Brushless dc motors: More power from a smaller package

Tag: Brushless Fan

Today’s brushless dc motors and gearmotors combine many of the best features of both ac and dc drive systems. Just like ac motors, brushless dc motors eliminate brush maintenance, dust and brush-generated electromagnetic interference. In addition, brushless dc motors provide a much wider speed range than inverter-driven ac motors. Brushless dc motors also run more quietly than their brush-type counterparts. Moreover, brushless motor construction makes the motors more thermally efficient, resulting in greater power from a smaller package.

Taken together, these benefits equate to longer life for a brushless motor than for a comparable permanent magnet dc motor. Compared to inverter-driven ac motors, brushless dc motors provide the same long-standing performance advantages of their brush-type counterparts:

    * Higher starting torque Predictable performance (a linear speed-torque curve)
    * Wider speed range
    * Ability to run from a wide range of power supply voltages.

Because brushless dc motors are constructed with magnets, bearings, laminations, shields and processes similar to many widely available PMDC motors, they typically compare favorably in cost to PMDC motors. With the cost of controls continuing to go down, some OEMs are even finding it cost effective to swap brushless dc systems for older brush-type designs (see Brushless dc motors become increasingly common).

Designers are also increasingly taking advantage of the special performance capabilities inherent in the brushless dc design to substitute them for more costly high-end servomotors. Inverter-driven ac motors have also been used to replace older brush-type designs. However they lack the wide speed range of brushless dc motors.
Hybrid of ac, dc designs

AC induction motors were the earliest motors to be commercially available. Typically, these motors have cast aluminum rotors and windings on the outside that induce current in the rotor to create the electromagnetic fields necessary for rotor movement. As electronics and magnet technology progressed, brush-type PMDC motors appeared. These motors reverse the design of ac motors – the windings are on the rotor (called the armature) and permanent magnets or field windings are on the outside.

The brushes are pieces of carbon-copper composite graphite that rub on a portion of the rotor, called the commutator, to electrically connect it to the power source. The commutator segments are located so that as the rotor turns, current flows in the proper direction in the rotor winding to keep the motor going in the desired direction.