Some brushless dc servomotors offer the options and torque ranges of larger varieties (for the unit shown here, four-pole operation, continuous torque to 12.2 nNm, and external commutation control or versions with an integrated speed controller). The miniature units can also be equipped with any number of encoders.

Drawbacks: As with brushed motors, brushless varieties must overcome starting friction as well. Again, this is the sum of torque losses not depending on speed.

Dynamic friction is dependent on speed. In fact, dynamic torque friction is the only quantity defining torque losses that is proportional to speed for BLDC motors. A function of speed (as in metric units, of mNm/rpm) dynamic friction is due to both ball-bearing viscous friction, and Eddy currents in the stator originating from the magnet's rotating magnetic field.

The higher cost of construction makes BLDCs impractical for many applications. A designer can easily spend double for a brushless system, and lose the simplicity of a brushed motor to boot. The motors also require space for housing the control/drive electronics, which must be mounted somewhere if not integrated into the motor.

Also keep in mind that the motor can't be mounted too far away from the drive, because long cable runs tend to introduce noise into systems. (To compensate here, phase leads can be twisted and shielded from sensitive feedback leads to reduce noise.) This said, overall a designer can expect excellent linearity in BLDC motor speed-torque ratios.

For more information, visit micromo.com or call (800) 807-9166. Tune in next month for the second part of this two-part series.