Voltage relays, current relays, frequency responsive relays and latching relays

Relays are used to transmit information in a control circuit. Relays sense information from some sensing device and feed it into control circuit at an appropriate level. Pilot devices which are used in conjunction with relays can sense current, voltage, pressure, temperature, frequency and overload. Fault analysis becomes easy on using relays in control circuits. Voltage relay is a small contactor which changes its contact positions from open to close and close to open when sufficient voltage is applied across its coil. In current relay, contact position changes with a change in current in the coil. The minimum current required to close relay armature is called as pull in current. Drop out current is the value of current below which the relay no longer remains in a closed position. The difference in value of pull in and drop out currents is known as differential current. In frequency responsive relays, contact position is changed when the frequency of applied voltage to the relay coil falls below a pre-determined value. A latching relay is a specially designed relay which can be used in control circuits where the relay remains in energized position even when operating coil is de-energized. There are two coils-latch coil and unlatch coil for a latching relay. The two varieties of latching relay are mechanical latched relay and permanent magnet type latched relay. In permanent magnet type latched relay, both the latch coil and the unlatch coil are mounted on the same magnetic core.