HOW DO BRUSHLESS RC MOTORS WORK
Mar 23, 2020
In this post you will learn:
- The physics behind how motors work
- How brushed motors work
- How brushless motors work
- What motor parameters like Kv mean
- What motor descriptions like 1806 and 2204 mean
- The meaning of common motor terms like “commutation” and “torque”
How Do Brushless Motors Work?
Both brushed and brushless motors work on the same principle:
If you put a piece of wire with a current running through it and place that wire into a magnetic field, a force will act upon that wire. This principle is called the Lorentz Force.
In Figure 1, the top left shows the magnetic field lines between 2 permanent magnets.
The top right shows the magnetic field lines around a circle, which represents the cross-sectional area of a piece of wire that has current flowing out of the screen.
If you place the wire inside the magnetic field between the two permanent magnets, you get the third picture on the bottom. The 2 sets of magnetic field interact and cause the field lines to “bunch up” underneath the wire and spread out above the wire. This causes a force to act on the wire in the upward direction.
To review, we can see that we can cause a force (and therefore torque and movement) on a wire. All we have to do is put that wire in a magnetic field and then apply a current to the wire.
With a little ingenuity, we can figure out how to make the wire rotate whenever we want just by causing current to flow in the wire…
Look at the top picture in Figure 2.
We have the same north and south magnets.
We have 2 current-carrying wires this time instead of 1.
But this time I’ve add 4 extra parts in the foreground of the picture: 2 brushes and two pieces of metal that are attached to the current-carrying wire.
These two pieces of metal are called the commutator. The brushes make electrical contact with the commutator so that if a voltage is applied to the brushes (depicted in the picture as a + and a – sign), it will cause current to flow as indicated in the picture.
In the top picture, the wire in red has current flowing into the computer screen and the wire in green has current flowing out of the computer screen.
Based on the Lorentz force, we can see that a force will cause the green wire to move upwards and a force will cause the red wire to move downwards.
These 2 forces end up twisting the wires so that they end up in the position shown in the bottom picture of Figure 2. A twisting force like this is called torque.
The bottom picture shows that the commutator bars have moved with the wires and are no longer touching the brushes. This causes current to stop flowing in the wires. Inertia will cause the wires to continue to rotate until the commutator bars are touching he opposite brushes than the ones they were touching at the start. This causes current to flow the opposite direction through the wires (the red wire has current flowing out of the page and the green wire has current flowing into the page).
The motor will continue to rotate until current is no longer applied.
Most brushed motors end up having more commutator bars so that the current can switch directions more often. This causes a more steady torque to be applied to the motor, which makes the motor run smoother.
The problem with brushed motors is … it has brushes. And a commutator.
(“Commutation” is just a fancy word for causes current to flow in the opposite direction. Likewise, a commutator is a mechanical device that switches the flow of current through wires.)
Both these parts of a motor are made of metal (because electricity has to flow through them). When metal rubs on metal it causes parts to wear down.
It also causes friction and extra energy losses.
Because of these and other problems, engineers thought it would be great if there was a motor that had all the advantages of a DC brushed motor but none of the disadvantages that come along with the commutator and brushes.
Eventually they figured out how to do this and they called this type of motor a brushless motor. It goes by other names, too, like: BLDC, brushless DC, permanent magnet brushless DC and many others. There are almost too many names and that causes a lot of confusion. For our purposes here, we can just call it a brushless motor.
The main difference between the brushed and brushless motor is the method they use to switch currents in their coils.
As I mentioned above, brushed motors use mechanical means of commutation: a commutator and brushes.
Brushless motors, however, use electrical means of commutation. They require a control that decides when to switch the direction of current in the wires based on the relative position of the magnets and the wires. Sometimes this is just called a brushless control, but in the RC world, this is called an brushless ESC (electronic speed control).
Brushless ESC’s do 2 things: they regulate the speed of the motor and they determine when to cause current to flow in different coils in a brushless motor.
Types of brushless motors
All brushless motors have the same parts:
- A stator, which is the stationary part of the motor and contains all the magnet wire and coils
- And a rotor, which is the part of the motor that rotates and contains permanent magnets
There are 2 main topologies that these can be arranged in: the inrunner and the outrunner.
The inrunner brushless motor has the rotor on the inside and the stator on the outside.
The outrunner has the stator on the inside and the rotor on the outside.
RC motor manufacturers describe the differences between motors by listing a number of motor parameters. The most common of these are diameter, length, weight, Kv motor constant, and Ro.
Here is a brief description of these different parameters.
Size – Motors are usually defined by the outer diameter and length of the stator. Diameter and length are usually specified by a 4 digit number. This might be a number like 2208, where the “22” indicates the the diameter of the stator in mm and “08” indicates the length of the stator in mm.
Weight – This is straightforward, but weight is very important for quadcopters because it impacts flight time, the size of props needed, among other things. This is usually measured in grams.
Kv – Kv stands for voltage constant and gives an indication of how fast the motor will rotate with no load on it for an applied voltage. This constant is measured in RPM’s/Volt. Brushed and brushless motors have a speed that is proportional to the voltage applied to the motor. So if your motor is 1000 Kv and you apply 5 Volts to it, the no-load speed should be 5000 RPM. Similarly, if your motor is 950 Kv and you apply 6 Volts to it, the no-load speed should be 950 RPM/Volt x 6 V = 5700 RPM. (I will note that Kv is a lot more complex than I’ve indicated here. I plan to have another post about it in the future.)
Ro – this is merely the resistance of the motor. Resistance has an effect on efficiency, torque and many other aspects of a motor.
# of poles – The number of poles of a motor is typically equal to the number of magnets found radially located around the rotor. So a 2 pole motor would have 1 north magnet and 1 south magnet. A 6 pole motor would have 3 north magnets and 3 south magnets. The # of poles can have an effect on efficiency, speed, and a number of other motor parameters.
What Kind of Motors do Quadcopters Use?
It depends. A lot of the smaller “nano” quads tend to use brushed motors. One example is the Cheerson CX-10. But as quadcopters start to get bigger, they become more and more likely to use outrunner brushless motors.
They use all different sizes, depending on much thrust is needed.