Drives on 230 3-phase with a high leg.

Not really my problem, but my city has asked for help. They have two 15 hp 230V 3-phase motors pumping make up water into their water tower. The incoming power is 230V 3-phase with a "high leg", i.e. one leg is 220V and two other legs are 120V. They are using GE drives (AF300's I believe) rated at 15hp, 230V to operate the pumps. Apparently the saleman was good and said these drives would operate without incident. They frequently (daily) have, surprise, Overvoltage faults, and sometimes have over or under current problems as it seems the drives don't like the non-uniform incoming power.
My initial idea would be to double the size of the drives and run off of 230 1-phase (2 low legs) and use the drive to change to 230 3-phase. The city also has been told by a few power conditioning vendors that they could not solve their incoming power issues for the drives unless they are oversized. Anyone out there have any experience with this type of 3-phase power that could lend me some advice?

What you should have is a Delta with neutral system. The 3 phases should be 240vac with 120vac between neutral and 2 legs and 208vac between neutral and the third leg. Any inverter should work as long as you use the 3 phases without the neutral....naturally the legs must not be unbalanced by the use of the neutral.

Are the voltages you list phase-to-phase or phase to ground? My guess is the voltages are phase to ground. If they are you probably have a delta transformer with a center tap in one of the delta legs. That works slick as an easy source for 120AC but drives tend to be a bit unhappy with this arrangement.

If the voltages are phase-to-ground my first reaction would be to put a delta/wye transformer upstream of the drives. Make sure you wire the X0 connection of the transformer according to the drive manufacturers recommendations. 40kVA worth of transformer should be enough for both drives unless you are running both drives together near rated capacity for extended periods. This should give you a common reference point for the voltage to the drives which should help with the trips.
If your voltages are phase-to-phase I'm not sure how you want to approach this. Your idea of and obversized drive running a single phase supply probably is as good as any.

Keith

I had to put a line conditioner (total of four) on the input and output of two drives that were in the same cabinet to solve this problem on this very brand and model of drive. The customer would not change brands. This also cleared up the problems with the PLC and Display panel. A large transformer, as kamenges said, will also help.

good advice listed above.

Ron's picture shows it well.

The picture shows what I call full, or real 3-phase. In rural areas, often they only run two phases down the road, so you wind up with open delta, where there usually is no windings on the B-C phases (this varies with area).

The down side of open delta, is the high, or wild leg, can float voltage wise all over, depending on load. I have seen it from 165-288 volts. For a farmers grain dryer, it is just fine. For precision electronics, well........

anyway, more food for thought. If you get a chance, do some research on Ron's site.

regards.....casey

The problem with the high leg is that it is not a constant voltage and has eaten up 3 of these drives in the past year. Plus everything is outdoors and the city's not crazy with putting in a large transformer in a 4X box and pay for a higher electric bill with the transformer loss. The voltage of the high leg swings from 180 to 210 but averages right around 200V most of the time.
I realize in an ideal situation any VFD would work, I am an EE and understand the principles. I was just hoping someone had a better ideal than a placing a transformer to regulate the voltage and protect the drive.

[I/Originally posted by LJBMatt:[/I]
The problem with the high leg is that it is not a constant voltage and has eaten up 3 of these drives in the past year. Plus everything is outdoors and the city's not crazy with putting in a large transformer in a 4X box and pay for a higher electric bill with the transformer loss.

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Does anyone else see the financial folly in this statement? The city is willing to take on the financial burden of three blown drives a year but isn't willing to take the one-time hit of a transformer (or other corrective action) and a little tramsformer loss.
I know the terms 'government' and 'fiscal responsibility' don't necessarily go together. But this seems particularly short-sighted.

LBJMatt, I know you can only do what they will let you do. But you really have to persuade these people they will have to spend some money to save some money. You have to be talking about at least $10,000 a year in drives. You can burn alot of electricity for $10,000. Assuming you run both motors full bore 24/7/365 your total energy cost over a year is about $25,000, assuming 80% composite efficiency and 0.95 PF. No matter what you do you won't increase your energy usage by 50%, which is what it would take to pay for the three drives you blow a year.

Keith

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The problem with the high leg is that it is not a constant voltage and has eaten up 3 of these drives in the past year.

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There is something wrong with any CITY/UTILITY Power that fluctuates. The wild leg SHOULD NOT fluctuate. i. In a Delta system you have 3 phases that are 230/240 volt and a neutral that is taken from the center of one leg. This will give 230/240 between any phases, 120v neutral to 2 phases and 208v neutral to 3rd (wild leg) phase

SOMEONE needs to look at the 120v legs to see if they have the wild leg unbalanced.

As far as a 3 phase motor is concerned there is no wild leg...its
230/240vac. IF the neutral/120vac part of the system is being used then SOMEONE needs to check if the phases are balanced...all the time.

You may need to add line conditioners prior to the drive as was mentioned.

IF the "wild leg" voltage fluctuates with or without the motor then SOMEONE has a PROBLEM....CHECK ALL CONNECTIONS.

The voltage should be CONSTANT....AT ALL TIMES, if not THERE IS A PROBLEM.

NOTE: A load...motor etc may drop the voltage but it should ALWAYS BE CONSTANT....no major fluctuations on ANY PHASE.

I am not a plc guru but this electrical stuff I do know....FLUCTUATIONS ARE NOT ALLOWED...NEVER.

Generally, AC drives tolerate power imbalance well including one-leg-grounded delta sources. I hate them having almost lost my life due to one some years ago and will not start up any DC drive on them.

But AC drives seem to work ok, the only exception I have run into is the ground fault detection. Nuisance trips occur frequently and the fault function needs to be turned off. Otherwise, no problems in my experience.

As Ron points out, the voltages should not be jumping around. The cause of that, if it is observed, needs to be found and corrected.

One more thought, did anyone analyze WHY the drives failed?

Recently I did some work at a plant that replaced an Indramat servo drive 4 times in approximately a year. Indramat did an analysis that showed the controller being destroyed not the actual drive. Using an Oscope I determined that an AC voltage was riding on top of the 24vdc control power. After some trial and error investigation I found a grounding problem, once the drive was properly grounded the AC voltage went away and the 5th drive has not had a problem.

I suggest that someone have the drive(s) analyzed to determine cause of failure.

I would also attempt to determine why the wild leg fluctuates, use more than one meter to determine if it truly is.

I am getting this information second-hand from the contractor house that the city uses and from the city themselves. They don't have cash to spend money on me to investigate the situation.

Being from Canada, I was just curious as to why the utility is not providing balanced 3 phase to a city installation??? If it is your problem, ie something in the cities network is unbalancing the system and causing wild voltage swings on one phase, then is the utultiy not complaining about the imbalance you are causing??? Here, if you ask for 3 phase the utility brings you balanced 3 phase. I had it brought to my garage for a large welder and a large air compressor.