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My first experience with DC motors was for a small elevator I built for a science project in school. At the time, I had no idea that this would be just the beginning of my journey with motors.
The motor worked perfectly during testing, but it failed when it mattered most. I used wood to build the elevator shaft and a pulley system with strings to lift a cardboard box up and down. (This was before I learned about gear and pulley ratios, so my elevator was more like an ejector seat than a real elevator.) For the motion control, I used a battery, a switch, and a DC motor. In short, I didn't have enough power for the final demonstration because the battery ran out right before the presentation. In hindsight, I should have changed the battery just before the demo. Still, I passed the class since someone saw the elevator working and vouched for me.
That was my first encounter with a DC motor. Can you guess which type I used? We’ll come back to that later.
**Types of DC Motors**
There are two main types of DC motors: brushed and brushless. Both are DC permanent magnet motors, as they both use a segmented permanent magnet rotor. The difference comes from their names—brushed motors use brushes to commutate their windings, while brushless ones don’t. These motors are typically used for speed control applications.
**Driver or No Driver?**
Brushed DC motors are also known as self-commutated DC motors. Their design allows them to operate without a drive circuit, which I’ll explain later. Brushless DC motors, on the other hand, cannot self-commutate and require a driver circuit that uses transistors to direct current to different winding coils.
**Design & Operation**
A motor energizes a set of electromagnets in its stator in sequence to create rotation with its permanent magnet rotor. A north pole on the stator attracts the south pole on the motor. This is the basic principle behind all permanent magnet DC motors, though the way they do it differs.
To understand how these motors work, let’s look at their design. Here’s what brushed and brushless motors look like inside. The image below shows a brushed motor with permanent magnets in the stator instead of the rotor. Sometimes, the magnets can be in the rotor depending on the manufacturer. Having the winding coils in the rotor doesn’t allow heat to dissipate as well as having them in the stator.
[Image 1: Brushed Motor Structure]
[Image 2: Brushed Motor Construction]
The top-left image shows the commutator and brushes. The bottom-right image shows the same motor from the front view. An electrode in the form of brushes and a commutator is set inside the motor. The commutator rotates with the rotor, and the stator remains stationary. In this motor, there are two permanent magnet poles—north and south.
When the power supply is connected to the stationary brushes, a specific set of electromagnets (coils) in the rotor are energized, attracting the next magnet pole and repelling the current one from the stator. As the rotor turns to the next set of electromagnets, the brushes mechanically switch to the next set. This process repeats until the power is disconnected. The direction of the motor can be changed by switching the polarity of the power supply.
The next image shows a brushless motor with its permanent magnets on the rotor instead of the stator, which is the type we manufacture. One advantage of this design is that the stator winding coils, which generate the most heat, can dissipate it faster than if they were in the center.
[Image 3: Brushless Motor Structure]
[Image 4: Brushless Motor Construction]
The top-left image shows the rotor, stator, and Hall Effect IC in the back of the motor. Unlike brushed motors, brushless motors use a dedicated driver circuit to monitor feedback from the motor and use transistors to electrically excite the stator poles to rotate the rotor. They are also known as brushless DC motors or BLDC motors. Oriental Motor refers to them as "brushless motors" because we offer these motors with either AC or DC input drivers. The bottom-right image shows the front side of the motor, which has six stator poles (electromagnets) and four rotor poles (permanent magnets).
The Hall Effect IC senses the permanent magnets in the rotor as it rotates, converting analog signals to digital, then sends the data back to the driver circuit. The driver then uses this information to determine the proper timing for phase excitation. The feedback is also used to regulate the motor speed.
The image below shows how a driver's power circuit turns specific winding coils on and off using transistors. We are showing a 12-step transistor excitation sequence with U, V, and W windings in the motor. After 12 steps, the cycle repeats.
[Image 5: Control Circuit Output]
[Image 6: Brushless Motor Switching Sequence]
Most of our brushless motors now have 10 poles. The output resolution of the Hall Effect IC is calculated as the number of Hall Effect ICs multiplied by the number of rotor poles, so that’s 3 ICs x 10 poles = 30 pulses per revolution. Some brushless motors, such as the BXII Series, offer an encoder for applications requiring higher resolutions.
**Feedback**
Another obvious difference between brushed and brushless motors is that brushless motors require feedback to function properly. Feedback signals from the Hall Effect IC provide rotation data and are essential for proper timing of phase excitation.
[Image 7: Brushless DC Motor Construction and HE Sensor Output]
Advanced brushless motor drivers may offer features not available in simple brushed motor controllers, such as stored speed profiles and RS-485 communication. The feedback and current sensors in brushless motors can provide a torque limiting function useful for tensioning applications. Although initial costs are higher for brushless motors, their benefits make them a strong choice when selecting a motor.
**Speed Control Performance**
Both brushed and brushless motors offer similar performance. Their speed-torque curves are the same, as shown below. For brushed motors, speed and torque can be controlled by varying the input voltage. However, increased voltage can sometimes lead to excessive heat and reduce the motor's duty cycle.
[Image 8: Rotational Speed-Torque Characteristics of a Brushless DC Motor]
Brushless motor drivers optimize the speed-torque curve for consistent performance. For brushless motors, the driver’s excitation sequence must speed up to rotate the motor faster.
[Image 9: High Performance Speed Control]
**Summary/Comparison**
You probably guessed that I used a brushed motor in my elevator project. While brushless motors are far superior, a brushed motor got the job done for my simple one-off project. Plus, I didn’t know how to build a driver, and I needed to keep costs low.
Here’s a summary of the differences in benefits between brushed and brushless motors:
[Image 10: Brushed vs. Brushless Motors Comparison Chart]
While brushed motors are simple and cheaper to operate, they are typically used in applications where long-term life or maintenance isn’t a major concern. The brushes are always in contact, leading to wear over time and periodic replacement. This could complicate design changes due to the need for motor access during maintenance. Sparks from brush commutation limit the environments where brushed motors can safely operate.
In contrast, the only components in contact inside a brushless motor are the ball bearings, so they don’t require periodic maintenance. Brushless motors are also quieter and last longer than brushed DC motors. Brush commutation is a major source of electrical and audible noise that can interfere with other electronic signals or require noise reduction measures. Because of their higher efficiency, brushless motors can be more compact and offer a better torque-to-weight ratio and torque-per-watt ratio. Additionally, the Hall Effect sensors in brushless motors regulate speed to within +/-0.2%, and with encoders, this improves to +/-0.05%.
Brushless motors are becoming increasingly popular compared to brushed motors. While brushed motors are still common in household appliances and automobiles, brushless motors are more versatile for a wide range of applications, from conveyors to AGVs.
**FYI**: Here's a short article comparing brushless and brushed motors to AC motors.
[Image 11: Brushless DC Motor vs. AC Motor vs. Brushed Motor]
Enjoy a short video about our brushless motors.
[Video Embed: Brushless Motors]
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