Types of motor overload relay

A motor overload relay is an electro-mechanical relay that is operated by heat developed in the relay.When the level of current in a circuit reaches a preset value, the increased temperature opens a set of contacts.

Overload relay should install in our motor starter application for extra protection.Overload relay can avoid serious damage for electric motor when overload happen to our system with proper setting.

The increased temperature opens the contacts through a bimetallic strip or by melting an alloy that activates a mechanism that opens the contacts depend on our setting for overload relay.This time i explain detail about common types of overload relay for motor control.

Types Of Overload Relay

1) Thermal overload relays

  • Melting alloy

These are probably the most popular type of overload protection.The motor current passes through a small heater winding and under overload conditions.

The heat causes a special solder to melt allowing a ratchet wheel to spin thus opening the control circuit contacts.It must be reset by hand operation and Heater coil and solder pot in one unit  non-tamperable.

  • Bi-metallic strip

This design uses a bimetal strip associated with a current-carrying heater coil.When an overload occurs, the heat causes the bimetal to deflect and actuate a tripping mechanism which opens a set of contacts in the control circuit interrupting power to the coil and opening the power contacts.

Most relays are adjustable over a range from 85% to 115% of their value.They are available with ambient compensation. An ambient compensated devices’ trip point is not affected by ambient temperature and performs consistently at the same value of current.


2) Magnetic Overload Relays

A magnetic overload relay is an electro-mechanical relay operated by the current flow in a circuit.

When the level of current in the circuit reaches a preset value, the increased magnetic field opens a set of contacts.

Electromagnetic overload relays operate on the magnetic action of the load current flowing through a coil.

When the load current becomes too high, a plunger is pulled up into the coil interrupting the circuit.The tripping current is adjusted by altering the initial position of the plunger with respect to the coil.


  1. Debasish says:

    To the pand clearly described.oint

  2. Julibeth Wamara says:

    i like your explanation but how can i have your charting live b`cause same time i would like to communicate live
    eg.Facebook what`s sup and so on?

  3. Dumisile says:

    How do I use these overload relaysbin instrumentation.I’m still an apprentice please help.And also give me some of the helpful information relating to the cold rolling mills in regard to instrumentaion.


  1. […] Overload Relays are generally of two types one is “Electro-magnetic” and another is “Thermal”. I have already given details about Thermal Overload Relay in past. In this post, you will find details about “Electro-magnetic Overload Relay” which is somewhat outdated now days, but still some old Air Compressor units are having these types of Relays. The Electro-magnetic Overload Relays consist of a coil or solenoid carrying the line current (or a portion of it through current transformers) with an armature which when attracted sufficiently operates the release circuit or latch. A time lag feature is in corporate to guard against unnecessary operation from temporary Overloads due to the normal operation of the machines which are being driven. Time lag is obtained by means of Dash pot (otherwise the action would be practically instantaneous) fitted to the Oil Circuit Breaker (OCB) controlling the stator circuits.   Construction and Adjustment of Dash pot. The time lag consists of BODY CASTING, MOVEABLE PLUNGER, and OIL RETAINING SCREW CUP. The moveable plunger comprises the core of the Overload release, to which is fitted a specially shaped brass disc moving in a restriction in the body casting, by turning the screw cap the position of the disc within the restriction is varied. The time lag are dispatched dry and must filled with special time lag oil (SAE-60 grade) provided in separate containers in the Circuit Breaker. While fitting remove screw “A”. It is not necessary to remove the bottom cup “B” and on no account should screw “C” be removed. Adjustment of the Overload setting is made by slackening screw “D” and sliding the body casting up or down, until the top face coincides with the calibration marking required on the calibration tube, screw “D’ is then tightened. The bottom rotatable cup “B” is now described reset themselves after operation within few second. This type of Overload Relays are very simple in operation but due care must taken for accurate operation. All the marks “A”,”B”,”C”&”D” are embossed on respective spaces, you have to indentify first before working on “Electro-magnetic Overload Relay”.  Some other useful articles… Valve: How Solenoid Valve Works? How to Dry out Electric Motor? Starters: Trouble shooting and Rectification of Automatic Starters. […]

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