Spikes from Hifi speakers when switching lathe off

We've noticed that we sometimes get an annoying 'pop' from the hifi speakers when the lathe is switched off and less frequently when switching on.

I was thinking that wiring a capacitor across the motor switch might cure this... as well as protecting the switch contacts.

The snag is that I've not been able to fnid much specific advice about this such as...

• whether it will work
• what capacitor would suit a 1/4 HP single phase motor
• whether it will be OK with an NVR switch which switches both live and neutral and where to wire it.

I've seen plenty of them used on appliances with brushed universal motors but am not 100% sure whether there are any different considerations for a (more powerful) squirrel cage motor.

To be clear... we're talking about switch spike suppression here, not start or run capacitors.

You are switching a somewhat inductive load and inductive spikes are quite likely when switching off. May get switch on ones too especially if the switch is bouncing. Anyway it can't do any harm to connect a mains-rated suppression cap of say 47 nF across the switch contacts, but make sure it is mains rated. Should be available from RS or Parnell/CPC.

You are switching a somewhat inductive load and inductive spikes are quite likely when switching off. May get switch on ones too especially if the switch is bouncing. Anyway it can't do any harm to connect a mains-rated suppression cap of say 47 nF across the switch contacts, but make sure it is mains rated. Should be available from RS or Parnell/CPC.

Adding just a capacitor across the switch brings new problems. Ideally you should use a series resistor/capacitor network. This is sometimes called a snubber. An example is this:

https://uk.farnell.com/kemet/pmr209mc6100m100r30/cap-0-1-f-20-paper-radial/dp/9717412?

Using any capacitor or snubber across a switch safely assumes there is another means of isolating it from the supply, for example the lathe has a 13A plug. Any capacitor must be at least a X rated type.

Robert G8RPI.

Mine does the same to the LCD TV on the wall in my workshop. Has done for many years. My soldering heat gun does it too. Never did any harm.

Thanks for the answers so far. I've found some info on X and Y rated capacitors... So X is line to line (i.e. live to neutral) and is split into X1, X2 and X3 with different spike voltage ratings.

I'm unclear what is meant by 'across the switch' because the NVR has 4 line terminals. Do we mean between the live and neutral output terminals? ... or between the live input and output terminals? I'm guessing the former but I'd rather not guess.

Then there is the question of voltage and capacitance required. Is there a way to figure out what is needed for a 1/4 HP motor? Or maybe copy the ratings chosen by a reputable manufacturer.

As it is a problem at switch off, it is likely contact arcing. Cure for this is a snubber (X rated as it does not connect to ground) across the contacts. This is between Live in and live out. If the NVR is double pole with a contact in the neutral you will need a second snubber between neutral in and out. Note that having two neutra terminals does not automatically mean it is double pole. IT might just be the connection for the holding coil and a passthrough connection.
The other options are:
A X rated capacitor between live and neutral at the supply side of the NVV.
A block filter (Xand Y capacitors and inductor) on the supply side of the NVR.

Robert G8RPI

Andy, If you have little or no experience of mains voltage work I would just put up with the click or noise on the speaker ! You could do more harm than good. Noel

Thanks Noel. I have plenty of experience of mains wiring but I know enough to know that I don't know the answer to this particular question.

... and thanks Robert too. The NVR is indeed double pole.

https://uk.rs-online.com/web/p/push-button-switches/3985279/

That's a nice NVR, btter than than the cheap far eastern ones.

You would need two RC snubbers, one between 13 and 14 and the other between 23 and 24.

The snubbers work by limiting the rate of voltage change across the contacts as they open. Fast rates of change = lots of interference.

Putting a capacitor across the supply does not stp the interference being generated, but stops it going down the mains cable. If it goes down the cable it can cause two forms of interference. The cable can act as an antenna ad radiate the interferebce or it can be conducted down the mains and into the stereo via it's mains input.

If you arre anywhere near Cambridge drop me a PM I may be able to help more.

Robert G8RPI.

Thanks Robert. I'm about 90 minutes from Cambridge so hardly local.

TBH I thought when I asked the question that there would be a commonly used and simple answer that I just wasn't finding... after all I've seen plenty of capacitors wired next to switches when I've dismantled old power tools, hoovers and so on in the past and even transplanted one to replace a fried one in some hedge trimmers.

Alas it seems that there isn't a well known answer (like... for 1/4 HP use this capacitor and that resistor) for lathe motors.

I will be seeing a friend of mine who does electrical and electronic design for a living later in the week so I will pick his brains on the subject. In the meantime the popping speakers will continue.

Run the lathe via a mains filter salvaged from a scrapped washing machine, etc?

Some hi-fi gear 'back in the day' was very susceptible to radio-frequency interference, leading to a lot of articles in the mags about how to reduce such susceptibility, usually by ensuring good screening, and severe filtering of inputs to block everything outside the audio frequencies. I think things are better these days.

Thanks. It did occur to me that washing machines and tumble dryers are more likely to have the sort of motor that we use for lathes and therefore might offer some useful opportunities for copying or reclamation.

I've never looked into this aspect of any of our previous large appliances. Once pronounced dead, they generally have to be taken away immediately to make room for a new one so I've never had chance to look for potentially salvageable bits.