With the development of technology, the application scope of motors is becoming more and more extensive, especially the use of motors in daily life and production is becoming more and more important. However, accidents of motor burning also occur from time to time. In real life, motor burning is mainly caused by the following reasons: overheating of the stalled coil, excessive load, motor overload, phase loss, etc. Installing a thermal protector is one of the effective ways to protect the motor. Thermal protector, also called temperature switch, temperature control switch, etc. It is a temperature switch that uses a bimetallic strip as a temperature sensing element. When the appliance is working normally, the bimetallic strip is in a free state and the contact is in a disconnected state. When the temperature rises to the action temperature value, the bimetallic element is heated to produce internal stress and quickly acts, opens the contact, cuts off/connects the circuit, and thus plays a role in thermal protection. When the ambient temperature drops to the reset temperature of the protector, the contact will close again to connect the circuit, and the motor will work again.

How to choose a suitable protector is crucial, otherwise it will not only fail to play a protective role, but will cause the motor to fail to work or even burn due to incorrect selection.

1. Determination of voltage and current

• Motors have different voltage ranges: 380V, 230V, 220V, 110V. DC motors also have 36V, 24V, 12V, etc. Protectors with different rated voltages are selected according to different working voltages. In principle, the actual application voltage should be less than or equal to the rated voltage of the protector. For example, a 380V motor cannot use a protector with a rated voltage of 220V, while a protector with a rated voltage of 220V can be used in a 110V motor.

• The current of the motor is divided into three types: starting current, normal working current, and stall current. The rated current of the selected protector should be greater than the maximum value of the three currents, and a margin should be left. For example, if a motor has a normal working current of 1A, a starting current of 1.5A, and a stall current of 2A, the rated current of the selected protector should be at least 2A. It is best to choose a rated current of 3A or 5A to keep a margin. Some motors have a starting current greater than the rated current of the protector, but the starting time is short, while the normal operating current and the stall current are less than the rated current of the protector. In this case, this protector can also be selected. This is because although the starting current is greater than the rated current, due to the short time, the thermal effect of the current has not yet had time to heat up the double-chip and cause the protector to protect, thus preventing false operation.

2. Selection of protection temperature

The selection of protection temperature is related to three factors: insulation grade of enameled wire, installation location and protector type.

• The protection temperature should be lower than the maximum allowable temperature. For example, the maximum temperature of Class F is 155 degrees. The actual selection can be 145 degrees or 150 degrees.

• The installation position is divided into built-in coil and tied to the outside of the coil, because when the motor is blocked and heated, the overheating phenomenon spreads from the inside to the outside. If the protector is buried inside, the protection temperature can be slightly lower than the maximum temperature of the enameled wire; if it is tied to the outside of the coil, the temperature difference between the inside and the outside should be considered, and a protector with a lower temperature should be selected. For example, if the insulation grade is Class F enameled wire, and the temperature difference between the inside and the outside of the coil is 20 degrees, a protector of about 155-20=135 degrees should be selected.

3. Selection of protector

• Limited installation location: Coil embedded type If the coil is blocked or abnormal, the heat usually starts from the inside of the coil and gradually conducts to the surface of the coil. If it is placed on the surface of the coil, there will be delays. Or the same model has more winding turns, squeezing the internal space, resulting in the inability to install the protector. ST01/ST11 button type: ST01 U10 is directly wave soldered on the circuit board, and the protector microphone head is close to the coil surface.

• Anti-molding pressure after embedding: The current solution in the industry is to dig a groove and insert it after shaping or use a fake protector to shape it inside before putting it in. The real protector will damage the coil and delay production efficiency. ST01 300N /ST11 450N /ST06 500N

• Vacuuming and painting: In order to provide insulation performance, the stator is placed in a paint tank and immersed in a vacuum, and the pressure reaches -0.09mpa. Especially water-based paint has good fluidity and is particularly easy to enter the inside of the protector. The paint enters between the contacts, causing product failure.

• When using a thermal protector with current passing through the double-piece, the advance amount of early tripping caused by current heating needs to be considered during design. Moreover, the advance amount is different under different ambient temperatures, which makes the design more painful. ST01/ ST06/ STH6/ ST11 are pure temperature protectors. The working current passes through the reed, not the double piece, and has little effect on the tripping temperature. The design is relatively simple.

• When the current passes through the double-piece protector, the double piece has a very slow creeping process when approaching the tripping temperature, resulting in too little contact pressure, increased contact resistance, and micro-cutting when reaching the tripping temperature. Due to the small opening distance, the arcing phenomenon is serious when tripping, resulting in limited life.

• For SAFTTY ST01, ST06, and ST11 products, the contacts are welded or placed on the reed, and the double-piece and the reed are independent. However, when the temperature is close to the tripping temperature and the double-piece creeps, the contact pressure does not change, and the contact resistance does not change, so it does not cause arcing during micro-cutting. When the double-piece reaches the tripping temperature, it quickly reverses, driving the moving contact on the reed and the static contact on the cover to separate, and the arcing and ablation phenomenon is not obvious.

• The insulation layer of the protector is sharp after heat shrinkage, which can easily cut the enameled wire, causing a short circuit or insufficient insulation withstand voltage. The outer shell of ST01 U4, BW-B, and BW-E is epoxy-encapsulated, round and smooth, and will not cause damage to the enameled wire.

• Explosion-proof motor protector, due to poor sealing performance, arcing problem when the protector is activated, ST01 U4 epoxy encapsulation.