Old electromechanical stabilizer (w-Variac inside) how it works ??

[kagliostro]

Hi friends

at a flea market I got an old electromechanical stabilizer

it is well build, have a good alluminium case and a variac inside

for this reasons I got it and the intention was to reuse the variac and the good case

on it there are other parts as two transformers a line filter and other stuff

Well as I'm curious and not experienced at all to that I would like to know how it works

first for learning someting of new (for me)

second to try to understand in which way I can reuse the big transformer that is there (no writes on it)

To contact the manufacturer (not very far from my town), was impossible, because they ceased activity
 
so, looking to the circuit I draw a schematic

a basic one without the feedback circuit

(there is also an electronic circuit that manage the variac thru an electric motor)

Can someone help explaining how this stabilizer acts

and also tell me if I can think to use the transformer somehow ???


AS ALLWAYS MANY THANKS IN ADVANCE

Kagliostro

[RicharD]

Most likely the 1st transformer is simply a 1:1 isolation transformer.  The motor that is controlling the variac is probably a DC servo driven by a comparator circuit.  There should be a voltage tap on the output of the variac that feeds the comparator.  If VAC exceeds 220V it drives the servo south, (CCW) and if it falls below 220V, it drives the servo north (CW).  It could be done with a stepper motor too.

[kagliostro]

thanks Butterylicious

I agree with you about feedback control unit (there is a dc motor)

but I'm disoriented about the transformer

first time I look to the circuit I was thinking like you about an 1:1 isolation transformer

(I was hoping it was that)

but when I draw the schematics I saw that the use was not that

(there is a direct connection between the input and the output)

not galvanic isolation between input and output

till naw I'm not able to understand well the real use of the trasformer in that schematics

at the moment the stab is disassembled

as I had to remove oxide from the pivot of the variac which otherwise was going to fatigue

may be if I reassemble all and measure voltages (at the transformer windings ends) I can understand more

at the moment it seems to me that the variac is used to drive the "primary"

(winding 1 and 2) of the transformer

in a way that at secondary of the transformer there is allways a 220v~

in that way of thinking I can assume the transformer is someting near a 1/2:1 transformer

where 1/2 is'nt an exact 1/2 because the tap in the variac seems to be near 1/2 of the winding but not exactly there

in my simply conception the schematic that I put here naw will be easy to understand and give galvanic isolation to the stabilizer

but if they don't do it in such way I think there is surely a reason

I forgot to say that the stabilizer is rated for imput between 170v~ and 260v~

thanks another time

Kagliostro

[kagliostro]

looking to my second schematic (an idea of a stabilizer) I sense someting

in my second schematic is possible only to have regoulation if input voltage increase but not if it decreases (with an usual variac)

and that could be the reason for which they do that schematics (the first I posted)

I arrived near to understand something or not ??

can someone help ?

Thanks

Kagliostro

[PRR]

> Most likely the 1st transformer is simply a 1:1 isolation transformer.

I'd bet on something like 120V:24V. It wants to make maybe +/-10% adjustments.

Then the center-tapped variac can apply 120V in-phase or reverse-phase, adding or subtracting 24V from the input voltage, to keep the output voltage "right".

Re-draw like this (below):

> regoulation if input voltage increase but not if it decreases (with an usual variac)

The usual Variac is wound (in the US) with 0V, 120V, and 140V taps. We feed the 0V and 120V taps with fairly constant 120V, we can get from zero to 140V out.

But your regulator does NOT want zero output. If we only use the (US voltage) 100V to 140V range, then 70% of the core must be wound but is never used by the output.

So they use the full 100% range of the variac, and step-down to give 10% add/subtract.

Another trick is the center-tap. The variac could be smaller/cheaper if it only went 0%-100%, and we used a phase-reverse switch to change from boost to buck. However consider the case where output is supposed to be 220V and input is wandering 219V to 221V. That phase-reverse switch would be clacking a lot. And it needs more brains to decide which way the switch should go. You also have to reverse the wires to the motor. So I guess they decided it was simpler to center-tap the variac and let the motor slide smoothly from maximum buck, through no-change, to maximum boost.

[PRR]

> stabilizer is rated for imput between 170v~ and 260v~

Ah, then it is more than +/-10%. IF the output is 220V, then it gives +30% and -18%. If the output is supposed to be 210V then about +/-24%.

> I can reuse the big transformer

It appears to expect up to (about) half the input voltage. You say the input is 260V max. So the transformer is not wound for much more than 130V input. Also, if the complete unit will take 170V in and give 220V out, the transformer secondary is 50V maximum.

It is roughly 130V:50V. I may be a little bit off because of the asymmetric boost/buck. However it probably will not accept much more than 130V input, nor safely deliver more than 50V output. With your 220V wall-power, this is not very useful.

It is still handy to have a steady "wall"-voltage on the bench for exact power-output readings. I have to correct my 111V wall to 120V for comparison to other published specs.

I do have a "voltage stabilizer" but it is much less nice than yours. Mine has six 8V taps on a 120V winding, and six transistor-switches to select taps. It will take a "120V wall" as low as 86V and bring it up to "about 120V". However it really gives 115V to 123V, the nearest tap which gives "about 120V". It was made to run data electronics in a factory or other place with poor power. Data electronics which quit on 85V are perfectly happy with anything 115V to 123V. However my analog measurements will differ 9% if I happen to get 115V and want to normalize to 120v; a moment later the tap may change and give me 123V and a too-high power rating.

[kagliostro]

Many thanks PRR

this evening I'll study your answer with attention to learn the more I can from your explanation


P.S.: very nice old stab gif    !!!

Kagliostro