When I was in college we had something called cooperative education or “coop
jobs”, basically a semester in industry as a paid intern. At my coop job you
typically arrived at 8am, grabbed coffee from the machine, and turned on the
power strip at your lab bench.
They never tired of sticking an electrolytic capacitor into one of the outlets
on your power strip so it would explode.
Almost as much fun as a banana potato.
From: AF <af-boun...@af.afmug.com> On Behalf Of Bill Prince
Sent: Wednesday, February 7, 2024 10:16 AM
To: af@af.afmug.com
Subject: Re: [AFMUG] OT fun trick running a variac in reverse
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
<mailto: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.
--
AF mailing list
AF@af.afmug.com <mailto:AF@af.afmug.com>
http://af.afmug.com/mailman/listinfo/af_af.afmug.com
--
AF mailing list
AF@af.afmug.com
http://af.afmug.com/mailman/listinfo/af_af.afmug.com
