Types of electromagnetic relays and applications

An electromagnetic relay works by the force produced by an electromagnet energised by current or voltage from CTs and PTs. There are two main types: attracted armature relays and induction relays.

1. Attracted armature relays

They work on electromagnetic attraction: force F ∝ I², so they operate on both AC and DC. They are fast (5–50 ms) and are used as instantaneous relays.

a) Hinged armature type

     ┌──────┐   armature (hinged)
     │ coil │ ┌───────────────────┐
     │ (I)  │ │                   ●─ contacts
     │ core │ └─┬─────────────────┘
     └──────┘   spring ↑

The coil current produces flux; when it exceeds pick-up, the armature is pulled towards the core against a spring and closes the contacts.

b) Plunger type (solenoid): a plunger is drawn into a solenoid when current exceeds the setting.

c) Balanced beam type: a beam pivoted at the centre is pulled by two coils (e.g. current coil on one side, voltage coil on the other); it trips when one force exceeds the other. Used as an impedance (distance) relay.

Applications: instantaneous overcurrent and undervoltage relays, auxiliary and tripping relays, differential relays, impedance relays.

2. Induction relays

They work like an induction motor/energy meter: two fluxes displaced in time and space induce eddy currents in a rotating disc or cup and produce torque T ∝ φ₁ φ₂ sin α. They work only on AC.

a) Shaded pole type: the pole face is partly covered by a copper ring (shading ring), giving two fluxes with a phase difference.

b) Watt-hour meter (wattmetric) type: two separate electromagnets (E-shaped upper and U-shaped lower) on either side of the disc; upper coil can be fed by voltage and lower by current, so it can act as a directional relay.

     ┌──── E-magnet (upper coil) ─────┐
     ════════════ disc ═══════════════ → contact
     └──── U-magnet (lower coil) ─────┘ + brake magnet

c) Induction cup type: a hollow aluminium cylinder (cup) rotates inside a four- or eight-pole stator; very low inertia and high torque, so it is fast and sensitive. Used for directional, mho and reactance relays.

Applications: IDMT overcurrent and earth fault relays (induction disc), directional overcurrent relays, distance relays (mho, reactance), reverse power relays.

Summary