A commutator is a rotary electrical switch that periodically reverses the direction of the current between the rotor and the external circuit.
Why is a Commutator Needed in Motors and Generators?
In motors and generators, a commutator serves a crucial role:
A motor’s windings receive electric current from a commutator. By reversing the direction of the current in the spinning windings every half turn, a constant rotational torque is generated. The commutator in a DC motor is typically made of copper.
In Generators: In a DC generator, the commutator converts the alternating current from the generator’s windings to unidirectional direct current in the external load circuit. The commutator inverts the direction of the current with each turn, acting as a mechanical rectifier.
What is Commutation?
In DC machines, commutation refers to the process of reversing current. For DC generators, the commutator converts induced AC in conductors into a DC output. In DC motors, commutation is used to switch the direction of the current before applying it to the motor’s coils.
Working of Commutator
A commutator is a split rotary ring, often made of copper, with one segment attached to each end of the armature coil. Key points about its working include:
Commutator segments and armature coils must have the same number. Spring-loaded brushes make contact with the commutator when it rotates, providing voltage to the commutator segments and armature coils. Switching the direction of the current in the revolving windings every half turn generates a constant rotating force or torque. In a generator, the commutator reverses the direction of the current picked up from the windings, acting as a mechanical rectifier to convert AC to unidirectional DC in the circuit.
Types of Commutation
To avoid defects, there are three types of commutation methods:
- Resistance Commutation
- EMF Commutation
- Compensating Winding
Ideal commutation is achieved when the current reversal is completed during the commutation time. Sparking at the brush contact and overheating, which can damage the commutator surface, occur if the current reversal is not finished within the commutation time.