Brushless DC Motorsmo

Brushless DC Motors

Conventional dc motors have traditionally been used in applications where dc

power sources are available, such as on aircraft and automobiles. However, small

dc motors of these types have a number of disadvantages. the principal disadvantage

is excessive sparking and brush wear. Small, fast dc motors are too small

to use compensating windings and interpoles, so armature reaction and L di/dt effects tend to produce sparking on their commutator brushes. In addition, the high

rotational speed of these motors causes increased brush wear and requires regular

maintenance every few thousand hours. If the motors must work in a low-pressure

environment (such as at high altitudes in an aircraft), brush wear can be so bad

that the brushes require replacement after less than an hour of operation!

In some applications, the regular maintenance required by the brushes of

these dc motors may be unacceptable. Consider for example a dc motor in an artificial heart-regular maintenance would require opening the patient's chest. In

other applications, the sparks at the brushes may create an explosion danger, or

unacceptable RF noise. For all of these cases, there is a need for a small, fast dc

motor that is highly reliable and has low noise and long life.

Such motors have been developed in the last 25 years by combining a small

motor much like a permanent magnetic stepper motor with a rotor position sensor

and a solid-state electronic switching circuit. These motors are called brushless dc

motors because they run from a dc power source but do not have commutators and

brushes. A sketch of a small brushless dc motor is shown in Figure 1, and a

photograph of a typical brushless dc motor is shown in Figure 2. The rotor is

similar to that of a permanent magnet stepper motor, except that it is nonsalient. the

stator can have three or more phases (there are four phases in the example shown).

The basic components of a brushless dc motor are

1. A permanent magnet rotor

2. A stator with a three-, four-, or more phase winding

3. A rotor position sensor

4. An electronic circuit 10 control the phases of the rotor winding

Brushless dc motors are available only in small sizes, up to 20W or so, but

they have many advantages in the size range over which they are available. Some

of the major advantages include:

1. Relatively high efficiency.

2. Long life and high reliability.

3. Little or no maintenance.

4. Very little RF noise compared to a dc motor with brushes.

5. Very high speeds are possible (greater than 50,000 r/min).