# BLDC motor controller- Hall sensor placement

1. ## BLDC motor controller- Hall sensor placement

How exactly does the angle between each hall sensor affect the speed of a 3 phase BLDC motor. Is there any formula connecting the two?

It was observed that speed reduced with increse in angle. But i couldn't understand why. Is it because of the programming done in the controller? How exactly is speed calculated from three hall sensor output combination through programming?

2. ## Re: BLDC motor controller- Hall sensor placement

The Hall Sensor does the job of a commutator of a DC motor.
i.e., to switch the windings at appropriate time.
So the sensor placement position affects the speed.

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3. ## Re: BLDC motor controller- Hall sensor placement

Seems to me the speed should be determined by our control system. I've done experiments building a BLDC motor from a 3-magnet spinning toy, powered by a coil of wire, photosensor and 2 transistors. A motor under load wants to stall. This cannot be permitted. Motion must be maintained. The magnet needs to be pulled along from coil to coil. Thus we ought to sense when a magnet is at a certain distance from a coil, where it 'does the most good' to energize the coil and attract the magnet. That is how to maximize torque and maximize efficiency.

Mine is not an expert answer. An expert might reply and tell what sensor positions give best results: either (a) detecting which coil has a magnet nearby, so that we know which coil to turn on. Or (b) to detect location of a magnet between coils, so we know at what point to turn on a coil.

4. ## Re: BLDC motor controller- Hall sensor placement

But u didn't mention about the relation between angle and speed. Is it possible to switch a 3 phase bldc with 2 hall sensor ? What is the programming logic?

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5. ## Re: BLDC motor controller- Hall sensor placement

how does it affect the speed. How can you prove it?Can u explain the logic behind it

6. ## Re: BLDC motor controller- Hall sensor placement

Do you have a commercial BLCD with two sensors or are you designing your own one? Using three (digital) hall sensors is the usual scheme because it allows a simple direct relation of sensor signal to motor control. With only three digital sensors you can't determine the initial position exactly and don't achieve optimal starting torque.

Generating three phase motor voltages based on two sensors isn't completely impossible but involves a large software overhead. It's not simple switching logic but estimating rotor position and speed and generating switch events based on this information.

7. ## Re: BLDC motor controller- Hall sensor placement

Question for you: How acquainted are you with basic electrical motor theory?

In any motor, to generate Torque, the rotor's magnetic field should be at right angles with the stator's magnetic field.
Torque is what rotates a motor, all the other motor quantities derive from it.

In a conventional DC motor, the commutator does that for you.
In an induction AC motor, the field induced on the rotor automatically does that for you.

But in a BLDC, you have to synthesize it. Since the rotor will attempt to align itself with the stator, causing the rotation, you will have to continually and dynamically adjust the rotor's field for the motor to continue rotating.
Thus you have to know at all times the rotor's angular position. This is done at a minimum, with a quadrature encoder, which tells the DSP microcontroller the rotor's Cartesian coordinates. The DSP, from that information and the user's commands, will then synthesize the rotating magnetic field by continuously changing the vector sum of three-phase voltages.

You have to study basic electrical motor theory, and its control methods. There is no other way you will fully understand.
In these days, it is as simple as doing a Goggle search.

Try for instance this web page:
https://www.renesas.com/en-us/soluti...lgorithms.html

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8. ## Re: BLDC motor controller- Hall sensor placement

The second sentence should read: " the rotor's magnetic field should be ideally at right angles with the stator's magnetic field."

9. ## Re: BLDC motor controller- Hall sensor placement

Hello!

I felt lazy to make a drawing on a simple case, but I found that:

There is a description of the simplest possible BLDC. You have 3 sensors and their information
will generate 3 square signals as shown on the figure's top left. We'll call the coils black coil,
yellow coil and red coil.
Let's imagine that when you detect that south of the rotor is facing the black coil (exactly like
the drawing), let's imagine that you generate a north in the black coil. What would it do?
It wouldn't move at all.
Now from this position, imagine that you can turn your hall sensors set. There will be a point
where the motor starts to spin. Slowly first because the timing would be quite bad, and then if
you move further, the speed would increase until an optimal point as indicated by the coil
signals graphics on the left. I suppose that you can understand that the angle has an influence
on how optimal the switching will be.
In fact, there are other ways to spin a BLDC, there are even solutions without hall sensors
by sending a 3-phase signal in the 3 wires. This is a pure openloop system.
And there are also solutions using a high resolution encoder for a close loop system. In this
case, you measure the angle, and you calculate the 3 phases and their sine values to feed
into the motor. These days, you can have (relatively) cheap encoders with a resolution
around 20 bit (1 million values per turn) with which you can have a very smooth movement.

Dora.

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