460 volt Delta Issues, Please help

Guys,
I just started a new job at a company in Charlotte NC that has been here for many years. The transformer feeding part of the plant is an old 460 volt delta without a neutral or ground. I have been researching and am seeing where people are saying that the newer VFD's don't like the Delta voltage. I have lost 6 Powerflex drives in two days, 3-1hp 40p's 2-3hp 40p's in one system and one 25hp pflx-70 on another for no apparent reason. I am looking at adding line reactors to try and help the issue but, I am wondering if anyone has any info on why this is happening or can explain a little to me so that I can understand what is happening and what to do. Never dealt with a Delta before so kinda scratching my head on this one.
Thanks in advance for any help,
Eli

As long as your supply voltage matches the drives ratings, it should be fine. might be worth contacting rockwell?

We have a lot of older PF40’s and when we get an unexpected or even planned outage, you can almost bet on at least one if not a few dying. I believe earlier models had an issue with a component in the power supply.

Did you have a power outage or blip? If you don’t have line reactors, that is the first thing I’d install. Cheap insurance.

I would look at the supply transformers to make sure one has not opened.
One transformer winding could open and with other inductive loads the systems would still think it has a 3 phase supply but the legs would be out of balance.
I have seen line start motors burn up because of similar conditions.
I once had a dual substation feed 7200 v 3 phase supply that lost one of the supply legs
The combinations of the transformer banks generated the 3rd phase and sent it back to supply (Primary side). it actually ran motor load on other services.
really tough to trouble shoot all voltages looked right ( Reasonable value) until we took the substation transformers off line then we found the problem.

From what I understand there isn't anything electronically wrong with a delta supply...until something starts pulling it around. Since there is no ground reference the whole thing can be pushed and pulled to about any voltage level. That is when the issues start.

When you ground a phase that can bring in its own issues. You do have a solid ground reference. But one of the phases relative to ground will end up quite high...significantly higher than any phase can get in a grounded wye supply.

I have never taken the time to look into this but I have been told that delta supplies can also create issues with harmonics in drive applications. It is not so much that the delta design creates the harmonics as the design does very little to damp them out. But, as I said, I never really studied that to find out how much truth there is to that.

As I recall AB specifically indicates that center grounded wye supplies are their preferred supply type. But as others have said, I have operated drives off of delta supplies both grounded and ungrounded and not had any issues. Hard to say if your case is caused by the supply. Given the issues a delta supply brings I don't think line reactors will help any...unless the noise thing isn't just someone blowing smoke. But they can't hurt and they are pretty cheap.

Keith

Do the failed drives have the MOV jumpers removed?

Rather than buy input reactors, buy input 480/460 transformers with a 460/277 wye secondary. Arrange the output windings in wye (star) and ground the center of the wye.
You will have no more trouble.

To help you understand the issues a bit, since the delta secondary of the distribution transformer is floating (no ground), any leakage from any other voltage source can cause the network to go to extremely dangerous levels (I have personally measured delta networks at 1575VAC to ground and, in another plant, at 800VDC to ground. Also, as mentioned above, a ground fault on one phase will cause the other two phases to rise to 460VAC to ground.

Many AC drive brands including AB have input protection MOV's designed for balanced phase-to-ground voltage of around 400VAC. Clearly, the Manufacturers instructions will have to state that input movs be disconnected when installed on floating delta systems.
That leaves the inputs of the drives completely unprotected from surges and spikes.]

By using input transformers as suggested, you convert the delta power to grounded wyed power while simultaneously gaining a rather excellent transient filter in the transformer--better than a line reactor, in fact.

Generally, you will find the same precautions mentioned in the instruction manuals in the section discussing connections to the input power terminals.

Rather than buy input reactors, buy input 480/460 transformers with a 460/277 wye secondary. Arrange the output windings in wye (star) and ground the center of the wye.
You will have no more trouble.

To help you understand the issues a bit, since the delta secondary of the distribution transformer is floating (no ground), any leakage from any other voltage source can cause the network to go to extremely dangerous levels (I have personally measured delta networks at 1575VAC to ground and, in another plant, at 800VDC to ground. Also, as mentioned above, a ground fault on one phase will cause the other two phases to rise to 460VAC to ground.

