DIRECT CURRENT STATIONARY MOTORS 77 



should be placed on the commutator, and the speed 

 again taken with the same voltage and field current 

 as before; this procedure being repeated for all pairs 

 of adjacent brushes. For a direct current generator, 

 the speed should vary directly with the voltage if a 

 potential curve is taken as described. This method 

 should never be employed unless it is impossible to 

 drive the machine as a generator, as it is very difficult 

 to read the tachometer sufficiently accurately. 



With no load, normal voltage, and full field, a speed 

 reading is taken, the brushes being shifted so that when 

 full load is on, the speed is not less than 5 per cent 

 below nor more than 2 per cent above normal speed. 

 At the end of the speed run the machine is loaded, the 

 brushes shifted if necessary, and the commutation 

 noted. 



On compound wound motors, a shunt is adjusted 

 across the series field to give a speed within a few per 

 cent of the correct speed at rated load. Speed curves 

 and running light should be taken with the series field 

 disconnected. 



Running light should be taken at hot full load speed. 



Commutating Pole Motors 



The electrical neutral on commutating pole motors 

 is determined by shifting the brushes until the same 

 speed is obtained in both directions with the same 

 value of field current. This position of the rocker arm 

 is marked. In double speed motors of this type, the 

 neutral should be obtained at the high speed. 



Machines sometimes hunt with full commutating 

 pole field, thus preventing the location of the neutral 

 from being obtained. In this case, the field current 

 should be slightly shunted, even if commutation is 

 affected. Good commutation is rarely obtained in 

 the unstable condition. 



In testing motors sent out as single units, of which 

 the direction of rotation is not known, the electrical 

 neutral should be located by shifting the brushes at 

 no-load, till a position is found that will give the same 

 speed in both directions of rotation. The fibre brush 



