How does a brushless motor control the speed of the motor?

Preface

Nowadays, many devices are equipped with brushless motors instead of brushed motors. Why? Because the performance is better!

Introduction

Like repels like, opposites attract

It can be seen that as long as the polarity of the magnet is changed, the motor can rotate

How do you change the polarity of a magnet?

The right-hand screw rule can determine which end is the N-level according to the direction of current flow.

As long as the direction of the current changes, the polarity also changes.

Now, we use a powered coil to replace the magnet.

How to change the direction of current?

If the four switches are connected as shown in the figure, the direction of the current will change when different switches are closed (this circuit looks a bit like the letter 'H', so it is called an H-bridge circuit)

Now, replace the switch with a MOS tube and you can use a microcontroller to control it.

How to control the speed of the motor?

If the switch is closed and open, the speed of the motor will change accordingly, right? (The motor will not stop immediately when the power is cut off due to inertia, and it will only slow down when the power is cut off)

This is called PWM (pulse width modulation) as shown in the figure.

Duty cycle = high level/period × 100%

That is, changing the duty cycle means changing the high level time, that is, changing the speed.

How to detect the rotor position?

Here we have to mention the Hall element. When the Hall element faces the N level, it outputs a high level, and when it faces the S level, it outputs a low level.

But when it is rotated 90 degrees, the Hall element faces the boundary, and then it becomes disordered.

The solution is to add another Hall element

In this way, even if another Hall element faces the boundary, there will be no disorder and the rotor position can still be detected.

Why is there a star connection?

If you energize the W coil (to generate the N pole) and wait for the rotor to rotate 120 degrees (assuming clockwise rotation), then energize the U coil, wait for the rotor to rotate 120 degrees, then energize the V coil.

The disadvantage of this method is that each coil is independent and only one coil is energized at the same time.

If two coils are energized at the same time, will the rotation be faster?

Next, connect these three coils and energize every two coils. This will increase the torque generated and increase the speed. By continuously switching the direction of the three-phase current, the rotor can continue to rotate at high speed.

When the Hall element detects how much the rotor rotates, it changes which line is energized or deenergized.

As shown in the figure

Summarize

The essence of a brushless motor is to switch the direction of the current according to the right-hand screw rule to generate rotation, and detect the position of the rotor according to the Hall element to switch the direction of the current in time. In order to be more efficient, a star connection is used.