What is a DC Motor and Its Working Principle?

DC motors play an integral role in almost every modern industry. Whether you’re looking for a motor for your next hobby application or a larger, more complex work-related application, chances are you’re going to need some sort of DC motor.

What is a DC motor? How does it work and what are its most common applications? Are there different types of DC motors? Our experts answer these questions and also highlight the advantages of opting for a DC motor over an alternating current (AC) motor.

What is a DC Motor?

A direct current (DC) motor is a specific type of electric machine that turns electrical energy into mechanical energy. Since these motors work with direct current, this energy is converted into mechanical rotation. Magnets on the rotor are attracted and repelled, which ultimately causes rotation.

Intro to DC Motor Parts

Before we get to the working principle of a DC motor, it’s important to take a look at the various parts that make up these motors. Here’s a breakdown of the DC motor construction:

• Stator: DC motors feature a stator that forms the main body of the motor. It provides protection and support to the motor. A rotating magnetic field moves the rotor or armature in then provided in the magnetic field. Additionally, the stator is also the static part of the motor that contains the filed windings and receives the required electrical supply through the terminals.
• Magnets: The magnets that are used in DC motors are more commonly referred to as permanent magnets. This is because their magnetic fields are always active. That means that opposite magnets attract each other while similar ends will repel. Magnetic fields run from south to north poles and this means that the most powerful part of the field is always the end point. Since two magnets form a very strong field, DC motors always have two magnets.
• Shaft: Typically, the shaft is rotated by the windings and the commutator and is made of hardened steel to withstand varying application loads. A plastic moulding joins the commutator bars to the shaft and the torque is produced when the winding energy is transferred to the shaft which is turn supported by the stator. The shaft protrudes through the stator which then connects the motor to the application you’re using it for.
• Terminals: DC motors always have two terminals—positive and negative. If the positive wire is connected to the positive terminal and the negative wire to the negative terminal, you’ll have a motor that rotates clockwise. Reversing them creates a counterclockwise motor. Terminals create the DC power supply for the motor and are usually connected to the brush arms and brushes on the inside of the back cover. Unlike AC motors, DCs don’t rely on AC mains as a supply voltage source.
• Rotor: The armature or rotor is made up of multiple disks that are insulated from each other using laminated sheets. As part of the motor, the rotor creates the mechanical revolutions to make the motor work properly. Disks on the rotor are made as small as possible to create more efficiency.
• Coil windings: The coil windings are wrapped around the rotor and produce a robust and powerful electromagnetic field. Coiling the wire enables each turned section to have the same magnetic field strength. Adding more coils will ensure a much smoother rotation as two coils often jam and stop the motor.
• Brushes: If a DC motor relies on brushes it’s because they provide the coils with the required power. Essentially they’re pieces of carbon metal that act like springs. One side features a pin that supplies the power to the motor, while the other side features conductive carbon material. The brushes are pushed against the commutator and are held in place by the brush arms. They’re also connected directly to the electrical supply or the terminals.
• Commutator: Made of small copper plates, the commutator is situated on the shaft and rotates every time the shaft rotates. This direct rotation of the rotor enables the poles of the power supply to the coils to change. The coils, in turn, are connected to two different commutator plates. These plates are electrically isolated from each other but remain connected by the coils. Since positive and negative terminals are connected to two commutator plates, current flows and a strong electromagnetic field is generated.

Below is a short clip that shows how a DC motor works.

DC Motor Applications

DC motors have a wide range of applications. In fact, you don’t have to look very far to find DC motors in your environment—whether you’re at home or work. Some of the more common applications are:

1. Electric vehicles
2. Conveyor systems
3. Cranes
4. Pump drives
5. Machine tools
6. Diesel electric locomotives
7. Elevators
8. Dishwashers
9. Small home appliances
10. Audio and video equipment
11. Computer equipment
12. Medical equipment
13. Industrial machinery
14. Radio-controlled hobby models
15. Robotics

Different Types of DC Motors

Different types of DC motors accommodate different applications and uses. The key difference is how the rotor is powered. Let’s take a more detailed look at the different types of DC motors and how they work.

Brushed DC Motor

Brushed DC motors feature a carbon brush and commutator that produce the current. These are connected to the rotor and can either be separately excited or self-excited. Additionally, the stator contains the motor components and the magnetic field. Coil winding can run in series or parallel to create a series-wound DC motor or a shunt-wound DC motor.

The commutator serves as an electrical switch that reverses the current between the external power source and the rotor. This method applies steady current to the windings which start rotating and create the necessary torque by reversing the direction of the current.

Brushless (BLDC) Motor

As the name implies, a brushless DC motor (BLDC) works without the carbon brushes found in brushed motors. Instead of working with brushes, a permanent magnet is driven by direct current power and an electronically controlled communication system. This process produces rotational torque as the phase currents change.

The electrical communication is the differentiating factor that separates it from its brushed counterpart. BLDC motors include stator windings, a magnet motor and a sequence of coils. Current carrying conductors remain in place as the permanent magnet rotates around it.

