Overload Relay: Types, Connection Diagram and Applications

An overload relay is an electrical device used to protect an electric motor from overheating. So it is essential to have sufficient motor protection. An electrical motor can be operated safely with the help of overload relays, fuses otherwise circuit breakers. But overload relay protects the motor whereas the circuit breaker otherwise fuse protects the circuit. More purposely, fuses, as well as circuit breakers, are intended to detect the overcurrent within the circuit, whereas overload relay is intended to detect overheat if an electric motor gets heated. For instance, an overload relay can explore without the tripping of a CB (circuit breaker). One does not restore the other. This article discusses an overview of overload relay, types and its working.

What is an Overload Relay?

An overload relay can be defined as, it is an electrical device mainly designed for imitating the heating prototypes of the electric motor, as well as breakups the flow of current when the heat detecting device in the relay attains a fixed temperature. The designing of an overload relay can be done with a heater coupled with generally closed connections that unlock when once the heater acquires too hot. The connections of an overload relay can be connected in series as well as placed among the motor & contactor itself to avoid the motor from restarting when the overload trips.

Overload Relay Types

Overload relays are classified into two types namely thermal overload relay and magnetic overload relay.

Thermal Overload Relay

Thermal overload relay is a protective device, and that is mainly designed to cut the power whenever the motor uses too much current for an extended time period.

To achieve this, these relays include an NC (normally closed) relay. Once extreme current supplies throughout the motor circuit, then the relay gets open because of the improved temperature of the motor, a temperature of the relay, otherwise detected overload current, based on the type of relay.

Thermal Overload Relay
Thermal Overload Relay

Overload relays are related to circuit breakers in construction as well as an application; however, most of the circuit breakers disturb the circuit if overload happens even for a moment. These are equally designed for calculating the heating profile of the motor; thus, overload should happen for a complete period before the circuit is broken up. Thermal overload relays are classified into two types namely solder pot as well as bimetal strip.

Magnetic Overload Relay

Magnetic overload relay can be operated by detecting the magnetic field strength which is generated by the flow of current toward the motor. This relay can be built with a variable magnetic core within a coil that holds the motor current. The flux arrangement within the coil drags the core up. As the core increases far enough, then it trips a set of connections on the summit of the relay.

Magnetic Overload Relay
Magnetic Overload Relay

The major difference between thermal type as well as magnetic type relays is that magnetic type overload relay is not responsive toward ambient temperature. Generally, these are used in the areas where extreme changes exhibit within ambient temperature. Magnetic overload relays are classified into two types namely electronic as well as dashpot.

Overload Relay Connection Diagram

The wiring diagram of an overload relay is shown below, and the connections of an overload relay symbol may seem like two opposite question marks otherwise like ‘S’ symbol. The overload relay working/function is discussed below.

Although there are several types of overload relays available in the market, however, the most frequent type of relay is the “bimetallic thermal overload relay”. The designing of this relay can be done by using two dissimilar kinds of metal strips, and these strips can be connected mutually as well as enlarge at diverse rates while heated. Whenever the strip is heated at a particular temperature, then the strip can twist far enough for breaking this circuit.

Overload Relay Wiring Diagram
Overload Relay Wiring Diagram

Whenever the flow of current toward the motor is more than what the heaters is charged for, the overload explores later than some seconds. The classes of overload relay can be classified into three types based on the duration of relay explore. Class 10, Class 20, and Class 30 overload relays can be explored later than 10 secs, 20 secs, and 30 secs correspondingly. One main security characteristic of this relay is that stops the motor from instantly restarting. For instance, when the overload relay explores within a bimetallic relay, then the NC (normally-closed) bimetallic connections will unlock the circuit until the strip gets cool. If anyone tries to push the start switch to shut the contactor switches, then the motor will not be switched on.

Overload Relay Applications

The applications of an overload relay include the following.

  • An Overload relay is extensively used to protect the motor.
  • An overload relay can be utilized for detecting both overload conditions as well as fault conditions & then declare trip commands for a protective device.
  • Overload relay has developed into microprocessor systems as well as solid-state electronics.
  • Overload relays deactivate the device whenever it pulls extreme current.

Thus, this is all about an overview of the overload relay. From the above information finally, we can conclude that these are electromechanical overload protection relay devices used for the circuits. These devices provide consistent protection for motors while the failure of phase otherwise overload occurs. Here is a question for you, what is the function of the overload relay?

Image Sources: Temco Industrial

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