--- title: "Types of electromagnetic relays and applications" subject: "Switchgear and Protection" chapter: 7 key: "semester_6/switchgear_and_protection:electromagnetic-relay-types" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/electromagnetic-relay-types site: BCT Engineering --- # Types of electromagnetic relays and applications Switchgear and Protection · Chapter 7: Protective Relays - Asked in: 2073 Magh, 6 marks; 2079 Chaitra, 8 marks (asked 2 times) - Video: [Types of Electromagnetic Relay | Protection and Switchgear | SNS Institutions](https://www.youtube.com/watch?v=6ZVmwvlyhHU) — M.Poornima SNS - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/electromagnetic-relay-types - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-7 ## Question With neat diagram explain the various types of electromagnetic relays and their field of application. ## Short answer Electromagnetic relays are either attracted armature types (hinged, plunger, balanced beam) operating on AC/DC, used for instantaneous overcurrent, voltage and differential protection; or induction types (disc and cup) working only on AC, used for IDMT overcurrent, earth fault, directional and distance relays due to their torque ∝ I² and inverse time characteristic. ## Answer 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** ```text ┌──────┐ 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. ```text ┌──── 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 | Type | Supply | Speed | Main applications | |---|---|---|---| | Hinged armature | AC/DC | Instantaneous | Overcurrent, auxiliary, tripping relays | | Plunger | AC/DC | Instantaneous | Over/under-current, voltage | | Balanced beam | AC/DC | Fast | Impedance, differential relays | | Induction disc | AC | Time-delayed (IDMT) | Feeder overcurrent, earth fault | | Induction cup | AC | Fast, sensitive | Directional, mho, reactance relays | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What are the two main types of electromagnetic relay? A. Static and digital B. Thermal and magnetic C. Attracted armature and induction D. Plunger and gas operated Answer: C. Electromagnetic relays are classed as attracted armature types and induction types. (Key point: Two main types) 2. Which statement is correct? A. Attracted armature relays work on AC and DC, induction relays only on AC B. Both work only on DC C. Induction relays work on AC and DC, attracted armature only on AC D. Both work only on AC Answer: A. Attracted armature force is proportional to I^2 so it works on AC or DC; induction relays need alternating fluxes with a phase displacement. (Key point: Supply of attracted armature vs induction) 3. What produces torque in an induction relay? A. Spring tension alone B. Two time- and space-displaced fluxes inducing eddy currents in the disc/cup, T proportional to phi1 phi2 sin(alpha) C. A single flux in phase with the current D. Gas pressure Answer: B. Two displaced fluxes create eddy currents in the rotor and give T proportional to phi1 phi2 sin alpha. (Key point: Induction torque) 4. Which relay type is matched with the correct application? A. Induction disc: instantaneous tripping relays B. Hinged armature: IDMT feeder overcurrent C. Induction cup: fast, sensitive directional, mho and reactance relays D. Balanced beam: reverse power relay Answer: C. The cup has low inertia, giving fast sensitive operation; induction discs are for IDMT overcurrent and earth fault, armature types for instantaneous duties. 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