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

As a result, you might set the speed of the machine, only to find it is running too fast an hour later. 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. Unfortunately, these circuits don't completely resolve the problem. 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. 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. The common brush-type SCR speed controls rely on an indirect measure of armature speed by reading the back-EMF generated by the motor. These sensors (usually the Hall Effect type) give a direct measure of rotor speed, rather than relying on indirect methods. You get precise control because the brushless DC motor gives the controller feedback from the motor's commutation sensors. Applications requiring especially reliable speed regulation benefit from using brushless DC motors. Looking at these factors more closely, you'll see the brushless DC motor can do some interesting things for you. 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. Understanding these differences can save you much grief. 1 and 2, on page 64 (of the original article), you can see more differences between a brushless and brush-type motor.

This view shows that the size and shape of the magnets clearly differ from a brush motor's. The photo (right) shows a cutaway of a brushless DC motor.

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