DC Motor Noise? - Page 1
Forum section: electronics / beginners

author: JSanders
author_rank: Contributor
timestamp: February 21, 2025, 04:53:53 pm
content: Have a system of sensors, relays, servos and DC (brushed) motors all controlled through an Arduino & PC. Main problem is I’m a software guy playing with electricity. The DC motors power vacuum pumps and at times, not always, appear to be causing the servos to jitter.A bit confused thinking if there really is a noise problem why wouldn’t it be happening all the time? I’ve had things running for days without the servos going nuts, then all of a sudden it goes south. The DC motors are powered through there own transformer separate from the servos.Any suggestions on what’s going on here or how to isolate or filter any motor noise would be appreciated.Thanks.

author: benj38
author_rank: Frequent Contributor
timestamp: February 21, 2025, 07:02:22 pm
content: Brushed DC motors can spew a considerable amount of noise, both electrically and electromagnetically.To see if it is electromagnetic radiation or not, try to move the controller electronics far away from the motors and see if it helps.If it is electrical noise, injected to the circuit through the wires connected to the motors, this can be filtered. Try a 0.47uF, or larger, 100V ceramic or film capacitor (not electrolytic!) across these wires. More elaborate filters can be used, but this is an acceptable start.If you have an oscilloscope to monitor things as you experiment, it would make your life easier.

author: Grandchuck
author_rank: Super Contributor
timestamp: February 21, 2025, 07:04:13 pm
content: Any chance that there is an occasional RF emission from some gadget or such? I have experienced this.

author: JSanders
author_rank: Contributor
timestamp: February 21, 2025, 08:58:19 pm
content: Tried replacing wires between the motors & transformer with some shielded cable I had lying around, didn’t seem to work but don’t know how well I ended up shielded things. Tied the cable shield directly to ground and the problem still popped up. Did the same with the servo wires, but no luck. Have a bunch of caps, all electrolytic, always seems to work out that way. Where’s the best place for the caps, right up next to the motors or back at the transformer, guessing closer to the brushes the better?As for the outside RF idea, turned off the cell and all my computers / printers are hard wired but went ahead and unplugged the Wifi router anyway, no help. Can’t think of what else might be interfering.Been looking for an excuse to get an oscilloscope, would I be looking for noise on the servo signal line, power lines or both?The transformers used for the servos and DC motors are both plugged into the same power strip. Don’t know if that’s making things worse or not. Was thinking about ordering a battery to power the servos but that wouldn’t isolate the signal line.

author: benj38
author_rank: Frequent Contributor
timestamp: February 22, 2025, 09:49:47 pm
content: Quote from: JSanders on February 21, 2025, 08:58:19 pmThe problem is most probably noise injected electrically from the motors through their supply lines to the controller electronics. Forget about the "same power strip", and powering with a battery. These are not your suspects. The shielding experiment did nothing as expected.Placing the filter capacitor is indeed better at the motor side of the wires, but it should work at either end. If you probe with a scope I would start by looking for noise on the DC motors supply lines.

author: TimFox
author_rank: Super Contributor
timestamp: February 22, 2025, 11:35:32 pm
content: The noise from a brushed motor (especially without capacitor) can be especially strong if driven by a current source from a DC servo, since the voltage will spike when the brush circuit goes open between commutator segments.

author: Benta
author_rank: Super Contributor
timestamp: February 22, 2025, 11:50:29 pm
content: This is the method used by RC model hobbyists since decades to avoid noise (RF and otherwise) from permanent-magnet "canned" DC motors <300 W.It suppresses electrcal noise caused by brush arcing and commutation.It should be placed as close to the motor as possible, ideally directly on the motor.For small motors (<30 W) you can halve the capacitor values.

author: JSanders
author_rank: Contributor
timestamp: February 25, 2025, 06:40:38 pm
content: Thanks for all the help, ordered some caps, planing on trying the circuit in the above post.Have one of the vacuum pumps open and the brushes / commutator are throwing more sparks then expected. If sparks are coming from collapsing fields when the brushes are breaking contact with the commutator, would something like a snubber diode also help in reducing noise or would that mess with how the motors function?

