SWITCHING EQUIPMENT 



511 



made. The rapidity with which the air escapes, that is, the time 

 intervening between the start of the moving part and the com- 

 pletion of contact, is a function of the power behind the com- 

 pression moving part, which in turn depends on the magnitude 

 of the electrical force actuating the relay coil. The size of the hole 

 through which the air escapes can be varied so that different time 

 elements may be obtained for disturbing forces of the same mag- 

 nitude, and different time curves for the same range of disturb- 

 ance. 



In the induction type overload relay (Fig. 316), the actuating 

 forces are due to the interaction of induced currents in a moving 

 metal element with the induc- 

 ing magnetic field. A lami- 

 nated iron core is surrounded 

 by one or more windings, and 

 in the air gap of the core is 

 pivoted the moving element, 

 usually a light aluminum 

 disc. When current is passed 

 through the main windings, 

 eddy currents are induced in 

 the disc which tends to rotate 

 and close the contacts after a 

 predetermined angle of motion. 

 The retarding force is pro- 

 duced by having the same disc 

 pass between the poles of 

 permanent magnets, in which 

 case the eddy currents induced by these will retard the motion. 



The relays are designed for use in the secondary circuit 

 of current transformers, and the normal rating, or continuous 

 current-carrying capacity, is 5 amperes. Taps are provided in 

 the relay winding, and by inserting a metal plug in a current 

 tap plate, settings 4, 5, 6, 8 and 10 amperes may be obtained, 

 these figures representing the lowest current values required to 

 close the relay contacts. Any tap setting, multiplied by the 

 ratio of the current transformers, gives the corresponding primary 

 or line current. 



A time-current index plate is provided as a guide for deter- 

 mining the settings of the relay, and the current values are 



FIG. 316. Induction Type Overload 

 Time Limit Relay. 



