The magnetic circuit system of the current relay is considered according to IW=C, i.e., when the coil inductance changes due to the action of the armature during the operation of the relay, it will not affect the loop current value. Current relay is a relay that reflects the change of current, its attraction coil has fewer turns and thicker wire diameter. It can pass a large current and is used in series with the load.
Voltage relays are relays that reflect changes in voltage and have a large number of turns of attractor coils with a fine wire diameter. It is used in parallel with the power supply. The current is determined by the larger impedance of other circuit elements in the loop, and the current relay coil impedance has a negligible effect on the impedance of the entire loop.
As a result, a typical current relay coil has fewer turns of wire, lower inductance and resistance, and therefore higher coil current. A constant value of current is supplied to the current relay coil.
The input signal to the coil of a voltage relay is a relatively constant voltage value, generally the supply voltage is applied directly to the coil or distributed to it through a network at a constant voltage value. Therefore, the loop current depends mainly on the coil impedance and generally does not involve other loop components. In order to minimize its shunt effect on other branch circuits, generally thin wires, more turns, inductance and resistance are larger, the coil current is notlarge.
When selecting a current or voltage relay, relative circuit conditions are required. Current relays require constant-current source circuit conditions, i.e., the loop has a large impedance in series with it, and its own impedance has little effect on the loop current. Voltage relays require a constant voltage. If a current relay is used as a voltage relay, because of its small coil resistance, it is easy to burn out the coil or even cause a short circuit in the power supply. If a voltage relay is used as a current relay with the coil connected in series in the line, its large impedance will significantly change the original circuit parameters, and the relay will not operate because the coil does not receive enough current.