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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