Preventing mains spikes from cheap water pump

Question

I bought a pack of add-ons for my DIY CNC that has a water cooled spindle. One of the items was the water pump. But when I switch on the pump, a fairly substantial crack can be heard in any speakers that are turned on on the same circuit, as well as a temporary dip of brightness in any switches on lights.

Something is obviously no bueno.

So without investing in a much more expensive pump (I'm not even sure I'll use the water cooling), could I add a large enough shunt capacitor to this pump to attempt to alleviate this?

I must admit, I'm not entirely sure if the purpose of shunt capacitors. I only know I've soldered a bunch into electronics projects seemingly for this kind of purpose.

The pump is connected directly to 240VAC mains power.

First question: is this an AC or DC pump and where is it getting its power from? — vir, Dec 19 '22 at 21:50
Please click the [Edit](https://electronics.stackexchange.com/posts/646982/edit) link below your question to answer winny and vir. — rdtsc, Dec 19 '22 at 22:06
A big capacitor will smooth real nice... if it's DC power. You probably don't want to smooth AC. — Harper - Reinstate Monica, Dec 20 '22 at 08:29

score 2 · Accepted Answer · edited Dec 21 '22 at 12:53

Filtering for AC power lines normally comprises of X-type shunt caps, MOVs, TVS diodes and common mode choke inductors. The shunt caps allow higher frequency voltage differentials to pass back to the source of the noise, rather than the mains source.

(Image source: CUI Inc - EMI Filter Components: what they are and how they work)

My advice would be to search for a commercial EMI filtering unit to plug into the power socket, because doing anything with mains voltage carries significant risk to life from high voltage and current, to potentially burning your house down.

I googled some dedicated EMI filter boxes for this sort of purpose, and they start from about 10 spacebucks. Maybe there are also some sort of soft start modules for large electric motors to inhibit the lights dimming, but I don't really know anything about them.

score 2 · Answer 2 · answered Dec 21 '22 at 13:01

A capacitor alone will not help with a 240VAC pump. Sparking and the resulting EMI is to be expected when an inductive load such as a motor or similar is switched with a mechanical contact. An RC snubber (a smallish capacitor with a resistor in series, typically something like 100nF + 100Ω) across the pump motor terminals might help a bit with the speaker noise, but it won't and can't help with the lights dimming. It is very important that both the capacitor and the resistor be properly rated for this application.

Typically small (you can hold them easily in one hand) submersible pumps are used for this application (and similar applications such as cooling 40-60W CO2 laser tubes) and they don't draw enough startup current surge to affect lighting in a properly wired mains circuit. It's pretty hard to make solid pronouncements without a lot more information on the pump size and type, but this does not appear to be a problem that is on-topic here, perhaps more in the realm of the DIY stack exchange.

I would not suggest running a CNC spindle without cooling if it is designed for such.

Preventing mains spikes from cheap water pump

2 Answers2