This article explains the commutator in a DC generator, its construction, its purpose, and how it works. It also covers the difference between slip rings and commutators and answers common questions about them.
The commutator in a DC generator is an important part that delivers unidirectional current to the load. It converts the alternating current (AC) produced by the machine into direct current (DC).
What is a Commutator?
A commutator is a ring-shaped component made of copper segments, with insulation between each segment. It’s attached to the armature in a DC generator. Its main purpose is to convert the alternating current (AC) produced in the armature into direct current (DC) for the output.
In simple words, the commutator acts like a switch. It changes the connection of the armature winding to the circuit whenever the current direction reverses. This ensures the current always flows in one direction in the external circuit. Thus, the commutator acts as a mechanical rectifier.
Definition of Commutator: A commutator is a rotating electrical switch used in DC generators and motors to convert alternating current (AC) induced in the armature windings into direct current (DC) for external circuits.
Construction of a Commutator
The commutator is cylindrical and is mounted on the DC generator shaft. It has copper segments separated by mica or similar insulating materials. The insulating materials prevent segments from touching each other and from the shaft. Thus, the insulation between segments electrically separates the commutator from the armature shaft and prevents electrical short circuits
The fixed carbon or graphite brushes rest against the moving commutator segments. The brushes transfer the generated current from the rotating commutator to the external circuit.
The commutator changes the direction of current flow within the winding as the shaft rotates. After the shaft completes half a turn, the connections ensure that the current flows through the winding in the opposite direction compared to the initial flow.
Working of Commutator
When the armature of the DC generator rotates in the magnetic field, an alternating EMF is induced in the windings due to Faraday’s Law of Electromagnetic Induction. This induced EMF periodically changes its direction when the armature conductors move from one pole of the magnetic field to the opposite pole.
The commutator segments are connected to the armature winding in such a way that they reverse the connections to the external circuit every half-cycle. This switching ensures that the current flowing through the external load always flows in the same direction.
Carbon Brushes rest on the rotating commutator. These brushes maintain electrical contact while allowing the commutator to rotate freely. The commutator reverses the connection to the brushes when the current direction in the armature winding reverses. Thus, the commutator converts the alternating current into a direct current.
Let us understand the workings of the commutator with the help of the diagram.
Imagine a commutator designed as a cylindrical metal ring split into two segments, labeled as C1 and C2, with a thin mica layer separating them. This setup ensures electrical insulation between the segments. The commutator is mounted on the rotor shaft and electrically isolated from it.
In this arrangement, the ends of the armature coil sides, labeled AB and CD, are attached to segments C1 and C2, respectively. Two stationary carbon brushes rest against the commutator’s surface. These brushes facilitate the transfer of current from the armature to an external load.
This design ensures that the commutator consistently connects the armature coil side positioned under the south magnetic pole (S-pole) to the positive brush and the side under the north magnetic pole (N-pole) to the negative brush. This dynamic connection is important for maintaining a unidirectional current flow to the external circuit.
Case 1
In this setup, the coil sides AB and CD are positioned under the N-pole and S-pole of the magnetic field, respectively (refer to the figure below for clarity). The coil side AB is connected to segment C1 of the commutator, which in turn is linked to point x of the external load R. Similarly, the coil side CD is attached to segment C2, which is connected to point y of the load R. As a result, the current flows through the load in the direction from y to x.
Case 2
After the loop completes a 180° rotation, the coil side AB comes under the influence of the S-pole, while the coil side CD moves under the influence of the N-pole (refer to the figure for better understanding). This reversal causes the current in the coil sides to flow in opposite directions. However, since the commutator segments C1 and C2 also rotate by 180°, segment C1 now makes contact with the positive brush, and segment C2 connects to the negative brush.
This arrangement ensures the commutator reverses the coil’s connections to the external load. As a result, the coil side AB is now linked to terminal y, while the coil side CD connects to terminal x. Despite the reversal within the coil, the direction of current through the load remains consistent, flowing from y to x. This mechanism enables the generator to deliver a steady, unidirectional current to the external circuit.
As a result, the alternating voltage induced in the armature winding is converted into a direct voltage at the brushes and ensures a consistent DC output across the load terminals.
Slip Rings and Commutators
Both slip rings and commutators are used to transfer current between rotating and stationary parts of electrical machines, but they differ in design and function:
| Aspect | Commutator | Slip Rings |
|---|---|---|
| Function | Converts AC to DC | Transfers AC without conversion |
| Structure | Cylindrical with segmented copper pieces | Continuous, smooth rings of conducting material |
| Application | Used in DC generators and motors | Used in AC machines like alternators and motors |
| Current Flow | Produces unidirectional current | Allows alternating current |
In summary, slip rings maintain the AC nature of current, while commutators rectify it.
Frequently Asked Questions – FAQs
1. What is the purpose of a commutator in a DC generator?
The commutator ensures the unidirectional flow of current by rectifying the alternating current induced in the armature windings.
2. How does a commutator differ from a slip ring?
A commutator rectifies AC to DC, while slip rings allow the transfer of AC without conversion.
3. What materials are used for commutator brushes?
Brushes are typically made of carbon or graphite, as these materials are durable and provide good electrical conductivity.
4. Why is commutation important in DC machines?
Commutation maintains unidirectional current flow in the external circuit and ensures smooth operation of the machine.
5. What causes sparking in the commutator, and how is it prevented?
Sparking can occur due to poor commutation, brush misalignment, or worn-out segments. Interpoles and compensating windings can minimize sparking.
Conclusion:
The commutator is an important part of the DC generator which converts the alternating current into direct current. It is important to understand its construction, purpose, and operation for the efficient operation of DC generators.