Protection Of Asynchronous Motors(1)

There are two main types of motor protection: thermal relays with current detection, motor protection circuit breakers with thermal-magnetic trips, electrical and solid state relays, motor protection circuit breakers with electronic trips, and Soft starter; the temperature detection type that directly detects the temperature of the motor winding includes bimetal temperature relay, thermal protector, detection coil and thermal resistance temperature relay, etc., but because it needs to be directly embedded in the motor winding, the price is more expensive and maintenance is difficult, etc. The reason is that it is only used in some frequent operation occasions.

Finally, it is pointed out that no matter what kind of protection device is used, the coordination between overload protection device and motor, overload protection device and short-circuit protection device must be considered.

The protection of asynchronous motors is a complex issue. In actual use, the appropriate protection device and starting equipment should be selected according to the capacity, type, control method and power distribution equipment of the motor. The relationship between the protection and control of the motor The protection of the motor often has a certain relationship with its control mode, that is, there is control in the protection, and there is protection in the control. For example, when the motor is started directly, a starting current of 4-7 times the rated current is often generated.

If it is controlled by a contactor or a circuit breaker, the contacts of the electrical appliance should be able to withstand the making and breaking of the starting current. Even a contactor that can be operated frequently will cause increased contact wear and damage to the electrical appliance; type circuit breaker, even if it is operated infrequently, it is difficult to meet the requirements. Therefore, in use, it is often used in series with the starter in the main circuit. At this time, the contactor in the starter is used to carry out the assessment of switching on the starting current, while other electrical appliances only carry the assessment of the overload current of the motor that occurs during normal operation. , As for the protection function, it is completed by the matching protection device.

In addition, the control of the motor can also use a non-contact method, that is, a soft start control system. The main circuit of the motor is connected and disconnected by a thyristor. In order to avoid continuous losses on these components, vacuum contactors are used to carry the main circuit (parallel to the thyristor) load in normal operation. This kind of control has program control or non-program control; near control or remote control; slow start or fast start and so on. In addition, depending on the electronic circuit, it is easy to achieve various protection functions such as electronic relays.