Selection and Application of Electronic Motor Protector

[China Aluminum Industry Network] Analysis of Electronic Motor Protectors We know that thermal relays have the advantages of simple structure, low cost, small size and easy to use. However, the thermal relay has a single protection function, low accuracy, unstable operation, and a short heating time constant. Simply put, all thermal relays are good, that is, the protection performance is not reliable, which is its achilles heel. It is precisely because of this that an electronic motor protector with reliable protection performance has emerged. While showing its vitality, it is experiencing a process of gradual maturity. My company's production environment is relatively harsh, high temperature, more dust, moisture, continuous production, high motor damage rate.

We have used several forms of motor protectors and have obtained some experience. Below from the reliability, ease of use and economical aspects of the electronic motor protector, talk about their immature experience. Electronic motor protector (motor protector) reliability At present, the thermal relays of industrial and mining enterprises are commonly used, but the phenomenon of damage to the motor is also widespread. According to investigations, more than 90% of the failures of asynchronous motors are due to overheating of the stator windings, and nearly 60% of them are caused by phase failures. This not only indicates that the thermal relay is unreliable as a phase-failure protection, but also shows that the phase-failure protection The necessity. In addition, due to the impact of the starting current of the motor, the thermal relay also causes its own phase failure to occur occasionally. Therefore, the novel motor protector must have a phase failure protection function. As mentioned earlier, the thermal relay has a small heat-generating time constant, which is not suitable for motors with heavy inertia and heavy load start-up. The method of short-circuiting the thermal relay at startup is often used. This not only makes the control system complicated in structure, increases in cost, but also exists. The protection of the dead zone.

In order to avoid misoperation during start-up, the setting current is increased to make the protection virtually useless, and most of the detection current transformers of the electronic motor protector use a speed-saturation current transformer, so it is generally allowed to have a starting time when cold. Long; rapid overload feature when hot. This is exactly matched with the actual protection requirements of the motors of industrial and mining enterprises, and can more reliably protect motor overload. The detecting element of the thermal relay is a bimetal. Due to overcurrent impacts such as starting current and overload, it is easy to cause the bimetal to have a fatigue effect, resulting in offset of the scale value and unstable operation. This is difficult to find at the production site. Later overload caused no action. The electronic motor protector uses a current transformer as the detection element, so there is no heating problem. The operation stability of the electronic motor protector is a qualitative leap compared to the thermal relay. With the use of more advanced electronic motor protectors (motor protectors), the detection elements generally use current transformers, and a core-through structure has emerged. Since the introduction of the core-type motor protector (motor protector), due to its many advantages compared to the thermal relay, it has attracted a great interest of the electrical industry. The core-through structure is not only easy to use, but also completely isolated from the main circuit, which will not affect the main circuit and increase its own reliability. At the same time, the problem of heating of the connection terminals is completely solved, and the interchangeability is good. However, currently only large-size motor protectors (motor protectors) are used at home and abroad, while small-size ones can only use wiring. Electronic motor protectors (motor protectors) solve the problem of a single function of the thermal relay and create new drawbacks that require a working power supply. Although the requirements for the electronic circuit amplification and driving need working power are reasonable, the application of the motor protector (motor protector) brings a series of problems. For example, the user must look for the power terminal before use and understand the power supply voltage. Grades and other inconveniences. In addition, because the motor protector (motor protector) needs to work continuously, the failure rate due to fluctuations in the voltage of the power grid, interference, and self-heating is also high. According to the incomplete statistics of the used electronic motor protectors (motor protectors) in our factory, more than half of the motor protectors (motor protectors) have their own faults in the power supply, which has greatly dampened the use of electronics by electrical workers. Motor protector (motor protector) enthusiasm.

In addition, the electronic motor protector (motor protector) has two insurmountable contradictions due to the use of electromagnetic relays as actuators. First, use the relay's normally open contact to implement the protection program, which features the contact is always open when the main loop is not working, so the contact must be serially connected to the contactor's self-protection loop, so that even the user feels inconvenient installation, and can not Circuit used for automatic control; Second, use the relay's normally closed contact action to implement the protection scheme, although its control contact can be directly connected to the control loop like a thermal relay, but due to its automatic reset, it can not be applied to automatic Control System.

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