High Leg 3 phase and vfd's

VaMike

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Join Date
May 2008
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VA
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I know this has come up many times but I've yet to hear a good way to deal with the vfd issue. Here's where I am. I have a very old Haas milling machine that I completely rewired. Machine has run fine for years until I moved my shop. This shop has high leg 3 phase 240. L1 and L3 are 120 to ground/neutral and L2 is about 210 to ground. Phase to phase is fine.
But, as covered many times on the net, the issue lies in that high leg when connecting to a vfd. First issue, I get a overvoltage fault because of that high leg charging up the capacitor way too high. Currently this has a AD G3 7.5 hp low voltage drive.
I purchased a rotary phase converter that was supposed to be really really smart and be great for vfd. Totally false as it generated 3rd leg was at 274 volts. Even worse so this was a waste of time.

So I now have this silly idea of using a 20 hp AB powerflex 755 that I have laying around. This is a 480 drive so my thinking is that the overcharging part will be fine. The unbalanced currents should work because I have 27 amp drive.

I'm pretty sure I can disable the undervoltage fault that certainly will occur. I can also program the name plate data for low voltage. This drive does allow for the removal of the ground screws so it will better tolerate the open delta 3 phase. On that note, I could add a 1:1 delta wye transformer as well.

Any thoughts on what I might be completely not understanding.
 
I would personally go with the isolation transformer. It more directly solves your problem and, if the PF755 ever fails, the overall cost will be less even though you have a drive you can use. I'm also not fully convinced you can configure your way out of the undervoltage case.

Keith
 
Thanks for fast reply. I have read several older post around the net saying thats the answer but no one ever gets back and says that alone fixes the problem.

It seems just a 1:1 delta wye transformer will still pass the high leg through at the 210volts or so. So the high leg would still be high relative to the new ground and still over charge the capacitor. Thats why I was thinking of the high voltage drive. But perhaps I don't understand completely.
 
The reason that the one leg of the delta is high is due to the way the delta is grounded. To get the two 120VAC legs to ground they center-grounded one of the legs. This results in a vector voltage sum of 210V on the other leg. Granted, a delta transformer winding would often not have ANY ground reference, which is sometimes just as bad.

A wye transformer has a designed, rigid ground point at the joining point of the three phases, which should be grounded by the way. That gives you a well defined ground reference for all three legs and the voltage to ground will be less than the 210V you are seeing now. This is why wye supplies are usually recommended for drive supply applications.

An isolation transformer will transform the phase voltages correctly irrespective of ground. How you ground (or not ground as the case may be) each side will then determine the leg voltage to ground of each leg.

Keith

BTW, the phase to ground voltage of a properly referenced 3-phase 240V wye transformer should be about 139V.
 
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I think I follow you. I have a 3 phase step down delta wye transformer. Its 480 to 208 on secondary. Think I'll connect this to my incoming power and see what that high leg looks like compared to the other 2. I know they will all be too low but should show me what I will get proportionally if I buy the 1:1 transformer. I would think that 210v to ground really needs to come way down or the bus is just going to overcharge on the low voltage drive.
 
Okay,
I get it now. The 139v is indeed bringing it way down. I'm guessing the current low voltage drive will have a shot at tolerating that.
Many thanks. I'm gonna try the isolation transformer and existing drive.
 
Keith left out one term, "separately derived neutral," which is how the NEC refers to the center point of the wye secondary. It is solidly bonded to ground and the three secondary phase voltages refer to this point through their respective windings, 120 volts L-N and 280 volts L-L.

I've never hears of 139/240V wye before. It's more likely that the OP's previous shop had 120/208V power. 240V delta to 120/208V wye transformers are not hard to find.

Mike
 
Originally posted by Mike Lamond:

I've never hears of 139/240V wye before.

The 139V is the leg to ground voltage that is a natural consequence of the wye connection with a separately derived ground. It isn't a transformer ratio. I just didn't say it in the form people would be used to seeing. It is no different than 480/277 or the 208/120 relationship you you listed. The leg-to-neutral voltage of a 3-phase wye transformer should be the phase-to-phase voltage divided by the square root of 3 (roughly 1.73).

