Shunlongwei Co. ltd.

IGBT Module / LCD Display Distributor

Customer Service
+86-755-8273 2562

Motor protection components: three types of overload protection of thermal relays

Posted on: 12/30/2021

In the era of electrical automation, motors play an important role in both production and life. During their use, protective measures are also indispensable. Among them, common thermal relays and motor protectors are the most commonly used protection weapons.

The working principle of the thermal relay is that the current flowing into the heating element generates heat, which causes the bimetallic strips with different expansion coefficients to deform. When the deformation reaches a certain distance, it pushes the connecting rod to disconnect the control Circuit, thereby making the contactor When the power is lost, the main circuit is disconnected to realize the overload protection component of the motor.

Today, as an example, the thermal relay is used as the overload protection component of the motor, and there are three forms:

1. Bimetallic type. Use the bimetallic piece to be heated to push the lever to make the contact move.

2. Thermistor type. It is made using the characteristic that the resistance value changes with temperature.

3. Fusible alloy type. Use the overload current to generate heat, melt the fusible alloy and make the relay operate.

Small size, simple structure and low cost are its advantages. It is suitable for long-term or intermittent operation of AC motors overload and phase failure protection, not suitable for frequent jog operation, let alone short-circuit protection. It has temperature compensation, automatic and manual reset functions.

With the development of people’s needs, thermal relays have fewer functions, no phase protection, poor ventilation to the motor, borehole sweeping, locked rotor, and long-term overload; the shortcomings of frequent start-up and other failures that do not have a protective effect are becoming more and more obvious.

Therefore, the motor protector is widely used. It is based on the principle of detecting the change of the line current (including adopting, positive sequence, negative sequence, zero sequence and overcurrent), which can detect phase failure or overload signal. In addition to the phase failure protection function, it also has the function of overload and locked rotor protection.

The advantage is that the wiring is simple, the main line only needs to pass through the threading hole of the body, there is no high-current wiring, and the contact failure is reduced. Most of the control circuit has four terminals, two power supply wires, and the other two terminals are connected in series with the contactor coil circuit, and some have only two control terminals, which is more convenient.

The disadvantage is that after equipment failure occurs, most of its control terminal protectors will automatically reset, and the operator may start the equipment again. Two-wire loads (mostly 220 volts), such as small air compressors and small water pumps operating in single phase, cannot be protected by protectors, while thermal relays can easily do it. In addition, most motor protectors on the current market do not provide a set of electrically independent contacts for alarm or other Circuits. Compared with the thermal relay, it provides a set of electrically independent normally open contacts on its body for designing alarms or other circuits.

Now, motor protection circuit breakers are also used by many people. The rated current is 630A and below (that is, the protected motor is 315kW and below). It is used to start and disconnect the motor infrequently when the motor is short-circuited or under-voltage. It can be protected (automatically cut off the circuit).

Its instantaneous trip current is 10 times the rated current, taking into account the motor’s starting and overload protection. When the motor starts, the current is larger than the rated current. The circuit breaker for motor protection will not trip if it allows a little overcurrent when it is started.

The Links:   SKIIP83AHB08T2 BSM400GA120DN2SE3256