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 <[email protected]> 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 > [email protected] > http://af.afmug.com/mailman/listinfo/af_af.afmug.com >
-- AF mailing list [email protected] http://af.afmug.com/mailman/listinfo/af_af.afmug.com
