Re: variac

To: tesla-at-poodle.pupman-dot-com
Subject: Re: variac
From: Tesla List <tesla-at-stic-dot-net>
Date: Tue, 18 Mar 1997 23:54:50 -0600
Approved: tesla-at-stic-dot-net
Subject: 
           Re: variac
      Date: 
           Tue, 18 Mar 1997 08:13:20 -0800
      From: 
           "David E. Sharpe" <sccr4us-at-erols-dot-com>
        To: 
           Tesla List <tesla-at-pupman-dot-com>
References: 
           1


Tesla List wrote:
> 
> Subject:
>         Re: variac
>   Date:
>         Mon, 17 Mar 1997 13:33:17 -0500 (EST)
>   From:
>         Esondrmn-at-aol-dot-com
>     To:
>         tesla-at-pupman-dot-com
> 
> In a message dated 97-03-15 06:57:00 EST, you write:
> 
> << 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??
> 
(snip)

How far you want to load a transformer or VARIAC depends on how much
life degradation due to hot spot heating you are willing to accept.

According to the National Electric Code (1993 but 1996 has not been
amended in transformer section), the primary can be fused up to 250%
of nameplate, provided the secondary is likewise fused at no more than
125% of rated secondary current ( Article 450-3(b)(2) ).  This applies
to two winding transformers under 600VAC. Above 600VAC, the fusing
can be as high as 300% (primary over 600V) and 125% (Secondary under
600V  [ Table 450-3(a)(1)].

With Autotransformers (read VARIAC's), if the autotransformer has a
rated input current less than 9A, fusing to 167% is allowed, if more
than 9A, overcurrent protection should be set for 125% or next available
trade breaker or fuse size (Article 450-4 (a) exception).

The reason I mention this information is the above information is based
on 24 hour a day operation, and is a conservative engineering design
(remember the saying, "When the going gets tough, the tough get going?")

If the transformer is being turned on for several minutes and allowed
to cool down (by ambient or forced air cooling), higher device rating
can be risked.  According to an old GenRad Variac reference, 24 hours
continuous rating is 100% (obviously), 1 hour is 120%, 1/2 hour is
133%, 15 min. is 150%, and 2 minutes is 225%.  Another manufacturer
indicated that loading up to 225% is permissible if the duty cycle is
no more than 15% (2 minutes on, 15 minutes off).

You should obviously monitor the Variac (and transformer's) temperature
while running, and run times should be kept short, until tolerance to
load level and rate of thermal rise is determined. I have also done
extensive testing of a pair of 60:1 120/7.2kV potential transformers,
and the units have linear characteristics until 250% (again).  Above
that loading, efficiency goes into the toilet, and i^2r internal heating
goes to the moon.

I generally avoid exceeding 225-240% nameplate loading on my equipment,
and monitor the thermal rise closely.

I'm sorry about earlier incomplete posting and hope this information
is helpful.  :^)

DAVE SHARPE, TCBOR

"THE DIFFICULT WE DO NOW, THE IMPOSSIBLE TAKES A LITTLE TIME!! "