m.f. But while the armature 

 is accelerating this counter e.m.f. is small, and some other 

 means must be employed to limit the starting current. 

 This is the function of the motor-starting rheostat, or, 

 as it is frequently called, the starting box. 



A starting rheostat consists of a variable resistance, 

 which may be gradually cut out as the motor speeds up 

 and which can be cut out altogether when the motor has 

 reached nearly normal speed. The total resistance of the 

 starting box must be of such a value that, when it is con- 

 nected directly across the motor supply line, the current 

 which flows through it will not be greater than about 150% 

 of the full-load current for the motor. The starting box is 



