Nominal voltage too high for vfd

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Twoskinsoneman

Twoskinsoneman

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Location
West Virginia, USA NEC: 2020
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Facility Senior Electrician
We had a vfd installed for a pump. I did not do it. I got called because the vfd won't start due to high system voltage. The voltage on the 480v system has always been high. At 505v this is the first time I've seen a vfd that wouldn't accept it. Rather than deal with the system voltage admin wants to just fix this particular problem. Is a buck boost transformer the best option for getting this wrapped up? It's a small 8.5FLA pump. 7.06kva buck transformer for this touchy vfd?
 
Twoskinsoneman said:
We had a vfd installed for a pump. I did not do it. I got called because the vfd won't start due to high system voltage. The voltage on the 480v system has always been high. At 505v this is the first time I've seen a vfd that wouldn't accept it. Rather than deal with the system voltage admin wants to just fix this particular problem. Is a buck boost transformer the best option for getting this wrapped up? It's a small 8.5FLA pump. 7.06kva buck transformer for this touchy vfd?
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The voltage is too high. Its over 5%. You should advocate fixing the system voltage. It might be as easy as reconfiguring the transformer taps.
 
Twoskinsoneman said:
We had a vfd installed for a pump. I did not do it. I got called because the vfd won't start due to high system voltage. The voltage on the 480v system has always been high. At 505v this is the first time I've seen a vfd that wouldn't accept it. Rather than deal with the system voltage admin wants to just fix this particular problem. Is a buck boost transformer the best option for getting this wrapped up? It's a small 8.5FLA pump. 7.06kva buck transformer for this touchy vfd?
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Do you have PFC capacitors in use that may be raising the voltage a bit high?
 
If you just want to get'er done, a buck boost is pretty reasonable.

I would use a 3 transformer configuration to keep the neutral balanced. 3x 200VA 277:16V transformers should do nicely.

Jon
 
Unlike a lot of other equipment excessively high voltage limits VFD modulation index and the ability to control the DC bus voltage (active front ends). So it hurts performance. A VFD actually does better on low voltage than high voltage.

Only reason not to tap the transformer down is if you have a soft bus or a VD problem.

A 5% line reactor won’t cause goofy voltages and is better than buck boost. It has the side benefit of improving surge protection, decreasing short circuit current, and reducing line side harmonics.
 
Thanks a lot guys. Moving the transformer taps is still my preference but it doesn't sound like I'll get my way.
Paul, the line reactor is an interesting idea that didn't come to mind. Thanks for that.
 
Twoskinsoneman said:
We had a vfd installed for a pump. I did not do it. I got called because the vfd won't start due to high system voltage. The voltage on the 480v system has always been high. At 505v this is the first time I've seen a vfd that wouldn't accept it. Rather than deal with the system voltage admin wants to just fix this particular problem. Is a buck boost transformer the best option for getting this wrapped up? It's a small 8.5FLA pump. 7.06kva buck transformer for this touchy vfd?
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Is it 505V everywhere at the plant?

The utility may have the wrong tap on their XFs if they own them and delivering secondary voltage.
 
Just to throw another option out there… I’m guessing that’s a 7.5HP motor. Those VFDs aren’t that expensive. If it’s a cheapo VFD, it may be worth it to look into just using a different manufacturer of VFD. I thought a lot of them were ok at +10% voltage, which would work for this. And then maybe you could use the existing VFD somewhere else where the VD may lower the voltage enough to be in that range.
 
bwat said:
Just to throw another option out there… I’m guessing that’s a 7.5HP motor. Those VFDs aren’t that expensive. If it’s a cheapo VFD, it may be worth it to look into just using a different manufacturer of VFD. I thought a lot of them were ok at +10% voltage, which would work for this. And then maybe you could use the existing VFD somewhere else where the VD may lower the voltage enough to be in that range.
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I agree. A lot of cheap Asian drives and some European based drives are really made as 400V for use in their counties, then they “stretch” that rating to include 460V because that’s what they believe we want since that’s the typical motor voltage. But most American based drive mfrs know that our distribution voltage is actually 480V and can be +5% to 504, then they make their drives accept +10% to 528V.

ABB, who sells in both markets, has 400V “class” drives but also 500V class for use here. Rockwell drives are all built to 480V +10%. It might be simpler to just get a VFD that is OK with 505V.

The line reactor is a good option no matter what though. I typically don’t recommend a 5% reactor because of the voltage drop, in this case you actually want that. If it were me, I would get a 5% reactor first, see if that fixes it because you already have this drive. If that doesn’t work, get a better drive, but keep the reactor fir its other values.
 
Won't a line reactor cause a load dependent variable voltage drop?

I'd not thought through the wider voltage tolerance of different VFDs. That sounds like the best advice of all.

Jon
 
winnie said:
Won't a line reactor cause a load dependent variable voltage drop?

I'd not thought through the wider voltage tolerance of different VFDs. That sounds like the best advice of all.

Jon
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A very good point. It all depends on how/where the VFD is sensing the overvoltage. The 5% reactor will definitely drop the voltage when the motor is running.
But if the VFD is sensing the applied AC before enabling the rectifier network that drives the DC bus it will not help.
Similarly, if the DC bus voltage goes too high when the VFD is turned on but not actually driving the motor yet (i.e. if you leave the VFD on and control the motor with the VFD inputs) then the VFD will never get past the bus precharge to the state where the motor is running and the input voltage is dropped.
 
retirede said:
See post #9. A lot of cheap drives from China are 400V “world” designs that they cheat and simply slap a 460V label on for the US market. We don’t know if that’s the case here, but a possibility.
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I don't know the Chinese market. Many of the VEDs we used were Vacon or Siemens. And CEGELEC in UK.
 
Besoeker3 said:
I don't know the Chinese market. Many of the VEDs we used were Vacon or Siemens. And CEGELEC in UK.
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Yes, and because Vacon and Siemens sell extensively in North America (and are ethical companies), they pay attention to those kind of details. A lot of the cheap no-name drives being sold on Amazon and AliExpress now at impossibly low prices are not well designed, nor do the sellers care. They are after cheap manufacturing in high volume to take advantage of buyers who only care about price, but just let the buyer beware as far as specs or QC.
 
I'd check the VFD manual and see if it has any settings for the input voltage or tolerance.

If not, I'd tell the owner to throw that VFD In the trash and get something brand name.
 
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