Overload Relays: Principle of Operation, Types, and Connection

Overload Relay: 

Overload relay is an electrical device designed as the heating prototypes of the electric motor. It breaks up the flow of current when the heat-detecting device in the relay attains a fixed temperature. An overload relay device can be prepared with a heater linked with generally closed connections that unlocks when the heater gets extremely hot.

Principle Of Overload Relay Operation

When excess current is drawn into the motor, it causes an overload. This causes overheating of the motor and may damage the motor. Thus, it is required to protect the engine, circuits, and branch circuit components from overload conditions. Overload relays protect the motor and branch circuit components from an overload. Overload relays monitor the flow of current in the circuit. Suppose the current rises above a specific limit over a particular time. In that case, the overload relay will trip, performing a supplemental contact that suspends the motor control circuit. This leads to the de-energization of the contactor and dismissal of the power to the motor. Without any power, the motor and motor circuit elements do not overheat. Overload relays can be manually or automatically reset, and the motor can be restarted.

Working Of An Overload Relay and connections:

Since the overload relays are wired in with the motor in series circuits, the current also flows through them, and whenever the current overflow, overload relays trip. This opens the circuit between the motor and the power source. The overload relay can be reset manually or automatically after a planned time. After the cause of the overload is identified, you can restart the motor. 

Connection of an overload relay seems like two opposite question marks or like 'S.'

Types of Overload Relays:  

Bimetallic thermal overload relays: 

Bimetallic thermal overload relays are made up of two metals with different coefficients and thermal expansion properties. A winding is wrapped around the bimetallic strips to carry the current. When overheating occurs due to the excessive current, these bimetallic strips bend to one side, Which activates the trip.

As the bimetallic strip heats due to the excessive current, the two metals expand at different rates because of varying coefficient and thermal capacity, causing the strip to bend toward the lower thermal coefficient expansion strip. It actuates a usually closed contactor when the strip turns, causing it to open, stopping the current flow. When the overload relays cool down, and the metal strips have reverted to their regular positions, the circuit resets to restart the motor.

Eutectic thermal overload relays: 

In these kinds of overload relays, eutectic alloys are used, a blend of metals with different melting and solidification points. They are placed in a tube and connected to a heater winding. When the excessive current flows through them, it heats the alloy. When the alloy is at a specific temperature, it melts into a liquid—activating the trip. Eutectic thermal overload relay can be reset only when the alloy has adequately cooled and returned to solid-state.

BCH - Thermal Overload Relay

Electronic overload relays:

These Overload relays measure the current and do not rely on temperature changes. They're also less prone to false tripping, making them more accurate than previously mentioned overload relays. Electronic overload relays provide data like TCU percentage( thermal capacity utilization), FLA percentage (full-load amps), time-to trip, RMS, and ground-fault.

Electronic overload relays can also protect motors against phase loss.

BCH - Electronic Overload Relay