Function of capacitor at DC motor terminals

[abdulbadii]

What actually is function of capacitor shorted across a DC motor terminal, how is its working mechanism on which condition it's really required ?

[wasedadoc]

Interference suppression.  DC motors with commutators produce sparks.  Google Marconi and his early spark transmitters.

[David Hess]

I second wasedadoc's statement; the capacitor is for suppression of the noise from the commutator.

[MikeK]

https://www.pololu.com/docs/0J15/9

[CatalinaWOW]

The capacitor helps a lot.  Remove it and put an AM radio near the motor.  Since the worst noise is at harmonics of the commutation frequency you may have to tune the radio or adjust motor speed to hear it easily.  Motor speed is best because then you are sure the motor is the culprit.  Re-install the capacitor and repeat.  My experience is many dB of improvement.