Machine Design

Brushless DC Motors

Despite their superior qualities, brushless motors still run second to brush types in motion-control applications. Brushless systems are usually preferred, however, where their benefits outweigh their higher price. And they cost less than brush types in some applications when energy, maintenance, and downtime costs are included in the comparison between the two approaches.

Amplifier costs for both systems have dropped in recent years, moreover, and are forecast to drop further. Thus, the cost differential between the two approaches is an increasingly smaller percentage of total system cost. The situation has brought a large number of competitors into the market, helping to reduce prices for brushless systems. A result is that brushless motor sales have risen rapidly in the last few years. And sales of brushless motors continue to increase.

Conventional brushless motors come in a variety of configurations. The most widely used look much like brush-type motors. But brushless motors have a wound stator that surrounds a permanent-magnet rotor, an inverse arrangement from that for brush motors. And stator windings are commutated electronically rather than through a conventional commutator and brushes.

Brushless motors generally contain a three-phase winding, although some operate four phase. Brushless motors powering small fans and other constant-speed equipment are often two phase.

Power for brushless motors generally is a trapezoidal ac wave form, but some of the motors operate with sine waves. Trapezoidal-powered motors develop about 10% more torque than those on sine-wave power. Sinusoidal-powered motors, however, exhibit less torque ripple and operate smoother at low speed. Thus, sinusoidal-powered motors are often used for machining, grinding, coating, and other operations calling for fine surface finishes.

Because they have no commutator, brushless motors are more efficient, need less maintenance, and can operate at higher speeds than conventional dc motors. High efficiency and small size are especially important for military, aircraft, and automotive applications, and for portable instruments and communications equipment.

The cost of both brush-type and brushless motors likely will rise because of increasing costs for iron, steel, copper, aluminum, and magnets. The increases will be partly offset by use of neodymium-iron-boron magnets. The magnets are often more powerful than samarium-cobalt magnets and promise to be much less expensive.

Amplifiers for brushless motors are more complex than brush types and generally call for two additional solid-state power switches. These switches account for most of the cost differential between the two types. But switch cost continues to drop, in part because of increased use of MOSFET and insulated-gate type switches. Costs also are dropping for ICs used in commutation, feedback interpretation, and PWM circuits. The lower costs reduce but do not eliminate the price differential between amplifiers for the two types of systems.

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