CHAPTER XI 

 DIRECT CURRENT STATIONARY MOTORS 



The connections and wiring of all motors should be 

 carefully examined, with particular reference to t-he 

 field. At starting, the speed of the machine must be 

 carefully followed with a tachometer, and the circuit 

 breaker immediately opened if the speed rises above 

 the prescribed limit. 



With the starting rheostat or water box in the off 

 position, the terminals of the rheostat or box must be 

 attached across the open main switch, with the circuit 

 breaker closed; the lower terminal being attached first. 

 The field switch should then be closed and the pole 

 pieces tested with a piece of iron for excitation. The 

 resistance across the main switch should then be grad- 

 ually cut out and, if the speed is all right, the main 

 switch closed. 



If the motor runs above normal speed the wiring 

 should be carefully examined to see that the field is 

 connected across the circuit. Sometimes by mistake 

 the field is connected across the main switch; in which 

 case as soon as the starting resistance is cut out the field 

 current falls rapidly and the motor speeds up excessively. 

 To test for wrong connection, read volts field during 

 starting and, if the field is wrongly connected, the volts 

 field will drop as the starting rheostat is cut out. 



If a potential curve cannot be taken on a motor 

 with a multiple wound armature by running it as a 

 generator, a "motor potential curve" may be taken 

 by the following method: The machine is run as a 

 motor with the field self-excited, the field current is 

 held constant, and a constant voltage is applied to the 

 armature, using only two adjacent sets of brushes on 

 the commutator. A careful reading of the speed is 

 then taken. The brushes on the next pair of studs 



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