Brushless motors are used worldwide for their basic benefit of optimizing performance per package size. No other technology can match it. In addition, brushless brings tremendous advantages to many applications, combining the best features and benefits of past successful designs while incorporating unique advantages.

Industry Pushed Developments

Fig. 1 – Exploded view of brushless motor.
At the dawn of the industrial age, the predominant machine was the AC induction motor, which is basically a constant speed device. Thus when applications required different speeds, the pulley would be changed, resulting in significant production down time—it took a long time to change pulleys.

Industry needed a more efficient method to attain different speeds. And, it wasn’t until the development of the DC motor that this would be accomplished. By simply changing the applied voltage, motor speed could be varied. DC motors remain in use today in applications needing variable speed control.

The next design was the DC PM (permanent magnet) motor. Since the stator field is generated with permanent magnets the field remains constant, this provides linear and predictable speed-torque characteristics, that is, speed is dependent upon applied voltage, and torque delivered is dependent on current. Since PM torque/current relationship provides high torques, these motors are used in applications where accurate positioning control exists.

The next motor technology enhancement was the brushless motor. This design has tremendous advantages. It combines the best benefits of both AC and DC designs, plus brings its own unique advantages to applications. It combines the long life of the induction motor and linearity of the permanent magnet motor, plus adds higher speed range capability (productivity), size weight reduction (compact design), and improved torque capability (precision). Therefore, it provides machine designers with a competitive edge in their marketplace by increasing production capability, improving machine reliability, and increasing life.

Brushless Concept

Brushless motors are used in markets worldwide from medical to electronics, from robotics to security, from cash dispensers to industrial automation, from instrumentation to automotive, as well as many other applications.

A brushless motor is similar to other motors; the design consists of a housing, windings, permanent magnets, and a shaft. (See Figure 1) Note, all motor technologies have a means for switching power between electrical windings, termed commutation, that result in shaft rotation. The AC motor uses alternating power, and the DC PM motor uses a mechanical commutator. However, brushless is a PM motor that does not incorporate a mechanical commutator in the design. This is the basic concept that makes a brushless motor uniquely different than other motor technologies; it utilizes “electronic commutation”.

Fig. 2 – Electronic commutation of brushless motors.
Actually, the brushless motor has no part in “commutation”. It is accomplished by the control, that is supplying power running the motor, which accomplishes the “electronic commutation”. Figure 2 illustrates the concept of how electronic commutation works. For this example, the brushless motor consists of a rotor with permanent magnets, stator with windings and a small feedback device (that consists of a small magnet and three Hall sensors). (See Figure 2)

As the rotor shaft turns, the small feedback magnet passes by and causes the Hall sensors to turn “on” and “off”. This provides information about rotor/shaft position. Of course, other feedback devices may be used to detect rotor/shaft position, however this was chosen for simplification. This rotor position information is fed into the control’s logic circuitry.

The logic circuitry uses rotor/shaft information to turn “on” specific power devices applying power to specific stator windings, thus generating a stator magnetic field. The PM rotor follows it. The Hall sensor feedback information allows for maintaining the optimum angle for rotation and for maximizing torque.

Note that commutation information for servo positioning applications may be obtained via Halls, encoder, or resolver feedback, and for non-servo simple speed control applications via back-emf.

Features and Advantages

Brushless motors provide a long, maintenance-free, trouble-free life. This is because, as indicated, there is no mechanical commutator, no parts (i.e., brush/commutator) to wear which limit life. Of course life is also dependent on some other application factors, such as shaft loading (axial and radial), application profile (accel, duty cycle, and environment), etc. Brushless motors can provide a long life of 10,000 to 15,000 operating hours.

Medical Design Briefs Magazine

This article first appeared in the July, 2015 issue of Medical Design Briefs Magazine.

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