Difference between Brushed and Brushless DC Motors

I will never forget my first project involving electric motors.

I built a small elevator for a science project in elementary school. Sure, it worked fine during the testing phase, but failed miserably when it came to calculations. I built the elevator shaft out of wood and used a pulley system with ropes to lift cardboard boxes up and down. (This was before I learned about gear/pulley ratios, so my elevator was more like an ejection seat than an elevator.)

For motion control, I used batteries, switches, and DC motors in my project. Long story short - because I was so focused on testing, my battery actually died before the demo. In hindsight, I should have replaced the battery before the demo. The teacher still gave me a good grade because someone witnessed the elevator working and vouched for me.

That was my first experience with DC motors. Can you guess what kind of DC motor I used?

Types of DC Motors

There are two types of DC motors - brushed and brushless. They are both DC permanent magnet motors as they both use a segmented permanent magnet rotor. These motors are often used in speed control applications.

With or without driver?

The first difference comes from their names. One uses brushes and one does not. A brushed DC motor is also called a self-commutated DC motor. Its design and construction allow it to operate without a drive circuit, which I will cover later. A brushless DC motor cannot self-commutate, so it requires a drive circuit that uses transistors to direct current to the different winding coils of the motor.

Design and Operation

The motor energizes a set of electromagnets in its stator in sequence to create rotation through its permanent magnet rotor. The north pole on the stator attracts the south pole on the motor. This is the operating principle of all permanent magnet DC motors. The way they do this is different.

To understand why these motors behave the way they do, we need to understand their design.

Here is a look at the internal structure of a brushed motor and a brushless motor. In the image below, we show a brushed motor that uses permanent magnets in the stator instead of the rotor. Sometimes, the permanent magnets may be in the rotor, depending on the manufacturer. By having the winding coils in the rotor, the heat is not dissipated as it would be if the winding coils were in the stator.

The upper left image shows the commutator and brushes. The lower right image shows the same motor from the front view. Inside the motor are electrodes and a commutator in the form of brushes. The commutator turns with the rotor, while the stator is stationary. In this motor, there are two permanent magnet poles - north and south.

When power is connected to the stationary brushes, a specific set of electromagnets (coils) are energized in the rotor, attracting the next magnetic pole and repelling the current pole of the stator. Once the rotor rotates to the next set of electromagnets, the brushes mechanically switch to the next set of electromagnets in the rotor. 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 image below shows a brushless motor with the permanent magnets located on the rotor rather than the stator, which is the type we make. One benefit of this design is that the stator winding coils, which generate the most heat, can dissipate the heat more quickly than in a motor where the coils are located in the center.

The upper left image shows the rotor, stator, and Hall Effect IC on the back of the motor. Unlike brushed motors, brushless motors use a dedicated drive circuit to monitor the motor's feedback, and the driver uses transistors to electrically excite the stator poles, causing the rotor to spin. They are also called brushless DC motors or BLDC motors. Oriental Motor uses the term "brushless motor" because we offer AC or DC input drives for these motors. The lower right image shows the front of the motor. This motor has 6 stator poles (electromagnets) and 4 rotor poles (permanent magnets).

The Hall Effect IC senses the permanent magnets in the rotor as it spins, converts from analog to digital, and sends the data back to the drive circuit. The drive then uses this data to determine the correct time to energize the phases. Feedback is also used to regulate the motor speed.

The following diagram shows how the power circuit of the drive uses transistors to turn specific winding coils on and off. We are showing a 12-step transistor excitation sequence with the U, V, and W windings in the motor. After 12 steps, the cycle repeats.

Most of our brushless motors today are 10-pole motors. The output resolution of the Hall Effect IC is the number of Hall Effect ICs x the number of rotor poles, so 3 ICs x 10 poles = 30 pulses per revolution. Some brushless motors, such as the BXII series, offer encoders for applications that require higher resolution.

feedback

Another distinct difference between brushed and brushless motors is the need for feedback to work properly. The feedback signal from its Hall Effect IC provides rotation data and is necessary for the correct timing of the phase excitation.

Advanced brushless motor drivers may offer some unique features not available in simple brushed motor controllers, such as stored speed profiles and RS-485 communications. Feedback and current sensors in brushless motors can provide torque limiting capabilities that can be used in tensioning applications. Although brushless motors have a higher initial cost, their advantages should be considered when selecting a motor.

Speed ​​control performance

Both brushed and brushless motors have similar performance. Their speed torque curves are shown in the figure below. For brushed motors, speed and torque can be controlled by varying the input voltage to the motor. However, increasing the voltage can sometimes increase heat excessively and reduce the duty cycle of the motor.

Brushless motor drives limit their speed-torque curve for optimal performance, so you can expect the same great performance every time. With brushless motors, in order to make the motor spin faster, the drive's excitation sequence needs to speed up.

Summary/Comparison

You must have guessed that I used a brushed motor in my elevator project.

While brushless motors are much better, brushed motors will do the trick for my simple one-off project. Plus, I have no idea how to build a driver, and I really need to keep costs low.

The following summarizes the differences between brushed motors and brushless motors.

While brushed motors are simple to operate and less expensive, they are typically used in applications where long life or maintenance is not a major concern.

The brushes are in constant contact so friction will eventually wear them out and they will need to be replaced regularly. This may require unnecessary changes to the design as the motor will need to be accessed for maintenance.

The only parts that touch inside a brushless motor are the ball bearings, so regular maintenance is not required.

Brushless motors are also quieter and have a longer lifespan than brushed DC motors. Brush commutation is also a major source of electrical and audible noise, which can affect other electronic signals or require noise reduction measures.

Sparks generated by brush commutation limit the environments in which brushed motors can operate safely.

Since brushless motors offer higher power efficiency, these motors can be more compact due to the high torque-to-weight ratio and higher torque per watt.

Finally, the Hall Effect sensors in the brushless motors regulate the speed to about +/- 0.2%. For encoders this drops to +/- 0.05%.

Brushless motors are more popular than brushed motors. While brushed motors are still commonly used in home appliances and automobiles, brushless motors are more versatile and suitable for a wide range of applications from conveyor belts to AGVs.