I will never forget my first engineering project involving motors. I built a small elevator for a science class back in elementary school. Of course, it worked great during the testing phase, but it failed to perform when it counted.
I used wood to build the elevator shaft frame, and I used a pulley system with strings to lift a cardboard box up and down. This was before I learned gear/pulley ratios, so my elevator was more like a ejector seat than an elevator.
For the motion control, I used a battery, a switch and a DC motor for my project. Because I was so focused on testing, my battery actually ran out of juice before the demonstration. In hindsight, I should have switched the battery just before the demonstration. The teacher still gave me an OK grade since someone did witness the elevator working and vouched for me.
That was my first experience with a DC motor. Can you guess which type 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 since they both use a segmented permanent magnet rotor. These motors are typically used for speed control applications.
Driver or No Driver?
The first difference comes from their names. One uses brushes and one doesn't. Brushed DC motors are also known as self-commutated DC motors. Its design and construction allows it to operate without a drive circuit, which I will cover later. Brushless DC motors cannot self-commutate, so they require a drive circuit, which uses transistors to direct the current to different winding coils of the motor.
Design & Operation
A motor energizes a set of electromagnets in its stator in a sequence to create rotation with its permanent magnet rotor. A north pole on the stator will attract the south pole on the motor. This is the operating theory for all permanent magnet DC motors. The way they do it is different.
To understand why these motors behave like they do, we need to understand its design.
Here are what brushed motors and brushless motors look like inside. In the image below, we show a brushed motor with permanent magnets in the stator instead of the rotor. Sometimes, the permanent magnets can be in the rotor depending on the manufacturer. By having the winding coils in the rotor, heat doesn't radiate as well as having the winding coils in the stator.
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 commutator are set inside of the motor. The commutator rotates with the rotor, and the stator is stationary. In this motor, there are two permanent magnet poles - north and south.
When the power supply is connected to the stationery brushes, a specific set of electromagnets (coils) are energized in the rotor, which attracts the next magnet pole and repels the current pole of the stator. Once the rotor is rotated to the next set of electromagnets, the brushes mechanically switches to the next set of electromagnets in the rotor. This process keeps repeating until the power supply 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 make. One benefit of this design is that the stator winding coils, which produce the most heat, can dissipate the heat faster than a motor with its coils in the center.
The top-left image shows the rotor, stator, and the hall effect IC in the back of the motor. Unlike brushed motors, brushless motors use a dedicated driver circuit to monitor the feedback from the motor, and the driver uses transistors to electrically excite the stator poles to rotate the rotor. They are also known as brushless DC motors or BLDC motors. Oriental Motor uses the term, "brushless motors" since we offer these motors with either AC or DC input drivers. The bottom-right image shows the front side of the motor. We have 6 stator poles (electromagnets) and 4 rotor poles (permanent magnets) in this motor.
The Hall Effect IC senses the permanent magnets in the rotor as its rotated, convert from analog to digital, then sends the data back to the driver circuit. The driver then uses the data to determine 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 with 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.
Most of our brushless motors are now 10 pole motors. The output resolution of the Hall Effect IC is # of Hall Effect IC x rotor poles, so that's 3 ICs x 10 poles = 30 pulses per revolution. Some brushless motors, such as the BXII Series, offers an encoder for applications that require higher resolutions.
Another obvious difference between brushed and brushless motors is that one requires feedback in order to work properly. Feedback signals from its Hall Effect IC provides rotation data and is necessary for proper timing of phase excitation.
Advanced brushless motor drivers may offer some unique features that are not available to 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 that can be useful for tensioning applications. Although initial costs are higher for brushless motors, their benefits should be considered when choosing a motor.
Speed Control Performance
Both brushed and brushless motors offer similar performance. Their speed torque curves are the same as below. For brushed motors, speed and torque can be controlled by varying the input voltage to the motor. However, increased voltage can sometimes increase heat too much and reduce the motor's duty cycle.
Brushless motor drivers limit its speed torque curve for the best possible performance, so you can always expect the same great performance every time. For brushless motors, the driver's excitation sequence needs to speed up in order to rotate the motor faster.
You must have guessed that I used a brushed motor in my elevator project. While brushless motors are superior, a brushed motor got the job done for my simple one-off project. Plus, I really needed to keep the costs low.
Here's the summary of the differences between brushed and brushless motors.
While brushed motors are simple and cheaper to operate, they are typically used in applications where long term life or maintenance is not a major concern. The brushes are always in contact, so friction will wear them down eventually, and they will need to be replaced periodically. This could dictate unwanted changes in design since motors need to be accessed for maintenance. The only components in contact inside a brushless motor are the ball bearings, so they do not require periodic maintenance. Brushless motors are also quieter and last longer than brushed DC motors. Brush commutation is also a major source of electrical and audible noise that can affect other electronic signals or require noise reduction measures. Sparks from brush commutation limits the environments brushed motors can safely operate in. Since brushless motors offer higher power efficiency, these motors can be more compact due to high torque to weight ratio and more torque per watt. Lastly, the Hall Effect sensors in brushless motors regulate the speed to about +/-0.2%. With encoders, this is down to +/0.05%.
Brushless motors are becoming more popular than brushed motors. While brushed motors are still commonly used in household appliances and automobiles, brushless motors are more versatile for a wide range of applications from conveyors to AGVs.
Learn more about the advantages of brushless motors with these white papers.
If you haven't watched this yet, here's a short video about brushless motors.
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