When you see the term “thermal protection” or “thermally protected” used in the description of an electric motor, it refers to a device found within the motor or motor compressor that is designed to prevent dangerous overheating that can cause motor failure.

The Purpose of a Thermal Protector

This overheating generally occurs when the motor is overloaded, when a bearing seizes up, when something locks the motor shaft and prevents it from turning, or when the motor simply fails to start properly. A failure to start may be caused by faulty start windings in the motor.

The thermal protector consists of one or more heat-sensing elements built into the motor or motor-compressor, plus an external control device. The thermal protection is present to turn the motor off when excessive heat is generated within the motor circuitry, halting the temperature climb before it can burn up the motor.

These thermal protectors reset themselves once the motor cools down to a safe operating temperature. There is usually a visible red button located on the wiring side of the motor— usually, though not always, located opposite of the motor shaft. On motors equipped this way, you must press the reset button to reset and restart the motor. On other motors without a reset button, the reset happens automatically as the motor cools.

Having a motor shut down because it tripped a thermal limit is inconvenient, but it certainly is better than having to replace a motor because it overheats. And the shut-down can alert you to problems with the motor or connected devices, or to the load attached to the motor. When a motor fails to start or overheats during operation, it may indicate that the motor has reached the end of its useful life and must be replaced. But often the fault does not lie with the motor at all. There could be an obstruction on the load attached to the motor, leading to excessive load that causes heat to build up in the motor—tripping the thermal protector that saves the motor.

Examples of Motors With Thermal Protectors

A sump pump is a motor for which this scenario often occurs. If a sump pump pumps water filled with debris, pieces of debris may get caught in the impeller and lock the rotation of the pump motor, causing it to overheat very quickly. On a pump equipped with thermal protection, the device will trip and shut down the circuit to the motor windings. This will allow the motor to cool down and may well save it from complete failure. Alerted to the problem because the motor shuts down, you may be able to clear the impeller and keep your sump pump motor operating for quite some before it needs replacing.

The same scenario may be true on other electric motors that process variable loads, such as garbage disposers, washing machines or vacuum cleaners. Without thermal protection, such motors can be more prone to burn-out.

“Thermal protection” is generally a good feature to look for when buying equipment with electrical motors. By protecting the motor from overheating, it can greatly extend the life of the motor.

  1. Logistics Service. We have contracts with TNT, FedEx,UPS and some forwarders, which can offer you door to door service. The cost based on freight prepaid is 30 percent to 50 percent lower than the cost based on freight collect. Most of the cargos are transferred from H.K . We do this is to cut down the cost of the shipment for our customers and make their goods more competitive in price.
  2. Providing all kinds of Accessories. Lead wire with additional length, connectors, terminals, brackets for easy mounting etc. are available for our thermal protectors. We have complete supply chain and even prepare stocks of many kinds of materials such as electric wire, silicone rubber wire, teflon wire, AMP & Molex teminals and connectors from Taiwan or Mainland, and so on. Customization is our strong point.
  3. Total solution of Overheat. We have good business relationship with near 30 manufacturers specializing in thermal protector, thermal cutoff, thermostat, resetable fuse, thermal sensor. Some of them authorize us as their sole agent in the world market, some of them OEM for us. Some of them are our distributors in domestic market. We can offer our customers total solution to over heat and purchase various kind of products for thermal control for our customers. Moreover, we offer inspection service to make sure the quality for the imports of our customers.

Testing be done in the oven that the precision of constant temp is ±1℃. When testing, the thermocouple or thermometer should be place nearest to samples. During temperature rising ,when the temperature reaches 10℃ less than rated temperature, the temperature rising rate should be less than 0.5℃per minute and the testing current should be no more than 0.01A.

  1. Due to plenty of types of thermal protector, it is much of headache to choose which thermal protector is suitable. Hereafter are the three forms for thermal protector selection of three main industries (battery pack, motor and ballast), what you only do is to fill the form and send it to sa8@saftty.com, we would contact you!Thermal protector for battery packInstallation:In recharging circuit □In discharging circuit□In both recharging and discharging □;

    Other installation________________

    Recharging current: ________ A;  Recharging time: _____mins

    Discharging current:________ A; Discharging time: _____mins

    Maximum current in abnormal situation: _____A; Abnormity time: _____mins

    Technology requirements to protector:

    Opening temperature: _________ ℃;Opening precision: ________ ℃(normally ±5 ±8 ±10℃)

    Reset temperature:_________℃; Reset precision: ________ ℃(normally ±10~15℃)

    Resistance: < ______  mΩ;

    Lead wire:_______________

    Thermal protector for motors

    Rated voltage: ________ V ;Working current: ________ A;

    Insulated level: _________

    Starting current: ________A;    Starting time: ________A

    Ambient temperature:  ________℃;

    Current of overload:_________ A; Temperature rising rate: ________℃/mins;

    Installation and varnish way:

    Installation:built-in□ out-lay□;

    Varnish: drop varnish□   vacuum varnish process □(pressure_____MPa)

    no drop varnish □   Immersion varnish□

    Technology requirements to protector:

    Opening temperature:________ ℃;Opening precision: ______℃(normally ±5 ±8 ±10℃)

    Reset temperature:_________ ℃; Reset precision:  ________ ℃(normally ±10~15℃)

    Cycles: ________  ;

    Lead wire:__________

    Thermal protector for lighting, ballasts and transformers

    Rated voltage:_______ V ;Working current:  _______ A;cosФ:____     ;

    Starting current: _____A;   Maximum current of overload:________ A;

    Ambient temperature:  _______℃; Temperature rising rate: _________℃/mins.

    Installation and varnish way:

    Installation: Embeding into the coil □; Banding in the coil □;

    The surface of power tube □; Other places_____

    Varnish: drop varnish□   vacuum varnish process □(pressure       MPa)

    no drop varnish □   Immersion varnish□

    Technology requirements to protector:

    Opening temperature:________ ℃;Opening precision:  __________℃(normally ±5 ±8 ±10℃)

    Reset temperature:__________ ℃; Reset precision: _________ ℃(normally ±10~15℃)

    Cycles: ________ ;

    Lead wire:________________


Theory of Protection

The operating principle of the thermal protector is simple and effective. At the heart of the protector is a bimetal snap-action disc. When the temperature of this disc reaches its pre-calibrated temperature it snaps open, resulting in an open circuit. This temperature is reached during a fault condition, caused by either an increase in ambient temperature, an increase in current flowing through the disc, or a combination of both. After thermal protector breaks the circuit, the system cools and it automatically resets allowing power to be restored to the circuit.

Opening Temperature

The temperature at which thermal protector makes the circuit open.

Reset Temperature

The temperature at which thermal protector reset the circuit as it cools.

Maximum Contact Ratings

The cycles of thermal protector reverts the circuit from open to reset when it works

under rated current and voltage.


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