AC brushed motor, caps & inductors

[NeilP]

I have just started tinkering with old Singer sewing machines, and had a problem with the capacitor in a carbon stack speed controller
https://www.eevblog.com/forum/repair/carbon-pilestack-sewing-machine-foot-control-emi-suppression-capacitor/

I read the above and it got me thinking about the original AC motor.(1940’s machine)

It does produce a little interference / crackling on my radio in workshop..  I run an old Amateur Electronics circuit valve amp I built 40 years ago at school .. 


So I looked at original AC motor and as far as I can see, no caps or inductors for any sort of suppression.  Many of the sewing forums advice is ‘just remove them, they were for the days of analogue TV & AM radio’.   So possible these have just been removed.

But… my theory knowledge in electronics is totally zero.. 

So please can any one tell me how / what values I’d need to fit some suppression to this little carbon brush AC motor.  240VAC 75 Watt.

Might do the other pale colour motor too once black one done.


Thank you.

[amyk]

Check the mica between the commutator bars. With long-term wear they can start to protrude a little and cause additional arcing and interference.

[NeilP]

Thanks
Yes, I took the brushes out, a very simple procedure on these motors, and did check/clean scrape between them.  All good there. 

This machine is 1940, one owner and I am thinking very little used

I’d suspect these are the original brushes too

I now have found a few diagrams of inductors and caps on motors, so does look a simple job. 
Just to settle on the values.

[MarkT]

Many of the sewing forums advice is ‘just remove them, they were for the days of analogue TV & AM radio’.

Ignore that dumb advice - you might need to replace old caps that are failing, EMI suppression is more important today than ever, and excess arcing will destroy the motor over time as well as play havoc with EMI.

High frequency stuff coming off brushes usually only needs 1nF to 10nF Y2-class ceramic capacitor (*) to ground on each brush connection, but first ensure the motor body itself is securely earthed (+) and you have a 3-pin plug - essential anyway to prevent it being a death-trap.  Adding a ferrite to the motor lead next to the motor is another good mitigation measure for EMI.

(+) internal windings in motors can and do become detached, then shorting to the metal work of the motor, so all mains universal brushed motors needed to be properly earthed unless galvanically isolated from the rest of the machine.
(*) for instance: https://cpc.farnell.com/vishay/wko102mcpcj0kr/capacitor-class-y-1-0nf-400v/dp/CA05844

[NeilP]

Thank you for some rational advice!

Might need to add a separate earth wire as I think it is on ly two core.  but I do have some old looking fashioned style look cable that is three core

Thanks