Thanks, I forwarded this note to someone there.
From: Jan-GAMs Sent: Saturday, August 13, 2022 8:57 PM To: [email protected] Subject: Re: [AFMUG] OT A Schematic question RF vii in New Jersey had some experience with fixing inductive welders. rfvii.com Mr. Steve Barber, probably retired by now. From what I've read so far, I think you're on the right track. jan On 8/13/22 19:38, Chuck McCown via AF wrote: I bought a large induction furnace off of Ebay. Said it was 11 kW. I have other units that are 7 kW and this one heats much less and slower than my other units. I measured the input power and it is less than 4 kW. Manufacturer has dropped off the face of the earth. No docs, so I drew the schematic. I then tested the diodes and SCRs. All test well. The off sheet connector in the middle of the drawing is where the heating inductor is connected. So just draw an inductor across that gap. We made a coil with more windings and got a bit more power. Tested the huge oil filled big assed cap with an LCR meter 33 uF. Calculated the resonant frequency of our heating coil and that cap and it is 20 kHz. So that series resonant section appeara to be resonating around 20 kHz but the machine is a 10 kHz machine. Did that cap go down in capacitance? I doubted the cheap LCR meter so I did an RC time constant test and came out with 38 uF. To resonate with the heating coil alone I would need about 85uF. All things point to that cap losing capacitance. It does have some external oil stains on it. So perhaps I will discovered the issue when the new cap arrives. Have not tested the voltage across it but the ebay unit is 1500 volts. Looks like new old stock. It may have issues too. My question is the following: I cannot see where those diodes will ever be forward biased. I know they appear to be freewheel or kickback diodes in the drawing. But I keep simulating the operation in my head and I cannot see the kick back. I also don’t know what the black coil does. I went out and double checked the connections this afternoon. This is how it is connected. These SCRs and diodes are large ceramic hocky puck devices that are all compressed together in a stack. So if anyone here has switch mode power supply experience or simply are better than I am at understanding the circuit I would love to hear it. I am sure the diodes are necessary, but I sure cannot see it. -------------------------------------------------------------------------------- -- AF mailing list [email protected] http://af.afmug.com/mailman/listinfo/af_af.afmug.com
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