author: Benta
author_rank: Super Contributor
timestamp: February 25, 2025, 08:11:08 pm
content: Don't go there.Snubbers are fine for protecting your transistors, but will not suppress brush arcing. Nothing will.The caps I showed are for suppressing the electrical noise from brush arcing.Brush arcing is a fact of life in small canned DC motors, live with it.

author: JSanders
author_rank: Contributor
timestamp: February 25, 2025, 09:15:34 pm
content: Got it, thanks Benta.Unrelated to the noise issue, when an armature winding is de-energized inducing back EMF, does that in turn create an opposite magnetic field? If so, is this what ‘regulates’ the speed of a DC motor for a given voltage?

author: Benta
author_rank: Super Contributor
timestamp: February 25, 2025, 09:54:33 pm
content: Quote from: JSanders on February 25, 2025, 09:15:34 pmArmature windings are not "de-energized", just switched aka commutated. Most small DC motors are delta-wound, so current is always flowing.Back EMF is generated from the movement of the armature in the magnetic field, not from inductance. The inductance is negligible compared to the mechanical effect.

author: janoc
author_rank: Super Contributor
timestamp: February 25, 2025, 11:13:20 pm
content: The capacitors themselves may not be sufficient. A common solution was to put a choke in series with each contact of the motor and a capacitor across it.I had this problem with one of the cheap yellow "TT" gearmotors I have used some years ago to build a model for a school. The moment the motor came on, the MCU lost Bluetooth connection because of the conducted EMI getting on the power supply rail. The 3 capacitors didn't do anything at all, no matter what values and which configuration I have used. I was watching the noise on the scope and there was huge amount of wideband interference going on.I wound two air-core inductors, about 10 turns of magnet wire each, and soldered them directly to each of the contacts of the motor. I have put a 10n capacitor across the motor pins. That has completely eliminated the problem - the power rail was pretty much clean, with only some minimal noise.

author: Benta
author_rank: Super Contributor
timestamp: February 25, 2025, 11:45:54 pm
content: Yeah, additional inductors can improve the situation even further, but are mostly overkill.I suspect your problem back then was more likely a supply voltage dip due to motor startup current. A DC motor is pretty much a short circuit when the armature is not moving.

author: JSanders
author_rank: Contributor
timestamp: February 26, 2025, 11:02:00 am
content: Would an inexpensive 10MHz Oscilloscope be able to help troubleshoot the motor noise issue? Not sure it would ever get much use beyond this one project.https://www.amazon.com/gp/product/B0DHL6LJ88

author: Refrigerator
author_rank: Super Contributor
timestamp: February 26, 2025, 01:36:43 pm
content: Quote from: Benta on February 22, 2025, 11:50:29 pmYou sometimes see this in cheap RC toys and other appliances that have brushed motors in them. Most of the time it's just a single ceramic across the brush terminals.

author: Refrigerator
author_rank: Super Contributor
timestamp: February 26, 2025, 01:47:06 pm
content: Quote from: JSanders on February 21, 2025, 04:53:53 pmIt really depends on how you are applying power to your motor. Without rotation a brushed motor is basically a veeery low resistance resistor.They suck enormous current at startup. Combine that with arcing and noise from the brushes and you will have a source of noise that will quite effectively mess with any electronics susceptible to EMI.If you're playing with servos on an arduino also check if the arduino itself is working correctly. From my experience, controlling servos with the servo library on an arduino nano can result in some jitter.If you can't make your circuit more immune to noise, then you can try to soft start your motor. If you're using a transistor to switch the power to the motor directly, then this could be as easy as adding an RC circuit to the gate/base of your transistor. Of course this only works if your transistor is big enough to handle the transient power dissipation.If your circuit instability occurs the entire time that the motor is running you might want to add filters on the motor as others have mentioned.Quite unusual that the motor would cause noise issues while on a separate power supply, perhaps this could be a grounding issue?Do you have a scope to measure noise?

