Commutator
A cylindrical part seated on the rotor shaft, made up of copper segments insulated from one another. By making contact with the brushes it continuously reverses the direction of the winding current during rotation and thereby keeps the torque always in the same direction. It is the mechanical switching mechanism of the brushed DC motor.
As the rotor turns, each segment comes under the brush in turn; the current direction in the winding is reversed and the angle between the magnetic field and the rotor field is preserved. It is the mechanical counterpart of electronic switching. As the number of segments increases the torque ripple decreases and the rotation becomes smoother.
The condition of the commutator directly affects the current you measure on the driver side:
- Dirty or oxidised surface: Contact resistance rises, the current increases at the same load and current-sensing stop is triggered early.
- Carbon bridge between segments: The windings are partially shorted, the motor heats up and loses power.
- Commutator worn oval: The brush bounces, arcing and interference increase.
Maintenance is generally simple: de-energise the motor, clean the carbon dust, dress the surface with fine abrasive paper or a commutator stone and open up the slots between the segments. In systems with a current threshold such as AntiSwitch the threshold has to be recalibrated after maintenance, because the operating current drops.
The context in which this term is used: DC motor and driver fault diagnosis matrix