# Harmonic Filter Fuse



## JRaef (Mar 23, 2009)

On a harmonic trap filter like that, the fuses are only there to protect against a fire if either a capacitor shorts or the reactor fails. It's usually the caps that fail though, reactors are just wire and steel. The fuse size used in the filter is based solely on the UL listing of the filter unit, there is no real "load" on the filter, the caps are in parallel to the motor, not in series. But the fuse also has to be able to withstand the inrush current that happens when the caps charge themselves up every time the unit is energized. So if you put in fuses that are too small, they will blow right away.

but too large might be a problem too. What happens is that aside from the reactor and caps, there are "bleeder resistors" in the caps that also serve to soften the current inrush. If those resistors burn out, the fuses blow soon after. Most likely if the caps test good, that's what had happened. So if you replace the resistors and then replace the fuses with some that are too large, you might damage the caps. 

Bottom line, you really need to use the right fuse, which will likely involve a phone call to the filter mfr. If they are out of business or something, fake it a little by calling another filter mfr and telling them you need the fuse size for their filter, based on the VFD HP size. They are likely all the same.


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## glen1971 (Oct 10, 2012)

Thanks Jraef for the help!! Definitely a great explanation of how it works!
I didn't think of metering the bleeder resistors. I'll have to get back there and check them before installing the new fuses. I had one of the plant guys check, and he found a hand sketch from 2004 (date when I saw the pencil marked amperage readings) and they are 80 amp fuses.
I can't find a nameplate rating on it at all. I'm guessing it was inspected and built to some design specs, which I may find on the sketch... 
The filter is on the 480 volt bus and was added when they kept having to replace low voltage VFD's. They did some checking (pre-2004) and when the 1,000 hp 2,300 volt VFD was running, it was causing a lot of harmonic issues.


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