Need Help Quieting EMI Brush Noise with Universal Motor - Page 1
Forum section: electronics / projects

author: SuzyC
author_rank: Frequent Contributor
timestamp: July 29, 2018, 09:29:45 am
content: I have made my own MCU design for controlling the speed of a washing machine universal motor using AC phase control with a triac, but the brush noise EMI is so high at the motor's highest speeds that at times it will reboot my MCU.This EMI is surely due to the brush arcing because the brushes are sllightly bouncing off the armature at high speeds.I managed to overheat the motor a few times testing my MCU controller with with heavy loads and slow startup speeds, and after a while the motor was making clicking noises that came from the brushes on the armature and was loud even at slow speeds.I removed the motor and just with my finger I could feel that that one of the armature contacts was slighly raised up and causing the clicking sound, and so I made all the contacts level again with all others by filing and sanding.Now the mechanically generated brush noise is even and although it increases with speed, the motor seems to be otherwise working well.In this type of washing machine universal motor, the brushes connect the rotating armature windings in series with the field windings.I obviously need to insert some sort of EMI filter.On my scope, especialy when the motor is running at high speeds, I can see on my scope many short bursts of approx 100nSec sinewaves that seem to be 2 to 5V or more in amplitude with my scope probe connected anywhere on my motor controller board, even though the controller board is located several feet away from the universal motor.These bursts can be seen anywhere near the MCU board with the scope, and the bursts seem to be even riding the 5V MCU VDD supply or any other MCU pin.I can even see the same noise bursts having several volts of amplitudes on my scope by just connecting the grounding wire to the tip of the oscilloscope probe and holding it near my controller board.I have some 2-in diam. EMI donut ferrite cores salvaged from other equipment that I hope I could use to make an EMI filter.I was thinking of creating a common-mode noise suppressor using these cores using two identical windings, but I am unsure of how to connect such a circuit to the brush circuit. I don't know if I could use some 250VAC polypropylene (.1uf to .47uf) capacitors I have salavaged form the mains EMI input filters of some old desktop PC power supplies as well to help make a circuit to quench the brush noise.It is easy to alter the brush circuit because the brushes are connected using push-on connectors on the outside of the motor.Any ideas?

author: timpattinson
author_rank: Contributor
timestamp: July 30, 2018, 12:35:40 am
content: Have you grounded the motor enclosure? (a good low impedance ground)Sent from my SM-G930F using Tapatalk

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 30, 2018, 01:43:41 am
content: Put small capacitors (Y type ceramic?) from motor terminals to its housing. It may be desirable to use some resistance in series with the caps (~50 ohms?). If more filtering is necessary, add a small choke in series with each terminal (about 1uH, after the caps), then more caps to ground.If you know specifically that the noise is diff mode, you can use caps from wire to wire, instead of to ground. Likely some is common mode, so will need caps to ground.If the motor is powered by a switching circuit, consider running conduit from the motor housing up to the circuit. Ground the conduit to the motor housing and to whatever circuit ground is (or if circuit common is live, use Y caps to achieve an RF ground). There's probably not much tolerance for capacitance on the motor terminals in this case, so better to just keep the noise bottled up tightly. I'm guessing this isn't so much the case, because the switching circuit itself would probably upset your MCU first. Or it's a nice filtered switching circuit, in which case capacitance at the motor will be fine.Tim

author: SuzyC
author_rank: Frequent Contributor
timestamp: July 30, 2018, 02:50:04 am
content: Thanks for your help!I've tried to hook up an EMI choke as shown in the attached file schematic,but I think I have one side of the bifiliar wound toroid choke wired backwards?The result so far is that the EMI is now not strong enough at the max speed of the motor to reboot the MCU.I wound 10-turns #14 AWG on a 2-in ferrrite core, bifiliar wound. See attached schematic.Perhaps, instead I should have made two non-magnetically coupled 10-turns #14 AWG chokes instead? Oris it that I just should reverse one of the bifiliar winding connections?The motor body is grounded to the washing machine metal frame with a #14 AWG wire.