How Does a Brushed DC Motor Work?

After understanding the construction of a brushed DC motor, the next step is to learn how it converts electrical energy into continuous rotational motion.

The operating principle of a brushed DC motor is based on a fundamental law of electromagnetism: a current-carrying conductor placed in a magnetic field experiences a force. By arranging conductors into coils and continuously reversing the direction of current at the correct moment, this force is transformed into continuous rotary motion.

Unlike a Brushless DC (BLDC) motor, which relies on electronic controllers for commutation, a brushed DC motor performs this switching mechanically using carbon brushes and a commutator. This simple yet effective mechanism has made brushed DC motors one of the most widely used motor types for over a century.

This chapter explains the complete operating process of a brushed DC motor, from power input to continuous shaft rotation.

(Illustration: Overview diagram showing the energy conversion process from DC electrical power to mechanical rotation in a brushed DC motor.)

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The Basic Operating Principle

Every brushed DC motor operates by converting electrical energy into mechanical motion through the interaction between two magnetic fields.

These magnetic fields are produced by:

• The stator, which provides a stationary magnetic field.
• The armature (rotor), which generates a magnetic field when current flows through its windings.

When these magnetic fields interact, they produce a force that causes the rotor to rotate.

This force is commonly referred to as electromagnetic torque.

(Illustration: Simplified diagram showing the magnetic interaction between the stator field and the energized armature.)

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Step 1 — DC Power Is Applied

The process begins when a DC voltage is applied to the motor terminals.

Electrical current flows through the following path:

1. Power supply
2. Carbon brushes
3. Commutator
4. Armature windings

As current flows through the armature windings, they become electromagnets.

The direction of the magnetic field depends on the direction of the current flowing through the windings.

(Illustration: Current flow from the DC power source through the brushes, commutator, and armature windings.)

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Step 2 — The Armature Generates a Magnetic Field

Once energized, the armature windings generate their own magnetic field.

The stator already provides a permanent magnetic field through either:

• Permanent magnets (PMDC motors), or
• Field windings (industrial DC motors).

The interaction between these two magnetic fields creates attractive and repulsive magnetic forces.

These forces generate rotational torque.

(Illustration: Magnetic field lines showing the interaction between the stator magnets and the energized armature.)

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Step 3 — Electromagnetic Torque Rotates the Rotor

The magnetic forces acting on the armature cause the rotor to begin rotating.

Initially, the rotor moves toward a position where the opposite magnetic poles are aligned.

If nothing else occurred, the rotor would stop once magnetic equilibrium was reached.

Continuous rotation therefore requires the magnetic polarity of the armature to be reversed repeatedly as the rotor turns.

This is the purpose of the commutator.

(Illustration: Rotor beginning to rotate as magnetic attraction and repulsion generate torque.)

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Step 4 — The Commutator Reverses the Current

As the rotor rotates, the commutator rotates with it.

The stationary carbon brushes remain in contact with different commutator segments.

This automatically reverses the direction of current flowing through the armature windings at precisely the correct position.

As a result:

• The magnetic polarity of the armature reverses.
• The attractive and repulsive forces continue acting in the same rotational direction.
• Continuous torque is maintained.

This automatic switching process is known as mechanical commutation.

(Illustration: Sequence showing the brushes contacting different commutator segments and reversing the armature current.)

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Step 5 — Continuous Rotation

Because the commutator repeatedly reverses the current, the rotor never reaches a stable magnetic position.

Instead, it continues rotating as long as electrical power is supplied.

The operating cycle repeats continuously:

1. Current flows into the armature.
2. The armature generates a magnetic field.
3. Magnetic interaction produces torque.
4. The rotor rotates.
5. The commutator reverses the current.
6. Torque continues in the same rotational direction.

This continuous cycle converts electrical energy into smooth mechanical rotation.

(Illustration: Circular flow diagram showing the continuous operating cycle of a brushed DC motor.)

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Why Are Brushes Necessary?

The brushes provide the electrical connection between the stationary power supply and the rotating armature.

Without brushes:

• Electrical current could not reach the rotating windings.
• The armature would not generate a magnetic field.
• The motor would not produce torque.

The brushes therefore serve two essential functions:

• Deliver electrical current to the rotating armature.
• Maintain continuous electrical contact during rotation.

Because they remain in physical contact with the rotating commutator, brushes gradually wear and require periodic replacement.

(Illustration: Close-up view of spring-loaded carbon brushes maintaining contact with the rotating commutator.)

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Why Does the Commutator Have Multiple Segments?

A commutator is divided into many insulated copper segments rather than being a single continuous ring.

This segmented design allows different armature coils to be energized at different times.

Benefits include:

• Continuous rotation
• Smoother torque output
• Improved efficiency
• Reduced torque ripple

In general, motors with more armature coils and commutator segments operate more smoothly.

(Illustration: Enlarged view of a segmented commutator with multiple copper bars insulated from one another.)

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Mechanical Commutation vs. Electronic Commutation

The most important difference between brushed and brushless DC motors lies in how current is switched.

Mechanical commutation makes brushed DC motors simple and inexpensive, but it also introduces friction, brush wear, electrical arcing, and maintenance requirements.

Electronic commutation eliminates these issues but requires additional control electronics.

(Illustration: Side-by-side comparison of mechanical commutation and electronic commutation.)

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Energy Conversion Process

The operation of a brushed DC motor can also be viewed as a sequence of energy transformations.

Electrical Energy
↓
Current flows through the armature windings

Magnetic Energy
↓
Magnetic fields are generated

Mechanical Energy
↓
Electromagnetic torque rotates the shaft

Some energy is inevitably lost through:

• Electrical resistance
• Brush friction
• Bearing friction
• Heat
• Mechanical losses

These losses explain why brushed DC motors generally have lower efficiency than brushless motors.

(Illustration: Energy flow diagram showing electrical input, useful mechanical output, and various energy losses.)

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Engineering Note

One common misconception is that the brushes are responsible for generating motor torque.

In reality, the brushes do not produce torque. Their primary role is to transfer electrical current to the rotating armature and enable the commutator to reverse the current at the correct time.

The actual torque is produced by the interaction between the magnetic field of the stator and the magnetic field generated by the energized armature windings.

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Key Takeaways

• A brushed DC motor converts electrical energy into rotational motion through electromagnetic interaction.
• DC current flows through the brushes, commutator, and armature windings.
• The energized armature creates a magnetic field that interacts with the stator field to produce torque.
• The commutator automatically reverses the armature current, allowing continuous rotation.
• Mechanical commutation is the defining operating principle that distinguishes brushed DC motors from brushless DC motors.