The main peripheral equipment of a frequency converter's main circuit includes circuit breakers, contactors, and the wire diameter of the main circuit (i.e., the wires between the power supply and the frequency converter, and the wires between the frequency converter and the motor). Below, we will provide a detailed analysis of how to choose the correct equipment.
1. Circuit breaker
When the frequency converter needs maintenance, or if it is not used for an extended period of time for some reason, disconnect the QF circuit to isolate the frequency converter from the power supply. Provide protection in case of faults such as short circuits on the input side of the frequency converter.
Selection Principles
(1) At the instant the frequency converter is connected to the power supply, the charging current of the capacitor can reach (2-3) times the rated current.
(2) The input current of the frequency converter is a pulse current, and its peak value may often exceed the rated current.
(3) The inverter is allowed to overload 150% for 1 minute.
Based on the above data, the DW15-400 circuit breaker can be selected. The rated voltage of the circuit breaker is 380V and the rated current is 300A, which meets the requirements.
2. Contactor
(1) Main function: The power supply of the frequency converter can be conveniently controlled by the push-button switch; the power supply can be automatically cut off when the frequency converter fails.
(2) Selection principle: Since the contactor itself has no protection function and there is no problem of false operation, the selection principle is that the rated current of the main contacts should be greater than 126.6A. A contactor with a rated current of 130A of the main contacts can be selected.
Based on the above data, Schneider's LC1-D150 meets the parameter requirements and can be selected.
3. Wire diameter of the main circuit
(1) The wires between the power supply and the frequency converter
Generally speaking, the method for selecting wires is the same as that for ordinary motors of the same capacity. Considering that the power factor on the input side is often low, the wire diameter should be determined according to the principle of "better to have a larger diameter than a smaller diameter".
(2) The wires between the frequency converter and the motor
Because voltage drops as frequency decreases, the proportion of line voltage drop in the output voltage will increase while the proportion of voltage received by the motor will decrease, provided the current remains the same. This could potentially cause the motor to be unable to drive the load and overheat.
