For several years now, we have seen brushless motors begin to dominate cordless tool drives in the professional tool industry. That's fine, but what's the big deal? Does it really matter as long as I can still drive that wood screw? Well, yes. There are significant differences and implications when dealing with brushed and brushless motors.
Table of contents
- How brushed DC motors work
- A look inside the brushed motor
- Inside the brushless motor
- Cost of Brushed DC Motors vs Brushless DC Motors
- Brush vs. Brushless Motor Efficiency
- Brush and brushless motor torque
- Deep Dive into BLDC Motor Technology
- final verdict
How brushed DC motors work
Before we dig into brushed and brushless motors with both feet, let's go over the basics of how DC motors actually work. When it comes to driving motors, it's all about the magnets. Magnets with opposite charges attract each other. The basic idea of a DC motor is to keep the opposite charge of the rotating piece (rotor) attracting the stationary magnet (stator) in front of it, so there is a constant pull forward. Think of it as putting a Boston Cream Donut on a stick in front of you while running – you'll keep trying to grab it!
But how do you get that donut to move? Complex solutions start with a set of magnets with a permanent charge (permanent magnets). A set of electromagnets change their charge (reverse polarity) as they spin. This creates a persistent situation where there is an oppositely charged permanent magnet to make it move. Also, the "same" charge experienced by the solenoid coil when it changes pushes the coil away. How the electromagnets change polarity makes a big difference when we compare brushed and brushless motors.
A look inside the brushed motor
Inside a brushed motor, you'll find four basic components. You have permanent magnets, armature, commutator rings and brushes. Static permanent magnets form the exterior of the mechanism and do not move. We call it a stator. One magnet has a positive charge and the other has a negative charge. This creates a permanent magnetic field.
An armature is a coil or series of coils that turns into an electromagnet when you apply electricity to it. This part also rotates (we call it the rotor). Typically, copper is used for the rotor coils, but sometimes you can use aluminum.
The commutator rings are secured to the armature coils in the form of two (2-pole configuration), four (4-pole configuration) or more. They rotate with the armature. In the end, the carbon brushes stay in place and deliver charge to every part of the commutator.
everything is in the armature
Once the armature is powered, the charged coil pulls towards the oppositely charged permanent magnet. It moves from one brush connection to the next as the commutator ring above it is also rotating. When it gets to the next brush it receives a polarity reversal. Now it wants to move towards another permanent magnet while being repelled by the same kind of charge. The commutator arrives in time to form a connection with the positive brush and follow the negative permanent magnet. The brushes work in pairs, so the positive coil will pull towards the negative magnet, while the negative coil will pull towards the positive magnet.
It's like I'm an armature coil chasing a Boston Cream Donut. I got close, but then changed my mind and went for a healthier smoothie (my polarity or cravings changed). After all, donuts are loaded with calories and fat. Now, I'm chasing smoothies while being pushed away from Boston cream. When I got there, I realized the donuts were more delicious than smoothies. As long as the trigger is pulled, I change my mind every time I get to the next brush, all the while chasing the object of my love like crazy. This is the ultimate use of ADHD. Plus, there were two of us there, so Boston Cream Donuts and smoothies were always enthusiastically pursued by one of us, but indecisive.
Inside the brushless motor
In a brushless motor, you lose the commutator and brushes while gaining an electronic controller. The permanent magnets now act as the rotor and rotate inside, while the stator now consists of stationary electromagnetic coils on the outside. A controller powers each coil according to the charge needed to attract the permanent magnet.
In addition to moving charges electronically, a controller can provide a similar charge against a permanent magnet. This pushes the permanent magnet as the same charges are opposite each other. Now, due to pull and push, the rotor is moving.
In this case, the permanent magnets are moving, so now they are me and my running partner. We no longer change our minds about what we want. Instead, we know that I want a Boston Cream Donut and my partner wants a smoothie.
Electronic controls keep our respective breakfast delights moving in front of us, and we're chasing the same thing all the time. The controller also puts what we don't want behind to provide a push.
Brushed DC motors are relatively simple, and the parts to make them are inexpensive (although copper isn't getting any cheaper). Because brushless motors require an electronic communicator, you're essentially starting to build a computer into your cordless tool. This is what is driving the cost of brushless motors up.
Brush vs. Brushless Motor Efficiency
Due to their design, brushless motors have several advantages over brushed motors. Much of this has to do with brush and commutator losses. Friction is also created because the brushes need to be in contact with the commutator to transfer the charge. Friction reduces achievable speed and builds up heat. It's like riding a bicycle with light brakes. If you use the same amount of force with your legs, you will be slower. Conversely, if you want to maintain speed, you will consume more energy in your legs. You're also heating the rim from frictional heat. This means that brushless motors run cooler than brushed motors. This gives them greater efficiency, so they convert more electricity into electricity.
Carbon brushes also wear over time. This is what causes sparks inside some tools. To keep the tool running, the brushes must be replaced from time to time. Brushless motors require no maintenance.
While brushless motors require an electronic controller, the rotor/stator combination is more compact. This leads to the opportunity to obtain lower weight and more compact size. That's why we see many tools like the Makita XDT16 Impact Driver pack an ultra-compact design with a lot of power.
Brush and brushless motor torque
There seems to be a misunderstanding about brushless motors and torque. Brushed or brushless motor design by itself doesn't really indicate the amount of torque. For example, the first Milwaukee M18 FUEL impact drill had lower actual torque than previous brushed models.
Ultimately, however, manufacturers realized something very critical. The electronics used in brushless motors can provide more power to these motors when needed.
Since brushless motors now feature advanced electronic controls, they can sense when they start to slow down under load. As long as the battery and motor are within temperature specifications, the brushless motor electronics can request and receive more current from the battery pack. This allows tools such as brushless drills and saws to maintain faster speeds under load. This makes them faster. Usually much faster . Some examples of this include Milwaukee RedLink Plus, Makita LXT Advantage, and DeWalt Perform and Protect.
These technologies seamlessly blend the tool's motor, battery and electronics into a cohesive system for optimal performance and uptime.
Deep Dive into BLDC Motor Technology
Commutation — changing the polarity of the charge — starts the brushless motor and keeps it turning. Next, you need to control both speed and torque. Changing the voltage to the stator of a BLDC motor can control the speed. Modulating the voltage at a higher frequency gives you greater control over the speed of the motor.
To control torque, you reduce the stator voltage when the motor's torque load exceeds a certain level. Of course, this introduces key requirements: motor monitoring and sensors.
Hall effect sensors provide an inexpensive method of sensing rotor position. They can also detect speed by timing when and how often the sensor switches.
Editor's note: Check out our What Are Sensorless Brushless Motors article to learn how advanced BLDC motor technology is changing power tools.
The combination of these benefits has another effect – a longer lifespan. While brushed motors and brushless motors (and tools) within a brand typically have the same warranty, you can expect brushless models to last longer. This can often be years beyond the warranty period.
Remember when I said electronic controllers are essentially building a computer into your tool? Brushless motors are also a breakthrough for smart tools to impact the industry. The Milwaukee One-Key technology wouldn't work if it weren't for the brushless motor's reliance on electronic communication.