unbalanced phases

[jt1971]

I need a little info.

This is my problem :
Have nemerous sewage pump stations (10Hp & 15Hp duplex Units w/ motors wound to 230V) at the end of two long utility company 2 phase runs. Power company has installed center tap grounded open delta transformers to provide 3 phase, 240V, 60Hz service at each location soley for the operation of these stations. What I am getting is unbalanced phases - what I was typically seeing is 228V, 242V, 238V (or something close - workable), three days ago voltages were @ 245V, 244V, 263V.. after a conversation they tweeked something somewhere and it looks like 246V, 228V, 268V since yesterday. The motors do not have Hp to de-rate past 90%, and the phase unbalance is tripping the thermal overloads - as I do not have the choice to not run them. My recent conversations with the utility provider says that at this location, they try to keep the voltage within 5% avg, 10% max, but unbalanced may be 4% - yeah right..

Provided my motors don't burn out by the time a response comes - What information can I pick from your brains as far as the next logical step?

??Somebody said something about using some sort of line regulator, or voltage conditioner? Is one available for the service voltage present, and at what cost? Any info / help would be appreciated.

 

[charlie b]

Re: unbalanced phases

Lacking any specific details, my best guess is that the problem lies within the installation, and not within the utility?s distribution system. But it is clear to me that what you are looking at is a design problem, and not a code-compliance problem. Therefore, what is needed is a design solution. We can?t solve a problem without having detailed information about the installation (e.g., one-line diagram, list of all motors and their points of connection, conductor sizes, and overcurrent protection sizes). If I were to take on a project of this type, I would also want to inspect the installation, to verify that it was constructed in accordance with any drawings that you may be able to provide. I think you are going to have to call for the services of an EE to help you on this one.

 

[charlie]

Re: unbalanced phases

Charlie, I think I may disagree with you on this one. If the motor load is large enough on an open delta bank, the load will tend to collapse the delta a little and cause an unbalanced voltage situation. I am guessing the electric utility has either too small of a bank or the third phase is to far away to string it in economically to add the third transformer.

I believe the onus is on the electric utility to do the testing to see if the problem is on their side.

 

[pierre]

Re: unbalanced phases

The question and answers are well worded, and very informative My question is, how would the voltages that JT has posted affect the motors installed to those circuits?

Pierre

 

[charlie]

Re: unbalanced phases

Pierre, the line voltages have to be reasonably close or the motor will fail at full load. If you have a voltage imbalance, you can derate the motors and still be fine. However, a large voltage imbalance is normally the fault of the serving electric utility. In other words, the load on one primary phase may be a lot more than on the other and cause low voltage on the output of one of the transformers or the three phase load may be too large for the open delta bank. The latter is what we have problems with because we have to many unexperienced engineers. We keep our lines balanced with load.

Going back to my original statement, "I believe the onus is on the electric utility to do the testing to see if the problem is on their side."

 

[iwire]

Re: unbalanced phases

If you do a Google search for motor unbalanced voltage you can find a lot of info on the effects to motors.

Here is a quote from
this site.

With 3% voltage unbalance, a motor can produce 90% of its rated horsepower without overheating. Operation of a motor when the unbalance exceeds 5% is not recommended. When unbalanced voltages exceed 5%, the temperature rise is so fast that protection by derating is not practical

They sell motor protectors so you may want to look at other sites too.

 

[arcandspark]

Re: unbalanced phases

Regulators are quite expensive but if the problem ends up being yours instead of the utilities, they might be the answer.

Those that I have experience with regulate the amps to a specified range. Would this not solve your problem?

good luck.

 

[arcandspark]

Re: unbalanced phases

You might also try using VFD'S and adjusting the parameters so as to make up for the imbalance.

I'm not sure this can be done but it might be worth investigating.

Good Luck

 

[tom baker]

Re: unbalanced phases

The usuall solution for voltage imbalace is to rotate the leads at the motor, keep the same rotation, but a to b, b to c, etc.

You state its 2 phase from the utility so that rules out adding the third transformer to the open delta. A VFD may be a solution, and the cost of small VFD's is very affordable. And for a sewage application, it eliminates water hammer, and if a dry pit application then there is no drive shaft shock.

And if the pumps start and stop a lot, then you eliminate contact wear. Plus VFD's are way cool to set up and program. My latest system is device net to the drives, with ethernet connected panelview and PLC to fiber optic. Can't work on it with out a laptop though!

 

[pierre]

Re: unbalanced phases

Bob
I performed the google search and I found some very good sites. Some way over my head

Pierre

 

[jt1971]

Re: unbalanced phases

Above post mentions rotating the phases on the motor to solve for unbalanced phases, a to b, b to c, c to a. A couple of questions:

What exactly does this do within the motor? And how could this effect voltage characteristics?

This might be a stupid question, but does it matter where the high leg(ie. 208V to ground) of the open delta connects to the motor?

The voltage unbalance I am looking at here is seen on the supply side with no pumps in operation, as far as balancing loads within the facility any other way - n/a.

[ January 20, 2004, 09:57 AM: Message edited by: jt1971 ]

 

[charlie]

Re: unbalanced phases

Why are you not contacting the serving electric utility? If the voltage problem is at the service point, it is their problem and they are required by most state laws to fix it. Cost doesn't enter into the equation, they are required to provide balanced voltages and bear the cost of repairing their installation.

 

[jt1971]

Re: unbalanced phases

I have contacted the power company!! Every day for the last two weeks - is not really getting me anywhere, because they say that they have 10%+/- of nominal line by tariff; with unbalanced voltage maximum at a company "guideline" of 4% - I believe the unbalanced phases is what is killing me (@ 8% calculated by NEMA standards, w/ IEC formula not much different)and it appears that that's all it is - a company guideline. Until last week they have been beating around the bush about getting on site. With the most recent spike, they just have not returned my calls - for 5 days now.

What I am trying to do here is get a little info. - just to see what possible options are available and also to see what information I can pick out of your brains about the situation I am in. So far what I have read in the posts - very informative, and may be put to good use once the finger is pointed in the appropriate direction.

[ January 20, 2004, 09:33 AM: Message edited by: jt1971 ]

 

[charlie]

Re: unbalanced phases

I believe it is time for you to contact the state utility commission. Give them the names of the people you talked to, the dates and times, their response to you, who came out and their response, the dates and times of calls you made that got not response, copies of all correspondence and reports, etc. Also, give them your interpretation of their attitudes when you did have conversations with them.

If your documentation is decent, you will get action . . . I promise. I also assume they will only do the minimum to get by the rules they have filed with the commission if what you are saying is correct about them.

 

[barrydmi]

Re: unbalanced phases

I agree with Charlie on contacting the state utility commision. Try placing a data logger on on the line side and record these voltage changes, at least to have documentation when facing the utility people. A VFD will not like the line power doing that, and will most likely fault.

 

[barrydmi]

Re: unbalanced phases

I agree with Charlie on contacting the state utility commision. Try placing a data logger on on the line side and record these voltage changes, at least to have documentation when facing the utility people.

 

[jt1971]

Re: unbalanced phases

Amazingly, we pulled voltages a couple of days ago and the voltages have corrected themselves (according to the utility they did nothing). The utility is going to place a monitor on to confirm.

I hope that is the last time it happens.

I still have a question from an earlier post - "The usuall solution for voltage imbalace is to rotate the leads at the motor, keep the same rotation, but a to b, b to c, etc." Could anyone explain?