"Balancing" or "matching" two variable autotransformers (stacked)

Thread Starter

oidium45

Joined Apr 24, 2010
138
Hello,
I have a powerstat 136BU-2 variable autotransformer. I took it apart, cleaned it up and built an enclosure. It has occurred to me that there may be a need to balance or match the pair for the rare occasion where I may want to use them together. Does anyone have data on how to properly accomplish this? Is it even necessary? The output voltages of each section appear to be pretty close. Within 0.1V or so of each other.
 

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crutschow

Joined Mar 14, 2008
27,231
If by "use them together" you mean connect the outputs in parallel, that's generally not a good idea.
If one output voltage is slightly different from the other, then a large circulating current will flow in their windings.
And if the two outputs get somehow out-of-phase, very large currents will flow, even if their voltages are the same.
 

Thread Starter

oidium45

Joined Apr 24, 2010
138
If by "use them together" you mean connect the outputs in parallel, that's generally not a good idea.
If one output voltage is slightly different from the other, then a large circulating current will flow in their windings.
And if the two outputs get somehow out-of-phase, very large currents will flow, even if their voltages are the same.
That was my concern. After doing some research it appears there is a choke that can be purchased to resolve that issue. The intent was to either run series or parallel depending upon needs.
 

Hymie

Joined Mar 30, 2018
976
It strikes me that the variac you have is for use on (to generate) a split anti-phase systems often found in the USA, supplying a 240V phase to phase voltage (allowing adjustment of the 240V).
 

Delta Prime

Joined Nov 15, 2019
1,022
Hello there :)
If one output voltage is slightly different from the other, then a large circulating current will flow in their windings.
The 'circulating currents' you speak of are a form of reactive load.
Ideally, with identical output voltages and identical internal impedances,
any reactive load supplied by the system is split equally between the
sources. And if there is no reactive load at all, then their would be no
reactive current (i.e. 'circulating current') flowing between the units
After doing some research it appears there is a choke that can be purchased to resolve that issue.
How would that work?
Chokes have the disadvantage that the final output voltage has poorer
regulation, because as load on the bank increases, the voltage drop on the
chokes causes the output voltage to drop more than if they weren't there. A
balancing transformer can have low inductance so the voltage
won't drop as much with applied load, yet still limit circulating currents.
By putting a balancing transformer in series with both variacs, any phase
di fference in variac output voltages cause circulating currents as before.
But the currents themselves cause the final output voltages to match through
both the internal reactance drop as before, but now also the balancing
transformer. The transformer forms a sort of 'proportional feedback' such
that a much smaller current is needed to bring the two final outputs equal;)
For the thread starter ...FYI Auto transformers used in parallel must be ganged together.
 

crutschow

Joined Mar 14, 2008
27,231
The 'circulating currents' you speak of are a form of reactive load.
I'm referring the current that flows in the secondaries of two transformers in parallel when one has an output voltage higher than the other.
That current, I believe, is determined by the voltage difference divided by the total secondary winding resistance, so that would not be reactive.
 

Delta Prime

Joined Nov 15, 2019
1,022
Hello there sir. :)
That current, I believe, is determined by the voltage difference divided by the total secondary winding resistance, so that would not be reactive.
So if the two transformer secondaries are slightly different voltages, and
the final output voltage of both circuits must be the same (they are
directly connected), then the voltage drop in the internal reactances must
be different. For that to be true, the two variacs' internal reactances
must have different current flows, hence 'circulating currents'.;)
 

crutschow

Joined Mar 14, 2008
27,231
Hello there sir. :)

So if the two transformer secondaries are slightly different voltages, and
the final output voltage of both circuits must be the same (they are
directly connected), then the voltage drop in the internal reactances must
be different. For that to be true, the two variacs' internal reactances
must have different current flows, hence 'circulating currents'.;)
Sorry but that does not make sense to me.
What "reactances" are you referring to?
And the current flow in both secondaries would necessarily have to be the same with no load, so how can they be different? :confused:
 
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