author: JSanders
author_rank: Contributor
timestamp: February 26, 2025, 03:56:35 pm
content: Don’t believe it’s a ‘startup’ issue as the servos jump around the entire time the pumps are running. If I keep the vacuum pumps turned off, all is good ruling out any potential issues with the Arduino. The pumps are controlled through a relay triggered by a pressure sensor connected to a vacuum reservoir.Don’t have a scope but mentioned the possibility of picking up an inexpensive one above (post 13).https://www.amazon.com/gp/product/B0DHL6LJ88

author: Picuino
author_rank: Super Contributor
timestamp: February 26, 2025, 04:58:32 pm
content: Quote from: JSanders on February 21, 2025, 04:53:53 pmThe problem you mention may be due to noise. It may also be due to an overflowing variable. For example an integral action that does not have a software limitation.Check that there are no overflowing variables.

author: JSanders
author_rank: Contributor
timestamp: February 26, 2025, 05:55:59 pm
content: Again if the vacuum pumps (DC motors) are turned off there are no problems with the servos.

author: schmitt trigger
author_rank: Super Contributor
timestamp: February 26, 2025, 05:56:22 pm
content: Momentarily replace the suspect motors with a power resistor.I am very sure that the motor’s noise is indeed the issue, but as others have mentioned, it could be also a software issue. Prove or disprove the theory.Back to the motor: at least for me, solidly grounding the motor’s case has helped with radiated noise.EDIT: you haven’t mentioned the load’s voltage. If it is 12V a vehicle’s headlight (incandescent) will provide a nice inrush current and a safely dissipate over a couple dozen watts.

author: JSanders
author_rank: Contributor
timestamp: February 26, 2025, 06:14:38 pm
content: The vacuum motors are quite small, 3 volts DC, don’t know the actual current draw but guessing it’s minimal as they are used just to create negative pressure. They are not being used to evacuate a chamber or move an actuator. Will definitely try grounding the motor case - thanks.

author: janoc
author_rank: Super Contributor
timestamp: February 26, 2025, 08:07:21 pm
content: Quote from: Benta on February 25, 2025, 11:45:54 pmNo, it really was not. The MCU was not crashing/resetting, everything was running fine. But the Bluetooth was unusable unless the device was right next to the laptop when the motor was running.Sorry mate, I know what I see on the scope when I see almost a volt of wideband noise on the power rails.And if you think those inductors are "mostly an overkill" I suggest you actually try it with a cheap motor that has a cheap and nasty commutator - like most toy motors. Look at the conducted noise using a scope and you will see that the capacitors do practically nothing (they help to suppress radiated EMI but not conducted) against it. Add the chokes - and most of the crap is gone.

author: schmitt trigger
author_rank: Super Contributor
timestamp: February 26, 2025, 08:34:48 pm
content: Excellent advice from an old National Semi application data book with respect to RFITake note in particular, of the very last tip.

author: Benta
author_rank: Super Contributor
timestamp: February 26, 2025, 09:22:38 pm
content: Quote from: janoc on February 26, 2025, 08:07:21 pmModerate your tone, please. This is not "X".I've worked enough with 480/600/700/800 size motor in RF environments (35/40 MHz and 2.4 GHz) to know what I'm talking about.

author: shabaz
author_rank: Frequent Contributor
timestamp: February 26, 2025, 09:42:19 pm
content: I was playing around with a brushed DC motor a while back, and grabbed some 'scope traces at the time. I only measured to 50 MHz.Photo shows 100 nF and 10 nF capacitors, and a couple of ferrite cores (quite large ones because that was all I had around at the time).I no longer have the detail concerning the precise testbed setup. I didn't capture the scenario of capacitors+no ferrites, simply because I didn't consider a scenario where I wouldn't fit at least one; just seems wise, to add one anyway, for the little cost, but I don't often play around with brushed DC motors to know otherwise.

author: Benta
author_rank: Super Contributor
timestamp: February 26, 2025, 09:57:18 pm
content: Nice plots, thanks.The effect of the filtering is very obvious.

