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/diy/ - Do It Yourself

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798188 No.798188 [Reply] [Original]

Hey guys, stupid question here. I have an old AC motor for a table saw and the starting cap is toast. A replacement is almost $50 though because its xboxhueg. I was wondering if I could salvage the caps out of an old receiver or something and build a bank of them that could perform the same function as a single large capacitor?

If so, what would I need to put together to make it work? The cap is 600μF and 125v AC. Can I just wire up a bunch of 125v caps in parallel until they add up to 600μF or is their more too it than that?

>> No.798202

The caps in your pic are polarised electroytic. They will explode if you try to use them for AC. Large capacitors salvaged from things like TVs and stereos are always going to be polarized. An AC cap will say "AC" on it, show a ~, or at the very least not have polarity markings.

>> No.798205

>>798188
Physicist here. Ctotal = C1 + C2 + C3 (etc) when capacitors are wired in parallel. Do Not Wire In Series Using This Method. So in example, if you have this 600microF capacitor that's broken, you can replace it (in parallel, remind you) with a 200microF, 300microF, and 100microF, IN PARALLEL.

>> No.798208

>>798188
You need a non-polarized capacitor. You can parallel multiple 125V non-polarized capacitors until you get to the required capacitance (600uf), but I think this would be cheaper (seriously? $50.00?)
http://www.intertexelectronics.com/MSC125V124-Capacitor-Motor-Start-110125V-124-149MF-P1943.aspx

>> No.798210

>>798202
Yeah, thats just a google image result for 'capacitor bank'.

>>798205
Okay. So match the voltage, make sure its AC, wire in parallel, and nothing else?

>> No.798211

>>798210
Not the guy you posted to, but yes. You can parallel.

>> No.798213

>>798210
Yep, wire in parallel, make sure the capacitors are supposed to be used for AC, and that should be it. Easy peasy.

>> No.798222

Okay, cool. I got the motor free because it wouldn't start. Guy that gave it away said it hums and starts if you turn the shaft a few times but had no clue how to fix it. Opened it up and the cap had popped its top, looks like years, maybe decades ago (it came out of a cabinet saw that went into storage for unknown reasons at an unknown date prior to 1995). No problem, caps are easy to replace. Well, the largest cap I ever had to buy was for a 3/4th HP motor and it was $15 shipped. Its $48 plus shipping for a 2.5 HP motor cap though. Sticker shock.

Heres hoping I can find 125VAC caps in some old receivers or TVs or something.

>> No.798309

>>798202
What can you salvage AC caps from? Aside from other large motors?

Op, personally I'd find a pump repair shop, someone that replaces this size motor. Beg them to pull a salvage cap from a motor removed from service. Beg poverty. Have fun.

>> No.798320

>>798188
weird theory 4a :
surely you could wire diodes in series with some DC caps . .. .
But then the DC caps would still need a Woltage [as my Indian lecturer K. das Gupta used-to say] rating that at least equals the PEAK Woltage [diggin' it now] of your AC : So 240V >> 339V peak . . ..
AND : the Diodes would also need that 339 as a 'reverse-bias' rating . . . . .
Upshot : just buy a suitable AC Cap.

>> No.798322

>>798309
You might find some in the AC side of CRTs and computer PSUs.

If you're feeling ghetto, you can pair DC caps back-to-back, though this halves the capacitance.

>> No.798397

by sticking two polarized caps back to back, you get a non-polarized cap at half the capacity. then you can parallel the couples to get higher capacity. no, it wont blow up.

https://electronics.stackexchange.com/questions/21928/can-you-make-a-non-polar-electrolytic-capacitor-out-of-two-regular-electrolytic

another possible solution is to start the motor manually since starting is all the capacitor is needed for.

>> No.798415

>798397

>another possible solution is to start the motor manually since starting is all the capacitor is needed for.

Oh great, now I got the thought of someone salvaging an entire lawnmower pull-start assembly and rigging it to a table saw stuck in my head...

>> No.798527

I see Taiwanese 600uf motor starting caps for $15-$30 online, so I'm not sure it's really worth jerry rigging a replacement.