capacitordc motor
I have found an article that suggests using one, two or three capacitors to filter DC motor noise.
Why is it better to add capacitors between motor terminals and a casing as opposed to putting all three of them across the terminals?
Does the metal case somehow contain the EMF when connected to the terminals?
I'm fishing around a bit and this graph is what I found. It tells you that the sensitivity of the device optically can be significantly changed in the presence of quite small variations of noise on the power line feeding it and this could be magnetically coupled or just a high-frequency loading effect the motor has on the battery which in turn gets dumped onto the sensor via its power wires: -
The graph tells me that as the "ripple" voltage on the power supply wires gets higher in frequency, the "effect" it can have becomes more dominant. Shown are ripple disturbances of 1kHz and 10kHz and I can envisage a scenario of 10 mV at 100 kHz having an effect. Even higher in frequency, I would say 1MHz at 1mV is likely to cause problems.
On this basis I'd consider having a local voltage regulator on the device just so the device power supply is kept as clean as possible. I don't know how long the cables are feeding the sensors but maybe if they could be shortened this might help too.
The capacitors you may have tried should not be electrolytic but rather they should be ceramic 100nF types. I'm aware that my answer may be missing the real problem but I'm just reporting worries on reading the data sheet.
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.
Best Answer
As for 2 and 3, you may be able to filter out very high frequency emissions this way. Just don't waste large capacitors here since you need them across + to -. You will probably be in the 100pF to nF range for 2 and 3, if any and depending on how the motor is grounded.