VFD blown line side fuses

[Davebones]

We had a VFD in our wind tunnel that has not been used in a couple months . Today they went to power up and had no power . I found all three line fuses blown . Checked the Line side no short there or the 5 hp motor . Couldn't see anything obvious in the VFD and nothing read to ground . Removed the motor leads and powered it back up and it blew the fuses . Know the VFD is bad so we'll replace it . I've never seen it but I'm curious if you had a motor that was shorted can that cause the VFD line side fuse to blow when you first power it up ?

 

[kwired]

We had a VFD in our wind tunnel that has not been used in a couple months . Today they went to power up and had no power . I found all three line fuses blown . Checked the Line side no short there or the 5 hp motor . Couldn't see anything obvious in the VFD and nothing read to ground . Removed the motor leads and powered it back up and it blew the fuses . Know the VFD is bad so we'll replace it . I've never seen it but I'm curious if you had a motor that was shorted can that cause the VFD line side fuse to blow when you first power it up ?

Most do not send any power to the motor when first energized. Most anymore do have protection built in that will sense load side fault and will shut down output before it reaches damaging levels, though a sudden fault while running could possibly still cause some damage, but starting a motor usually is applying low volts and low frequency and ramping it up and you can somewhat easily catch a fault before it is at a high current level doing this.

The input side of the drive goes to a rectifier first, where DC current is created, then it is converted back to a pulsing current that to the motor will look close enough to real AC current that the motor will function. Chances are one or more diodes or a capacitor that is part of the rectifier circuit is what has failed. On small "off the shelf" drives probably about as well to just replace the drive. On larger drives, like at least more than maybe 15 Hp, or some specialty drive like one that is intended for 240 single phase input and 480 three phase output it may be worth having repairs made.

 

[Jraef]

Not usually. Most decent VFDs have electronic short circuit and ground fault protection for the LOAD SIDE, so when something happens there, that acts faster than the line side fuses would.

What most likely happened is that on small drives, there is a little current limiting resistor in the DC bus capacitor circuit that is used to limit the charging inrush current of the capacitors when first energized. That "pre-charge resistor" is then bypassed a second later once the capacitors are charged up. If the bypass contact welds closed, then the pre-charge resistor is no longer functioning and the inrush of the capacitors causes the fuses to clear. One thing that often causes that is if the VFD is operated by someone opening a contactor/starter ahead of it every time they run the machine. VFDs are generally not made for that and the manual will warn you not to do it, this is why.

As to repair or replace, it will be best to just replace it. On most small 5HP VFD, finding and replacing that pre-charge bypass relay is no simple task, it is often now just a surface-mount component on the single electronic board that is the VFD. Replacing a comonent on a surface mount wave soldered board is not for the novice electronics repair person, and that's assuming the VFD mfr will share the PCB drawings with you to be able to identify it.

 

[__dan]

Generally when vfd takes out all of the line side fuses, I would suspect first that the input rectifier diodes are failed shorted, and you can usually do a diode check test pretty quickly with a doide test check function on a DMM (and the factory manual). Trying to reapply line power to it will only make it go bang again. If you see the line side fuses blown, do the diode check test first.

Smaller vfd's up to a pretty large size are completely assembled by automation and no human touches them until they come out of the cardboard box, making their labor component of cost very low, and with competion this is carried through into the wholesale cost. As soon as you touch them and add the specialized trade labor cost, plus the huge factory markup on repair parts because now you are locked in to that unit, cost to repair can exceed cost to buy competitive new, pretty quickly.

The hp size were this tradeoff happens keeps going up over time.