~Ques. How is the inverse time element introduced by this arrangement?~
Ans. The retardation is due to eddy currents induced by moving the disc or drum through the magnetic field. The reaction thus induced varies inversely with the magnitude of the force with which the disc or drum is urged through the field and hence inversely with the abnormal condition.
~Ques. What are the ordinary limits of adjustment for inverse time limit relays?~
Ans. From one-half second to 30 seconds, depending upon the time setting and magnitude of the overload current.
[Illustration: FIGS. 2,313 to 2,321.--General Electric time limit overload circuit opening relays with covers removed. The construction of this relay is similar to that of the inverse time limit relay, except that it has a compression spring interposed between the plunger and diaphragm. The plunger compresses the spring and further motion is prevented by a stop, making the relay practically independent of the amount of the overload, only the stored energy of the spring, if the overload continue, applies power, dependent on its own mechanical strength, to the diaphragm. The time limit therefore becomes practically a constant for any given setting under ordinary conditions of overload or short circuit. If, however, the overload come on slowly so that the spring is not fully compressed at once, the time limit will vary slightly. If the scheme of selective operation make it necessary to take care of a creeping load of this character, two relays may be used and definite time limit positively secured. In this case, an instantaneous circuit closing, overload relay would be used and a definite time limit relay, provided with a direct current coil in circuit with the closing contacts of the first relay. The time limit relay would be of the circuit closing type and control a direct current trip coil on the oil switch.]
A setting of from two to six seconds is ordinarily used, depending upon the requirements. Where selective operation is desired a minimum setting of two seconds is recommended.
~Differential Relays.~--In this type of relay there are two electromagnets. In normal working these oppose and neutralize each other. Should, however, either winding become stronger or weaker than the other, the balance is upset, the magnet energized, and the relay comes into operation.