Weird Transformer Load Configuration 2

[vds002]

One of our heaters has a transformer that had load connected between primary and secondary windings. Can anyone give any thoughts on what does this type of connection mean... any ideas would help...

 

[davidbeach]

Looks like an unsuccessful attempt at using a transformer as a buck or boost autotransformer, but it's hard to tell.

Is that exactly how it is wired?

What is the line-neutral voltage of the source?

What are the voltage ratings of the two transformer windings?

What is the rating of each of the two loads?

 

[waross]

What are the loads?

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

 

[vds002]

Each of the load is four heating elements in series.
The Line voltage is 500V 1000A.
I have no details of the transformer winding.
Yes this is how the load is wired.
Just trying to make some sense out of the configuration.

 

[sibeen]

The load at the top will receive a lower voltage than the load at the bottom.

 

[waross]

Are the heaters working properly? I suspect a mistake either in the wiring or in the diagram.
Is this the only such circuit or do you have other similar installations to compare with?

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

 

[rbulsara]

I have no details of the transformer winding.

How did you come up with the diagram?

Perhaps it is not a transformer. They could be just two reactors, not a transformer. One load has one reactor in series, other has two.

Or just an error somewhere or missing information. What are the wattages? Application?

Can you measure voltages across each element of windings and loads? Amps through them?



Rafiq Bulsara

http://www.srengineersct.com

 

[potteryshard]

Might that be a 1:1 transformer with a winding in parallel with each load group? (Same connections to load; different transformer internal connections)

With the high-voltage high-current loadss you are showing, it may be that a 1:1 transformer was used to stabilize the voltage drops across the groups of loads.

 

[waross]

Are the heaters working properly or have they failed?

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

 

[vds002]

potteryshard's you have nailed it... This is exactly what our requirements were for the system and we asked the vendor to design it so it delivers EQUAL high voltage and high current to both the loads but if one load becomes open the voltage and current for other load should not be effected....

CAN ANYONE SHED SOME LIGHT ON HOW THIS TRANSFORMER CONFIGURATION IS ACHIEVING THIS....

P.S. There might be several ways to do what we requested the vendor but I only what to discuss what our vendor has designed and try to understand the system.

 

[rbulsara]

It is NOT. The wiring diagram is wrong. What you "require" can be achieved by standard wiring, that is two loads in parallel. That is how almost all loads are connected to a source.

1:1 transformer only gets you isolation. What does it have to do with "high" voltage, which will be the same on either side, unless it is equipped with an on load tap changer.

Do not ask a question like a puzzle or a quiz. Post all the pertinent information that you think will be helpful to others to comment on the issue.

Rafiq Bulsara

http://www.srengineersct.com

 

[jghrist]

Rearrange the circuit as shown on the attached drawing. The transformer would be subtractive polarity 2:1 ratio. The transformer forces the currents through the heating elements to be equal.

If the heating elements were the same resistance, there would be no voltage across the transformer windings. If one resistance was lower, there would be a voltage across each transformer winding that would reduce the current through the lower resistance element and increase it through the higher resistance element.

 

[potteryshard]

This is what I thought the manufacturer had done. It is essentially the same as a preventative autotransformer on a LTC. Note that this arrangement is the same as the OP's drawing except that the assumed transformer connections are rotated.

 

[vds002]

jghrist ....I sincerely appreciate your efforts ....
BUT IF ONE OF THE LAODS R1 or R2 were to become open would it double the current trough the working load..

 

[davidbeach]

Actually, if R2 becomes open, no current flows across V2 and therefore no current can flow across V1. Current can not flow in one winding if it can't flow in the other, the amp•turns values have to match.

 

[davidbeach]

I hate to respond to my own post, but...

If R1 is open there will also be no current through R2. Look at your equation (Ia+Ib)/2=Ib. It is true under all conditions. If Ia is zero, the only solution is Ib=0. If Ib is zero, the only solution is Ia=0. Trust the math.

 

[waross]

It may help to remember that these transformers are acting as current transformers. If the resistive load on a current transformer is reduced, the transformer increases the voltage in an attempt to maintain the current.
If the transformer is chosen so that the voltage across the windings is approximately 1/2 line voltage, then in the event that one load goes open the transformer becomes a very low impedance in series with the remaining load and the voltage across the remaining load will be close to line voltage.

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

 

[davidbeach]

Sorry Bill, but I have to disagree. Amp•Turns have to match on both sides. In a series connected transformer worrying about voltage across the transformer leads to many false paths. For example in a CT, the most common example of a series transformer, the secondary current is proportional to the primary current by the ratio. If the CT is shorted, full current flows and no voltage is developed. If the CT is open circuited, the current still flows, but enough voltage is developed the force all of the secondary current through the magnetizing branch. This can result in thousands of volts across the secondary while still having essentially zero volts across the primary.

In this case if R2 is open, the zero current in Ib will cause full line-neutral voltage to appear across V1 because Ia will also be zero. If R1 is open then Amp•Turn balance requires Ib/2=Ib, for which the only solution is Ib=0.

All of this assumes an ideal transformer. In reality there will be some magnetizing branch current and Ia will only be very close to zero for R2 open and Ib will only be very close to zero for R1 open.

 

[waross]

Thank you for your reply, David.
The part I am having trouble with is the case when R1 goes open. This leaves us with a series circuit, One winding, R2 and the second winding in series. Since the same current must flow in all parts of a series circuit, and since the turns ratio has been suggested as 1:2, the amp turns must differ between the windings.
What am I missing?

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

 

[davidbeach]

Amp•Turns can't differ, therefore the only current that meets the requirement is zero. Ignoring losses and magnetizing current there will be no current if either load is open.

Consider a normal step up or step down transformer connected line-neutral on one side. If there is a short circuit on the secondary there will be a large amount of current on the primary, limited only by the impedance of the transformer. But when the secondary is open circuited no current (ignoring losses and magnetizing current) flows on either side. Why isn't there current on the primary? Because there isn't current on the secondary and the Amp•Turns have to balance. Same in this case. However much it might look like a circuit exists, if current can't flow in one winding it can't flow in the other.