Reverse Energy Relays.—The chief object of this species of relay is to protect the generator. When so used, the overload adjustment is set at the maximum value to give overload protection only at the maximum carrying capacity of the generator and a sensitive reverse protection to prevent a return of energy from the line.
Fig. 2,305.—General Electric direct current solenoid control relay. Solenoids for operating large switches, etc., frequently require comparatively large operating currents in the "closing" coils. This necessitates the use of relatively heavy leads between the control switch and the solenoid and is the cause of severe arcing at the control switch, especially with solenoids of high inductance. These objectionable features can best be eliminated by the use of a suitable control relay located near the solenoids. The control relay consists of a solenoid plunger and switch, the latter insulated from the frame of the relay. It operates satisfactorily on one-half the rated voltage and requires only a very small operating current. The terminals of the switch and the relay coils are independent. The relay can be wound for operation on 125, 250, or 600 volt circuits.
Reverse Phase Relays.—This type of relay is used chiefly to prevent damage in case of reversal of leads in reconnecting wiring to two or three phase motors.
Time Element.—It is often inconvenient that a circuit breaker should be opened immediately on the occurrence of what may prove to be merely a momentary overload, so that time lag attachments are frequently provided, particularly with relays. These devices, which may form part of the relay or may be quite distinct from it, retard its action until the overload has lasted for a predetermined time—several seconds or more.
Fig. 2,306.—Alternating current series reverse phase single pole, circuit closing, two contact relay for 600 volts or less. This type of relay is used chiefly to open motor circuits for elevators to prevent damage in case of reversal of leads in reconnecting wiring to two or three phase motors. The relay is provided with a dust proof metal cover.
Ques. What should preferably govern the time lag?
Ans. It should depend on the extent to which the overload is reduced as the time elapses.