VFD's have a large capacitor bank that needs to be charged before operations can commence. The bank is usually sized right around a point that's problematic for the supply power system sized to run any particular VFD.
When you close power onto the bank it looks like a dead-short initially. This causes very large inrush currents that stress everything in the link up to the capacitors. It also can dim the lights in a facility.
To deal with this, especially with drives bigger than a few hp, precharge circuits are used. It is a timer, relay, and resistor that limits the inrush from theoretically infinite to at a maximum of whatever the resistor will allow. After the bank is mostly charged the timer times out and the resistor is removed from the circuit connecting the mains directly.
If the system is complaining about 'pre-charge open' then it likely means the resistor is toasted and failed open or the relay for whatever reason is no longer conducting. It can be:
1) failed resistor
2) burned away contact(s) on the relay
3) loose connection
4) failed relay coil
5) or worse, a failure in the controls that prevents the relay from ever closing
Take extreme caution around capacitor banks on this type equipment. Make absolutely sure the capacitor bank is fully discharged before any checking is done.
Keith Cress
kcress -
