From magnetic field to rotation: an article to understand why DC motors rotate

In the previous article, we have already had a preliminary understanding of what a DC motor is, what parts its basic structure consists of, and its wide range of applications in life and industry. In this article, we will explain in more depth "Why can a DC motor rotate and what is its working principle".

 

We already know that the rotation of a DC motor requires electric current, a magnetic field, and a complex coil structure, but how do electricity, magnetism, and coils react to each other, and what physical laws allow a seemingly stationary component to begin to rotate continuously?

 

We will explain these issues one by one in the following content, so let's get started.

 

 

To truly understand why DC motors can rotate, we need to know a very basic law of physics - Ampere's law.

 

Basic principles of electric motors: Ampere's force law (F = BIL)

There is a law in physics that says:

When current passes through a wire and it is in a magnetic field, it will be acted upon by a magnetic field.

 

The magnitude of this force is determined by the following formula:

F = B × I × L × sinθ

F: Force

B: Magnetic field strength

I: Current intensity

L: Wire length

θ: Angle between current direction and magnetic field direction

This force is what we often call "Ampere force".

 

It's not mysterious, just like when you put a magnet close to a conductive coil, you will feel a "pushing" or "pulling" force, which is the interaction between the electric current and the magnetic field.

 

In simple terms: current passes through a magnetic field → force is applied to the wire → the wire moves

 

This is the basis for the motor to move.

 

How does a DC motor turn this force into "continuous rotation"?

Earlier we said that a wire is subjected to force. But in the motor, it is not a wire, but a group of coil windings - we call them armature coils, which are installed on a rotor that can rotate freely.

 

The current flows from the power source into the coil, the coil generates force, and the rotor starts to rotate. Here is a question:

If the force is only applied once, the rotor will only rotate once and then stop, and cannot rotate continuously?

 

Yes, so there is a very important structure designed inside the DC motor - the commutator.

 

The function of this small component is to automatically switch the direction of the current in the coil during the rotation of the armature. The advantage of this is that although the current changes direction, the "force direction" in the magnetic field remains consistent, allowing the rotor to continue to rotate.

 

You can think of the commutator as a switch that "constantly flips" during rotation. It works with the brushes to always keep the current "flowing in the right direction" to maintain stable rotation.

 

 

The reason why the DC motor can "move" stably is not only because of the current and magnetic field, but also due to the coordinated work of a series of precision components inside it, including the "armature coil", "commutator" and "brush" . For a simpler understanding, the explanation here will be based on the brushed DC motor.

 

1. Armature coil: the "track" of current

In a DC motor, the armature coil (also called the rotor winding) is the direct carrier of the Ampere force. When current enters the motor from an external power source, it is through these coils distributed in the slots that the force is applied in the magnetic field. Since the coils are symmetrically distributed on the rotor, these forces will cooperate with each other to form a stable and balanced rotational torque (Torque).

 

It can be understood as follows:

Each section of wire is like a "track" where the current runs, and the magnetic field acts as a referee to exert "driving force". When multiple coils are combined together, they are like a team, running in circles rhythmically and eventually generating continuous torque.

 

In addition, the more armature coils there are, the smoother the motor runs and the smaller the output torque fluctuation.

 

2. Commutator and brushes: the magician who reverses the current

It is not enough to have current flowing through the coil - in order to keep the armature under constant force in the same direction, the direction of the current must be reversed every half turn, which is the job of the commutator.

 

The commutator is a structure of copper plates fixed to the shaft that keep contact with the brushes on the stator. As the rotor rotates, the brushes slide over different copper plates, causing the current to "auto-reverse". This is why the force on the wire remains in the same direction even after the coil has turned half a turn.

 

In other words, the commutator is like a system that automatically adjusts traffic lights to ensure that the current "flows smoothly" and maintains the rotation rhythm.

 

So why are brushes and commutators often the fastest wearing parts?

Because they are in a state of continuous contact and friction, they are prone to sparking and heating at high speeds and high currents, and their lifespan is limited under long-term operation. Therefore, in high-performance motors (such as brushless DC motors), people use electronic commutation to replace this part of the structure.

 

 

A DC motor is not just about "turning", it can also "turn fast", "turn violently", and even maintain stable output under different loads. So, how are the speed (RPM) and torque (Torque) of the motor controlled? We can understand it from the following aspects:

 

1. Relationship between voltage, current, speed and torque

The output characteristics of a DC motor are closely related to the input voltage and current:

 

Voltage determines speed

Under the premise that the load remains unchanged, the speed of the DC motor is roughly proportional to the voltage.

