THE CONTINUOUS CURRENT MOTOR 



175 



Necessity of " No-voltage Release." The " no-voltage 

 release " permits the rheostat arm to fly back to the " off " 

 position if the line voltage drops below a certain value. 

 The object of this release is always to open the armature 

 circuit when the supply line becomes " dead." When this 

 occurs (as, for example, when the station circuit breaker on 

 a feeder blows) the motor im- 

 mediately slows down and stops. 

 If the line is again made alive 

 (circuit breaker re-set) an exces- 

 sive current will flow through 

 the stationary armature and 

 may injure it. Of course, the 

 over-load release would open the 

 circuit under such conditions, 

 but these over-load releases are 

 designed only to break currents 

 of about the same magnitude as 

 the full-load rating of the motor. 

 As was shown on page 172, the 

 current which flows through a 

 stationary armature when it is 

 connected directly to a normal 

 voltage line is many times the 

 full-load current and if the small 

 circuit breaker on the front of 

 the starting box were depended 

 upon to break such large currents 

 it would soon be damaged. 



Connections for Starting Rheostats. The solenoid which 

 operates the no-voltage release is connected (in series with 

 a suitable resistance) directly across the supply line, and 

 the solenoid which operates the overload release is con- 

 nected in series with the motor. The connections of such 

 a rheostat to the motor and supply line is shown in Fig. 1 06. 

 In starting a motor the arm must not be moved over too 



FIG. 106. Connections of 

 " Over-load release " Mag- 

 net and " No-voltage re- 

 lease " Magnet of a Motor- 

 starting Rheostat. 



