The little motor that could: The rise of the BLDC motor

Thanks to the development of stronger permanent magnets capable of producing higher torque levels, the brushless DC (BLDC) motor is quickly displacing the single-phase induction motor that traditionally dominated most low-power markets. The BLDC also compares favorably to the brushed motor; it is more reliable, more efficient and generates lower acoustic and electrical noise, all at a lower cost, thus making it the motor technology of choice for appliance control.

Actually this versatile motor is not a DC motor at all; the wound stator is supplied with an alternating current, which can be of various waveforms. The permanent magnet rotor rotates synchronously with the rotating MMF set up by the stator. Unlike the induction motor, there is no slippage between the stator and rotor fields.

BLDCs have been the focus of innovative electronic control solutions. For example, 'sensorless' solutions that monitor the 'back EMF' to measure the relative position of stator and rotor are replacing solutions that rely on physical, Hall-effect sensors. This simplifies motor construction, reduces cost, eliminates the wiring and connections to sensors and thus improves reliability in dirty, humid environments.

Another innovation is to provide continuous sinusoidal output voltages to the motor phases, which eliminates the torque ripple and commutation spikes associated with the simpler trapezoidal commutation. Reduced acoustic noise is a desirable attribute in appliance motors.

This reference design for pedestal fans and other small appliances from Texas Instruments enables engineers to design sensorless control for 24V/2A BLDC motors, with sinusoidal commutation. It is a compact single-layer design of 2-inch diameter and an integrated controller with protection against over-current, under-voltage and over-temperature conditions.