· Voltage reduction → speed reduction

· Voltage increases → speed increases

 

Current affects torque

The greater the current, the stronger the Ampere force generated through the coil, and the greater the output torque.

· More current → more torque (but also more prone to overheating)

 

This is why electric vehicles require more current when accelerating, while the current decreases when cruising at a constant speed.

 

2. How does the motor "self-regulate" under load changes?

When the load driven by the motor becomes heavier (like two people sitting on an electric bicycle), the movement of the rotor will encounter greater resistance and the speed will naturally decrease. At this time, the back electromotive force of the armature coil will decrease, causing more current to flow into the motor, which will automatically increase the output torque, resist the load and maintain rotation.

 

This "adaptive" mechanism is one of the reasons why DC motors are very practical.

 

3. PWM control: a variation of voltage control

In current motor control, the power supply voltage is not adjusted directly. Instead, a method called PWM (Pulse Width Modulation) is used to simulate the "variable voltage" effect.

 

In simple terms:

The controller switches the power on and off quickly, allowing the motor to operate in a high-frequency "on-off-on-off" switching cycle.

By adjusting the "on" time ratio (duty cycle), different average voltages can be simulated.

 

For example:

50% duty cycle ≈ half voltage supply → speed is about half of full speed

90% duty cycle ≈ high voltage supply → speed close to full speed

 

PWM not only has precise control but also reduces energy loss. It is the core means of modern DC motor control systems.

 

 

In the previous content, we used the brushed permanent magnet DC motor as an example to explain the working principle, but in fact, the "DC motor" is not a single structure. It can vary in design forms based on commutation methods, magnetic field sources, etc.

 

So, do these different types of DC motors work in the same way? What are the key differences? Let's take a look.

 

1. Brushed vs. brushless: Differences in commutation mechanisms

Brushed DC Motor

Commutation method: Rely on mechanical commutator + brush to complete the reversal of current direction.

Features: simple structure, easy to control, low price, but the brushes are easy to wear and require regular maintenance.

 

Brushless DC Motor (BLDC)

Commutation method: Electronic commutation, through the position sensor and controller to determine the rotor position and change the energized coil.

Features: high efficiency, long life, low noise, suitable for scenarios requiring high performance (such as drones, power tools, electric vehicles, etc.).

 

Summary of core differences:

project	Brushed Motor	Brushless Motor
Commutation method	Mechanical commutator	Electronic Control
Maintenance frequency	high	Low
Service life	Relatively short	Longer
cost	Low	Higher
Control Difficulty	Low	Medium to high

 

2. Permanent magnet vs excitation: different sources of magnetic field

Permanent Magnet DC Motor (PMDC Motor)

· Magnetic field source: Permanent magnets are used, with stable magnetic field and compact structure.

Advantages: small size, high efficiency, commonly used in micro motors, portable devices, electric vehicles, etc.

Disadvantages: The magnet has limited heat resistance and the magnetic field strength cannot be adjusted.

 

Excited DC Motor

· Magnetic field source: The magnetic field is generated by the excitation coil, which can be series excitation, parallel excitation, compound excitation and other structures.

Advantages: The magnetic field is adjustable, suitable for applications requiring large starting torque or variable speed, such as industrial lifting equipment, elevators, etc.

Disadvantages: more complex structure, larger volume, slightly higher energy consumption.

 

Magnetic field difference comparison:

project	Permanent Magnet Motor	Excitation motor
Magnetic field source	Permanent magnets	Excitation coil
Magnetic field adjustability	Not adjustable	Adjustable
cost	Relatively low	Slightly higher
Application Scenario	Small and portable	Industrial, heavy duty

 

By comparison, it can be seen that although different types of DC motors differ in commutation mechanisms and magnetic field sources, their core principles are the same: using the force exerted on the current-carrying conductor in the magnetic field to form torque, thereby driving rotation.

 

 

At this point, I think you have a complete understanding of what a DC motor is and the whole process of why a DC motor can rotate. From the physical principle (Ampere's law), to the coordinated work of key components (armature coil, commutator, brush), to the differences in the working mechanisms of different types of motors (brush/brushless, permanent magnet/excitation), it can be said that DC motors are a technology that "seemingly simple but contains sophisticated design".

 

 

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