Commutators

What is a Commutator?

A commutator is an electrical device used in electric motors and electrical generators to periodically reverse the current direction. The commutator reverses the direction of current by working between the rotor and the external circuit. All direct current machines like DC motors, DC generators, and universal motors use a commutator for their working. The commutator generally converts the alternating current generated in the armature to the direct current.

Construction of a Commutator 

A commutator is attached to the armature in the form of a ring segment. A commutator comprises a set of copper segments that are mounted around the rotor’s circumference and a set of spring-loaded brushes that are fixed to the machine’s stationary frame. Two or more fixed brushes are connected to an external circuit, which can be either a current source for a motor or a load for a generator. Commutator segments are connected to the armature’s coils, with the number of coils (and commutator segments) varying depending on the machine’s speed and voltage. Hundreds of segments can be found in large motors. The commutator’s conducting segments are all isolated from one another. Mica was used in early machines and is still used on huge machines. Other insulating materials are employed as well. In early machines, brushes constructed of copper wire strands were employed to make contact with the commutator surface. These hard metal brushes scratched and grooved the smooth commutator segments, necessitating commutator resurfacing. As the copper brushes wore out, dust and brush fragments could become lodged between commutator segments, shortening and lowering the device’s effectiveness. Gauze brushes were more expensive than strip or wire copper brushes because fine copper wire mesh or gauze offered better surface contact with less segment wear.

The commutating plane is where a brush makes contact with the commutator. The brush contact area is not a tiny line but a rectangular patch across the segments to conduct enough current to or from the commutator. The brush generally spans 2.5 commutator segments. When it comes into touch with two adjacent segments, it connects them electrically.

Working Principle of a Commutator

The commutator is a current reversing device used in all direct current machines. The rotating shafts of the direct current machines consist of two contact bars in a commutator. These contact bars are connected to the armature windings. The commutator connects the rotating armature of the motor to the static external circuit of the motor.

In a DC motor or generator, the armature is placed between the magnets, and the armature has a set of commutators pressed by the brush.

When a magnetic field is passed from the north pole to the south pole, and the current is going inward in the armature, the force will be applied upward. The direction of the current is now reversed, so the magnetic field is acting in the same direction, and the current flows in an outward direction. Thus, the force acts in a downward direction, creating torque, which rotates the motor’s armature.  When the current induced in the armature current is alternating, the AC is converted to a DC by using the brushes pressed on the commutators. When the current flows in a positive cycle,  the brushes are attached to the commutator to have a positive amplitude. When the current changes direction, the brushes are detached from the commutator and do not transmit any current. Again, when the current comes in a positive amplitude, the brushes are attached to the commutator and transmit the current. In this manner, only a DC with all positive amplitudes emerges from the circuit.

Conclusion

All direct current machines like DC motors, DC generators, and universal motors use a commutator for their working. The commutator generally converts the alternating current generated in the armature to the direct current.