An autotransformer is in fact a transformer. It works just like a
transformer with two windings (primary and secondary).
The difference is the autotransformer shares the same winding for both
primary and secondary. The core must be magnetized just as it is in a
regular transformer. The core will have losses just like a common
transformer.
If of course the output tap on the autotransformer is set to the same place
as the input tap then there will be little transformer action and mostly
just core magnetizing loss.
However the further away from the input tap the secondary tap is, the more
transformer action there is and losses go up accordingly.
If the output tap is halfway between the input tap then you have transformer
action like you would have with a two winding transformer with different
turns on each.
73
Gary K4FMX
> -----Original Message-----
> From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Ros
> Sent: Wednesday, July 22, 2015 10:15 PM
> To: amps@contesting.com
> Subject: Re: [Amps] HV transformer and Variac
>
> First, the Variac is not a transformer in the classical sense, it is an
> autoformer or auto-trans-former. It will not lose efficiency. Here is a
> schematic of one and perhaps you can see why it won't lose efficiency.
>
> A common transformer has less efficiency than an autoformer due to core
> loses, eddy currents, hysteresis, more heat (I2R losses), etc.
>
>
>
> Ros, W5BRY
>
>
>
>
> On 7/22/2015 8:28 PM, Steve Bookout wrote:
> > Hello all,
> >
> > Thanks to everyone for the informative responses. Although I've now
> > got more info to use in order to figure out what I am going to do, I'm
> > not sure what that will be.
> >
> > It's really just a bit 'hot' for my needs and if I turn it down with a
> > variac, regulation may suffer. I do have a stack of two variacs on a
> > common shaft, and although they are large, I don't know what their
> > actual specs are. Something for me to check.
> >
> > 73 de Steve, NR4M
> >
> > On 7/21/2015 03:03 , Steve Bookout wrote:
> >> Hello all.
> >>
> >> Question for the experts.
> >>
> >> How does using a Variac to control the output of a HV transformer
> >> affect things like efficiency?
> >>
> >> I think it would be reasonable to think that a transformer operated
> >> at less than rated input voltage would be less efficient than one run
> >> at full primary voltage. I mean, it cost 'something' just to make a
> >> transformer act like a transformer.
> >>
> >> I ask because I have a perfectly fine 'pole pig' transformer rated at
> >> 5 KVA. 7200 AC volts on the output (reversed) is a bit much for my
> >> needs, so was wondering what the trade-off would be if it were run at
> >> considerably less than 220 input.
> >>
> >> I can see this is a better xformer for a 4cx10000, than anything I
> >> would be using, but I have what I have.
> >>
> >> 73 de Steve, NR4M
> >> _______________________________________________
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> >> Amps@contesting.com
> >> http://lists.contesting.com/mailman/listinfo/amps
> >>
> >
> > _______________________________________________
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> >
>
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