Keith
 
Originally posted by VaMike:

I have a 3 phase step down delta wye transformer. Its 480 to 208 on secondary.

You can check the specs but I would expect that drive will operate just fine on 208V assuming you don't need full torque at full speed. Your bus voltage on 208 will be about 15% lower on 208 than on 240. This might cause you some speed limit issues.

Keith
 
I am not familiar with the AD G3 drives but from what you describe it sounds like your line voltage is higher then what you are reading maybe a motor shutting down is spiking the voltage.
VFD’s don’t normally use or care about the phase to neural/ ground voltage all they see it phase to phase. Your buss voltage should be about 415vdc 250 x 1.717 I would expect the overvoltage to be at about 450vdc
Before I would go to the expense of a rotary phase converter, I would change the VFD to a better quality one.
There is something wrong with the way they are sensing the buss voltage. It could be that the line voltage is higher then you are saying.
There is no natural in a VFD power supply so the voltage reading phase to natural has little meaning the phase to phase is the working voltage. You can have a voltage spike from an outside source kick it up maybe another motor shutting down.
Is it faulting overvoltage when you first apply power, when it’s running or when you stop the VFD
It is important to know.
What is the name plate voltage for the VFD low voltage doesn’t do it for me need to see the
Some of the stuff coming in from overseas have a different voltage than we use here
Maybe a simple buck transformer will give you the correct voltage
For cost and trouble I would just install a VFD with the correct ratings and save the time and expense
 
Originally posted by GaryS:

VFD’s don’t normally use or care about the phase to neural/ ground voltage all they see it phase to phase.

You have to be careful with this one. While this is often true with the DC bus voltage it may not be true of the IPMs. The IPMs often have a ground reference to them that is usually tied to the drive baseplate or heat sink. The DC buss will float with the high leg and this can cause breakdown issues with the IPMs if the voltage gets too high. I learned that lesson the hard way...with AB Pf70 drives no less.

Keith
 
Most drives have cap’s, and sometimes LC networks connected buss to ground and phase to phase along with outputs phase to phase and phase to ground they will have no effect on the overall buss voltage.
Those are filter caps to suppress electrical noise
The buss caps may be stacked in series to get the voltage level necessary but on 240 volt vfd I don’t think so the power caps are usually 480vdc.
The overvoltage detection it strictly buss voltage it doesn’t look at the buss to ground
In this case the vfd would have an 3 phase full wave bridge rectifier with a precharge resister on the front side of the buss and IGBT’s for the output most likely an all in one module
And given the brand they are not putting a lot of filtering in.
Any vfd that would use the ground as a current carrying conductor would not be allowed to sold in the US
I would still just replace it with a better VFD it’s not worth the time
 
Upgrade to the GS4 and it will tolerate unbalanced delta. I think you also have to remove a jumper for emi filter when you do this. We I installed a pair of them about 3 years ago for some pumps fed by 460vac delta and they have not been any trouble.
 
Expanding on what I said above...The customer in our case was required to have a constant flow to a filter plant about a mile away from these pumps that were on motor starters...there was no budget for this. They blew up the water line once before they accepted our advice that they absolutely required VFDs.

I investigated the lowest cost VFDs quickly available and found the GS3 manual and warnings not to use it with unbalanced voltage, but the GS4 had the capability and was not much more expensive.

The GS4s were easy to set up and worked nicely with a ML1400 using Modbus RTU for control and feedback. If I remember right, the RFI jumper was on the left side of the VFD and accessible from outside the nema 1 enclosure. I simply pulled it out. I can't remember if I also had to edit a related parameter.

https://www.automationdirect.com/ad...quency_drives_(vfd)/high-performance/gs4-27p5

EDIT: I agree with the above recommendations about the isolation transformer being the better solution...and at such a small horsepower, it might not be too expensive. Sometimes finding space for the transformer and wiring to and from it can be a problem. In our case, we did fit the VFDs with line reactors inside the old starter panels. That was done to help protect the electrical systems including the power company from distortion, and had nothing to do with the corner grounded delta feed.
 
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