WEG CFW-11 Frequency Inverter for 3PH, 600V, large HP fan motor went down the other day. Initial inspection found that a 125A fuse on the line side of the inverter was blown. Did a thorough inspection of unit and found nothing obvious. Replaced fuse and attempted to power up with the fan motor locked out. Display monitor on outside of panel failed to power-up. Checked fuses for the controls and verified that power was available to monitor yet it does not power-up.
Anyone familiar with this model? Any ideas what is preventing the display monitor from powering up?
Myself and another electrician took another look at it this morning. We traced the wiring coming off the "On","Off", "Bypass" switch and found that it went to the E-Stop. I checked the resistance of the conductors going back to the E-Stop while the other electrician operated it. I was getting strange results where the readings fluctuated up and down rapidly. We decided to open the E-Stop housing and test the conductors after isolating them at that end, too. As soon as my co-worker opened the housing a significant amount of water ran out onto the floor. The NC contact was corroded and the wiring was wet. Once this E-Stop was cleaned up and the damaged part was replaced we tried to run the fan motor and everything worked as it should.
How the water got inside the E-Stop is a mystery. And how did this cause the 125A fuse on the one phase of the line side to blow? Have some ideas but I'm not certain of anything.
If the wet E-Stop was cycling the bypass on and off repeatedly and quickly, plus you have what's called a "3 contactor bypass" where there is a line isolation contactor ahead of the VFD, you can end up in a situation where the pre-charge current limiting resistor of the drive is not in the circuit and when the drive is powered on again, the capacitor charging inrush current is so high that it clears a fuse (if you are lucky).
If you don't know this about VFDs, all of them have some means of avoiding the charging current of the caps from doing damage, collectively this is referred to as a "pre-charge" circuit regardless of how it is done. Capacitors charge instantly so when first energized, the are pulling current at the available fault current rate for an instant. This can be so fast and high that it damages the diodes on the drive and/or clears the fuses. So the pre-charge circuit limits the charging current for the first second when a VFD is energized by the line. Usually this is done with a little resistor (current limiting resistor) in series with the caps on the DC side of the rectifier. But that resistor can't be in the circuit all the time or it will burn out, so a second after you first power up a drive, a relay or small contactor closes to shunt (short) around that resistor. If you have a line contactor in front of the drive and that contactor chatters, that little shunt contact doesn't have time to respond and the caps pull the full current, which might blow the diodes or hopefully, clear the fuse first.