What is a brushless motor, and why is it marked as a good thing? Are “brushed motors” common?


What is a brushless motor, and why is it marked as a good thing? Are “brushed motors” common?

In: 101

Brushed motors have a cheaper controller, but the trigger takes the full amps of the tool. A brushless motor means the controller needs a way to know the position of the rotor as it spins, so the controller can energize the proper electromagnet at the exact moment needed. They most often use Hall sensors.

This makes the brushless controller more complex and expensive, but…the best part is that the trigger can now be low voltage and low amps. When these cordless tools that are brushed will wear out, it’s most often the battery and the trigger.

Now, only the battery wears out, and even they last longer than before. Brushless tools will last longer, especially under heavy use.

A brushless motor is driven by an electronic device generating alternating current, which energizes electromagnet coils around the motor in sequence to continuously pull it around. It relies on a measurement for the rotor’s current position. A brushed motor is a simpler design where alternating current is generated by engaging with sliding contacts over pads on the rotor. As it turns, the “brush” slides over a new pad and breaks contact with the previous one, causign the motor turn a bit more. This is worse because there is wear and heating from the rubbing as well as radio and acoustic noise form the rapid switching and jerky movement. But a brushed motor doesn’t require expensive electronics for handling a high power.

Most cheap household appliances designed for intermittent use that need to run at high speed have brushed universal motors, such as electric mixers. Those devices that need to run for extended periods of time contain electronically driven or induction motors, for example a fridge or a fan.

To make an electric motor, you generally need two things: A magnetic field, and a changing magnetic field. The changing magnetic field “pushes” against the constant magnetic field and creates the rotation.

There are a few ways to do this. One is to wrap wires around parts of the rotor so that it can be made into an electromagnet. To do that, you need to supply electric current to something that is rotating. So there are electrical contacts on the rotor, and metal brushes that supply the power even as it turns. It’s a relatively cheap way to make a working electric motor and they are very common.

But as others have mentioned, brushed motors have a lot of drawbacks, such as creating electrical “noise” and wearing out. A brushless motor avoids most of these drawbacks, but they have their own set of disadvantages.

First understand how an electric motor works. Its basically magnets pushing against other magnets to make them spin. You have a spinning magnet in the middle called a rotor being pushed by magnets around the outside called the stator. As the rotor spins you need to change the magnetic field around in order to keep it aligned properly to keep applying force.

One of the easiest ways to do this is with brushes and a commutator. Since the rotor is spinning, you can make the rotor out of electromagnets (coils of wire) and have contacts that use the rotation of the rotor itself to keep changing the magnetization of the rotor. These brushes rub against the commutator in physical contact. Thats great, except physical contact means that the brushes wear over time and have to be replaced. They are usually made out of something like graphite that wears into dust (which can also be a problem).

The alternative is a brushless motor. Since it isn’t using brushes, it can’t rely on the physical contact against the spinning rotor to change the magnetization. It also means that the center probably can’t be an electromagnet, and permanent magnets usually aren’t quite as strong. To do this requires extra sensors to know where the rotor is and some more complicated control circuitry to power it properly. The upside is that there are no brushes to wear out, and this extra control circuitry allows you to do fancier stuff to control the motors more precisely.

There are also induction motors which are also brushless but use a different mechanism. They induce magentism in the rotor, which isn’t normally magnetic, but don’t have the same kind of control as what we usually call ‘brushless motors’ as they just make use of the constant changing of their AC power source so they run at a preset speed.

The brushes on brushed motors tend to make them louder, cause sparking, add friction, and wear out.