Reduction of DC motor noise

Question

I am designing a circuit with a DC motor 12V DC Reversible Gearhead Motors - 70RPM and some other stuff including a MCU and a LASER, all driven from a single 12V source and am concerned about big HF noise ripple from the motor (electrical rather than radiative but no harm in reducing both).

I haven't worked much with motors much before, however from reading the articles in this community and some searching elsewhere on the internet, it seems there are a few techniques for dealing with this noise and I was wondering if I could get some educated response on the validity and drawbacks of a few of the techniques that I have encountered.

1. Small capacitors (1 or 10nF) connected across the terminals in a variety of combinations including between Vcc/Gnd, two between Vcc/Gnd with the middle connected to the case exterior, and a combination of the above two. Non-polarised if the motor needs to run both ways.

2. Directly grounding the case of the motor.

3. Choke inductor in series with the Vcc of the motor.

4. Employing a more complex filtering topology close to the motor.

5. Twisting and shielding the cables of the motor and physically isolating them from the remainder of the circuit.

6. Keeping the ground of the motor separate from the ground of the remainder of the circuit and connecting it directly to the terminals of the power source if possible (or as close as possible if not) to avoid ground loop problems (star grounding?)

7. Enclosing the motor physically inside a metal case (and grounding that case).

8. Using large (1000uF+), low ESR electrolytic capacitors connected as close as possible to other sensitive equipment between their Vcc and Gnd (Anode to Vcc, Cathode to Gnd), or placing these large capacitors next to the power source itself on all of the lines leading out.

9. Running some of the other equipment through a linear regulator (Not sure if these are particularly good at rejecting HF noise)

10. Placing diodes next to the power source for different lines leading to different systems.

Looking for a generic answer regarding the effectiveness of the above techniques and perhaps more regarding protection from DC motor noise, not something specific to that motor as that project is actually over, now I am just curious and think it would be useful to have this information available in one spot for future projects and other interested people.

Answer 1

You should always put a capacitor across the motor terminals even if your circuit is not affected, because brush arcing creates rf noise that can interfere with other equipment (eg. AM radios). The usual recommendation is to install two 0.1uF ceramic capacitors, one connected from each motor terminal to the case. This 'grounds' the case to rf without the danger of having an exposed DC connection.

Ripple can be a problem for sensitive equipment which has poor power supply rejection, but normal filter capacitors and regulators will usually eliminate it. Another concern is the current spike and voltage dip that occurs when starting the motor. This motor has a stall current of only 390mA, so provided your 12V supply can handle that you shouldn't have to worry about it. Just make sure that the motor and its control circuit is wired directly to the power source, and run separate wires to the other devices.

Answer 2

Regarding your points to reduce the noise:

1. Small capacitors (1 or 10nF)

That's correct, except of the mention of capacitor polarity: anyways the capacitors must be ceramic, designed for working under high frequency, not electrolytic or paper even if motor will work only in one direction. Place these capacitors as close as possible to motor, and to the motor driver if you are using a PWM driver.

Using large (1000uF+), low ESR electrolytic capacitors connected as close as possible to other sensitive equipment between their Vcc and Gnd (Anode to Vcc, Cathode to Gnd), or placing these large capacitors next to the power source itself on all of the lines leading out..

Most likely using large capacitors will only be partially effective, primarily during start/stop/reverse of motor. Better noise protection is - to make separate power supplies for power circuit and for control part even if both require the same 12V. Your p.9 is exactly about this.

Twisting and shielding the cables of the motor and physically isolating them from the remainder of the circuit.

The major cause of noise transmitting by motor (through cables and by air) is the ignition of brushes. So if your motor is not brand new please check the brushes and connector conditions, and grind in the connector if needed.

Also plan your wire topology as a star(with power supply in center) with rays (parts of your scheme) and try to avoid the making of chain contained of consumers.

Star:

consumer2 <---wires---> PowerSupply <---wires--> consumer1

Chain:

PowerSupply <---wires--> consumer1 <---wires---> consumer2

Answer 3

Your motor is relatively low current, so unless you have a good model of your motor, the best approach is experimental.

Leave room on your board for choke inductor. Have small capacitor directly soldered on the motor terminal. Have sufficient decoupling on the power lines that supply the motor driver.

Then try with a few values for the capacitors and for the choke inductor and measure the noise on the supplies with a scope (or a spectrum analyser if you have one).

Answer 4

Several points that somehow were not yet mentioned.

1. The whole power path from your bridge to the motor must be carefully shielded. The shield, if possible, should be connected to motor's enclosure. On your board side the shield must be connected through capacitors to the ground, and somewhere in the system it should be connected to the ground directly.
2. Near the bridge you should have input capacitors to provide power during the switching. I mean small caps to react quickly and several large caps that will have enough energy to hold the voltage even during ling current peaks. Normally it's hundreds of microfarads.
3. Use common mode chokes of better ready made filters on your power line to virtually only allow DC current. All switching effects must be limited to the bridge and cable towards the motor.
4. Of course, good layout is very important. Always think about where the current goes, reduc magnetic flux, remember that in switching system it's not just the motor current that is switched, it's alse bridge transistors gates, boost capacitors, and sometimes other stuff.

Also ferrites around the cable are a good idea.

Good luck!

Answer 5

currently the Arduninos are very sensitive to this type of noise, the lcd are much more... The main solution for this type of situations, is a; snubber they are wide recomended to be used in conjunction with inductance loads as motors.

Some articles with subber desings and calculators

Answer 6

Same here, I found that cheap drones are really bad and it is normally one motor which is the culprit. The really strange thing is that its typically no weaker than the rest but adding ceramic capacitors helps a lot. I found a physically larger part seems more reliable, ended up using the ones from old flat screen LED backlight PCBs.

Also relevant, this helps with "mad drone disease" which is normally the RF interference problem.

Answer 7

I have had great success with a motor which at first failed EMC using 10nF between poles and 10nF from each pole to housing.

Instead we made a PCB which were to be mounted directly on to the solder tabs of the motor. 3 wires from the PCB "gnd" to motor housing. One X2Y capacitor directly between the poles.

EMC lab said they had never seen such improvement on a failed brushed motor.