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Large Variac Help


From: 	Bert Hickman[SMTP:bert.hickman-at-aquila-dot-com]
Reply To: 	bert.hickman-at-aquila-dot-com
Sent: 	Sunday, January 11, 1998 2:14 AM
To: 	Tesla List
Subject: 	Re: Large Variac Help

Tesla List wrote:
> 
> From:   Barton B. Anderson[SMTP:mopar-at-mn.uswest-dot-net]
> Sent:   Saturday, January 10, 1998 2:09 PM
> To:     Tesla List
> Subject:        Large Variac Help
> 
> I have a Variac I think is shorted. Here's the specs:
> GE, Input 120/240V, 50/60Hz, 1PH, Output 0 - 240/280V,
> 30 Amps.
> 
> There's 7 taps and #3 is the variable output tap. My
> problem:    I'm measuring winding dc resistance and
> the Fluke reading 0.6 ohms at pins 1 and 4 (full
> winding). Measuring all the other taps from tap #1
> common as well as all other combinations is below 0.6
> ohms. I don't have a lot of experience with variacs,
> but something tells me this isn't right. I first
> expected a wire short, but from all the other tap to
> tap measurements, there would have to be multiple
> shorts between each. This does not seem probable. It's
> got me a bit confused. The winding core is epoxied to
> a metal base, but there's no short between base to
> windings. There's no burnt smell and no visible burnt
> wiring. I assumed the amount of wire wound would have
> to read larger than 0.6 ohm dc resistance? Has anyone
> with a large variac measured dc resistance with the
> same results? I don't feel comfortable plugging this
> in yet. Do I have a reason for concern?
> 
> Any comments greatly appreciated,
> Bart

Bart,

Sounds about right! A 30 Amp variac is wound with fairly heavy gauge
wire, which will have a very low DC resistance. If you are concerned
that it might be shorted, connect inputs 1 and 4 in series with a couple
of hundred watt light bulbs. After the initial turn-on surge (due to
core magnetization), the lights should either go out or get very dim as
the inductive reactance of the winding at 60 Hz kicks in and limits the
current flow. Once you've confirmed that it isn't shorted, hook it up
without the series lightbulbs and begin using it for your coil!

Safe coilin' to you!

-- Bert --