THE CONTINUOUS CURRENT MOTOR 



173 



Power Supply 



sometimes designed so that it takes a current not greater 

 than the full-load current of the motor. 



Example of a Proper Starting Rheostat. Suppose that a 

 starting box is desired for the 100-volt, 40-ampere motor 

 mentioned in the previous paragraph. If the current in 

 the armature circuit is to be 40 amperes at the start, the 

 total resistance of the armature 

 circuit (armature resistance and 

 starting box) must be 110/40 = 2.75 

 ohms. As the armature resistance 

 is 0.2 ohm, the total resistance of 

 the starting box must be 2.75 0.2 = 

 2.55 ohms. The wire of which the 

 starting box is made must be of 

 sufficient size to carry safely 40 

 amperes during the short time 

 required for the acceleration of the 

 motor armature. 



This resistance of 2.55 ohms 

 would have taps at about five 

 points so that it could be gradually 

 cut out as the motor speeds up. 

 The steps are not even; the above 

 rheostat would probably be divided 

 into steps of 2.55 ohms, then 1.7 F IG . 104. Connection of 

 ohms, 1.0 ohm, 0.4 ohm, and 0.2 Resistance for Starting 

 ohm. The connection of the arma- a Motor, 

 ture to the line through this variable 



resistance is shown in Fig. 104. The wires of which the 

 starting box resistance is composed are imbedded in sand 

 in the best type of starting boxes; they are then enclosed 

 in fire-proof material so that if the operator keeps the 

 starting resistance in the circuit for a longer time than that 

 for which it was designed, thus overheating and possibly 

 melting it, no fire risk occurs. 



A starting box of good design and manufacture is shown 



