Single Phase Transformer Question 2

[ThePunisher]

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.

 

[wayne440]

Does your 230V supply have a ground reference?

 

[ThePunisher]

The 230 V power source is solidly grounded.

 

[waross]

If that is two separate transformers your voltage will be inversely proportional to the secondary loads for small load variations.
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
--------------------
"Why not the best?"
Jimmy Carter

 

[ThePunisher]

HI Bill, is the concerns you mentioned above going to occur even of the transformers are totally identical?

 

[ThePunisher]

HI Bill, are the concerns you mentioned above going to occur even if the transformers are totally identical? I am trying to understand how these concerns (saturation, inverse-proportionality of voltage ad loads, etc) going to happen...I am doing a little research but it is not gonna harm if you can give me a bit of crash course of what you are implying.

Thanks and my apologies if I am asking too much.

 

[waross]

The effective impedance of the transformer primaries may be determined by dividing the voltage by the current. As the current is mostly dependant on the load, when the loads are not equal, the impedances are not equal. When you connect unequal impedances in series the voltage drop across each is inversely proportional to the load.
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

 

[itsmoked]

I tried exactly that in college except the outputs were really 15V. It didn't work AT ALL. It was a disaster. One transformer dropped all the voltage.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[stevenal]

Lets try a simple example. The load on A cycles off while B continues to be connected to load. The secondary of T1 sees an open circuit, so the primary of T1 also sees an open circuit. In comparison, T2 is a low impedance. Overall, the 230 V source will see an open circuit in series with a low impedance, so you will be dropping nearly 230 V across the primary of T1 and nearly 0 V across the primary of T2.

 

[waross]

With resistors yes; with transformers, no.
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
--------------------
"Why not the best?"
Jimmy Carter

 

[ThePunisher]

Thank you all for your comments. I thought when connecting the transformer primary windings in series like I shown, the voltage rating will be additive (115 + 115 = 230V)... it did not work the way I perceived it.

 

[waross]

Consider the transformers as two non-equal, non-linear resistances or impedances in series.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[stevenal]

Waross,

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.

 

[waross]

Stevenal; Thank you for pointing out my silly error. I have corrected the error.
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
--------------------
"Why not the best?"
Jimmy Carter

 

[ThePunisher]

Thank you all for your responses..it is clear to me now