3 phase Delta Trans. for Lighting

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shawn73

Member
Location
Napoleon, Ohio
I have been arguing at work with some individuals about transformers. More specifically a 45 Kva 480V 3 phase Delta primary with a 3 Phase Delta 240/120 (With a center tap)secondary. A transformer was purchased to provide power to a 200 amp panel board providing primarily 120V power to assembly line equipment. From what I have read and been taught a Delta-Wye configured transformer would have been correct to use in this situation. I always thought that all Delta secondary transformers with the lighting tap could provide the 120V voltage we needed, but only at 5% of the name plate KVA . I was also told that the nameplate KVA needed to be de-rated by 30% in this situation. It seems that running an entire 200 amp panel off of this center tap would cause an unbalanced situation in the transformer. I am having a hard time convincing people of this because there is already another transformer of this style running a 200 amp panel. In my opinion just because it's working - doesn't make it right. Am I right or confused?? Would a tranformer of this style be wired to 2 separate panelboards? One to provide some lighting voltage (5%) and the other panelboard to provide 3 phase 220V? I appreciate any information that could support my argument or correct my thinking. Thank you!
 

bob

Senior Member
Location
Alabama
Re: 3 phase Delta Trans. for Lighting

Shawn
When you use a delta connected transformer to provide power to this type load, the capacity is 58% of the original rating. When used in this manner you are only using 2 phases to supply the load instead of all 3. You said you already have a 200 amp panel connected in this manner. That may be true. Its one thing to connect load to the transformer and another to actually supply watts to the load.
You do not need to use 2 panels. There are 3 phase panels that will allow you to connect single phase load at 120 volts and still provide limited 3 phase power.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Re: 3 phase Delta Trans. for Lighting

Bob,
I don't agree. The 58% capacity is when supplying 3 phase loads from a two transformer, open delta bank. Shawn has a 3 phase delta/delta transfomer, and he is correct that the single phase load should be limited to 5% of the transformer rating. The following is from A Square D publication.
Q. Why is there a 5% limit for single phase loading on Delta-Delta connected transformers with 240/120 center tapped
secondaries?
A. Delta-Delta connected transformers are intended to supply balanced three-phase loads, such as motors. Unbalanced loading will cause a circulating current to flow in the windings.
This additional current is like a ?hidden? load within transformer windings and can severely de-rate or even overload the transformer. Adding a center tap on the secondary for a combination of a 240V three-phase and 120V single phase loads will create an unbalance. The amount of unbalance is limited to 5% to prevent excessive circulating currents.
Don
 

bob

Senior Member
Location
Alabama
Re: 3 phase Delta Trans. for Lighting

Don
You are absoutly correct. Thanks for the correction.
 

charlie

Senior Member
Location
Indianapolis
Re: 3 phase Delta Trans. for Lighting

If you really want to load a 120/240 volt, 3 phase, 4 wire secondary full delta transformer, add all the single phase at the rate of 2/3 of the single phase on the center tapped coil and 1/3 on each of the other two coils. The three phase load is spread equally 1/3 on each coil. If done in this manner, you can ignore the 5% rule but you will not be able to load two of the coils very much if you have a lot of single phase load in comparison to the three phase load. :D
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
Re: 3 phase Delta Trans. for Lighting

Charlie, I don't think that you can/should do this with a single core dry type transformer.

I found this formula for determing circulating currents in a delta-delta transformer in an old Square D publication #S-2:

%I = (100x%E)/(3x%IZ)

%I = circulating current in % of full load current
%E = percent unbalanced voltage difference in the delta
%IZ = percent impedance of the three phase transformer
 
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