David,
That's the reason for me always using the wattage in equals wattage out
scenario. The current varies with the load and should have a constant voltage
after the sag due to the transformer regulation. That is for a transformer
receiving the specified primary working voltage though. To be honest, I didn't
see you mention varying voltage and am thinking about getting glasses LOL. Of
course the voltage will vary by using a variac on the primary. Then one must
figure by the maximum power and current capability of the transformer because
when you have a voltage drop, you have a current rise. This meaning the magnet
wire in the transformer would have to be able to handle the current through it.
Let's say we have a 10 volt trnasformer at 2 amperes from the secondary
equaling 20 watts. The wire size in the secondary would be sized around 2 amps.
However, if the primary voltage is reduced to produce say 5 volts from the
secondary, the current will rise X2 or 4 amps to get 20 watts. So,
the power capability would have to drop since the secondary is only rated for
2 amps. From this, the transformers power handling capability will fall to 10
watts even though there's enough iron for 20 watts. In other words, it will
have to drop to keep from burning out the secondary. I wanted to mention this
so any would know that playing with the primary voltage can drastically change
the power of a transformer. Even though the voltage changes from a variac, the
transformer has a maximum limit on the current drawn from the secondary.
Best,
Will
From: "David Kirkby" <david.kirkby@onetel.net
To: "amps@contesting.com" <amps@contesting.com>
Subject: Re: [Amps] filament transformer
Date: Sat, 09 Jul 2005 05:07:05 +0100
>
> David Kirkby wrote:
> > John Irwin wrote:
> >
> >> hi
> >> I have a transformer 10vac at 20 amps. 110vac primary. Does
> >> anyone know how to calculate the primary ac line draw. I want
> >> to use a variac to put it on the money . Need to get the right
> >> auto transformer.
> >>
> >> thanks
> >> john kb9tc
> >
> >
> > The primary current will depend on the secondary current. With no load,
> > the primary will draw very little (just the magnetising current). So
> > unless you state the heater voltage and current, one can not give you a
> > value, but I can tell you how to calculate it.
> >
> > At full load the transformer will be somewhere around 90% efficient -
> > I'm sure Will could give you a better idea, but the exact value does not
> > make a lot of difference here. So you must allow 10% more for the losses.
> >
> > 20 A at 10 V is 200W. Plus another 10% (20W) and that is 220W.
> >
> > Current = power / voltage
> > = 220/110
> > =2A.
> >
> > So a 2A variac will *just* be OK if you need the full 10 V at the full
> > 20A. But personally I would go for something a little bigger (the next
> > size up) *if* your heaters really are going to be 10V at 20A, since I
> > don't like running any component at its maximum ratings.
> >
> > If the heaters don't need the full 200W the secondary can provide (i.e.
> > they need less than 10V, or less than 20A), then 2A would be fine, but
> > it is a bit tight if you really need the full 200W on the secondary.
> >
>
> Having thought about this again, there was a serious flaw in my argument.
>
> Ignoring losses, the primary current will depend ONLY on the secondary
> current, NOT the secondry voltage. Hence I was not right to work out the
> size of a variac based on the power required at the secondary, if the
> voltage at the primary is not its full value (110V) and so the secondary
> voltage a full 10V.
>
> At full load, a 2A unit should do (just), as I was correct to work out
> that current.
>
> BUT should your heaters need 0.1V at 20 A (just 2W, which I know that is
> not so), then the secondary is still going to supply 20A, and the
> primary will still draw 2A. The primary power will be low (2W + losses)
> and the primary voltage low (about 1V), but the primary current is still
> going to be 2A. So you would still need a 2A variac (in principle
> capable of providing 220W), despite the fact your heaters are only using
> 2W.
>
> So if you heaters need 20A, you need a 2A variac - irrespective of the
> heater voltage.
>
> If the heaters need 10A, a 1A one should do (just) - again irrespective
> of the hearter voltage.
>
> I hope I have not confused too many people there.
> --
> David Kirkby,
> G8WRB
>
> Please check out http://www.g8wrb.org/
> of if you live in Essex http://www.southminster-branch-line.org.uk/
>
>
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> Amps@contesting.com
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