Carbon pile/stack sewing machine foot control EMI suppression capacitor

[door king]

Hello all, 
I'm rebuilding the carbon pile assembly in a speed control pedal for an old Singer sewing machine.  I'm just thoroughly cleaning everything and swapping out the carbon discs.  The speed control mechanism also has a capacitor connected in parallel across the carbon pile rheostat.  The conventional wisdom states that the capacitor is there for EMI suppression to prevent interference with TVs and radios of the day.  When the capacitor fails, it can short which bypasses the rheostat and causes the machine to run whenever its plugged in regardless of pedal position.  For that reason, and because EM interference is apparently not an issue with modern-day appliances, the capacitor is usually removed and discarded whenever a pedal is opened up for any kind of service. 

I'm wondering if the capacitor also serves a second role perhaps protecting other components from arcing during a sudden release/engagement of the pedal, similar to the way a 'condenser' protects the points from arc damage in an old I.C.E. ignition system.  The capacitor in my machine has already been removed and I'm wondering if I should put one back in.  The carbon discs are hard to come by so I want to avoid frying them. 

The pedal is in series between the outlet and the motor.  The electrical guts of the pedal consist of the carbon pile rheostat, the capacitor, and a make/break switch all connected in parallel.  When the pedal is fully depressed the switch closes, bypassing the rheostat and delivering full power to the motor, effectively connecting it straight to the electrical outlet.  So the question is, when you jam on the pedal or release it suddenly, is there a potential for arcing at the switch contacts, or among the carbon discs due to the inductance of the motor? 

And... if you theory-minded members determine that the capacitor does serve a protective role, any idea what value I would need?  The original capacitor was long gone before I ever took possession of the machine.  I haven't been able to find much info online other than a few pictures of disassembled pedals, but the pedals pictured appeared to be UK market and I'm on the USA grid.  The values didn't seem to be consistent from one example to the next either. 

Thanks in advance,
DoorK

[amyk]

It could be to suppress EMI from arcing of the rheostat wiper.

[TheMG]

Carbon piles are pretty though, a bit of arcing from back EMF from the motor ain't gonna hurt it. The motor likely also has (or should have) its own suppression cap, for EMI/RFI reasons.

The very nature of how a carbon pile works can generate some small arcs, this the need for a capacitor to help suppress EMI/RFI.

I would put one in there. If any of your neighbors are ham radio operators or just like to listen to AM broadcast, they will much appreciate it.

A modern X rated suppression film capacitor will do, and those fail open not short, but they are also much longer lasting than the paper caps that were common many decades ago.

[floobydust]

Capacitor across the carbon-pile is to prevent it from arcing due to the motor's inductance and back-EMF. I could see the carbon lighting up and making high voltage.
Capacitor across the motor plus inductors is to lower EMI from the brushes arcing, so you can sew and listen to the radio. I see that in the SCR/TRIAC foot pedals.

I would use a vanilla X-capacitor 0.1uF at least 275VAC. Kemet has some with longer leads like R46 $0.51

But see pics Singer later used two capacitors, 0.1uF and 0.0047uF
pic taken from https://fixitworkshop.co.uk/2018/11/11/silent-singer-sewing-machine-pedal/

DON'T buy the Rifa PME271's (still for sale?!)  they fail bad, OMG they are in sewing machines too.

[door king]

Thanks everyone.  Life happened and the project got benched for a while, but the carbon pile assembly has now been rebuilt.  I individually sanded both sides of almost 120 little carbon discs and wiped them down with isopropyl.  Tedious. 

Now to move on to ordering a capacitor, but first a few more questions...

Capacitor across the motor plus inductors is to lower EMI from the brushes arcing, so you can sew and listen to the radio. I see that in the SCR/TRIAC foot pedals.

I looked into it and it seems my model (Singer "Touch & Sew" 603) doesn't have any capacitor(s) at the motor.  Not saying it wouldn't benefit from it, but I'm probably going to let that ride for the moment, unless someone contends there's an urgent need to have one in there.  (brush/commutator life?)

I would use a vanilla X-capacitor 0.1uF at least 275VAC.

How did you select that value?  Was there reason beyond the fact that that's what was in the pedal in the picture you shared?  It's a UK 240V pedal.  Will 0.1uF still be appropriate for a USA 120V pedal?  I'm not versed in this EMI suppression business.  Any explanation or links to a crash course would be helpful. 

Also, any recommendations on what filters to use to narrow down my DigiKey results?  "EMI supression" is not an option  and I'm not sure what I should be looking for.  I very well may buy the exact one you linked to, but I'd like to get an idea of where it resides in the broader landscape of options. 

But see pics Singer later used two capacitors, 0.1uF and 0.0047uF

I stared at that picture and other examples of the same style pedal online for some time.  There is a pair of contacts that are normally open which keep the controller open-circuit when the pedal is released.  The first event as the pedal is depressed is the closing of the contacts which completes the circuit through the carbon pile.  The 4.7 nF capacitor is wired across these contacts.  My pedal is a different style and it has no analog to the 4.7 nF cap.  In my pedal, the carbon pile is actually split into 2 banks and the circuit is closed/broken between the two banks.  I'd have to connect the cap to a moving plunger on one end and directly to graphite on the other, so that's a no-go.  I wonder if the design was changed to accommodate the second capacitor for further arc suppression. 

DON'T buy the Rifa PME271's (still for sale?!)  they fail bad, OMG they are in sewing machines too.

So, the results for a "singer sewing machine capacitor" search on ebay are almost entirely RIFA PME271 caps.    I'm really glad I decided to post my questions about this here on EEVB before I bought anything!  Thanks for the warning.  Is there a thread about these caps' reputation?  I did a search, but there were hundreds of results. 

[amyk]

Yes, the hundreds of results should be enough to convince you not to use those caps... here's a few notable ones:
https://www.eevblog.com/forum/chat/old-rifa-capacitors-and-a-disaster-story/
https://www.eevblog.com/forum/testgear/check-your-rifa-caps-in-your-agilent-bench-power-supplies-and-lessons-learned/
https://www.eevblog.com/forum/repair/rifa-madness/

Dave even did a video about it: https://www.eevblog.com/forum/blog/eevblog-1183-rifa-madness-(schaffner-repair)/