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Why do rc cars have capacitors on the motor? Answered

Being a hacker or at least a take-it-apart-er, I have seen what looks like one or more small capacitors (I think) soldered directly onto the motors of radio or remote control vehicles. Are these capacitors? I understand the reason for capacitors on large AC motors. Do they serve the same purpose on small DC motors?


They are there to reduce the electrical 'noise' given off due to the DC motor's commutators constantly making and breaking the circuit as they spin. The switching creates small electrical arcs which generate wideband RF.  Without them, the motors can create interference to cordless phones, TVs etc.

And yes, they are capacitors.
A useful little tip : If there's room, put in a larger capacitor across the motor - as large as will fit, but rated at least 25 volts.  This will make the car run more evenly because it supplies power to the motor if the brushes momentarily lose contact with the track due to dirt.

Have to stop quickly banging out an answer just before I log off ! - I read it as a Scalextric car question.  
But yes, they are capacitors, they do reduce the RF noise from the sparking and the reason they are there is different from why they're fitted to AC motors.

@AndyGadget: No matter the size of the capacitor if your brushes are losing contact with the Comutator you will still lose power. The Caps still transmit any power via the brushes so any dirt would still cause problems. A polarized Cap would be great for cars with only Forward ESC's and really big does not really mean better performance. They are there to bridge the gap if there would be any intermittent power coming form the ESC so unless you have a really crappy ESC that misses all the time you would not need anything to large..

The caps are really there to prevent any Radio frequency noise produced by the arcing to interfere with your radio gear.

just new here.i burn the capacitor on my motor and i cant read the code or the number in it. whats the best capacitor to buy?

 who knew that rc cars put out emissions

Agreeing with all previous comments, these things are not just confined to RC cars. Any brushed motor can generate RF, interfere with TV, radio, mobile phone, there are regulations governing these things.
Spark gap transmitters


And most directly, interference with the RC car's remote controls!

No, they are usually RF suppression filters. They could be just 100nF capacitors, or they could be networks of 100nF and 100 Ohm resistors.


+1. They are often applied to reduce radio frequency emissions. one of the less onerous prices that supplying the rest of the world with jobs making 10-15 years at a shot worth of dollar store crap has given us. Cheap junk produces copious amounts of noise.

They don't do the same job as on an AC motors.