How is brushed DC motor noise suppressed?
In a brushed DC motor the commutator arc produces both differential-mode and common-mode noise. For suppression an X capacitor is used between the terminals, a Y capacitor between each terminal and chassis, and a ferrite bead at the point closest to the motor. The ferrite is chosen so that it does not saturate at the operating current of the motor; using capacitors alone is inadequate in most cases.
The arc that forms continuously between the brush and the commutator segments is a broadband noise source. This noise spreads by two different paths and each one calls for its own measure.
- Differential mode: the noise circulates between the two power lines. The countermeasure is an X capacitor connected between the motor terminals.
- Common mode: the noise travels in the same direction on both lines and returns over the chassis. The countermeasure is a Y capacitor connected between each line and the motor body/chassis, plus a common-mode choke at the supply input.
The most effective arrangement is the LC filter formed by a series ferrite together with a parallel capacitor; a capacitor on its own, combined with the line inductance, often does not give the expected attenuation. Points to watch in practice:
- Mount the capacitors and the ferrite as close to the motor as possible, with short connections; long leads spoil the filter.
- Choose the ferrite according to the continuous operating current of the motor, not according to the starting or stall current.
- The motor body must have a good connection to the chassis; otherwise the Y capacitor has no return path.
- Separate the motor cable from sensor and communication cables, in practice leaving at least 15 cm between them.
The full topic: Installation, redundancy and field continuity