Single phase motor on three phase Delta
Single phase motor on three phase Delta
(OP)
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?





RE: Single phase motor on three phase Delta
Eng-Tips: Help for your job, not for your homework Read FAQ731-376
RE: Single phase motor on three phase Delta
RE: Single phase motor on three phase Delta
RE: Single phase motor on three phase Delta
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
RE: Single phase motor on three phase Delta
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
RE: Single phase motor on three phase Delta
RE: Single phase motor on three phase Delta
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
RE: Single phase motor on three phase Delta
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.
RE: Single phase motor on three phase Delta
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
RE: Single phase motor on three phase Delta
RE: Single phase motor on three phase Delta
RE: Single phase motor on three phase Delta
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
RE: Single phase motor on three phase Delta
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
RE: Single phase motor on three phase Delta
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
RE: Single phase motor on three phase Delta
RE: Single phase motor on three phase Delta
Thank you!
Chuck