I reviewed the Renesas BLDC Motor Control Evaluation Kit and you can find the written and video reviews on the Evalkits.com site. Unfortunately, much of the high-level information about brushless DC (BLDC) motors can leave you wondering how they actually work.

A standard brushed DC motor involves a rotor wound with electromagnetic coils that rotates inside a stator, or stationary form, that holds permanent magnets in place. A commutator on the rotor acts like a rotary switch that selects which of the coils to power. Apply power to the coils through the commutator and the electromagnetic field starts the motor rotating. The interaction of the magnetic flux from the permanent magnets and the electromagnets make the motor turn. Fixed brushes, often made of graphite or a similar material, conduct electricity to the spinning rotor's coils. You can see a simple animation of a brushed DC motor at: brushmotor.org/about_Brush_DC_Motor.aspx.

A brushless DC motor, on the other hand, reverses things and places the permanent magnets on the rotor and uses fixed coils to create electromagnetic fields that attract the magnets. The diagram below shows a simplified cross section of a BLDC motor and the electrical configuration of the three coils. By switching the current on or off in each coil and by reversing its direction, you rotate the motor clockwise or counter-clockwise.

Three sets of two coils each in the diagram above drive this BLDC motor. Bipolar voltages, say ±24V, drive coils in pairs: orange+ red-, orange+ blue-, blue- red+, orange- red+, orange- blue+, and blue+ red-.

BLDC motors have several advantages over their brushed siblings. They produce lower electromagnetic interference. Because the coils are on the outside, they can better dissipate heat. Electronic motor-control provides better regulation of speed. They have no brushes to wear out and need replacement. The brushes do not create conductive "dust" in the motor's environment.

But how do you properly operate the current "switches" when you don't have a rotating commutator and brushes?

You can use one of two methods. First, use Hall-effect sensors in the motor or second measure the back EMF in the coils.

Hall-effect sensors detect a magnetic field and let control circuits know the position of the magnetic rotor. Microcontrollers use this information to properly switch the current through external field-effect transistors (FETs) or insulated-gate bipolar transistors (IGBTs). (You can buy packaged modules that provide these motor-drive devices and supporting electronics.) The switching algorithm operates to keep the angle between the flux produced by the permanent magnet and the flux created by the electromagnets at about 90 degrees. That angle produces the maximum torque.

The Hall-effect sensors determine the location of the rotor so the microcontroller can "commutate" the coils in the proper sequence. For more information and the theory behind the Hall-effect sensor operation, see: Elevich, Leonard N., "3-Phase BLDC Motor Control with Hall Sensors Using 56800/E Digital Signal Controllers," AN1916, Freescale Semiconductor, 2005. www.freescale.com/files/product/doc/AN1916.pdf.

And: "3-Phase BLDC Motor Control with Hall Sensors Using the MC56F8013," 56F8013BLDCUG at www.freescale.com/files/dsp/doc/user_guide/56F8013BLDCUG.pdf.

The back electromotive force, or BEMF, technique measures the EMF produced in unpowered stationary coils. In effect, the rotating magnetic core acts like a generator and induces a small but measurable EMF in unpowered coils. The unpowered condition exists while the control electronics drive other coils. Thus, for a given coil, the EMF exists during the time between when the controller drives the coil with a positive voltage and when it then drives it with a negative voltage. The microcontroller needs to detect when this back EMF crosses through 0 volts, which occurs about 30 degrees before the next commutation (or coil switching) should occur.

Unfortunately, the back EMF signal can contain quite a bit of noise, so external circuits or the microcontroller must perform some filtering to determine the actual zero-crossing time. For a detailed description of the back-EMF technique and digital filtering, see: Condit, Reston, "Sensorless BLDC Control With Back-EMF Filtering," AN1083, Microchip Technology, 2007, at: ww1.microchip.com/downloads/en/AppNotes/01083a.pdf.

And also see: Torres, Daniel, "Sensorless Control of a Brushless DC Motor," Design News, June 2009, pp. M9--M10. www.designnews.com/article/279139-Sensorless_Control_of_a_Brushless_DC_Motor.php.

In my next blog entry, "Brushless DC Motors: Part 2," I'll describe why you might choose one commutation technique over the other for a given application.
--Jon Titus