What are soft charge resistors & soft charge contactors ? thanks

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dineshdeshmuk

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What are soft charge resistors & soft charge contactors ?

What do they really do in a frequency inverter (VFD) ?

thanks
 

Soft change resistor are nothing but series resistor inserted to reduce the inrush currents. Inrush current are common in any circuit with capacitive filter.
 
The soft charge resistor works in conjunction with the soft charge contactor to slowly charge the DC bus capacitors by minimizing the inrush current that happens when power is first applied to an inverter.

When input power is applied, the DC bus voltage starts to increase. Typically, the DC bus voltage will raise to a point that is above the under voltage detection point at which time the soft charge contactor will energize and thereby bypassing the resistor. :shock: If not for the soft start charge resistor damage to the input diode could happen.
 
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