Brushless DC Motor Control

Brushless DC motors are used in all sorts of devices nowadays. You'll find them in CD-ROM drives (spindle motor), video recorders (head drum motor and capstan motor), floppy disk drives (spindle motor), laser printers (mirror drum motor) and even in computer cooling fans. They're popular because, without brushes to wear out, they can be very reliable, with the usual failure mode being wear to the bearings. They also generate no electrical noise, unlike DC motors with brushes and a commutator, and they don't make sparks.

These motors work by having three sets of windings and three position sensing Hall-effect devices. They're driven by a three-phase power supply, just like large AC motors, but in this case the power is switched by a controller chip and the operating frequency is not limited to 50Hz. In fact, the frequency of operation is derived from the Hall-effect devices that detect the position of the rotor and act as an electronic commutator. There must be extra driver electronics to deal with reading the three position-sensing inputs and switching the currents throught the stator coils. I have a head drum motor from a 1989 video recorder that uses a Toshiba TA7736 chip to generate the three-phase drive.

My goal is to generate the three-phase drive with the Atmel AVR chip. Then, I'll have accurate speed control. One possible driver chip is the L298 that I originally planned to use for bipolar stepper motor control. One possible use is to spin a mirror drum for the laser message display.