Watt-hour Meter Relays

 Watt-hour Meter  Relays

The Watt-hour structure type relay works as an electromagnetic induction type of watthour – a meter so named, after it. 

It consists of two magnets: upper and lower electromagnets and aluminium disc, which is pivoted such that it rotates freely between the two electromagnets. 

In this relay, the upper magnet consists of two windings: primary and secondary windings, wherein the primary winding carries the relay current and the secondary winding is connected to the lower magnet. 

This primary current induces EMF in the secondary winding, so it produces a flux that lags behind the main flux by some angle. 

The phase difference of these fluxes produces torque on the disc, whose

magnitude is proportional to the product of these two fluxes multiplied by the sine angle between these two fluxes.