Many AC drive brands including AB have input protection MOV's designed for balanced phase-to-ground voltage of around 400VAC. Clearly, the Manufacturers instructions will have to state that input movs be disconnected when installed on floating delta systems.
That leaves the inputs of the drives completely unprotected from surges and spikes.]

By using input transformers as suggested, you convert the delta power to grounded wye power while simultaneously gaining a rather excellent transient filter in the transformer--better than a line reactor, in fact.

Generally, you will find the same precautions mentioned in the instruction manuals in the section discussing connections to the input power terminals.

A floating power distribution can reach voltages relative to ground in the order of thousands volts and that can perforate insulation and also blow up capacitors.

It seems that you need a grounding transformer.

http://electrical-engineering-portal.com/grounding-transformer-zig-zag-grounded-wye-delta

AustralIan said:

Do the failed drives have the MOV jumpers removed?

Click to expand...

Ding ding ding......

This is the correct answer. Read the AB manual about ungrounded service.

Thanks Guys,
We have just found that the MOV's Jumper has not been removed. We are trying that now.
Thanks for the help,
Eli

I work in a plant that is ungrounded delta. The plant was built in the 1950's. We have transformers converting delta to wye almost everywhere there is a motion system.

I had a parts balancer with intermittant jerkiness. That one I found a heater in a parts washer on the other side of the plant that was going to ground. When it did I got the jerkiness leading up to a fault. The heater cycled on and off so the balancer worked and didn't work right. I have a transformer just waiting to be installed for this one.

I had an rotary table that ran for months and then one day it mostly didn't want to. Replaced the inverter, some times it ran, most times it didn't. Fed the inverter with a delta/wye transformer and it worked and hasn't failed me yet. In that case I didn't have a shorted (grounded) leg in the plant.

Wes

Removing the jumpers is a fix for one possible problem, that if the MOVs becoming little bombs when the reference voltage to ground exceeds the MOV rating, which can (will) absolutely happen sooner if not later on an ungrounded, or even a corner grounded delta supply. This action however ALSO defeats the benefits that the line side MOVs provided and removing them voids the UL listing of the VFD.

This by the way is not unique to PowerFlex drives and has no bearing on the age, it is true of ALL drives on the market. All drives are design for solidly grounded eye systems and in fact, delta systems ONLY exist in North America. The thing is, SOME manufacturers don’t give you a way to remove that ground reference (the aforementioned jumpers), they simply tell you in the manual that you CANNOT use the drive on anything OTHER than a solidly grounded eye source. Only a few of the major mfrs, for whom North America is an important marketplace, provide the jumpers or some means of removing that ground reference point. But again, that only solves ONE ASPECT of the problems you will encounter.

So I’ll add a +1 to DickDVs post (although he already did his own anyway :whistler in that the only PROPER solution is to add what’s called a “drive isolation transformer” ahead of each drive, or a bank of drives if in the same area. 480 in, 480/277 Wye out to the drive. These transformers will also do the same as line reactors in other ways, so don’t bother with them at all, they will do nothing for you in this case, so save your money.

Side note; as of a few cycles ago, ungrounded delta systems are a code violation unless you EITHER ground one leg (corner grounded) , which defeats the purpose, or add a very expensive ground fault monitoring system for the entire service. Your facility may be “grandfathered” in, but if you ever pull a permit to make any changes you will be forced to fix that situation anyway. Your owner may want to consider just biting the bullet now when the economy is good rather than have to suffer that expense later when it isn’t, along with the constant cost of replacing VFDs and other power electronics or adding a bunch of smaller transformers. Ungrounded delta is a relic of the past, it has no value in the modern world.

jraef said:

Ungrounded delta is a relic of the past, it has no value in the modern world.

Click to expand...

+1 more...

We have a lot of it in our rural areas, and most of it has been converted to 'corner ground'. However, it's still no good for the filters in the drives.

BTW, virtually all of the 240/3ph is delta... YUCK!

jraef, can you give me a reference or two concerning floating delta no longer being "code". That would be an answer to prayer and end a lot of me preaching against it if true.

That stuff almost killed me and I have suggested it be outlawed for 15 years. And, now I get my wish! Wonderful!

I'll incorporate the references into my classroom materials.

Thanks!