What are Hall sensors used for in a brushless DC motor?

Question

I'm trying to buy a brushless DC motor for a hobby project, and I found a nice one, which lists the information table I pasted at the bottom of this message.

As you can see at the top of the table you can choose between one with, and one without a Hall sensor. So I was reading up on Hall sensors, and I read that Hall sensors can measure the strength and direction of an electromagnetic force. And (as far as I understand) this can be used for measuring the position and speed of rotation within a Brushless DC motor.

My questions are:

1. Am I correct in what I say above?
2. I also need a rotary encoder for my motor. Can a Hall sensor be used for the same purpose as a rotary encoder?
3. Can a motor with a Hall sensor be reversed or does that have nothing to do with it?
4. Or am I wrong here and is the Hall sensor used for something completely different?

Answer 1

The Hall effect sensors provide a positive indication of rotor position (it has the magnets on it for your type of motor). This allows the 3-phase coils to be driven appropriately (commutated) in sequence depending on rotor position. You could read this for some help.

Sensors add capability for both higher starting torque (much higher) and higher RPM.

In a sensorless 3-phase motor the torque at zero speed (high slip speed compared to rotor position) is much less than with a sensored motor (commutated in sequence with rotor). This is the main benefit for sensored drives but the controllers are specifically designed to support sensed commutation.

If you use the sensored motor then you can detect 6 states (with 3 sensors) and so could use them as a crude 60 degree encoder. You'd have to generate the position signal with an MCU unless your controller provided outputs.

Here's a paper discussing high resolution rotor position sensing using Hall effect sensors if you are willing to do some DSP work.

Whether you buy sensored or sensorless BLDC motors they can be driven CW or CCW.