Transformer sizing

[fluoronator]

I have two spare transformers, one is a 15kva 480d to 240d/120ct, the other is a 9kva 480d to 120d. I need to power a 120v panelboard. Since only one coil of the 15kva is center-tapped, can I actually get more total current at 120v from the 9kva using all three phases than the center tapped 15kva?
Second question, if the 480d to 240d/120ct is only being used to supply 120v, does the third phase on the primary need to be connected?

 

[Julius Right]

In order not to have circulation currents between transformers and the load be distributed among the
transformers working in parallel to their nominal power, when their are coupled in parallel the following conditions have to be met:
1. primary and secondary nominal voltages of the transformers have to be equal (U1nI=U1nII=U1;
U2nI=U2nII=U2). This condition is practically met by qualling the transformation reports, meaning kI=kII;
2. tri-phased transformers must have the same groups of connections.
3. The short-circuit nominal voltages of the transformers have to be equal, meaning:
ukIn% = ukIIn% their active and inductive components respectively be adequately equal;
4. identical marked terminals of the transformers windings be connected to the same network
conductor, both on the primary part and on the secondary part;
If the parallel coupled transformers comply with all the conditions, their vectorial diagrams at the load working built in relative units they come one over another. In this case, all the transformers are loaded with proportional loads with their nominal powers, and the transformers load currents are arithmetically summed.
Practically, only the second condition has to be thoroughly met, for the first and third conditions
tolerances being accepted.
Besides, the sum of their loads does not have to be higher than the sum of their nominal powers.

 

[petersonra]

I don't think he asked if he could parallel the two transformers.

I dont see how you can use the 480d to 120d xfme at all, given the requirement for a grounded conductor.

 

[jim dungar]

I have two spare transformers, one is a 15kva 480d to 240d/120ct, the other is a 9kva 480d to 120d. I need to power a 120v panelboard. Since only one coil of the 15kva is center-tapped, can I actually get more total current at 120v from the 9kva using all three phases than the center tapped 15kva?
Second question, if the 480d to 240d/120ct is only being used to supply 120v, does the third phase on the primary need to be connected?

What do you mean by 120V? Do you really only need 2 wires?

Each 'winding' is rated at 1/3 the capacity of the full transformer, in your case 5kVA and 3kVA.

 

[petersonra]

In most cases transformers that have a Delta secondary with a center tap between two of the lines are only rated for a small percentage of the full KVA to be used by the loads using the center tap.

 

[fluoronator]

I don't think he asked if he could parallel the two transformers.

I dont see how you can use the 480d to 120d xfme at all, given the requirement for a grounded conductor.

Thanks, I guess I was thinking about a corner grounded delta for this transformer. Wouldn't this give me 6kva? btw, I need to have your quote tattooed on the inside of my eyelids.

 

[fluoronator]

In most cases transformers that have a Delta secondary with a center tap between two of the lines are only rated for a small percentage of the full KVA to be used by the loads using the center tap.

Thanks, so I'd get 5kva or about 40amps at 120v?

 

[fluoronator]

In most cases transformers that have a Delta secondary with a center tap between two of the lines are only rated for a small percentage of the full KVA to be used by the loads using the center tap.

Thanks, so if I understand, using the 15kva transformer I can get 5kva of 120vac, using the 9kva with a corner ground I can get 6kva? Would a corner grounded delta be acceptable/appropriate/safe?

 

[jim dungar]

Thanks, so I'd get 5kva or about 40amps at 120v?

No. Each coil would be 5kVA at 240V. Since you only want to use 1/2 of the winding for 120V, you would only have 1/2 of the capacity or 2.5kVA.

 

[petersonra]

Thanks, so I'd get 5kva or about 40amps at 120v?

You would need to look in the literature, but for larger xfmrs it is typically more like 5 or 10%.

 

[jim dungar]

You would need to look in the literature, but for larger xfmrs it is typically more like 5 or 10%.

The loading restriction is based on the transfomer being used with both three phase and single phase loads.

Personally, I would not use only one winding of a three phase transformer for any extended period of time (i.e. measured in years). The core losses might be excessive compared to a true single phase unit.

 

[winnie]

If you have a 120V corner grounded delta feeding a standard single phase panelboard you are probably ok with some caveats:

L-grounded conductor voltage will be 120V, just fine for 120V loads, but L-L voltage is only 120V.

The grounded conductor is not a neutral, and will carry almost the sum of the L currents. No MWBCs allowed and be careful of neutral bus capacity.

Jon