The armature winding coils are switched electronically by different transistors and this force enables the rotor to move. With the absence of brushes, the BLDC motor boasts a quieter operation and also has a high efficiency rate.

Servo DC Motor

Another popular electric motor is the servo motor. Essentially, this motor is made up of four parts—a DC motor, control circuit, gearbox, and position sensing unit. It works when the gearbox turns high-speed input into a much slower practical speed. That means the control circuit functions as an error-detecting amplifier.

Feedback of the shaft’s position is fed through to the control circuit in a closed loop. Any mismatch between the reference position and its shaft will result in an error signal being sent to the error-detecting amplifier.

Compound DC Motor

This type of DC motor combines the features of the series and shunt field windings. Since the winding for the armature is connected in series formation, the winding field is connected parallel to that connection. Compound DC motors are subdivided into differential and cumulative. In the cumulative DC motors, the shunt field flux assists the series field flux as they both move in the same direction.

On differential compound DC motors, the series and shunt fields move in an opposite direction. Whether or not these two motors have a short or long stunt depends on the shunt field winding.

Self-Excited DC Motor

In this type of DC motor, the armature and field winding are connected and function with a single supply source. Connections can be parallel or series with a parallel shunt wound. If you’re opting for a series motor, the shunt will be series wound.

Here’s how the two versions stack up:

• Shunt DC motor: With a shunt wound DC motor, the armature and field windings are connected in a parallel formation. This essentially exposes the field coil to terminal voltage. While the power supply is the same, the electrical current is different. A shunt DC motor always has a constant speed and doesn’t vary even if the mechanical load changes.
• Series DC motor: On a series DC motor, the armature and field winding are connected in a series formation. This means the same electric current flows through both. Furthermore, a series wound DC motor can with either AC or DC power, making it universal. Since series motors always rotate in the same direction, irrespective of their voltage source, their speed tends to vary based on their mechanical loads.

Permanent Magnet DC Motor

Unlike some of the other motors, a permanent magnet DC motor features an armature winding but no field winding. In this instance, the permanent magnet is mounted on the inside of the stator core to create the required magnetic field.

A regular armature is made up of brushes and a commutator. Permanent magnet DC motors are typically smaller and less expensive.

Separately Excited DC Motor

With a separately excited DC motor, there are separate electrical supplies for the field winding and the armature winding. Since they’re separate, the field current and the armature current don’t interfere with the other’s actions. However, the total input power is the sum of these two fields.

Below is a detailed clip showing you the different types of DC motors.

Advantages of DC Motors

Understanding how DC motors work is only half of your deciding factor when picking components. Knowing what their advantages are will make it considerably easier to see why these nifty little motors are a go-to in so many industries.

• Startup torque: The top advantage of DC motors is without a doubt their high startup torque. This is especially effective for applications that require constant speed.
• Speed control: DC motors can monitor and control their speed. It’s this accuracy that guarantees the success of the application, especially in mills where consistent speed is a prerequisite. In these instances, you can also use an Assun DC motor encoder which is a mechanism that measures the speed of the rotor and provides essential closed loop feedback to the drive to provide precise speed control. Encoders are primarily used for speed regulations in robotics, drug delivery systems and electronic assembly equipment.
• Maintenance: Since DC motors have a relatively simple design, they’re easy to repair or even replace. Moreover, they’re deeply rooted in just about every industry which means that technicians and electricians are well versed in their operation and appreciate the little maintenance that goes along with them. Problems are simple to diagnose, and faulty components can be easily replaced.
• Cost-effective: Another significant advantage of DC motors, is their cost-effectiveness. Not only are some versions cheaper than AC motors, but they have a remarkably long life span. While some brushless and permanent magnet DC motors are initially more expensive, the cost is offset by their long lifespan.
• Installation: Overall, DC motors require fewer electronic rectifications in their power systems and also fewer general adjustments. As soon as the DC motor is installed, it starts feeding power directly from its power source, which means that it can be used immediately.
• No harmonic effects: Typically, harmonics in motors cause the performance of the system to become degraded and affect reliability. The damage caused by harmonics can result in metal components heating up and becoming unsafe to operate. Fortunately, the harmonic issue isn’t found in DC motors.
• Linear speed torque: A direct curve between the speed of a motor and its torque determines how fast the motor can spin. This is then the deciding factor about the amount of torque it can create. As DC motors create an exceptional amount of speed, the torque curve is more linear than other types of motors.

Conclusion

With the information we’ve shared, it’s easy to see why DC motors are preferred above other motor types for various home, mechanical and industrial applications. Since advantages include high torque and precision speed control, it’s no wonder that these motors are used in every industry from medicine to robotics!

Keep in mind that Assun’s motors—the brushed, brushless and servo options—are better suited to precise motion engineering. Furthermore, Assun’s encoders are designed for use with DC motors and servo motors.

If you’ve found this article informative, we urge you to share it with a friend or colleague who would benefit from an Assun DC motor!