The term pull-in voltage is associated with the operation of electromagnetic relays. It is the voltage required to cause the relay to operate. It's related to drop-out voltage, which is the voltage at which the relay returns to its rest position. The pull-in voltage is always greater than the drop-out voltage.
Magnet Circuit Reluctance
At rest, there is an air gap between the relay armature and the coil core. The magnetic circuit is open, therefore the pull-in voltage has to produce a current in the coil sufficient to overcome this reluctance to operate. Once closed, the armature and coil core are in contact and the magnetic circuit is completed. There is a resistance to release (this resistance is sometimes referred to as reluctance). Therefore, the drop-out voltage must be reduced substantially before the relay will return to the rest position.
The pull-in voltage and the drop-out voltage each have an associated operating current. This current flows through the relay coil and induces the magnetic field required to actuate the relay and change its state. This current is equal to voltage across the relay coil, divided be the resistance of the coil. Coil-current I = coil-voltage V / coil-resistance r.
The relay is a current-operated device. Voltage levels will change due to the temperature coefficient of the relay coil resistance, which increases with temperature rise.
The electromagnetic relay is a durable and reliable component, and if operated within its specified parameters, will give years of reliable use.