How to measure current draw of a brushed DC motor?

[BlackICE]

I have a brushed DC motor that is claimed to be 350W connected to a 600w 13.5V SMPS. I tried to measure the current draw using a oscilloscope and Hantek CC650 DC clamp probe. That show a lot of high frequency noise. So I tried measuring the voltage drop over a 0.00171 shunt that I had. That also proved to have a lot of noise. The noise readings mask out any useful measurements. I used another handheld DC clamp meter that maxs out at 20A. That one did read some values before displaying overload.

I found info like this on the web saying to add 10-100 nf caps across the brushes and both brushes to GND.  Will this really work? What are the best values to use for the caps? If it won't likely work I don't want to tear the machine apart to try.

https://forum.arduino.cc/t/measuring-noisy-dc-motor-currents/638872/6

[Benta]

Those caps are pretty much mandatory for small brushed DC motors.
They remove RF noise stemming from commutation and brush fire. The risk is high that radio communication can be disturbed ifyou don't mount them.
I normally use 100 nF directly across the poles and 2 x 22 nF from each pole to the motor case.

[beanflying]

Depends on the actual motor you are using but in days gone by we used to use a bunch of Mabuchi 540 sized motors for earlier R/C electric aircraft. At 6 or 7 cells (circa 7-8V fully loaded) and 20-25 amps the sparking wasn't to bad but push that to 12 and it became horrid. Typically we ran three Caps, the two lower ones to the can and the third between the terminals to reduce it. We did also run more upmarket larger Can brushed motors to around 30V and 50-60A but they were a very different beast in price and build quality, still used a similar capacitor filter on the backend as the smaller ones.

Are you running a filter on your CRO input? If not a lot of the high frequency hash will also be coming from your supply in your readings.