Details of Dalan Motor: The coordination of the protection device and the three-phase asynchronous motor is very important!

The protection of a three-phase asynchronous motor often has a certain relationship with its control method, that is, there is control in the protection, and protection in the control. Today, the editor of Dalan Motor will share the relationship between the motor and the protection device with everyone.

1. Coordination of overload protection device and motor

(1) The action time of the overload protection device should be slightly longer than the motor starting time. The characteristics of the motor overload protection device can only ensure its normal operation by avoiding the characteristics of the motor starting current; but its action time cannot be too long, and its characteristics can only play an overload protection role under the thermal characteristics of the motor.

(2) The instantaneous operating current of the overload protection device should be greater than that of the motor

There is a misunderstanding in motor protection. Use circuit breakers and fuses to protect the motor. Effective and simple is the thermal relay + contactor (or other devices that can cut off the power supply) for overload and phase loss protection. The action characteristics of the thermal relay bimetallic piece are matched with the motor heating, and there is a phase protection function. Choose a good quality thermal relay, which should be effective for motor protection.

The setting of the thermal relay: It is 1.05-1.1 times the rated current from books and studies, so it is no problem. However, in actual applications, in order to be more effective protection, it should be set according to the maximum current in operation.

The thermal relay setting method is as follows: reduce the setting current, let the motor run for a few minutes to trip, and then increase it to 1.1-1.2 times according to the current setting value. In this way, the failure caused by the inaccurate current scale of the thermal relay can be avoided.

2. There are roughly four types of circuits for motor protection:

(1) It is composed of thermal relay FR, contactor kM and circuit breaker QF with transient protection only, as shown in Figure 4. The contactor is used to start and stop the motor, the thermal relay is used to protect the motor from overload, and the circuit breaker with only instantaneous protection is to protect the motor from short circuit.

(2) Composed of thermal relay FR, contactor kM and fuse FS. The thermal relay protects the motor from overload, the contactor starts and stops the motor, and the fuse protects the motor from short-circuit faults.

(3) It is composed of a contactor kM and a motor protection type circuit breaker QF. The contactor is used to start and stop the motor, and the motor protection circuit breaker is used to protect the motor from overload and short-circuit faults.

(4) It is composed of a motor protection type circuit breaker. The motor protection type circuit breaker can not only start and stop the motor, but also protect the motor from overload and short-circuit faults. Among the above four types, two types 1 and 2 are suitable for more frequent starting-stopping the motor, the third type is suitable for general frequent starting, and the fourth type can only be used for infrequent starting and stopping.

－－－－- Editor in charge: Dalan Oil Pump Motor 02-Purchasing Consultant

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