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...

 

[sibeen]

Nice explanation, David.

I suppose the question is now, what does the circuit do?

It appears that if either load goes open circuit there will be no current flow in the other load.

This seems to be at variance with what was requested by vds002's company.

 

[potteryshard]

Guys...

When I posted my previous sketch,I marked the instantaneous polarity wrong, making it a bit hard to tell what I was talking about.

Looking at a corrected sketch, it can be seen that the 1:1 two winding transformer can be connected as a 2:1 autotransformer.

The autotransformer will split the supply voltage, and will thus stabilize the neutral point between the series loads. Should one of the heater strings open up, current will bypass the open string, but maintain the proper voltage on the remaining string.

With an autotransformer arrangement, we can't easily predict what the currents are through the individual windings. Some portion of the current will pass through the winding bypassing the open load and contribute directly to the remaining load. Another portion will be transformed at a 1:2 current ratio to make up the remainder of the normal current through the remaining load.

As well as maintaining design voltage to the remaining heater string after an open, this arrangement gives another benefit. Because the OP pointed out the each branch of the heater load is four series heater units, the autotransformer gives some protection against heaters failing shorted. If all 8 units were in an unstabilized string, one unit failing shorted would raise applied per unit voltage by 8/7ths, likely failing another, possibly to a cascading failure scenario. (8/6ths, 8/5ths, 8/4ths, etc.)

The auto will limit the per-heater voltage in the failed shorted half-string to 4/3rds nominal heater voltage, but a cascading failure will still be limited to only half of the total heaters.

 

[jghrist]

The autotransformer will split the supply voltage, and will thus stabilize the neutral point between the series loads. Should one of the heater strings open up, current will bypass the open string, but maintain the proper voltage on the remaining string.

Each string of heaters will have 1/2 of the L-N voltage across it. I don't see the advantage of this over just having two strings in parallel, with string each rated for full L-N voltage. No matter what happens to one string (except having a complete short), it won't affect the voltage across the other string. The autotransformer arrangement does nothing to stabilize the current. It will be (V_L-N/2)÷R.

 

[potteryshard]

Agreed... The putative series configuration may have been chosen to make standard available heaters work with availalble voltage. With a 500 V supply and 1000 A load, it seems odd from the get-go as far as design is concerned.

I'm just trying to puzzle out a reasonable interpretation of the original sketch. By moving the N connection from the right side winding to the left side it could also be simply a voltage matching transformer between supply and heater requirements.

 

[davidbeach]

So now it is 1:1? Assuming additive polarity, the current on each winding will flow toward the node between the loads. With both loads in place there is no current in the transformer. If either load is open, you have a situation where there would be a loop current through the remaining load driven by the current passing through the transformer winding in parallel with the open load. I'm not going to try to figure out where it stabilizes, but I doubt it results in constant output of the remaining load. Again, just put the two in parallel.

 

[vds002]

Thank you. I understand.