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Our sewing machine's foot controller is running continuously and I need to change the capacitor but I cannot find the same value capacitor i.e 0.05 μF and 250 volt AC.

I just want to know that can I replace it with 0.047 μF 250 VAC or 0.15 μF 275 VAC. Please advise me if I can use different capacitor value on this foot controller.

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I'd say the 0.047uF capacitor would be fine but do check it's X or Y rated for use on the AC power - just because it says it is 250VAC it doesn't mean it won't fail disgracefully and burn a hole in something (or worse).

I would also ask you to double check the value of the capacitor you are removing. I'd have guessed your sewing machine motor to be rated at maybe 100 watts and a cap I replaced recently on a 350 watt motor was 6uF. However, this was for an AC induction motor and something in the back of my mind makes me think that sewing machines use DC motors - if you can control the speed with a foot switch then it is a DC motor and the incoming AC is rectified - speed control could be via a thyristor/triac/SCR circuit OR via an extra field winding on the motor.

However, if it is an AC motor then the reason why the 0.15uF capacitor would probably be no good is that on many types of single phase induction motors there are two windings and, rotationally, they are spaced 90 degrees to each other. The winding fed from the cap has an inductance that together with the capacitor form a series tuned circuit at 50 Hz or 60 Hz or whatever AC frequency you operate at. A series resonant tuned circuit produces a voltage phase angle across the coil that lags the other coil by 90 degrees thus matching the mechanical position. This means that the rotor rotates one full rev in 20 ms (50 Hz) or runs at 3000 rpm.

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  • \$\begingroup\$ Thank you Andy for quick response.I will double check again tomarrow morning and follow your instructions. I only didn't understand X and Y rated which you have mentioned. And which one i can use \$\endgroup\$ – saj Apr 3 '14 at 19:48
  • \$\begingroup\$ Please wait for someone to confirm that this link has useful information on X and Y rated capacitors: www.justradios.com/safetytips.html. I found it informative. \$\endgroup\$ – Andrew Morton Apr 3 '14 at 20:12
  • \$\begingroup\$ @AndrewMorton - that's a really useful link dude. \$\endgroup\$ – Andy aka Apr 3 '14 at 20:57
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You can replace the capacitor with 0.047 uf 250V AC, class X(2) or Y(2) as Andy says, and that will be just fine.

However the capacitor is only for noise suppression, and this will have no effect on the controller running continuously.

Because the speed must be instantly controllable over a wide range, an AC induction motor would be useless on a sewing machine unless the foot controller was a varispeed inverter : I've never seen one on a sewing machine.

Sewing machine motors are generally "Universal" brushed motors running equally well on AC or DC.

The foot controller invariably turns out to be a combination of a switch (to turn it off at no pressure) and an extremely crude variable resistor, (on Singer machines I've seen from the 1920s to the 1980s; earlier ones didn't need electricity!) by varying the pressure on a pair of carbon (graphite?) rods; the whole lot adjustable for wear by various nuts and bolts. Details differ over the years (decades!) and may include another switch to bypass the resistor for flat out running.

My guess is that some inspection and thought applied to the disassembled foot controller will show what to adjust to allow the switch to open, and give a good range of control speeds.

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    \$\begingroup\$ If the capacitor blows it can short circuit the variable resistor and causing the machine to run continuously. I know this because I measured the resistance of the blown capacitor of my wife's sewing machine and it was 24ohm. \$\endgroup\$ – Stephen Paulger Oct 22 '15 at 9:45
  • \$\begingroup\$ That's true of the original capacitors and I've seen it happen too, which is why the recommended replacement is class X or Y rated, these are designed not to fail short. So that's a useful comment. However it's not the only cause of continuous running, wear in the footswitch requiring readjustment is possible and (IME) more likely. \$\endgroup\$ – Brian Drummond Oct 22 '15 at 11:27

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