Commutator – Definition, Construction, Working, Uses

A commutator is an essential component in electric motors, generators, and dynamometers. Its primary function is to reverse the direction of current flow in the windings of the machine. This ensures continuous rotation in one direction in motors and converts alternating current (AC) to direct current (DC) in generators.

As the armature rotates within the magnetic field, the commutator segments periodically change their connection to the brushes. This reverses the current flowing through the armature coils, maintaining continuous rotation in one direction in motors. In generators, the commutator converts the alternating current induced in the armature coils into direct current.

In this article, we will learn in detail about commutators, their function, types and limitations. We will also compare commutators with slip rings.

What is a Commutator?

A commutator is an electrical component found in electric motors, generators, and dynamometers. Its primary function is to reverse the direction of current flow in the windings of the machine. This maintains the desired direction of rotation or converts alternating current AC to DC.

In a DC motor or generator, the commutator consists of a series of copper segments attached to the ends of the armature coils. These segments are insulated from each other and arranged in a circular pattern around the shaft. Carbon brushes made of graphite press against the commutator segments and make electrical contact with them.

As the armature rotates within the magnetic field of the motor or generator, the commutator segments periodically change their connection to the brushes. This reverses the current flowing through the armature coils and the torque produced by the motor remains in the same direction. In a generator, the commutator serves a similar purpose, converting the alternating current induced in the armature coils into direct current.

Commutator Definition

A commutator is a rotary electrical switch in DC motors and DC generators that reverses the direction of current flow in the windings.

Commutator in DC Motor

In a DC motor, the commutator ensures the proper flow of electrical current through the motor windings, resulting in the generation of rotational motion. The operation of a DC motor is dependent on the interaction of the magnetic field of the spinning armature with that of the fixed stator. The attraction between the armature’s north pole and the stator’s south pole generates a force on the armature (and vice versa). The armature rotates as a result. The process of switching the field in the armature windings to give continuous torque in one direction is known as commutation.

Commutation in DC Machines

In DC machines, commutation is the process of reversing current. Both stationary brushes and the commutator are involved. Commutation is the process of changing the direction of a DC motor’s current before it is applied to the coils. Commutation in DC generators transforms the AC produced in the armature winding into DC.

The electrical switch that rotates and reverses the current flowing from the rotor to the external circuit is called a commutator. It is a split rotary ring , made of copper, with one segment attached to either end of the armature coil. Spring-loaded brushes that come into contact with the rotating commutator supply electricity to the armature coils and the commutator’s segments. For optimal transformation of current , there should be a constant moving contact between the brushes and commutator segments.

Commutator and Slip Rings

The difference between Commutator and Slip Rings are as follows:

Point of Difference	Commutator	Slip Rings
Define	It is a device that facilitates the motor’s connection to the power source.	A slip ring is an electromechanical device that uses electrical connections to transfer power and signals over a revolving surface.
Construction	Commutators have a cylinder-shaped configuration with several insulated copper segments connected to the rotor’s winding coils. The commutator segments selectively reverse the direction of current as the rotor rotates, resulting in a unidirectional current in the external circuit. This is accomplished by the stationary brushes staying in contact with the segments.	Usually, these are made up of one or more metal rings that are attached to the rotor and are in touch with the rings by stationary brushes. The electrical connection between the metal rings and the brushes does not break while the rotor rotates, enabling the continuous flow of power and electrical signals.
Functionality	Commutator changes the direction in relation to the electric field.	Slip rings is utilized in any electromechanical device that requires rotational motion to transmit a signal or power.
Use	DC motors usually use commutators.	Slip rings are used in generators to produce alternating current (AC).

Working Principle of Commutator

The principal objective of the commutation is to maintain constant torque applied to the armature in one direction. The commutator transforms the alternating voltage produced by the armature into direct current. The commutator regulates the direction of the electromagnetic fields, by turning the coils on and off. This is because electricity should always be “going away” from one side of the coil and moving in the other direction. This ensures that torque is always generated in the same direction.

