Selection of motor power switch
Thread Content
I have a low-voltage motor equipped with a comprehensive protector, the current protection setting is set at the motor’s rated current. The power switch is an air circuit breaker – how should its size be determined? I have a current level of 2.5 to 3 times the motor’s rated current, is that correct? Would it be okay if someone suggested using a larger air switch? Are there any standards or national regulations?
It’s not that large; generally, the rated current of a circuit breaker just needs to be slightly higher than the operating current. The instantaneous trip current should be set at around 12 times the value. Source: GB50055-93 Code for Design of Power Distribution for General Electrical Equipment.
Well-founded, great! ! Remind the poster to choose a “motor protection type” circuit breaker ； Otherwise, it might not be possible to avoid tripping due to the starting current! For greater precision, it is more accurate to use the motor startup parameters and compare them with the circuit breaker’s tripping curve for verification
When selecting a circuit breaker, two parameters need to be taken into account: 1. The current rating must be higher than the rated current of the motor, in order to prevent accidental tripping across levels; 2. The process should be as follows: first select the cable based on the motor, and then use that cable to determine the appropriate switch. If things are done as the original poster suggested, the cable will end up suffering greatly. No protection left! For a specific understanding, you can refer to the second floor.
The circuit breaker in the motor circuit is primarily used to protect the cables; protection for the motor can only serve as a backup measure. Therefore, the cable should be determined first, followed by the circuit breaker; then the coordination between motor startup and the circuit breaker needs to be verified. If it is not possible to handle the starting current, the capacity of the cable and the circuit breaker switch should be increased.
The topic is quite innovative; I’ve never heard of it before, hehe! Maybe everyone calls it differently; we usually call it a circuit breaker!