To say "it's about time" would be an understatement regarding the steady, growing, newfound popularity for brushless motors. These cleaner, smaller, lighter-weight wonders are steadfastly gaining ground on their century-old brethren, brush motors.
As recently as two years ago, brushless motors were significantly more expensive than brush motors. But thanks to advances in design and materials, brushless motors have experienced a dramatic drop in pricing. Today, the cost differential between the two motor technologies can be a mere 10 percent. The most notable converts are designers working with industrial applications. This was traditionally considered hard-core brush territory because "clean" operating environments are not of critical importance. But now the brushless motor is finding a new fan base, thanks to its declining cost barrier.
The most notable difference between brush and brushless is the absence of the familiar brush-commutator arrangement. For over a century, commutation for brush-type dc motors has been performed electromechanically via graphite brushes that contact a circular commutator mounted on the rotor.
Thanks to Hall sensors, which feed back the rotor position, the control circuitry always knows the exact moment to commutate (sequence) the motor phases. Most brushless motor manufacturers supply motors with three Hall-effect position sensors. Because the brushless motor lacks brushes, and thus this interface, brushless motors are cleaner, exhibit lower acoustic noise, are virtually maintenance-free and simply last longer.
So what's not to like? Despite all the growing excitement surrounding the brushless wonders of the motor world, work still remains to be done in the area of developing the aforementioned control electronics of the brushless motor's Hall sensors.
The development of motor drive boxes and motor drive cards that provide the designer with the microcontroller, the programmability and the driver-all in a single assembly-are in steep demand. This integration scenario, either in digital or analog mode, in essence performs the necessary commutation in each motor scenario. Without this integration, the brushless motor is rendered idle.
When it comes to selecting the best driver, pulse width modulation (PWM) ICs are gaining acceptance as the preferred technology. The selection of the fittest is based simply on efficiency. The disadvantage of the linear circuit becomes clear in the midrange, when the output level is in the vicinity of 50 percent. At these levels, the resistance of the pass element is equal to the load resistance, which means the heat generated in the amplifier is equal to the power delivered to the load! More simply stated, a linear control circuit exhibits a worst-case efficiency of 50 percent when driving resistive loads at midrange power levels.
Now consider a PWM solution. In a PWM control system, an analog input level is converted into a variable-duty-cycle switch drive signal. The process of switching from one electrical state to another, which is simply the difference between OFF and ON, is called "modulation," which is why this technique is called "pulse width modulation." Beginning at zero duty cycle, which is to say OFF all the time, the duty cycle is often advanced as the motor begins to rotate, until it is running at the speed and/or the torque required by the application. Again, what's not to like?
Microcontrollers that have the special functions (routines) necessary for controlling brushless motors, as well as the PWM IC drivers that deliver power to the motor and form the interface between the microcontroller and the brushless motor, itself, are beginning to emerge. But much work remains to be done in the marketplace to provide designers with the integrated, brushless control circuitry options they now desperately seek.
David Cox (firstname.lastname@example.org) is vice president of engineering at Apex Microtechnology Corp . He joined Apex in 2004 and holds a BSEE from the University of California at Berkeley, a MSEE from Stanford University and his PhD from Utah State University.