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I'm restoring a 1960s sewing machine (a Necchi Lydia 544, pure italian-made robustness). Inside of the foot pedal a capacitor is completely dead (gets hot and fume). I'm confident into changing it, but I am unclear which reference I should use.

The capacitor is rated 100'000pF (so 100uF) for 250V. The size is 28x17mm. No idea what '3'000 Vpr' means.

I believe this capacitor is a filtering cap. But I don't usually work with AC-stuff, so I prefer to ask for guidance. What is it I should look for? Any name to help me? Do I need a Motor capacitor? Should I also buy insulating sleeves for my cables?

See below pictures for more details.

Inside view of the pedal

References of the capacitor

What do you think?

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    \$\begingroup\$ 100,000 pF is 100 nF, not 100 μF. \$\endgroup\$
    – Hearth
    Commented Dec 2, 2022 at 4:24
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    \$\begingroup\$ replacing that capacitor will not fix a broken motor, all that capacitor does is prevent radio noise. \$\endgroup\$
    – Jasen Слава Україні
    Commented Dec 2, 2022 at 6:17
  • \$\begingroup\$ @Jasen The motor is fine, all the mechanical parts just needed a drop of oil and it went no problemo. But when I opened the pedal I indeed saw quite a bite of arcs when I pushed it slowly, so it matchs the expertise of the previous answers. \$\endgroup\$
    – apinto
    Commented Dec 2, 2022 at 18:39
  • \$\begingroup\$ That's good news. \$\endgroup\$
    – Jasen Слава Україні
    Commented Dec 2, 2022 at 19:43
  • \$\begingroup\$ i would guess that Vpr stands for "V proof" and is the voltage the capacitor was tested at (substantially higher than the working voltage). \$\endgroup\$
    – Peter Green
    Commented Dec 2, 2022 at 21:24

3 Answers 3

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Motors for household sewing machines are typically universal motors - they have brushes and run on DC as well as AC. A universal motor doesn't have a start or run capacitor like some AC motors

The capacitor is in the foot pedal, which makes it more likely it is for arc supression.

The value on the capacitor amounts to 100 nF. You'll need a capacitor rated for more than the AC line voltage - 300VAC is a common rating for such capacitors.

Since it goes across the line voltage, you'll need a capacitor that is rated to fail short so as to trip the fuse or circuit breaker. That's an X rating.

300VAC, 100nF, X1 or X2.

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  • \$\begingroup\$ Thank you very much. You nailed it, I found what I was looking for by searching '300VAC, 100nF, X1' :) \$\endgroup\$
    – apinto
    Commented Dec 1, 2022 at 18:48
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    \$\begingroup\$ From my investigations, X-rated capacitors are designed to fail SHORT and blow a fuse but Y-rated fail OPEN. recom-power.com/en/… vishay.com/docs/48251/… \$\endgroup\$
    – Magoo
    Commented Dec 3, 2022 at 16:50
  • \$\begingroup\$ @Magoo the capacitor is connected to the same outputs as the variator (not sure about the technical term). So you want a capacitor that fail short. Hence when it does fail, the machine will "think" the pedal is being pushed. Then you'll know smthg is wrong (the machine was indeed going when I wasn't even touching the pedal) If it fails open you will never know. And the 3kV of the arcs are going straight for the motor. \$\endgroup\$
    – apinto
    Commented Dec 3, 2022 at 20:49
  • \$\begingroup\$ @apinto: Yes, but JRE claims (second last paragraph) that an X-rating fails open. X-rating actually fails short; Y-rating fails open. \$\endgroup\$
    – Magoo
    Commented Dec 3, 2022 at 21:32
  • \$\begingroup\$ @Magoo: I've corrected the description. \$\endgroup\$
    – JRE
    Commented Dec 3, 2022 at 22:10
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100,000pF is 100nF, not 100µF. This is a film capacitor, likely used for transient suppression. You can replace it with another film capacitor of suitable voltage rating (I'd go for 300VAC or more). Any X-rated cap should work. It has to be X-rated because it'll be used across the line, which means that it should not fail catastrophically (and catch on fire) while also withstanding transients. X-caps are designed to fail safely without burning.

"3000Vpr" likely refers to the peak voltage rating of the capacitor, which suggests that it already is an X-class cap (which has failed).

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  • \$\begingroup\$ Thanks a lot for the reply! My bad for the 100nF. [edited, found it :)] \$\endgroup\$
    – apinto
    Commented Dec 1, 2022 at 18:27
  • \$\begingroup\$ Alright, glad you found it! To elaborate a bit: "X"-class capacitors are designed to not burst into flames when they fail (they fail safely). In your case, an X2 capacitor is sufficient. (X1 is more rugged than X2, which again is more rugged than X3). X2 is the most common type of X-class capacitor. They're cheap and widely available. Wire sleeves are needed if the wires themselves don't have insulation (or might get chewed up by sharp metal parts). \$\endgroup\$
    – Jonathan S.
    Commented Dec 1, 2022 at 18:28
  • \$\begingroup\$ Thank you very much for the explanations, very clear! Got it :) \$\endgroup\$
    – apinto
    Commented Dec 1, 2022 at 18:45
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    \$\begingroup\$ I suspect Vpr means Volt "picco-ripetitivo" (peak-repetitive). If it is used across the mains line it would make sense: it is rated to handle up to 3000V because of the HV spikes that could be present on the mains. This should be taken into account when choosing a suitable X-class capacitor replacement. \$\endgroup\$
    – LorenzoDonati4Ukraine-OnStrike
    Commented Dec 1, 2022 at 18:48
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    \$\begingroup\$ From my investigations, X-rated capacitors are designed to fail SHORT and blow a fuse recom-power.com/en/… hence I question your claim that X-rated... means that it should not fail catastrophically (and short out). It's designed to short-out - and X-caps cannot replace Y-caps, so "fail safely" depends on the application. \$\endgroup\$
    – Magoo
    Commented Dec 3, 2022 at 16:56
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The capacitor is rated 100'000pF (so 100uF) for 250V

No, 100,000 pF is 100 nF and not 100 μF.

I believe this capacitor is a filtering cap. But I don't usually work with AC-stuff, so I prefer to ask for guidance.

Looking at its position and trying to figure out how it's wired; it looks like it might be a switch-contact arc-suppression device. If you can establish that it is across the contact that activates the motor windings then a replacement should easily be found.

Alternatively, it might be a motor-start capacitor although 100 nF does appear to be too low to service this requirement.

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    \$\begingroup\$ Thanks! Indeed 100nF, my bad. \$\endgroup\$
    – apinto
    Commented Dec 1, 2022 at 18:12
  • \$\begingroup\$ Looking at the wire connection it looks like indeed a switch-contact arc-suppression device. The switch-contact makes indeed a lot of arcs. [edited, found it :)] \$\endgroup\$
    – apinto
    Commented Dec 1, 2022 at 18:29
  • \$\begingroup\$ @apinto I'd be looking to replace it with a polypropylene type. If it is across the switch then it need not be X or Y rated but, these are fairly easy to get hold of so it would be no bother using one. \$\endgroup\$
    – Andy aka
    Commented Dec 1, 2022 at 18:33

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