Wierd problem with a VFD
Wierd problem with a VFD
(OP)
I have a control panel with 3 VFD. Two of them are 2HP and the other one is a 5HP. The VFDs are 600V, 3PH. From the main I go through a fusebox that protects my VFDs. There is one fuse for each phase of each VFD, so there is 9 fuses in all. Nothing is plugged at the exit of the VFDs. As soon as I put the power ON, the 9 fuses blow. I have noticed that they blow only when the 5HP is plugged, so I took it out. The two other ones are ok. Because of that I thought the problem was my 5HP VFD, so I got another one. Same thing. As soon as I put the power on the 5HP, at least 2 of the 3 fuses blow instantly.
I looked everywhere for a short, and didn't find any. Because the client wanted the panel as cheap as possible, I did not put any filter before the VFDs. The only thing left was to put an L filter. After putting the filters on all the VFDs, the fuses do not blow anymore. Does anyone would have an explication? The line reactors (filters) are after the fuses and before the VFDs. I understand that they protect the drive, but because they are after the fusebox, they should have no consequence on the fuses. Why did this fix my problem? What was blowing my fuses?
Thanks for any help. All I want is to understand.
I looked everywhere for a short, and didn't find any. Because the client wanted the panel as cheap as possible, I did not put any filter before the VFDs. The only thing left was to put an L filter. After putting the filters on all the VFDs, the fuses do not blow anymore. Does anyone would have an explication? The line reactors (filters) are after the fuses and before the VFDs. I understand that they protect the drive, but because they are after the fusebox, they should have no consequence on the fuses. Why did this fix my problem? What was blowing my fuses?
Thanks for any help. All I want is to understand.





RE: Wierd problem with a VFD
Are the fuses fast or slow blow? Are they selected according to VFD installation manual? What make are the VFDs?
Gunnar Englund
www.gke.org
RE: Wierd problem with a VFD
I do not know the exact correlation between the current limiting resistor, buss caps and starting torque but I am fairly sure you want the caps big and resistance small which leads to high inrush.
Barry1961
RE: Wierd problem with a VFD
In your situation it is a new product. I am curious to know what manufacturer it is. Also like the previous posters asked....what fuses are being used and what size are they?
RE: Wierd problem with a VFD
Rather, having just worked thru a similar experience, I suspect that the power supply is one of those cursed floating delta or high resistance grounded systems. When you have more leakage current to ground than the drive fuse ratings, the virtual ground in the drives cause the fuses to blow on leakage current alone.
You can verify this by removing all grounding from the three VFD's and then turning on the power. If the fuses hold, its time to install a transformer with a wye secondary with the center of the wye properly grounded. The filters in the line probably reduced the leakage current just below the fuse clearance point but, as soon as you try to load the drives, the fuses will blow again.
RE: Wierd problem with a VFD
RE: Wierd problem with a VFD
The manufacturer of the drives is AC Tech, The fuses are fast acting (as suggested by manufacturer) and the size of the fuses are 10A for the 2HP and 15A for the 5HP. AC Tech suggested using reactors in the following situations:
1. The supply power is subject to disturbances such as surges, spikes, dips, transients, etc.
2. The supply power is very stiff (greater than 10 times the kVA rating of the connected VFDs)
3. Harmonics are a concern
I should've used reactors because the plant is not on a power supply, but plugged directly on the energy distributor's 600V. So the line is far from clean or constant. Because of money issues, I had to cut the cost as much as possible and did not put reactors in the beginning. It ended out costing more in the end.
What I didn't understand then, was the correlation between the fuses blowing and the reactors, but what you guys told me makes a lot of sense⦠thanks again.
RE: Wierd problem with a VFD
It was all good until..... it happened again. The fuses of the 5HP blow again. After 1 week of running I got called, the fuses are blown. Do you guys have any idea of what should I do next? A RLC filter on the input? Output reactors?
When I change the fuses, they hold, but for how long? I can't leave it like that, I have to find out why it happened again... Do you have an idea of what to do next?
thanks a lot,
Vince
RE: Wierd problem with a VFD
You say that harmonics are a concern. Does that mean that you are concerned or that the harmonics level really is high?
One thing that inverters don't like is voltage asymmetri. If one phase is down more than a few percent, the other phases take over the current. That usually means that one of the fuses don't blow (the one fed from the low phase). Check phase-phase voltages. They shall stay within about 2 percent from each other.
I hope that you are not using very fast "semiconductor" fuses. That shouldn't be necessary and is sometimes the reason for problems like yours. I would even suggest that you use slow-blow fuses. It is not common to have any overcurrent at all once the inverter has started (output is current limited). And since the fuses blow during start, I think they are to fast. Simple as that.
The next option is to talk to the manufacturer. He should be able to help you.
Gunnar Englund
www.gke.org
RE: Wierd problem with a VFD
I predict that you will read surprisingly high current in at least two of the three phases.
This current is leakage current from phase leg to ground elsewhere in the facility and finds a path from ground back to another phase pair thru the drive and the drive input fusing.
This only happens on floating delta or high resistance grounded wye power networks.
The solution is to install a drive isolation transformer with a wye secondary and the center of the wye directly connected to ground.
As I mentioned in the earlier post, you can test to see if ground leakage currents are the problem by lifting all ground connections from the drive frame by insulating the mounting and removing all ground wires. (I assume the drive is inside an enclosure for safety). If the problem goes away with the drive floating, the transformer is the answer.
RE: Wierd problem with a VFD
I am not saying they are doing anything wrong because I do not know anything else about their design or testing, but that has always bothered me. If you have noticed, most other manufacturers only sell 575V drives at above 5 HP, or if they have <5HP they are larger dimensions and more expensive than the AC Tech. That's because they have to use stand-alone diodes and IGBTs to get that voltage rating. IPMs allow for a smaller and less expensive design, but AFAIK, nobody officially offers an IPM rated for 575V use. I may be wrong, and someone please correct me if I am, but that's how I see it.
If a diode is shorted, and the new one shorted immediately upon energizing it, it is likely that your drives are not surviving the applied voltage, especially if it's a little over spec.
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RE: Wierd problem with a VFD
So if this gets resolved, I still suggest that you install reactors on the line side.
There are some really good technical commenst mentioned above by everyone, and I am interested to hear your feedbakc after measuring all they had to offer.
Let us know.