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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