How to Choose a Circuit Breaker for a Motor

Low-voltage circuit breakers are generally divided into two categories: plastic enclosures (also called equipment) and frames (also called universal).

When the motor circuit breaker operates by itself, it can be controlled manually (button control or knob control), and it can be remotely controlled when the contactor is connected; the protection of the motor is provided by the circuit breaker with integrated thermal relay electromagnetic equipment; All live parts have been protected, and the board in front cannot be directly touched with fingers; with the delinquent delinquency module, the circuit breaker can be disconnected in the case of delinquency; Lines can be remotely controlled; both open mounted and enclosed motor circuit breaker operators can be locked in the "N/C" position using 3 padlocks.

1. Selection principle of circuit breaker for motor protection

(1) According to the rated current of the motor, determine the overall value of the long-term operating current of the circuit breaker.

(2) Instantaneous rectification current: For the circuit breaker protecting the cage motor, the instant rectification current is equal to (8 to 15) times the rated current of the motor, depending on the type, capacity and starting conditions of the cage motor to be protected. For the circuit breaker of the wound rotor motor, the instantaneous overall current is equal to (3 to 6) times the rated current of the motor, depending on the type, capacity and starting conditions of the rotor motor to be protected.

(3) The recovery time of the circuit breaker's 6 times longer operating current setting value is longer than the actual starting time of the motor.

2. Quick check of circuit breaker specifications and models

For example, a Y160M-4 type 11kW motor should be equipped with a DZ5-50 type circuit breaker with a thermal release rated current of 25A from the quick look-up table.

3The circuit breaker tripper sorts out the formula of current speed calculation

"Motor instantaneous action, 20 times kW" "Thermal release, according to the rated value" The above formula refers to the circuit breaker that controls and protects a 380V three-phase cage-type AC asynchronous motor. Select 20 times the number of "kW". For thermal release, select it according to the rated current of the motor.

4. Scope of application

(1) The motor control and protection performance complies with IEC 947-2 and IEC 947-4-1 standards;

(2) Any equipment is easy to install, and it can be fixed with screws or tightly clamped on the guide rail.

Of course, for motors that need to be started frequently, if the probability of phase-break operation is not high or there is a phase-break protection device, it is more appropriate to use a combination of fuse and magnetic starter to control and protect, because this protection method It is easy to control from a distance.