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thingstodo · 06-14-2012, 01:51 AM

I set about doing something that I should have done when I got the big old VFD home - power up the DC bus and cycle the capacitors (slowly) a few times, from 0V up to maybe 100V, the 0V to 200V, 0V to 400V (that's about as high as I can get)

So I dragged out the VFD frame with the capacitors, made sure that the DC bus and it's capacitors were isolated from any loose metal parts, and connected the DC bus to the correct polarity on my smallest VFD. I used a 120Vac variac (sort of like a big volume control that can change its output voltage from 0V to 120V by turning a dial) to feed AC into the little VFD. Internally, the little VFD has a diode bridge to turn AC into DC with some ripple voltage. And there is a fairly large capacitor to smooth out the ripple to a reasonable value. This DC bus on the little VFD rose slowly in voltage. It gradually raised the voltage on the old and larger VFD bus and capacitors. I put a current-limiting resistor of about 680 ohms to limit the current from the the little VFD to the large VFD just to protect the small VFD from any short circuit conditions (if I blew up a capacitor).

The 100V charge went well. I kept it there for about 5 minutes. No special reason, just wanted to work it up slowly.

The 200V charge caused a small 'snap' like you'd get when an old-fashioned camera flash discharges. That sounded bad. Perhaps some corrosion on the contacts of a capacitor had it charge up quickly? Or maybe discharge the ones without corrosion into the one with corrosion?

The 400V charge did not go so well. At about 360V, there was a loud bang, then a few more snaps like the first one at 200V. I left things sit for a few minutes and there were 3 more snaps.

If each snap was a capacitor failing, then maybe half of them have failed. But no white powder was released ... at least not yet. I'm used to the smaller electrolytic capacitors failing with a ruptured side and a snowfall of white powdery stuff.

I guess I have to take the frame apart and see if the capacitors have shorted, opened, or are still OK. I'll need individual connections to each capacitor to check that, though. I'll likely have to clean the contacts on each capacitor to make sure that I can tell the difference between an open circuit and corroded terminals.

The capacitors are about 15 years old so they would not have been a reliable addition to SalvageS10, but I was hoping to do some testing with a few of them.

As usual, I guess we'll see.

06-14-2012, 01:51 AM	#119 (permalink)
thingstodo Master EcoModder Join Date: Sep 2010 Location: Saskatoon, canada Posts: 1,488 Ford Prefect - '18 Ford F150 XLT XTR Tess - '22 Tesla Y LR Thanks: 749 Thanked 565 Times in 447 Posts	Capacitors, aging, and sparks I set about doing something that I should have done when I got the big old VFD home - power up the DC bus and cycle the capacitors (slowly) a few times, from 0V up to maybe 100V, the 0V to 200V, 0V to 400V (that's about as high as I can get) So I dragged out the VFD frame with the capacitors, made sure that the DC bus and it's capacitors were isolated from any loose metal parts, and connected the DC bus to the correct polarity on my smallest VFD. I used a 120Vac variac (sort of like a big volume control that can change its output voltage from 0V to 120V by turning a dial) to feed AC into the little VFD. Internally, the little VFD has a diode bridge to turn AC into DC with some ripple voltage. And there is a fairly large capacitor to smooth out the ripple to a reasonable value. This DC bus on the little VFD rose slowly in voltage. It gradually raised the voltage on the old and larger VFD bus and capacitors. I put a current-limiting resistor of about 680 ohms to limit the current from the the little VFD to the large VFD just to protect the small VFD from any short circuit conditions (if I blew up a capacitor). The 100V charge went well. I kept it there for about 5 minutes. No special reason, just wanted to work it up slowly. The 200V charge caused a small 'snap' like you'd get when an old-fashioned camera flash discharges. That sounded bad. Perhaps some corrosion on the contacts of a capacitor had it charge up quickly? Or maybe discharge the ones without corrosion into the one with corrosion? The 400V charge did not go so well. At about 360V, there was a loud bang, then a few more snaps like the first one at 200V. I left things sit for a few minutes and there were 3 more snaps. If each snap was a capacitor failing, then maybe half of them have failed. But no white powder was released ... at least not yet. I'm used to the smaller electrolytic capacitors failing with a ruptured side and a snowfall of white powdery stuff. I guess I have to take the frame apart and see if the capacitors have shorted, opened, or are still OK. I'll need individual connections to each capacitor to check that, though. I'll likely have to clean the contacts on each capacitor to make sure that I can tell the difference between an open circuit and corroded terminals. The capacitors are about 15 years old so they would not have been a reliable addition to SalvageS10, but I was hoping to do some testing with a few of them. As usual, I guess we'll see.