The wattmeter-type induction relay is the classic overcurrent relay — the spinning-disc relay found in substations for decades before digital relays took over. It works on the same principle as an energy meter: two alternating fluxes, displaced in phase, induce eddy currents in an aluminium disc and spin it. This post explains its construction and its inverse-time characteristic.

Figure 5.1: Wattmeter-type induction relay construction and IDMT operating characteristic. (Schematic, not to scale.)

Construction

The relay consists of a pivoted aluminium disc, free to rotate in the air gap between an upper E-shaped electromagnet and a lower U-shaped electromagnet:

• The upper centre limb carries the primary tapped coil, energised by the CT secondary current I, producing flux Φ₁.
• A secondary winding on the upper core feeds the lower U-magnet coil, producing a second flux Φ₂, displaced from Φ₁ by an angle α.
• Fluxes Φ₁ and Φ₂ induce eddy currents in the disc, and the interaction of flux and eddy currents produces the driving torque.

Operating principle and torque

The driving torque is:

T ∝ Φ₁ · Φ₂ · sin α ∝ I²

Both fluxes are proportional to the relay current I, so the torque grows with the square of the current. A fault current ten times the pickup therefore produces a hundred times the torque, and the disc spins to close the trip contacts far faster. The plug setting bridge sets the pickup current, and the Plug Setting Multiplier, PSM = I / I_pickup, measures how many times over pickup the relay is operating.

The IDMT characteristic

The relay gives an Inverse Definite Minimum Time (IDMT) characteristic: operating time is inversely proportional to fault current at low PSM — a small overcurrent takes seconds, a large fault trips in a fraction of a second — and it becomes a definite minimum time at high PSM, because the magnetic core saturates and the torque can grow no further. The flat tail of the curve is the relay's fastest possible time, and it is what makes grading between upstream and downstream relays possible.

Key points to remember

• Construction: pivoted aluminium disc between an upper E-core (primary, CT current I → Φ₁) and a lower U-magnet (secondary winding → Φ₂, shifted by angle α).
• Torque T ∝ Φ₁Φ₂ sin α ∝ I² — torque grows with the square of the current.
• PSM = I / I_pickup measures how far above pickup the relay is operating.
• IDMT: operating time falls as fault current rises, then levels off at a definite minimum time when the core saturates.

Based on Power System Protection & Switchgear, Complete Examination Solution, Exam 07 May 2026, Q5(b).