Know About Thermal Overload Relays

The current flowing to the motor also flows through the overload relay as with the motor thermal overload relays are wired in series. Rating of thermal overload relays is done by their trip class.

Three types of thermal overload relays are there— bimetallic, eutectic, and electronic.

A bimetallic thermal protection relay is made of two metals that too with different coefficients of thermal expansion that are together fastened or bonded. Current is carried by a winding, wrapped around, or placed near the bimetallic strip.

The two metals expand at different rates as the current running through the relay heats the bimetallic strip, which results in the bending of the strip towards the side with the lower coefficient of thermal expansion. It actuates a normally closed (NC) contactor when there is bending of the strip that causes it to stop and open the flow of current to the motor.

The circuit, once the bimetallic relay has cooled, is automatically reset and to their normal state, the metal strips have reverted. The motor then can be restarted. Electronic designs can protect motors against phase loss which only occurs when one phase of current equals zero amps.

A Fuji electronic thermal overload relay is used by eutectic thermal overload relays, housed in a tube and connected to a heater winding. To the motor, the supply current flows through the heater causing winding and heating of the alloy. When a sufficient temperature of the alloy is reached, it transforms rapidly into a liquid.

But, the mechanical device releases when the alloy melts, opening the overload contacts., a eutectic thermal overload relay cannot be reset Like the bimetallic design until the alloy has cooled and returned to its original, solid-state.

Rather than relying on a heater mechanism electronic thermal overload relay measure current electronically, as a result, they are insensitive to changes in ambient temperature. They’re quite less prone to nuisance or false, tripping. Electronic designs can protect motors against phase loss which only occurs when one phase of current equals zero amps. On the remaining two phases this causes the motor to draw excessive current and leads to significant motor heating. Follow us on Facebook