I always used a potato.
bp
<part15sbs{at}gmail{dot}com>
On 2/7/2024 7:48 AM, Ken Hohhof wrote:
Variac is just an autotransformer with a variable tap. Not surprising
you can swap input and output. Watch out for voltage ratings though.
And wrong gender plugs.
I thought it was potato in the tailpipe.
---- Original Message ----
From: "Cameron Crum"
Sent: 2/7/2024 9:15:03 AM
To: "AnimalFarm Microwave Users Group"
Subject: Re: [AFMUG] OT fun trick running a variac in reverse
Ah the old 'variac in reverse' trick, similar to a banana in the
tailpipe.
On Tue, Feb 6, 2024 at 6:05?PM Chuck McCown via AF <af@af.afmug.com>
wrote:
Was testing a repair to a 480 volt induction heater today. One of
our employees decided to blow the dirt out of it, took the cover
off and got a copper tube across an inductor to case ground. It
was probably 800 VDC at that spot. Discharged the capacitor.
Sounded like a gunshot. Tripped a 125 amp 480 volt breaker at our
power service panel. Turning it off at the front switch just
turns off the control circuitry. Everything else is hot unless
you kill the breaker on the back of the unit. I think the kid is
still shaking.
In any event, took the power supply to the lab. Used a variac to
put 0 to 130 volts across each leg with a clamp on volt meter on
it as I tested. Never got past 10 volts and was drawing 3-5
amps. 3 phase bridge rectifier was totally shorted out. Exactly
as expected. These things take raw 480 VAC, rectifier, 800 VDC
cap and then on to the IGBT transistors that chop it into ac etc.
I was hoping it was just the rectifier.
So we got the replacement today. Put it in and started testing.
No current, all the way up to 130 volts. But the cap was
charging. So far looks good. Told my sons to take it back and
hook it up to 480. My son Frank said “just reverse your variac
and use it to step up”. I initially refused to believe it would
work. Then I thought through it a bit and decided that it
actually should work.
I started with the variac set at 130 volts output. Feeding 120
into the output gave us about 110 on the input (that was connected
across one phase of the induction unit). As I turned the variac
down the voltage went up. I got to 380 volts before we started
smelling that wonderful “Allen Bradley” wafting through the lab
and the variac started buzzing pretty bad. I think I got it down
to about 60 volts. But we got it high enough out (in?) that the
control transformer made enough juice to power the control
circuitry. It appears that the machine is fixed. Of course until
we actually try to use it we will not know for certain.
But the TL;DR is: You can run a variac backwards and make higher
voltages.
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