Driving a three-phase brushless DC motor with Arduino - Part 1. Theory

Typical CD/DVD Spindle BLDC Motor With 12 Magnetic Poles and 9 Wound Cogs

A disassembled CD/DVD brushless DC spindle motor

It may be a little easier to watch the movement of the magnetic field on the animation that follows. The animation can be paused on any step and you can move straight to the step you want if you hover your mouse above the step’s number in the navigation menu. Go ahead, play with it! We’ll be right here when you come back 🙂 While you’re at it, see if you can spot those three points with maximum attraction force in every step. They are always in different places but they are always there.

The south poles are marked with RED and the north poles are BLUE. The poles are so marked in accordance with the commonly accepted naming convention. The direction of the magnetic force is defined by the right hand rule: if the palm of the right hand follows the direction of the current (i.e. goes around the cog as the wire does, current flowing from HIGH to LOW), the thumb points in the direction of the magnetic field. The point where the magnetic field lines are originated from is called NORTH and the thumb is pointing in the direction of SOUTH.

The spots where the red dot on the winding corresponds with the blue pole of the rotor are the maximum attraction points.

The animation above was made based on several assumptions. One of them of course is the motor configuration – 12 poles, 9 cogs. Another one is that the least possible amount of MCU outputs is used to drive the motor and the driver circuitry is the simplest. I am going to have to save the circuitry for another post but suffice it to say here that the easiest implementation from the hardware stand point requires us to drive the winding either HIGH or LOW. That requires only three digital I/O pins and a very common H-bridge IC such as SN754410. There’s another possibility, too: we drive only two windings at a time and leave the third one open so no current is flowing through it and no magnetic force is generated, but that requires using 6 I/O pins and 6 discrete transistors. The benefit of the 6-pin scheme is better efficiency and torque due to lack of secondary, smaller magnetic forces pulling in the wrong direction.

Counting BLDC rotor poles using iron shavings on a piece of paper

Acknowledgements:
To continue your BLDC education and learn more than you ever wanted to know about BLDCs, head straight for Brian Mulder’s Electric Motors, Part 1-5 series of articles on the Southern Soaring Club Web site

Special thanks to Mike Alsup (malsup) for creating the JQuery Cycle plugin used in the animation

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