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Re: variac
Subject:
Re: variac
Date:
Sat, 15 Mar 1997 21:10:30 -0800
From:
"David E. Sharpe" <sccr4us-at-erols-dot-com>
To:
Tesla List <tesla-at-pupman-dot-com>
References:
1
Tesla List wrote:
>
> Subject:
> variac
> Date:
> Sat, 15 Mar 1997 01:33:15 -0600 (CST)
> From:
> c604313-at-showme.missouri.edu
> To:
> Tesla listserver <tesla-at-pupman-dot-com>
>
> Hello to all!
>
> It sure would be nice to hook up a variac transformer on the primary
> side
> of some step up transformers. My question is by how much can one skimp
> on
> the power rating of such devices??
>
> Consider banked neons totaling 15 kv at 150 ma = 2250 W impedance
> matched
> with the primary caps in an equidrive setup. Certainly one wouldn't need
> a
> 20 amperer variac running off of a 110 line. Could one get by with just
> a
> 10 amperer variac if running a coil for only short periods of time (10
> minutes) ?? What have you all been able to get by with safely?
>
> I've been enjoying all the discourse and expertise of the list!
>
> -Bert S.
Bert
General rule of thumb is to avoid loading variacs or transformers for
that matter over 250% for longer than 2 minutes. I wish it was
that easy, but class of insulation, cooling (air, oil or impregnated)
all effect the final outcome, which is shortened life.
As a sidebar, the National Electric Code does allow fusing the primary
on control/power transformers at 225% nameplate, PROVIDED THE SECONDARY
WINDINGS HAVE OVERCURRENT PROTECTION SIZED FOR CONNECTED LOADS. That is
a 1.2KVA transformer can be fused up to 25A on the primary (if fed
with 120VAC), provided secondary is fused for no more than 10A, if
supplying 120VAC to load. The reason for this is transformers like
motors, have horrific magnetizing inrush currents, fusing a transformer
at nameplate with a fast blow fuses will result in the transformer
eating fuses like popcorn.
DAVE SHARPE, TCBOR