Thermal Overload Relay Working Principle Explained!

Electrical motors make up a large percentage of power system loads. Market demands have compelled the motor control industry to evaluate motor protection technology continuously. Technology advancements now allow the motor control industry to offer several options for motor protection. Its effective working principle makes a thermal overload relay one of the best solutions for motor protection applications.

What is a Thermal Overload Relay?

Thermal overload relay provides protective features for 1 or 3-phase motors. The relay monitors the operating current of the motor and switched the contactor off, in the event of an overload situation. It also protects the motor from damage during a phase loss.

The thermal overload relay performs the following functions:

• Allows harmless temporary overloads (such as motor starting) without disrupting the circuit.
• Trips and open a circuit if the current is high enough to cause motor damage over a while.
• Can be reset once the overload is removed.

By selecting the proper type of thermal overload relay with the appropriate functionality, the motor can be protected from most damage caused by the following conditions:

• Bearing fault
• Rotor bar fault
• External failures
• Stator winding fault
• Shaft coupling fault

How Does a Thermal Overload Relay Work?

The contacts of a thermal overload relay close or open depending on the degree of heat produced by the current through the relay’s coil that is connected in series with the protected load. Since there is a definite time interval between the current flow and the heat produced by the coil, thermal overload relays are used for overload protection.

The heat generated on overload is proportional to the current squared. This heat is used either to melt an alloy, thereby permitting a ratchet wheel to turn and open a control contact, or to heat a bimetallic strip, causing it to bend and open a control contact.

Thermal overload relay is designed for current-dependent protection of applications with normal start-up conditions against impermissibly high rises in temperature as a result of overload or phase failure. An overload or phase failure causes the motor current to rise above the set-rated motor current. This current rise heats the bimetal strips within the relay via heating elements which, in turn, operate the auxiliary contacts via a tripping mechanism due to their deflection. These switch the load off via a contactor. The switch-off time is dependent on the ratio of tripping current to operational current Ie and is stored in the form of a tripping characteristic with long-term stability. The “Tripped” state is signaled using a switching position indicator.

The thermal overload relay has, as a standard, a normally closed (NC) contact that is manually reset after the current interruption. The more expensive thermal overload relays have an automatically reset NC contact and may also be equipped with a normally open (NO) contact for alarm or indication purposes.

Important notification:

Unlike their widely used name, thermal overload relays are not relays like electromechanical relays, but sensors detecting electric current. Furthermore, these relays could be considered or called “switches”, but are not switches although they perform indirectly the action of switching. Thermal overload relays can interrupt the auxiliary or control circuit of a power relay—that is, the real switch—causing its deactivation. From this point of view, a thermal overload relay is an integral part of any power relay supplying an electric motor. It is mechanically and electrically coupled with the power relay, and both of them constitute together a unified power device that is inserted in the power circuit of an electric motor. The relay acts then as a switching mechanism and the overload relay as a sensor detecting the motor current. For this reason, all the power relays manufacturers produce also the corresponding thermal overload relays.

Thermal Overload Relay Rating Plug

To understand the thermal overload relays clearly, you should know the functions of the buttons and terminals on the relay.

1. Connection for mounting onto contactors
2. Manual/automatic Reset button
3. Test button
4. Motor current setting
5. Stop button
6. Supply terminals

Technical Terms

Thermal overload relays are advanced products. For this reason, it is better to know the technical terms related to this product.

Trip classes

The trip classes describe time intervals within which the overload relay must trip with 7.2 times the operational current Ie for symmetrical three-pole loading from the cold.

Current setting

The thermal overload relay is adjusted to the rated motor current using a rotary potentiometer. The scale of the rotary potentiometer is calibrated in amperes.

Manual and automatic resetting

When manual resetting is selected, a reset can be performed directly on the device by pressing the “RESET” button. When automatic resetting is selected the relay will be reset automatically. A reset is not possible until the recovery time has elapsed.

Recovery time

After tripping due to an overload, it takes a certain length of time for the bimetal strips of the thermal overload relays to cool down. The overload relay can only be reset once it has cooled down.

Test function

The correct functioning of the ready thermal overload relay can be tested with the TEST button. The test button is used to simulate the tripping of the relay. During this simulation, the normally closed contact (95-96) is opened and the normally open contact (97-98) is closed whereby the over-load relay checks that the auxiliary circuit is wired correctly.

Stop function

When the STOP button is pressed, the normally closed contact is opened and the series-connected contactor is de-energized, and therefore the load is switched off. The load is reconnected via the contactor when the STOP button is released.

Status indication

The current status of the thermal overload relay is indicated by the status indicator.

Auxiliary contacts

The thermal overload relay has a normally open contact for the trip signal and a normally closed contact for switching off the contactor.

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