At the most basic level, there are brushed and brushless motors and there are DC and AC motors. Brushless DC motors, as you may imagine, do not contain brushes and use a DC current. These motors provide many specific advantages over other types of electrical motors, but, going beyond the basics, what exactly is a brushless DC motor?
It often helps to explain how a brushed DC motor works first, as they were used for some time before brushless DC motors were available. A brushed DC motor has permanent magnets on the outside of its structure, with a spinning armature on the inside.
They are more commonly seen in industrial tools and home appliances, such as drills and vacuum cleaners. The alternating current AC motor uses alternating current to induce its rotor to spin. For this reason, it is usually used when plugged into a wall outlet. An AC motor would need a transformer to run on batteries. On the other hand, the direct current DC motor is very similar to the AC motor, but it is wired so that it uses direct current instead of AC.
BLDC motors don't require brushes as the copper coils carrying charge are directly connected to the stator, unlike brushed motors that use brushes to connect the power supply with the rotor. The brushes themselves are also the main downside to brushed motors, since the brushes wear down and have to be maintained or replaced.
Brushless motors have all the characteristics required for great drone design: high efficiency, wide speed ranges and overall high speed-torque capabilities. They are also relatively affordable and require comparatively less maintenance.
A conductive wire coiled around a metallic base will not act like a magnet, but when a current flows through the wire, it will induce it to behave as a magnet would. This is commonly referred to as an electromagnet. If a negative current flows through that same wire, the magnet now has the opposite effect, it would attract another magnet instead of pushing it away.
The parts seen on the inner circle of figure 3 are the electromagnets, while on the outer circle we have the permanent magnets. To turn on the motor, you activate one of the electromagnets by delivering an electric current to its coils.
This will make the rotor start to spin as the permanent magnet experiences repulsion from the like-electromagnet and tries to align with an opposite permanent magnet on the stator. This will only make it spin for a short period of time as the electromagnet and opposite permanent magnet align. The next step is to power another electromagnet to keep the rotation from stopping, followed by the next electromagnet, and the next, and so on.
By delivering a three-phase current at a given frequency, the motor will spin at a speed equal to the frequency of that signal.
An ESC or Electronic Speed Controller controls the electric motor by delivering electric signals that are translated into changes in speed or even a reversal of the rotation. It uses direct current coupled with a switch system to achieve an alternate three-phase current. This output current can then be modified by changing the rate at which the switches open and close in the circuit.
Brushless ESCs need information on the current position of the rotor to be able to start the motor and choose a direction for the rotation. To determine its position, the ESC uses information from the last unpowered electromagnet to measure its induction. This induction varies depending on where the closest permanent magnet is and the closer it is to the electromagnet, the stronger is the magnetic field induced.
The throttle is used to vary the speed of the motor. There are several signal delivery protocols with different performances, the most common ones being Oneshot, Multishot and Dshot. The difference between them is the frequency of the signals delivered. Shorter frequencies allow a faster reaction time.
Furthermore, the Dshot protocol is different from the two others because it sends a digital signal instead of an analog signal. This makes the signal more reliable since it is less sensitive to electrical noise and is more precise with its higher resolution. Based on the sequence above it would seem logical that the motors performance would be rather jumpy, in the same way a stepper motor can often be seen to be taking fixed steps. The reason this can be avoided is largely down to the type of motor controller which is used and how it is set up.
At low speeds with a low number of poles in the motor it is still quite possible that a degree of jumpiness might be observed this is why many lower speed applications use stepper motors rather than brushless but at higher speeds and with the right type of controller in use this will be smoothed out by a combination of the mass on the shaft operating in the same way as a flywheel operates , the controller and the speed.
The permanent magnet is on the rotor and the electromagnets are on the stator. By powering the electromagnets in sequence it is possible to move the permanent magnet around and therefore turn the motor. Different motors have different numbers of poles and the more poles there are the smoother the motor will typically be. Now that we understand the basic principles of brushless motor performance and how they work, it is worth considering why we might choose to use brushless motors over brushed DC motors.
Brushes wear out over time due to friction. The typical lifespan for brushed motors is around hours whereas a brushless motor will typically last hours please note these numbers are only to be used as rules of thumb as there are a huge number of factors which will influence motor life.
Brushes cause sparks and electrical noise within the motor. The friction created by the brushes limits the top speed of the motor and can create heat issues. As the electromagnets are on the stator it is easier to keep them cool through heat sinking or other cooling techniques.
Electromagnets being mounted on the stator also makes it easier to increase the accuracy of control within the motor because they can be mounted very precisely and accurately. As you have probably worked out by now, it is impossible to turn a brushless motor without a brushless motor controller.
If you apply power to the motor in the same way that you might do with a brushed DC motor you simply lock one coil up and make the shaft very hard to turn. They make contact with two spinning electrodes attached to the armature and flip the magnetic polarity of the electromagnet as it spins.
With the advent of cheap computers and power transistors , it became possible to "turn the motor inside out" and eliminate the brushes. In a brushless DC motor BLDC , you put the permanent magnets on the rotor and you move the electromagnets to the stator. Then you use a computer connected to high-power transistors to charge up the electromagnets as the shaft turns. This system has all sorts of advantages:. The only disadvantage of a brushless motor is its higher initial cost, but you can often recover that cost through the greater efficiency over the life of the motor.
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