IDMT characteristics and induction disc relay

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).

 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.

   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.