Properties of a Commutator

Let’s talk about the properties of the various commutator parts, like segments, brushes, etc.

• The segments are secured around the rotor’s circumference, and the brushes are bolted to the machine’s stationary frame. The size of the apparatus and the requirements of the application usually dictate whether these segments are made of copper or another metal.
• Although industrial motors are intended to replace commutator segments, less expensive equipment typically takes their place.
• The number of coils in the armature, which is based on the machine’s voltage and speed, is connected to the commutator segments.

Applications of Commutator

Commutators are primarily used to convert alternating current into pulsating direct current in electric generators, electric motors, and direct current (DC) spinning machinery.

• In motors, commutators provide the rotor with power.
• Commutators produce a constant rotational force (torque) by flipping the direction of current in the motor’s armature’s moving coil.
• In generators, a unidirectional (direct) current is produced by reversing the coil’s connection to the external circuit.
• Commutator motors are also found in other lightweight motors and in a variety of electrical products, including electric drills and hoover cleaners.

Functions of the Commutator

Here are the main functions of the commutator:

• Reversing Current Direction: As the armature rotates within the magnetic field, the commutator segments periodically change their connection to the brushes. This reverses the current flowing through the armature coils and maintains continuous rotation in one direction. In generators, the commutator serves a similar purpose, converting AC induced in the armature coils into DC.
• Maintaining Electrical Contact: The commutator ensures continuous electrical contact between the armature coils and the external circuit. Carbon brushes press against the commutator segments and make electrical contact.
• Preventing Short Circuits: By providing individual segments for each armature coil, the commutator prevents short circuits that could occur if the ends of the coils were connected directly.

Construction of the Commutator

Here’s an overview of the construction of a commutator:

Commutator Segments : The commutator consists of a series of metal segments mounted on the shaft of the machine. These segments are typically made of copper or a copper alloy due to their good electrical conductivity. Each segment corresponds to one armature coil.

Insulating Material : Insulating material is used to separate the individual commutator segments from each other. This prevents short circuits between adjacent segments. Common insulating materials include mica, phenolic resins.

Mounting Mechanism: The commutator segments are mounted on the shaft by a clamping mechanism or being directly attached to the shaft. This ensures that the commutator remains firmly in place during operation.

Brushes: Carbon brushes press against the surface of the commutator segments and make electrical contact with them. These brushes are typically made of graphite due to its high conductivity and low friction properties. The brushes are mounted on stationary parts of the machine. These are spring-loaded to maintain constant contact with the commutator.

Operation of the Commutator

In this section we will try to understand the operation of the commutator through the operation of a DC generator.

Starting Position: The commutator segments are initially in contact with the carbon brushes, and the armature is stationary.

Rotation of Armature: When the armature rotates within the magnetic field of the permanent magnets, an emf is induced in the armature coils due to Faraday’s law of electromagnetic induction. This induced emf is sinusoidal in nature.

Commutation: The commutator converts this to unidirectional DC emf. Thus commutation is the process of converting AC to DC.

As the coil rotates, a sinusoidal emf is induced. When the polarity is about to change, segments A and B lose contact with the brushes for a moment. When they touch the brushes again, the direction of current flow is reversed. This ensures that the output voltage of the generator is always of the same polarity.

Output Voltage : This provides a steady output voltage from the generator, which can be used to power electrical loads.

Types of Commutators

Split Ring Commutators: Split ring commutators are commonly used in small DC motors, such as those found in household appliances, power tools, and small machinery.

• They consist of two separate conducting segments, usually made of copper, which are insulated from each other. These segments are mounted on the motor shaft.
• Carbon brushes press against the split ring commutator segments, providing electrical contact.
• As the motor rotates, the brushes make contact with different segments, effectively reversing the direction of current flow in the armature coils at the appropriate times, thus maintaining continuous rotation.

Segmented Commutators

Segmented commutators consist of multiple individual conducting segments, each corresponding to one armature coil.

