Single phase motor on three phase Delta 1

[garyvin]

I have a 5hp single phase 240v AC motor in a prosessor. I do not have the capacity for the whole thing to be supplied by a normal single phase 240v supply. I do however have grounded Delta three phase at 240v L-L. The mid tap of A phase is grounded giving the ability to pull single phase loads. Can I run the motor off of two legs of the three phase? The phase shift will not be 180 degrees between phases since it is three phase. Will this cause any problems with the motor? Like reduced capacity or pull higher current?

 

[jraef]

It's done all the time. Not a problem.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[stevenal]

faq238-777

 

[garyvin]

This is for my employer. The problem is we have had two motors on two separate pumps burn up in the past week. They were old so it might have been thier time. I just wanted to make sure it was not the power source.

 

[jraef]

Ah, that's another issue!

If you have 2 motors, and both are being fed from the same 2 phases of a 3 phase system, you may have created a somewhat severe voltage imbalance, and THAT may have shortened the life of your motors. When dividing up 1 phase loads on 3 phase systems, you must be careful to balance them.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[waross]

With apologies to jraef,
The original poster seems to have described a 4 wire delta system. This may be open delta with reduced capacity on "B" and "C" phases, in which case all loads except three phase loads should be on "A" phase.
If it is a normal balanced delta, half the current will be supplied by the "A" phase transformer and half will be supplied by the "B" and "C" phase transformers. If the transformer bank is fully loaded with single phase loads on one phase, the performance will be similar to a "Double Delta" connected generator and the capacity will be reduced to 2/3 of the three phase capacity.
"A" phase is often the preferred supply for single phase loads with 4 wire delta systems.
Voltage drop is a little more with a heavily loaded phase on a delta system, but it is usually not enough to cause issues.
If this is a three phase transformer with a delta secondary, the transformer will be more likely to fail as a result of unbalance than motors.
SparksAlot, be aware that one phase is what we call the "Wild" leg. "B" phase will have 208 volts to ground and to the center tap on "A" phase. If you inadvertently use a connection from the "Wild" leg to neutral you may have low voltage issues with 230 volt motors.
respectfully

 

[jraef]

I defer to your experience on this subject waross. I failed to notice his description of the 4 wire delta until I read your post and his again.

 

[waross]

Hello SparksAlot
Can you tell us any thing about your 3 phase supply.
Is your three phase panel heavily loaded or lightly loaded?
is it fed from two transformers or three?
Are the transformers the same size?
KVA ratings if you can get them. If not, and there are two transformers, is one transformer physically bigger than the other?
respectfully

 

[davidbeach]

waross, I have to disagree with your terminology. I'm sure you what you are talking about, but the terminology is a bit unusual. Using cuky2000's diagram in thread238-157326 as a reference (which has c as the wild leg instead of b - more on that in a moment), all 120V single phase loads should be phases a and b (a and c if b is the wild leg). Phases a and b both have the same capacity to feed these single phase loads. Single phase 240V loads can be between any two phases in a closed delta, but should not span the open leg of the delta. Three phase loads would obviously connect to all three phases.

If you had a 45kVA transformer and a 15kVA in an open delta, you could have 30kVA of single phase load on the 45kVA transformer and 15kVA of three phase load. Running the single phase above 30kVA would reduce the amount of 3 phase load capacity. If all you had was 120V and 240V single phase loads, all of the 120V single phase would have to be on the 45kVA (with the center tap) and the 240V single phase should be divided between the other two transformers, but not across the open leg, so in cuky2000's diagram you could have single phase 240V loads a-b and b-c, but no loads c-a.

I wonder if the damage to the OP's motors occurred because they were connected across the open leg.

Location of the wild leg - Utilities have traditionally arranged their metering equipment such that the wild leg is on phase c, as shown on cuky2000's diagram. Then to meet the NEC requirement of having the wild leg on phase b, the phases have to roll or transpose. They can roll such that a (cuky2000's diagram) becomes C (customer system), b becomes A, and c becomes B. In that case the open leg is B-C and single phase load should not be connected there. If, instead of rolling, they transpose, a becomes A, b becomes C and c becomes B. Now the open leg is A-B.

Deltas with three transformers are much easier to deal with.

 

[waross]

Hi davidbeach.
Yes my terminology is off.
By "A" phase, I meant the center tapped phase.
By "B" phase I meant the wild leg.
I'm away from home and my library. However that is no excuse for me. I should have checked the drawings that cuky2000 posted.
The main issue with loads across the open delta is transformer losses. The losses are double what they would be if the load was connected across one of the transformers. If you sketch out the vectors, you will see that the voltage drop is the same for a given load across the open delta as if the load was connected across one of the windings.

For loading on an open delta:
At 240 volts, 15 KVA delivers 62.5 amps.
A three phase load drawing 62.5 amps per phase will be 26 KVA Maximum
(3 x 15 KVA x 57%)
At 240 volts, 45 KVA delivers 187.5 amps. The three phase load takes 62.5 amps.
125 amps is left for single phase loads or 30 KVA
My preference, but by no means the only valid connection is to avoid the wild leg for single phase loads. However, when I run out of spaces in the panel and still have loads to connect, I will use the wild leg for single phase loads.
Likewise, on an open delta, I try harder to connect all the single phase loads on the center tapped winding.
In a new installation of course, we will all install panels with sufficient space for the number of circuits needed. It is the jobs installing one more piece of equipment in an old plant, where the panel is almost full of breakers already that the real challenges arise. Sometimes there are some spaces open on the wild leg. We should check our transformer loading, and we sometimes resort to a solution that is safe but that we wouldn't use in a new installation.
BTW congratulations jraef
respectfully

 

[garyvin]

The power source is three single phase pole mount type transformers. 50 KVA 4160(2400) primary wired in Wye and 240(120) secondary in Delta. The mid tap of "A" phase is grounded making a grounded Delta. A and C phase are ~124v to ground and B phase reads about 225v to ground. They all read about 235-238v Line to Line. It is wired in through A and C phase. There are no real single phase loads. Just a few three phase welders.

 

[davidbeach]

waross, my point was that you can't have a center tapped phase. You can have a center tapped winding, and the ends of that winding will be connected to two different phases. Those will be phases a and b in the reference diagram and will be A and C once the wild leg is rolled or transposed into B.

 

[waross]

My apologies davidbeach. I understand and accept your correction.
I think we all agree that the preferred connection is on the two phases that read 124 volts to ground. The system you have is commonly used to supply mixed loads comprised of 240 volt three phase and 120 and 240 volt single phase.
jraef said it in his first post:
jraef (Electrical) 20 Jun 06 14:41
It's done all the time. Not a problem

respectfully

 

[machmech]

Hello Waross

Waross you said "SparksAlot, be aware that one phase is what we call the "Wild" leg. "B" phase will have 208 volts to ground " Which is a lower voltage then the other two phases to ground.

Is this correct or do you mean what I call the stinger leg which always has higher voltage then the other two phases?
Am I mistaken on this terminology?

Chuck

 

[machmech]

Opps! Sorry Waross

I was thinking 240 v to ground, please disregard.

However my question is the "wild leg" the "stinger leg" one
in the same always has higher voltage then the other two?

Now my observations are that this "wild leg" is located center of the three conections in a panel. Is this a general proceedure/requirement ? or can the "wild leg" be located else where?

Chuck

 

[davidbeach]

Reread my 21 Jun 06 0:31 response about location of the wild leg. NEC requires it as B in the gear, but it may be elsewhere on the meter.

 

[machmech]

Hello David

Thank you!

Chuck