author: Paul Ebert
author_rank: Newbie
timestamp: February 27, 2025, 05:57:51 pm
content: I've got a similar problem, though my motor is quite small (somewhere between 3 and 10 watts). I'm also pretty much a software guy. What size inductors should I try. I'd rather not use ferrite beads because I have essential tremor so holding such things stable for soldering is a challenge (any soldering is a challenge). Is there some sort of nice small (non-SMT) component inductor I can buy from Digikey or something like that?I'm controlling my motor by using an arduino controlling a DAC feeding into a TI OPA548. The op amp has current limiting. Can I use that as a basic soft start? If not, could someone please suggest an appropriate simple circuit?

author: shabaz
author_rank: Frequent Contributor
timestamp: February 27, 2025, 06:59:16 pm
content: Hi,What's the precise issue you're experiencing?In general, the capacitor(s) are highly recommended by default.You might not need wound inductors per se (sometimes it can indeed be useful) you might just need ferrites, such as the clip-on variety, it would cause no harm, and could be effective.If not a toroidal ferrite (sometimes also called a bead, although it's larger), then you could pick any clip-on ferrite (several loops through it will be more effective).If issues are still experienced after this, then some further thought/experimentation will be needed.However, you may have an unrelated issue, so it's worth describing it, in case any other suggestion springs to mind. A circuit diagram and maybe a photo of the setup if possible, may also help.

author: Refrigerator
author_rank: Super Contributor
timestamp: February 28, 2025, 06:55:25 am
content: Quote from: Paul Ebert on February 27, 2025, 05:57:51 pmHigh current opamps are such an unusual concept to meBut yes, this will work just fine, you can even put an RC circuit in there, or interpolate in SW.This IC has thermal shutdown and current limit, so there's pretty much no way you can kill it. This also works as a protection for the motor in case it stalls or burns out, which would cause sustained high dissipation in the IC.Also wouldn't winding a couple loops of wire around a ferrite ring be easier than soldering? There are even clamp style ferrite rings that are even easier to use.I'm not familiar with tremors.

author: shabaz
author_rank: Frequent Contributor
timestamp: February 28, 2025, 01:31:33 pm
content: If you can share your use-case, that would be helpful. What are you using the motors for? It's unusual to use a DAC + Op Amp to control the motor, normally it's approximated by using PWM and a H-bridge instead (and possibly an inductor too), and it's usually cheaper/simpler/efficient to do this. This is not quite related to this thread though.

author: Paul Ebert
author_rank: Newbie
timestamp: March 02, 2025, 07:40:59 pm
content: Quote from: shabaz on February 27, 2025, 06:59:16 pmFirst, a bit about the application. This is for a DIY turntable motor controller. The provision of DC is as described. To control it, I've got a disk with alternating "pie slices" of white and black mounted to the underside of the platter. An optical sensor detects the edges as the platter spins. Software detects the time between edges and adjusts the DC voltage accordingly. This issue I was having was that the time between edges was quite variable. On the order of a few percent. Clearly not going to do for playing a record. I jumped into this thread because of this issue and the fact that I don't have any equipment (other than a cheap DVM) for hardware debugging. I, too, was struggling with the question if I should spend $400 or more on a scope that I might very well use once.Anyway, I've largely solved my problem (for now anyway). I was able to determine that the strobe disc I made wasn't very accurate. When I look at the same slice as the platter spins, the time is stable to a tenth of a millisecond or so. Not great, but maybe not a problem. The second realization was that I was only measuring with a microsecond resolution and this wasn't sufficient. When I switched it to microsecond (a simple SW fix), it was clear that the stability was an order of magnitude better than I could detect previously.Part of me still wants to get a scope. I could always sell it if I find I'm never going to use it again. On the other hand, the money would be better spent on a better cartridge.

author: Paul Ebert
author_rank: Newbie
timestamp: March 02, 2025, 07:51:10 pm
content: Quote from: Refrigerator on February 28, 2025, 06:55:25 amLOL (at myself). I was showing my ignorance here. I have a several decades recollection of ferrite beads being actual beads - about a millimeter or two - soldered directly to IC chip pins. I wasn't thinking about something that could be easily clipped around a pair of leads.Essential tremor looks like the tremor one often gets from Parkinson's, but doesn't have the other eventually fatal symptoms. It's an annoyance. Difficulty soldering is where I bump into it most impactfully.