Single Phase Transformer Question
Single Phase Transformer Question
(OP)
WE have 2 x 1 kVA ISOLATION TRANSFORMERS 115-15 V separately supplying power to two separate 115 V loads (load 1 and load 2). Our primary power supply is 230V.
I will connect the two primary windings series to be able to connect the transformer to the 230 V supply. Then I will connect the secondary windings and extend one more line to obtain a center tap and still get 115 V. I will connect the secondary to a common panelboard that will have two lines and one coming from center tap.
My question is whether the above idea is sound and do I need to ground the center tap connection and have a grounded conductor instead (Code compliance)? Will this undermine the "isolation service of the transformers"? See attached
Your comments are appreciated to ensure I did not miss anything.
I will connect the two primary windings series to be able to connect the transformer to the 230 V supply. Then I will connect the secondary windings and extend one more line to obtain a center tap and still get 115 V. I will connect the secondary to a common panelboard that will have two lines and one coming from center tap.
My question is whether the above idea is sound and do I need to ground the center tap connection and have a grounded conductor instead (Code compliance)? Will this undermine the "isolation service of the transformers"? See attached
Your comments are appreciated to ensure I did not miss anything.






RE: Single Phase Transformer Question
RE: Single Phase Transformer Question
RE: Single Phase Transformer Question
As the load variations become greater one transformer will saturate and somewhat limit further voltage swings.
Your secondary voltages will be unstable and if there is a large difference in loads, you may have heat losses in the saturated transformer.
You may be able to make it work by connecting the secondaries in parallel but it is not a good idea because of the implications of the various possible failure modes. Probably not code compliant.
As far as isolation and grounding. That leads to a quagmire of "What If?", "Ya But", and "It Depends".
What is the purpose of the isolation? That has a bearing on the grounding. Generally that would be a separately derived system and need to be grounded. However some isolated circuits in operating theatres must be ungrounded. Some circuits in some hazardous areas must be ungrounded. "It Depends"!
Bill
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"Why not the best?"
Jimmy Carter
RE: Single Phase Transformer Question
RE: Single Phase Transformer Question
Thanks and my apologies if I am asking too much.
RE: Single Phase Transformer Question
Once one transformer saturates the losses increase as the load on the heaviest loaded transformer increases. The transformer will stabilize the voltage across it at a point somewhere on the saturation knee point by wasting energy as heat in order to pass enough current to satisfy the heavier load.
Once unequal loads are connected to the transformers they are no longer equal systems.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Single Phase Transformer Question
Keith Cress
kcress - http://www.flaminsystems.com
RE: Single Phase Transformer Question
RE: Single Phase Transformer Question
When the voltage of T1 reaches about 120% of rated voltage (120% x 115 V = 138 Volts) the transformer will saturate. The resulting increase in current will raise the voltage of T2 to about 92 Volts. (230 Volts minus 138 Volts = 92 Volts).
Bill
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"Why not the best?"
Jimmy Carter
RE: Single Phase Transformer Question
RE: Single Phase Transformer Question
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Single Phase Transformer Question
In my defense, I did say it was a simple example. No need to get into non-ideal transformer aspects to show this connection won't work.
Didn't you mean to say T2 is 92 V and not T1?
Punisher,
It is not just the transformer rated voltage that is the problem here. You could use 230:230 rated transformers and the connection will still not provide a stable voltage to the connected load.
RE: Single Phase Transformer Question
Please forgive my unfortunate wording. I intended to indicate which post I was referring to to avoid confusion.
I wished to add to your information the non linear aspect of transformers subject to over-voltage.
Your post is accurate when the voltage seen by both transformers is below the saturation point. Please accept my post as additional information rather than a contradiction of your post.
Yours
Bill
Bill
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"Why not the best?"
Jimmy Carter
RE: Single Phase Transformer Question