1 ph. unbalanced load on 3 ph. 240/120 xfmr

[MIEngineer]

I am working in an application that both single phase 120/240 and 3 phase 240/120 are to be available for load testing equipment. There are two separate load banks, sized to draw no more than 10 amps at 1 phase 120/240 and 3 phase 240. The test will not be conducted simultaneously.

I am proposing installing a 15 kVA 3 phase 240/120 transformer. All the 120/240 load testing would be performed on phases A & C due to the high leg. Would this imbalance cause early failure of the transformer and therefore not be advised?

My other option is (1) 15 kVA, 3 phase, 240 V xfmr & (1) 5 kVA, 1 phase, 120/240V xfmr.

Thanks,

 

[skeshesh]

I don't see why it would. In my understanding the transformer life is most significantly a function of the thermal characteristics. There may be some small factor I'm overlooking, but I would think that the overall life would not be effected significantly.

 

[gar]

101215-1318 EST

I do not understand your question.

Do you currently have a 3 phase delta source with one secondary center tapped to provide 120-0-120, and 240 single phase, and 240 delta 3 phase? Usually described as having a wild leg around here.

Or do you just have a 3 phase source, delta or Y, of some voltage that requires you to supply a transformer or transformers to get the type of output you want?

What type of output do you want?
(1) Do you require 3 phase 240 delta with no specific ground requirement, and no 120. In other words could be floating relative to ground.
(2) Or, a 240/138 Y with the Y center point grounded.
(3) Or, a 208/120 Y with the Y center grounded.

Separately in the case of the delta
(4) Do you require 3 phase 120 delta with no specific ground.
(5) Or, a 120/69 Y with the Y center point grounded.

If there is no specific need for a particular ground, then you could use a Y secondary structure and center tap each secondary to get 240, and 120 as delta outputs. The voltage on each secondary line to neutral would be 138 for the total secondary and 69 for half of the secondary. 138 neutral to a hot line produces a 240 delta. And 69 line to neutral provides 120 delta.

If you do not overheat a transformer winding, then you do not shorten the transformer life. So an unbalanced load does not affect transformer unless something is overheated.

Also note that the general statement 3 phase transformer does not always mean a single transformer. Many times three single phase transformers used to perform the function of a true three phase transformer are referred to as a three phase transformer.

.

 

[skeshesh]

101215-1318 EST
Do you currently have a 3 phase delta source with one secondary center tapped to provide 120-0-120, and 240 single phase, and 240 delta 3 phase? Usually described as having a wild leg around here.

I'm pretty sure that's what he's talking about. He mentioned the phase unbalance being due to the "high leg" which I believe is the same as "wild leg".

 

[gar]

101215-1451 EST

skeshesh:

I also think he has high leg service, but all the other comments don't seem to provide a clear indication of his real question. Today in most locations I would expect to see at least a 25 KVA pole transformer, and in any location with 3 phase probably at least a total capacity of 100 KVA, and with an open delta probably an uneven split in transformer sizes. The one supplying single phase 120-0-120 loads would be the larger.

If he has a 240 delta with a high leg service, then he needs to do nothing to get 240 3 phase delta, and 120-0-120 single phase. Likely that 10 A is peanuts of a load for this part of the question..

If he needs 120 3 phase delta or Y, or 240 (maybe really 208) Y, then he needs transformers.

The original post is not at all clear on the needs.

.

 

[MIEngineer]

I am feeding from a portion of the building electrical. In the building currently is a 3 ph. 480Y/277V panelboard (main) and a 3 ph. 208Y/120V panelboard through a 480-208Y/120 transformer.

My application has equipment (meter sockets) mounted outside on a strut-rack. At the rack I require 3ph., 3W, 240V and also 1 ph, 3W, 120/240. I am proposing having a 3 phase, 480-240D/120V transformer, Sq. D dry-type, high/wild/stinger/bastard leg, inside the building and then running conduit to a 240D/120V panelboard mounted on the strut rack.

My question goes to when the 120/240V equipment is operating there will be minimal, < 1A load on phase B and up to 12A load connected to phases A & C, with neutral balancing out. The load is a load bank. Could this set-up cause an issue with the transformer?

Thanks,

 

[bob]

You should have no problem with the 15 kva transformer. Your max 3 phase load is a little over 4 kva and single phase load is 2.4 kva. You are no where near the 15 kva capacity. This type of installation is common when used to serve commercial and industrial loads.

 

[LarryFine]

Could this set-up cause an issue with the transformer?

Won't be an issue. A 15kva 3ph Delta transformer secondary is made of three 5kva secondaries, each able to carry that without regard to the load on the others.

Attempting to supply 15kva unbalanced from such a transformer would overheat it. Your 1ph application will place no more than 2.4kva on the tapped secondary.

Do remember to properly bond that center tap to the building electrode system.