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.
--
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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