• These segments are separated from each other by insulating material to prevent short circuits.
• Segmented commutators are commonly used in medium-sized DC machines, including industrial motors and generators.
• Carbon brushes press against the segmented commutator segments, providing electrical contact.
• Similar to split ring commutators, as the machine operates, the brushes make contact with different segments, facilitating commutation.

Hook-Type Commutators

Hook-type commutators consist of conducting segments connected by metal hooks or bridges.

• These bridges provide electrical continuity between adjacent segments while allowing for thermal expansion and contraction.
• Hook-type commutators are often used in larger DC machines, such as industrial motors and generators, where higher current and power levels are involved.
• Carbon brushes press against the hook-type commutator segments, providing electrical contact.
• As the machine operates, the brushes make contact with different segments, facilitating commutation.

Limitations of Commutator

Commutators are limited in a number of ways, such as:

• Deterioration: The sliding contact may cause the commutator and brushes to wear out. Sparking, dust, and decreased efficiency may result from this.
• Drop in voltage: A voltage drop brought on by the resistance between the brush and commutator may result in power loss.
• Inadequacy: Energy losses in the commutator and brushes might result in inefficiencies when the current direction is reversed.
• Speed: Only so fast can the commutator run before the brushes begin to bounce and break contact. This restricts the DC machines’ top speed.
• Voltage and current density: The greatest voltage and current density that commutators can switch has a limit.
• Machine dimensions: Enormous direct current devices.

Conclusion: Commutator

The commutator is a crucial component in electric motors, generators, and dynamometers. Its primary function is to ensure continuous rotation in motors and convert AC to DC in generators. It is constructed in insulated segments with carbon brushes making electrical contact. As the armature rotates within the magnetic field, the commutator periodically changes connections to the brushes, reversing current flow in the armature coils. This maintains rotation direction in motors and converts AC to DC in generators. Different types of commutators, like split ring, segmented, and hook-type, are used in various machine sizes.

Sample Questions on Commutator

Example 1: Describe the construction of a split ring commutator. What materials are typically used in its construction?

The split ring commutator consists of two separate conducting segments, often referred to as the “halves” or “rings.” These segments are mounted on the shaft of the motor and are typically circular or cylindrical in shape.

They are made of copper or a copper alloy due to their excellent electrical conductivity.

Compare and contrast segmented and hook-type commutators.

Segmented Commutators consist of multiple individual conducting segments, each corresponding to one armature coil. Hook-Type Commutators consist of conducting segments connected by metal hooks or bridges, providing electrical continuity between adjacent segments.

Segmented commutators are commonly used in medium-sized DC machines, including industrial motors and generators. Hook types are often used in larger DC machines, such as industrial motors and generators, where higher current and power levels are involved.

Segmented provides precise control over commutation and reduces the risk of sparking. Hook-type provides robust electrical contact and can handle high current loads.

Example 2: What are the advantages and disadvantages of segmented commutators?

Advantages:

• Precise control over commutation.
• Reduced risk of sparking.
• Suitable for medium-sized machines.

Disadvantages:

• More complex construction compared to split ring commutators.
• Require careful assembly to ensure proper insulation between segments.

Example 3: What are the advantages and disadvantages of hook-type commutators?

Advantages:

• Robust electrical contact.
• Suitable for large machines with high current and power requirements.
• Hooks allow for thermal expansion and contraction, ensuring reliability.

Disadvantages:

• Less precise control over commutation compared to segmented commutators.
• Higher risk of sparking under certain operating conditions.

Commutator FAQs

Is a commutator used in AC or DC?

Commutators are used in direct current (DC) machines, such as DC generators and DC motors.

Can a commutator convert AC to DC?

In DC generators, the commutator converts the induced AC in the armature windings into DC output

Can a commutator convert DC to AC?

No, a commutator cannot convert direct current (DC) to alternating current (AC). The primary function of a commutator in a DC motor or generator is to reverse the direction of current flow.

What is commutation in DC machines?

Commutation refers to the process by which AC generated in the armature winding of a DC machine is converted to DC. This is done by reversing the direction of current flow through a commutator.