Control Transformer voltage Question.

[Mporter4]

I am building a plc cabinet with starters,p/b, pilot lights etc.
I was going to purchase a 480v primary 240/120 secondary (2000va) enclosed.I was talking to a co-worker and he said by only using one leg of the 240v would make this an unbalanced system.
I think this would be correct but to purchase a 480v to 120v control transformer is going to run me about a $950.00.(SOLA HZ-122000) would I be okay with the 240/120 secondary??

 

[jim dungar]

It all depends on your transformer and your loading.

Your transformer is not going to be affected by a '100%' unbalance. However a 2000VA 120/240V transformer is rated for 8.33A per winding, which would mean you would only be able to supply 8.33 x 120 = 1000VA of load.

Many transformers are re-connectable, for 100% loading, as 120V 2wire, 240V 2wire, or 120/240V 3wire.

 

[ATSman]

I am building a plc cabinet with starters,p/b, pilot lights etc.
I was going to purchase a 480v primary 240/120 secondary (2000va) enclosed.I was talking to a co-worker and he said by only using one leg of the 240v would make this an unbalanced system.
I think this would be correct but to purchase a 480v to 120v control transformer is going to run me about a $950.00.(SOLA HZ-122000) would I be okay with the 240/120 secondary??

Normally if you parallel the 2 secondary windings to feed the 120V load (instead of a series configuration for 240V) then you utilize the full 2KVA capacity of the transformer.

 

[SG-1]

Do you have a model number for the transformer you want to use ? The second ( Sola ) transformer has an 80 degree C temperature rise. The transformer is mounted outside of the control box ?

 

[Jraef]

If the "enclosed" transformer means it is a distribution transformer, you will need a larger transformer for the same task as a Control Power Transformer. CPTs are specifically designed to provide short duration high inrush while maintaining good voltage regulation, something that is critical in that application. You don't want the inrush of a motor starter coil to cause other things in the panel to drop out. A standard (cheap) potted distributioin transformer is not designed for that. So if you calculated that you need 2kVA from a CPT, you have better buy a 3kVA distribution transformer.

 

[texie]

There is truth to Jraef's comment. Once again Jraef points out the finer points of motor control and all things related. We should all aspire to learn from him. Jraef, thanks for all your contributions!

 

[templdl]

Remember that a 480-240/120 transformer has a sengle winding secondary with a center tap (ct) and, yes, as such if you only need 120v for a single 120v circuit you will only have 1/2 of the kva on the transformer available.
As one of the other guys suggested get a transformer with a 120/240v secondary which has (2) 120v windings where X1-X2 is one winding and X3-X4 is the other. By simply connecting X1 and X3 together and X2 and X4 together you will be paralleling each 120v winding and have 120v available between the X1-X3 and X-2-X4 with the full KVA of the transformer.

 

[Cow]

There is truth to Jraef's comment. Once again Jraef points out the finer points of motor control and all things related. We should all aspire to learn from him. Jraef, thanks for all your contributions!

I agree, I learn a lot from Jraef's posts!

And I thought a transformer is a transformer is a transformer. Never would of guessed there was a significant difference.

 

[Jraef]

It comes from a lifetime of making mistakes, but learning from them.

"Note to self:

Self, DON'T do that again!"