According to the power of motor, how to select the contactor and switch?
Must first determine the breaking current, the general low voltage bus bar breaking capacity above 500 kva transformer with 50 kA switches, small transformer low voltage busbar can use 12.5 kA and the breaking capacity of the switch. Secondly according to the current power calculation, and generally current = power/voltage/power factor, if the three-phase motor is divided by 1.732 (because of calculation is single-phase current), choose the rated current higher levels of switch and the contactor. Then select the corresponding tripping characteristics depending on the type of load switch (including tripping accessories), feeder switch will generally choose electromagnetic, should choose the thermomagnetic motor switch type, high perceptual load should choose high grade tripping characteristics of the switch.
Select the circuit breaker as follows: The capacity of the motor is set to X (kW), the rated current of the motor as: 2 * X (A), in general, choose the circuit breaker rated current capacity shell frame is 4 * X (A), thermal rating of the breaker tripping current choice for (2.5 ~ 3.2) * X (A). For example: motor power 7.5 (kW), rated current of 15 (A), the capacity of circuit breaker should be 32 (A). Motor power (kW), 55, 110 (A) current rating, the rating of the breaker shell frame current capacity should be 250 (A), the circuit breaker rated heat tripping current choice for around 150 (A). The contact choice is the same as (3.5 ~ 4) * X (A). Note: the fan, pump circuit breaker and the contactor choose to be larger, because they have a longer start time, and the start torque is larger. 4, conductor selection: choose according to the rated current of the motor, generally 1.5 times the rated current, but want to consider laying environment, laying mode, multiplied by the appropriate factor.
Based on the capacity of the three-phase squirrel-cage asynchronous motor, the calculation method for selecting circuit breaker, contactor, heat component and conductor is as follows: 1, the capacity of the motor is set to X (kW), the rated current of the motor as: 2 * X (A), in general, with the motor nameplate rated current of the difference is very small, if not sure, can take the motor manual check. For example: motor power 7.5 (kW), rated at 15 (A); The motor power 55 (kW), the rated current is 110 (A).
According to the power of the motor, the current of the motor is about the value of the electric current, the ac contactor capacity should be more than twice as long as the electric current, and the circuit breaker is more than 1.4 times the electric current.
