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Old 30th Oct 2016, 12:08 pm   #41
G6Tanuki
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Default Re: "Bucking" Transformer.

My thought-experiment about autotransformer-wiring of a bucking transformer came about while thinking "if the mains voltage is already high, surely applying this to the primary of the transformer will overstress the transformer?"

(e.g. if you've got 255V 'overvoltage' mains and you put this straight on to the primary of a 240V transformer you have a 15-volt overvoltage so the transformer will mrun a bit hot and flustered?).

But if you have a 240V-primary transformer with a 15V secondary, wire the 2 windings in series - autotransformer-style - and you now have essentially a "255V primary" transformer which won't think it's seeing an overvoltage so should run cooler and have a longer life.
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Old 30th Oct 2016, 1:15 pm   #42
broadgage
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Default Re: "Bucking" Transformer.

To reduce a generous mains voltage, I would indeed wire as just described.

Connect the short (24 volt) and the long (230 volt) windings in series so as to give in effect a single 254 volt winding, which is very close to the actual mains voltage.
Then connect the load to the longer winding, thus reducing the voltage to about 90% of the input.

This produces almost the same result as applying the mains to only the mains winding, and then connecting the 24 volt winding in series opposition with the load. As posted above, this has the merit of not overrunning the 230 volt mains winding by subjecting it to 250 volts.
A "230 volt" transformer should in theory be fine up to 253 volts input as that is within the accepted tolerance for mains supplies, but best to be cautious IMO.

Great care must be taken with the original connections, if you get the 24 volt winding the wrong way round WRT the 230 volt winding, then you have in effect created a 206 volt winding which will be significantly overstressed at 250 volts.

With the 230 volt and the 24 volt windings in series across the mains, it is debatable as to which way round the mains should be connected.

Some state that the neutral should be connected to the 24 volt end, on the grounds that the 24 volt winding is thereby not subjected to mains voltage to earth.
IMO, this is incorrect since it results in the "neutral" into the load being live at 24 volts. What happens in the event of a short circuit between the "neutral" which is live at 24 volts, and the earth ? A 3 amp fuse in the plug could permit of 30 amps at 24 volts being produced and either destroying the transformer or starting a fire.
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Old 30th Oct 2016, 2:06 pm   #43
bikerhifinut
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Default Re: "Bucking" Transformer.

I'm going to be rewiring a new toroid soon. So what you say is wire it as a autotransformer and I will be stressing it less?
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Old 30th Oct 2016, 2:20 pm   #44
Alistair D
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Default Re: "Bucking" Transformer.

I can see one drawback of the autotransformer method. The voltage on the primary of the bucking transformer will now be affected by the transformer's regulation so that there will be a greater fall in the load voltage when going from zero to full load. Usually torroidal transformers have lower regulation percentages than other types so this may not be a problem in your case.

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Old 30th Oct 2016, 7:06 pm   #45
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Default Re: "Bucking" Transformer.

No it won't. If the load current is high, the transformer regulation will effectively drop the voltage to the original primary winding. This will make the secondary winding produce a lower voltage. So the supply will be bucked by a bit less! And this cancels out the regulation drop somewhat.
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Old 30th Oct 2016, 11:11 pm   #46
bikerhifinut
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Default Re: "Bucking" Transformer.

So I am safe to configure this new toroid (Vigortronix 160VA) as an auto transformer to supply a couple of mono Valve amps pulling 3 amps in total between them?
Andy.
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Old 31st Oct 2016, 10:01 am   #47
kalee20
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Default Re: "Bucking" Transformer.

Definitely, yes!

But if you are not sure of your connections, whether you have the phasing right etc, just use a 60W light bulb across input and then output. You should see a small but still noticeable difference in brightness between them.
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Old 31st Oct 2016, 11:53 am   #48
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Default Re: "Bucking" Transformer.

Quote:
Simply cut the complete set of windings in half and simply count the ends of the wires.
This also has the advantage in that you can count them again if you need to.
 
Old 31st Oct 2016, 2:20 pm   #49
bikerhifinut
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Default Re: "Bucking" Transformer.

Quote:
Originally Posted by kalee20 View Post
Definitely, yes!

But if you are not sure of your connections, whether you have the phasing right etc, just use a 60W light bulb across input and then output. You should see a small but still noticeable difference in brightness between them.
Wouldn't a voltmeter do the same job?
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Old 31st Oct 2016, 2:27 pm   #50
kalee20
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Default Re: "Bucking" Transformer.

Yes of course, though with a light bulb you can actually see the reduction in the grunt from your supply!
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Old 31st Oct 2016, 5:52 pm   #51
bikerhifinut
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Default Re: "Bucking" Transformer.

Wired as an Autotransformer.
Works a treat.
Dropped mains by 24V so it now varies between 224 and 230V, my mains is generally right on the upper limit.
I will wait until the house is quiet later on, we have builders doing their thing.
But initial impression is that the noisy mains transformers are running quieter. The toroid bucking or pseudo autotransformer isn't silent by the way but in its nice ABS box it isn't audible from the sofa.
Fingers crossed
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