VFD on high leg a bad idea?

[mark32]

Hey guys,

So I was doing some work for a customer in his warehouse when he asked me if I could check something for him. He had recently purchased a newly overhauled mixer and when put under load, either the on board breaker (Disconnect) would trip or something in the VFD would trip, I can't recall. He also said the mixer would "Skip" just prior to the unit tripping. The specs for the VFD: Input 230vac, Output 180vdc. There's 240v running to the machine, one leg is 120v to ground, the other is 208v to ground. Could this arrangement be a problem for a VFD? I really know nothing about them.

Thanks for the help!

 

[JFletcher]

Dunno much either except that I know of almost no single phase install that would use a high leg. Move that breaker to AC so each leg is 120V to neutral/ground and see if that fixes the issue.

 

[GoldDigger]

Was the VFD intended for use on single phase input?
When you talk about connecting to two legs of a three phase system you end up with single phase.
Assuming that the VFD does not use the neutral at all, you have 240V L-L, and that should be OK as long as any surg
You can often drive a three phase VFD from single phase as long as you reduce the output rating because of the higher current load on those diodes of the input bridge that are forced to carry all of the current.
If the device uses the neutral as part of the input to the bridge, then the 208V high leg could be a problem.

I am also not completely clear on why the output is given as 180V DC, unless the "VFD" is really a controlled DC pulse generator for a DC motor.
Do you have any additional information (brand, model, etc.)?

 

[Jraef]

Hey guys,

So I was doing some work for a customer in his warehouse when he asked me if I could check something for him. He had recently purchased a newly overhauled mixer and when put under load, either the on board breaker (Disconnect) would trip or something in the VFD would trip, I can't recall. He also said the mixer would "Skip" just prior to the unit tripping. The specs for the VFD: Input 230vac, Output 180vdc. There's 240v running to the machine, one leg is 120v to ground, the other is 208v to ground. Could this arrangement be a problem for a VFD? I really know nothing about them.

Thanks for the help!

That's not a VFD, that's a DC drive (VFD = Variable Frequency Drive so is putting out Variable FREQUENCY, which means it's for AC motors).

DC drives are simpler, they just rectify the AC to DC, usually using SCRs that vary the voltage by doing what's called Phase Angle Firing. To function correctly, the firing board inside of the DC drive has to reference where the "zero cross" point is between the two poles so that it has a reference point for determining the amount of "On" time of each SCR to get the desired voltage. Some designs do this by comparing it to a ground reference point. When hooked up to the High leg, the ground reference voltage difference is going to cause you problems because it messes up the detection of the zero cross point and the drive misfires. Fix it quick before you blow an SCR.

 

[mark32]

That's not a VFD, that's a DC drive (VFD = Variable Frequency Drive so is putting out Variable FREQUENCY, which means it's for AC motors).

DC drives are simpler, they just rectify the AC to DC, usually using SCRs that vary the voltage by doing what's called Phase Angle Firing. To function correctly, the firing board inside of the DC drive has to reference where the "zero cross" point is between the two poles so that it has a reference point for determining the amount of "On" time of each SCR to get the desired voltage. Some designs do this by comparing it to a ground reference point. When hooked up to the High leg, the ground reference voltage difference is going to cause you problems because it messes up the detection of the zero cross point and the drive misfires. Fix it quick before you blow an SCR.

Thanks for the replies, yeah Jraef that makes sense with regard to this unit not being a VFD, could have sworn it said VFD on it though, haha. The unit is an Emerson 2331 MKII:

http://www.galco.com/buy/Fincor/2331

I'm going back next week to remedy as the customer isn't in a rush to use the machinery.

Thanks again,

Mark

 

[Electric-Light]

Check with the drive manufacturer. They may have instructions or factory modification options. As you guys know, 208 wye has each leg at 120v(170v peak) in reference to ground where the high leg of a 240v delta has 208/(293 peak).

Semiconductors are quite easily damaged from surge and they often come fitted with a MOV clamp to absorb the surge. It's common to see an MOV clamp in something as primitive as consumer LED lamp which can take quite a few small surges and have a better chance of device survival than leaving the disposable LED ballast exposed to surge. MOVs act as a protective false ceiling below the device's ceiling and having a greater difference between the two increases protection but increases the frequency of accidental strike.

Each strike wears away MOVs and they fail by falling down and shorting out or blowing away (leaving drive electronics open to get struck by surge)

It's a compromise between best protection or flexibility (works with 208-230 delta, wye or corner ground).

 

[Electric-Light]

The input on a drive is something like this. T, S and R are phases. The MOV Z4 could clamp prematurely when R is connected to high-leg depending on the voltage rating of that MOV.

 

[Jraef]

The input on a drive is something like this. T, S and R are phases. The MOV Z4 could clamp prematurely when R is connected to high-leg depending on the voltage rating of that MOV.

You are still thinking VFD, a little Fincor DC drive is nowhere near this complicated.

The power schematic will look like the one on the bottom here;

 

[Electric-Light]

SCRs are surge sensitive too and they may use MOVs as well. A call to manufacturer and asking about using with the service doesn't hurt.

It might be that he's just overloading. Electrical speed control doesn't quite duplicate a torque converter or a transmission. You can bring the full torque to a lower RPM but you do not receive more torque in return as you do with a mechanical means.

 

[mark32]

So I went back and hooked the receptacle up to a and c. (All three phase conductors were present at the receptacle's location so it was a super easy fix) While there I noticed another mixer sitting on a pallet. I asked him about it and was told that it was also hooked into the same receptacle, in which it too failed to operate as intended, and now it doesn't work at all, oops. He wants to ship it back to the reburb hoping they'll fix it for free, (Roll eyes).

Jraef, you are correct once again, the unit does say DC Drive all over it, not sure why I thought VFD, in my defense I was distracted as a nearby worker had decorated his work station with some rather attractive pics, I'll leave it at that Anyway Jraef, can I just replace the DC drive with another identical unit? Is it easy as that?

By the way, I just spoke with the customer yesterday and he says the current mixer is working fine, so thanks again guys!

 

[Electric-Light]

So I went back and hooked the receptacle up to a and c. (All three phase conductors were present at the receptacle's location so it was a super easy fix) While there I noticed another mixer sitting on a pallet. I asked him about it and was told that it was also hooked into the same receptacle, in which it too failed to operate as intended, and now it doesn't work at all, oops. He wants to ship it back to the reburb hoping they'll fix it for free, (Roll eyes).

Jraef, you are correct once again, the unit does say DC Drive all over it, not sure why I thought VFD, in my defense I was distracted as a nearby worker had decorated his work station with some rather attractive pics, I'll leave it at that Anyway Jraef, can I just replace the DC drive with another identical unit? Is it easy as that?

By the way, I just spoke with the customer yesterday and he says the current mixer is working fine, so thanks again guys!

So what was wrong with it after all?

 

[JFletcher]

So what was wrong with it after all?

One can deduce from the previous posts and the one you quoted that the problem was 208V to a machine circuit designed for 120V. It had 240V line to line tho not 120-0-120, but 120-0-208. No bueno. Unless there is other pertinent information that was left out. OP and owner were (half) lucky, many times when that happens, the only fix is a smoke replacement kit (new equipment).

High leg services often have every third breaker skipped unless there are 3ph loads. and yes, apparently a bad idea to hook one up to a DC drive.