Heat is a crucial component in motor performance and longevity, and current travelling through the motor windings is one of the principal sources of motor heating. Because heating is an inherent condition of motor operation, it is critical to safeguard the motor against overheating, also known as thermal overload. In a recent piece, we discussed many types of sensors that can directly monitor the temperature of motor windings.
However, motor heating may be sensed indirectly in some circumstances, notably for AC induction motors, using thermal overload relays, which calculate motor temperature by measuring the amount of current given to the motor.
What is a thermal overload relay?
Thermal overload relays safeguard single-phase and three-phase motors. In the event of an overload, the relay checks the operational current of the motor and switches off the contactor. It also protects the motor in the event of a phase failure. The thermal overload relay has the following functions:
Enables temporary overloads (such as motor starting) without disrupting the circuit. Trips and opens a circuit if the current is high enough to cause long-term damage to the motor. Resettable once the overload has been removed.
Thermal overload relay technical jargon
1. Trip classes
The trip classes specify the periods during which the overload relay must trip with 7.2 times the operating current Ie for symmetrical three-pole loading from cold.
2. Current setting
A rotary potentiometer regulates the thermal overload relay to the rated motor current. The rotary potentiometer’s scale is calibrated in amperes.
3. Manual and automatic resetting
When manual resetting is selected, a device reset may be accomplished by hitting the “RESET” button. When automatic resetting is enabled, the relay is automatically reset. A reset is not feasible until the recovery timer runs out.
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4. Recovery time
It takes a specific amount of time for the bimetal strips of the thermal overload relays to cool down after tripping due to an overload. Only once the overload relay has cooled down can it be reset.
5. Test function
The TEST button can be used to verify that the ready thermal overload relay is working correctly. The test button is used to mimic the relay tripping. During this simulation, the usually closed contact (95-96) and normally open contact (97-98) are closed, allowing the overload relay to verify that the auxiliary circuit is connected correctly.
6. Stop function
When the STOP button is hit, the typically closed contact is opened, and the series-connected contactor is de-energized, thereby turning off the load. When the STOP button is depressed, the load is reconnected through the contactor.
7. Status indication
The status indicator displays the current status of the thermal overload relay.
8. Auxiliary contacts
The thermal overload relay provides a usually open contact for the trip signal and a normally closed contact for turning off the contactor.
Electronic overload relay vs. thermal overload relay
A thermal overload relay uses bimetals for safety. Electronic overload relays, on the other hand, combine integral current transformers, ASICs, and microprocessors with electromechanical design principles to provide a tiny, high-function overload protection solution.
Power is supplied to the integrated circuit as motor current passes via the inbuilt current transformers. By monitoring three-phase power, opening a normally closed contact, and dropping out the motor starter coil circuit, the ASIC/microprocessor may analyse current data and activate a trip mechanism under overload conditions.
Advantages of thermal overload relay
It prevents overheating of the motor. It may be utilized in single-phase or three-phase motors. Internal phase loss prevention is available on some overload relays. It employs an efficient working concept. It is an inexpensive gadget. (Price check) It may be attached directly to a contractor or a panel using a din rail adaptor. It features a troubleshooting internal test button.
Internal trip class selection buttons are available on some overload relays. It features an adjustable and broad current range. It has a reset button (Either manual or automated). It includes temperature compensation. It includes a trip-free system.
Disadvantages of thermal overload relay
It lacks a high switching frequency. After so much tripping, it’s essential to cool down. It lacks a direct braking function. To terminate the circuit, it should be used with other switching or protection devices. There is no short circuit protection. It is not suitable for heavy-duty starting. It operates slowly. It is not resistant to vibration and stress. (Only some varieties can perform well.)
What is the operation of a thermal overload relay?
Thermal overload relays are designed to safeguard applications with normal start-up conditions from excessive temperature rises caused by overload or phase failure. The motor current exceeds the rated motor current due to overloading or a phase breakdown. This increase in current heats the relay’s bimetal strips through heating elements, which operate the auxiliary contacts via a tripping mechanism due to their deflection. A contractor is used to turn off the load in these cases. The ratio of tripping current to current operating Ie determines the switch-off duration, which is saved as a long-term stable tripping characteristic. The “Tripped” status is indicated by a switching position indication.
The contacts of a thermal overload relay close or open dependent on the amount of heat produced by the current running through the relay’s coil, which is linked to the protected load. Thermal overload relays are used for overload protection because there is a temporal difference between the current flow and the heat produced by the coil. The creation of overload heat is proportional to the current squared. This heat is used to either melt an alloy, which allows a ratchet wheel to turn and open a control contact, or to heat a bimetallic strip, which causes it to bend and open a control contact.
Following the present interruption, the thermal overload relay’s normally closed (NC) contact is manually reset. Some more expensive thermal overload relays have an automatically resetting NC contact as well as a normally open (NO) contact for warning or signalling purposes.
Thermal overload relay rating plug
- Mounting connection for contactors
- Reset button (manual/automatic)
- The test button
- Motor current control
- The Stop button
- Power supply terminals
