34 DIRECT CURRENT MOTORS. [Exp. 



an appreciable time) before* the armature circuit is closed. The 

 armature circuit is then closed and this is commonly done by the 

 starting box lever. 



Bring the motor, unloaded, up to speed by cutting the starting 

 box resistance slozvlyf out of circuit until the whole resistance 

 is cut out. Note the ammeter during the process and the in- 

 crease of speed as indicated by the hum of the motor. The 

 starting box should be kept in circuit only during starting, for 

 (except in special cases) it is not designed for continuous 

 operation. 



If the motor does not now run at normal speed, the speed can 

 be increased by gradually varying the field current by means of 

 the field rheostat. Do not reduce the field current too much, 

 nor under any circumstances break the field circuit,$ or the motor 

 will run at a dangerous speed. 



Note the speed at no load for several excitations; also, when 

 facilities permit, for several supply voltages. (For example, 

 operate a no-volt motor with 55 volts on armature and on field; 

 with 55 volts on armature and no volts on field; but not with 

 no volts on armature and 55 volts on field.) 



12. Stopping. Motors are commonly stopped by opening 

 the supply switch and not by first opening the armature circuit. 



* ( na). If the starting box were made with sufficiently high resistance, 

 so as to properly limit the current irrespective of counter-electromotive 

 force, the armature circuit could be closed simultaneously with the field. 

 This, however, is not usual practice. 



t (nb). Starting boxes are sometimes made so that it is impossible to 

 manipulate them too rapidly. The " multiple-switch " motor starter, used 

 particularly in starting large motors, has a number of switches, thrown 

 successively by hand; these give good contact for large currents and re- 

 quire time for cutting out the successive sections of the resistance. 



J ( nc). Automatic Release. This danger is commonly guarded against 

 by a solenoid on the starting box which releases the lever and allows it to 

 spring back to the starting position when there is no current in the field 

 circuit. This also acts as a " no-voltage " release, giving protection against 

 damage which might occur were the current supply cut off and put on 

 again with the starting box resistance all out. 



