--- title: "IDMT characteristics and induction disc relay" subject: "Switchgear and Protection" chapter: 7 key: "semester_6/switchgear_and_protection:idmt-induction-disc-relay" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/idmt-induction-disc-relay site: BCT Engineering --- # IDMT characteristics and induction disc relay Switchgear and Protection · Chapter 7: Protective Relays - Asked in: 2072 Magh, 7 marks; 2076 Bhadra, 4 marks (asked 2 times) - Video: [IDMT RELAY || COMPLETE AND EASY EXPLANATION](https://www.youtube.com/watch?v=YSeE6cVaaJU) — Electrically Yours - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/idmt-induction-disc-relay - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-7 ## Question Explain IDMT characteristics and working principle of induction disc relay (with the help of a neat sketch). ## Short answer The IDMT relay's time is inversely proportional to current at low multiples and levels off at a definite minimum at high currents due to saturation. An induction disc relay uses two magnetically displaced fluxes from an upper electromagnet with tapped primary and secondary windings, producing a driving torque on an aluminium disc; plug setting and time multiplier adjust the pick-up and time delay. ## Answer An **IDMT (Inverse Definite Minimum Time) characteristic** is a relay time-current characteristic in which the operating time is approximately inversely proportional to fault current at low multiples of setting, but becomes almost constant (a definite minimum time) at high currents. It is usually obtained with an induction disc relay. #### IDMT characteristic - At currents just above pick-up the time is long; as current increases the time falls inversely. - At high currents (PSM > about 10–20) the electromagnet **saturates**, torque stops increasing, and the time levels off to a definite minimum. - This gives discrimination between relays in series and protects against both overloads and heavy faults. - Standard (IEC 60255) normal inverse curve: **t = TMS × 0.14 / (PSM^0.02 − 1)** (e.g. about 3 s at PSM 10 for TMS = 1). ```text Time │\ │ \ │ \ inverse region │ \ │ `-. │ `--.______________ definite │ minimum time └──────────────────────────── PSM 1 5 10 20 ``` #### Construction of induction disc (non-directional) relay - **Upper E-shaped electromagnet** with a primary winding (fed by the CT) on the central limb; the primary has **tappings** brought to a **plug setting bridge** to set the pick-up current (50–200%). - A **secondary winding** on the central limb is connected to windings on the two outer limbs of the upper magnet. - **Lower U-shaped electromagnet** with no winding of its own. - An **aluminium disc** pivoted between the two magnets, carrying a moving contact on its spindle. - A **control spiral spring** gives restraining torque; a **permanent brake magnet** gives eddy current damping so the disc speed is proportional to torque. - The disc's backward stop position is adjustable by the **time multiplier setting** (TMS 0.1 to 1), which changes the angle the contact must travel. ```text CT ─► plug setting bridge │ ┌─────┴──────┐ upper E-magnet │ prim+sec │ (secondary feeds │ windings │ outer limbs) └─┬──┬──┬────┘ ════════════════ Al disc ═══ spindle ─► contact └──┬────┬──┘ lower U-magnet │ brake magnet ``` #### Working principle 1. The CT current in the primary winding produces flux φ₁ in the upper magnet. 2. This flux induces an emf in the secondary winding, whose current produces flux φ₂ through the outer limbs and the lower magnet. φ₂ lags φ₁ in time and is displaced in space. 3. The two fluxes induce eddy currents in the disc, giving a driving torque **T ∝ φ₁ φ₂ sin α ∝ I²** (below saturation). 4. When the current exceeds the plug setting, the driving torque exceeds the spring torque and the disc rotates; the brake magnet controls the speed. 5. After travelling the distance set by the TMS, the moving contact closes the trip circuit, energising the breaker trip coil. 6. Larger currents give larger torque and faster rotation, so the time is inverse; at heavy currents the core saturates and the time approaches a definite minimum. **Applications:** overcurrent and earth fault protection of distribution feeders, transformers and motors, and back-up protection of lines. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What does the IDMT characteristic mean? A. Operating time is constant at all currents B. Operating time is inversely related to current at low multiples, becoming a definite minimum at high currents C. Operating time increases with current D. Operating time is zero above pick-up Answer: B. Time falls with increasing current, then levels off to a definite minimum when the magnet saturates. (Key point: IDMT meaning) 2. Why does the time level off at high PSM? A. The brake magnet stops working B. The spring becomes stronger C. The electromagnet saturates so torque stops increasing D. The TMS resets automatically Answer: C. At heavy currents core saturation limits torque, so disc speed and time stop improving. (Key point: Cause of definite minimum time) 3. How is driving torque produced in the induction disc relay? A. By the primary flux and the secondary-induced flux (lagging and displaced) inducing eddy currents in the disc B. By direct attraction of the disc to the core C. By the brake magnet D. By thermal expansion Answer: A. Flux phi1 induces a secondary current producing phi2 lagging phi1; their interaction with disc eddy currents gives T proportional to phi1 phi2 sin α. (Key point: Torque production) 4. What do the plug setting bridge and the time multiplier setting adjust? A. PSB sets travel distance; TMS sets pick-up current B. Both set the pick-up current C. PSB sets pick-up current (tappings); TMS sets disc travel to contact, scaling operating time D. Both set the spring torque Answer: C. The plug setting selects the primary tap (pick-up, 50-200%); the TMS changes the backstop angle so the contact travels less or more. 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