Capacitors across motor

Hi
Can anyone give me some information regarding putting a capacitor or diode across the motor terminals to stop any EMF interfearence value and type would be good and orientation . Possible stock number
Thanks
Les

Capacitors across motor terminals are common with brushed motors to cut down on brush arcing and radio frequency emissions since arcs are broad spectrum noise sources.

For noise they are generally small ceramic caps and are harmless to the motor control circuits.

Electrolytics are used on various motor controllers to give a place for inductive kickback to go because voltages can scream up when motor voltage is turned on and off rapidly (off is where you get the inductive kick) with a PWM control signal. Without either a diode or cap, voltages can pop the electronic switches. I think some also can dump switching currents back into the power supply lines but am not certain. But with big inductive loads and switching the motors on and off rapidly, those kicks need somewhere to go or else they find somewhere.

With H bridges, where you put caps and diodes really matters since the voltage gets reversed. Diodes will short things out if placed on the motor.

For more information look at H bridge motor control circuits like the Monster Moto or Pololu JRK. They include proper snubbing of kickbacks.

Without specifics of what you are referring to, voltage, current, specific motor controller, etc, nobody can give you part numbers or specs, though.

if you put a capacitor across the motor make sure it is not polarised

the diodes look like this on a h-bridge -- https://i.stack.imgur.com/4iGc7.png

which h-bridge are you using ?

I use the Pololu JRK 12v12 with my rig. It has built-in snubbing but I also have high current chokes on the motor wires to help attenuate the kickback since I’m running the Pololu’s beyond spec. Some have found the snubbing on Monster Moto and Pololu close but not quite good enough when run near or beyond limits.

Details are here: https://www.xsimulator.net/communit...-20-strut-jrks-wind-flying-driving-rig.11281/

The chokes are in the red round housings in the photos. Some links to JRK things. The things that limit the JRKs is the cooling on the small board and the amount of snubbing you get with the caps they used. The ST chip itself easily goes to 30A IIRC. I used big heat sinks and silicon thermal pads to handle the heat, and the chokes help with the snubbing. With that cooling the JRKs only warm a few degrees measured on the ST chip surface with an IR thermometer. I measured voltage across the chokes with a scope to see how much voltage spikes were getting attenuated and it was only around 10volts but that’s an additional 10V not slamming the H bridge.

And I know polarized caps well. One tantalum cap used in a high voltage circuit under a shield, inserted backwards at the factory, cost me a whole year of stress and frustration. It worked for a while as I was bringing up that part of the project but failed somewhere after that. I assumed that part was good since it has passed all tests when I was checking it out but after it failed that circuit was applying random high voltages to parts I couldn’t test to diagnose everything else that seemed to not work. It was only when I was going back over everything and leaving nothing to chance that I took the shield off that circuit and saw a shiny silver bead of tantalum hanging onto the side of the cap and the cap backwards from the markings on the board. Replaced it with the right polarity and everything just started working. The company that made that part had done a field fix for everyone who bought that component (ion trap mass spectrometer) but since we hadn’t bought ours they neglected to even tell us. Wasted a year of my time and nearly ended my career before it started. I was a little profane on the phone with the company rep once I found and fixed the problem.

wow that capacitor caused a lot of grief ! what industry do you work for ?

I have just been testing some motor drivers - one was from ST a vnhd7012ay - it worked pretty good but i am not getting good support from ST

in the end i opted for a 100A dual chanel driver - so far i am very impressed, the board seems to be very robust with lots of protection built in

That was back in school. Retired now. ;-)