Brushless DC motors offer definite advantages in certain applications. So, should you automatically choose a brushless DC? After all, you're familiar with DC motors. Well, comparing brushless DC motors to brush-type motors may have a significant impact on your final decision.

The photo (right) shows a cutaway of a brushless DC motor. This view shows that the size and shape of the magnets clearly differ from a brush motor's. In Figs. 1 and 2, on page 64 (of the original article), you can see more differences between a brushless and brush-type motor. Understanding these differences can save you much grief.

The sidebar "Factors To Consider When Selecting A Motor Type," on page 66 (of the original article), lists seven critical points everyone should be aware of before selecting a motor. Looking at these factors more closely, you'll see the brushless DC motor can do some interesting things for you.

Tight speed regulation. Applications requiring especially reliable speed regulation benefit from using brushless DC motors. You get precise control because the brushless DC motor gives the controller feedback from the motor's commutation sensors. These sensors (usually the Hall Effect type) give a direct measure of rotor speed, rather than relying on indirect methods.

The common brush-type SCR speed controls rely on an indirect measure of armature speed by reading the back-EMF generated by the motor. The back-EMF constant (or voltage constant) is a function of the motor's winding resistance, which in turn is a function of the winding temperature. As a motor warms up, its winding resistance changes, and this causes the measured speed to "drift." Some SCR controls have a temperature-compensation circuit. Unfortunately, these circuits don't completely resolve the problem. This is especially true when the motor is operating at the low end of its speed range, or if it is driving a very light load. As a result, you might set the speed of the machine, only to find it is running too fast an hour later.

However, a brushless DC motor and control system will give the same output speed regardless of changes in motor temperature.

Low audible noise. Manufacturers of medical centrifuges typically use series-wound brush-type motors because of their high operating speed (up to 15,000 rpm). However, at such high speed, the brushes make significant noise as they rub the commutator. The ventilated motors add to the overall noise, as their fans push air through dozens of tiny crevasses. Run a few of these motors in one area, and you get an idea of what the vacuum cleaner Olympics would sound like.

By comparison, a non-ventilated brushless DC motor at the same speed is virtually silent. We're starting to see these users switch to the brushless DC.

Low EMI. Brush-type motors produce electromagnetic interference (EMI) because of arcing at the brush centers. In applications having very stringent noise standards, like office equipment (or any machine that needs CE certification for Europe), you may have to add filter capacitors, line filters, or shielding to contain the EMI generated by a brush-type motor.

No brush maintenance. Brushes, by design, wear out faster than the motor. You have to replace them if you want to keep using that motor. After a few brush changes, it's not unusual for you to have a worn commutator. Since expected brush life is difficult to predict, the typical solution is to replace brushes well before they completely wear out. However, with a brushless motor, you don't have to endure this extra expense and hassle. Applications depending on an especially reliable drive can benefit from a brushless DC motor.

Small size. The heat-generating part of a brush-type DC motor is the armature winding, in the center of the motor. The heat-generating part of a brushless DC motor is the stator winding, which is close to the outside surface of the motor. Since it's easier for heat to dissipate from the outside of the motor than from the inside, the thermal resistance of the brushless DC motor is lower. Thus, you can produce more continuous power before exceeding the temperature limit than you could with a brush-type. This means a smaller brushless DC motor can provide the same continuous power output as a larger brush-type DC motor. For example, one manufacturer's one-eighth hp brushless DC motor fills 28 cubic in. of space and weighs 3.5 lb. Compare that to a one-eighth hp brush-type DC motor, which takes up to 48 cu in. of space and weighs 6.3 lb. Any product designed for portability can benefit from a lighter and smaller motor.

No brush dust. Brushless DC motors obviously produce no brush dust. This makes them desirable for clean-room environments. Applications could include food processing machinery or semiconductor handling equipment.

Lower rotor inertia. In a brushless DC motor, the bulky winding sits in the stationary part of the motor. In a brush-type motor, the winding is in the rotating part. Thus, the latter motor has to accelerate and stop a higher inertial mass (winding plus rotor). But, the one-eighth hp brushless DC motor mentioned earlier has a rotor inertia of .0072 oz-in-second-squared. A one-eighth hp brush-type motor has an armature inertia of just 0.24 oz-in-second-squared. This makes the brushless DC motor a good choice for positioning applications.

Suggested Reading EC&M Books: Understanding NEC Rules on Motors & Motor Controls, 2nd Ed. Order #6115; four-page supplement to the 1993 book, Order #4104S96. Practical Guide To Motors & Motor Controllers, Order #4570. For ordering information, call (800) 543-7771.

* Tight speed regulation; * Low audible noise; * Low EMI; * No brush maintenance; * Small size; * No brush dust; and * Low rotor inertia.