1) Determine the type and protection form of the circuit breaker according to the requirements of the line protection; determine the frame type, device type or current limiting type.
2) The rated voltage UN of the circuit breaker should be equal to or greater than the rated voltage of the protected line.
3) For circuit breakers with undervoltage release devices, the rated voltage of the undervoltage release is not less than the rated voltage of the line.
4) The rated current of the circuit breaker and the rated current of the overcurrent release should be greater than or equal to the calculated current of the protected line.
5) The limit breaking capacity of the circuit breaker should be greater than the effective value of the maximum short-circuit current of the line.
6) The protection characteristics of the upper and lower circuit breakers in the power distribution system should be coordinated, and the protection characteristics of the lower level should be below the superior protection characteristics and not intersect.
7) The long delay trip current of the circuit breaker should be less than the continuous current allowed by the conductor. The long delay current setting value is equal to the rated current of the motor. The return time of the 6 times long delay current setting value is equal to or greater than the actual starting time of the motor. According to the light weight of the load at the start, you can choose one of the 1s, 3s, 5s, 8s, 15s return time.
8) The thermal trip unit setting current of the circuit breaker thermal release device shall be consistent with the rated current of the controlled load.
9) The instantaneous trip setting current of the circuit breaker electromagnetic release device shall not be less than the normal working peak current of the load circuit.
10) Instantaneous setting current: For the circuit breaker that protects the asynchronous motor, the instantaneous setting current is 8 to 15 times the rated current of the motor; for the circuit breaker protecting the winding type motor, the instantaneous setting current is 3 to 6 times the rated current of the motor.
The main function of the circuit breaker in the main circuit of the variable frequency speed control system is isolation and protection. It is used to safely trip the inverter power supply when the main circuit of the inverter is faulty. The open circuit current can be calculated as follows:
IQN ≥ (1.3 ~ 1.4) IN
In the formula, IN is the rated current of the inverter.
The load end of the circuit breaker can be connected to one or more inverters and other loads. When the inverter or other load has a short-circuit fault, the circuit breaker can automatically cut off the power supply to prevent accidents from expanding. When the power grid loses power, it prevents electrical calls from automatically turning on the power of the inverter and causing electrical or mechanical accidents. When the variable frequency speed control system is overhauled, the circuit breaker can be used to cut off the power supply safely. The circuit breaker can use ordinary circuit breakers or high-performance motor-specific circuit breakers.
For a single motor, the instantaneous trip setting current Iz can be calculated as follows:
Iz≥KIST
In the formula, K is a safety factor and can be taken from 1.5 to 1.7; IST is the starting current of the motor.
For multiple motors, it can be calculated as follows:
Iz ≥ K (ISmax + ∑ In)
Where K is 1.5 to 1.7; ISTmax is the starting current of one of the motors with the largest capacity; ∑In is the sum of the rated currents of the remaining motors.
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