In order to operate, the induction disk relay torque is produced that acts on a metal disc to make contact,
according to the following basic current/torque equation:
Where
K – is a constant ϕ1 and ϕ2 are
the two fluxes θ is the
phase angle between the fluxes
The relay's primary winding is supplied from the power systems
current transformer via a plug bridge, which is called the plug setting
multiplier (psm). Usually seven equally spaced tappings or operating bands
determine the relays sensitivity. The primary winding is located on the upper
electromagnet. The secondary winding has connections on the upper electromagnet
that are energised from the primary winding and connected to the lower
electromagnet. Once the upper and lower electromagnets are energised they
produce eddy currents that are induced onto the metal disc and flow through the
flux paths. This relationship of eddy currents and fluxes creates torque
proportional to the input current of the primary winding, due to the two flux
paths been out of phase by 90°.
A restraining
spring forces the disk to rotate in the direction that opens the trip contacts
while current creates operating torque to close the
contacts. The net positive torque closes the contacts. The IPU relay setting fixes the value of the pickup
current. When the current applied to the
relay equals the pickup current, the contact closing torque just equals the
restraining torque and the disk will not move regardless of its position. If the applied current increases above the pickup current, the disk
will begin to rotate so that the trip contacts come closer together.