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.

Fig. 3 – Speed-torque characteristic.

Since the brushless motor stator field is produced by permanent magnets, the field remains constant at all motor speeds. This provides linearity to speedtorque characteristics, which makes it easy to design into applications. It’s simple to predict performance when loads vary. It’s simple to predict performance when voltage/current becomes limited, such as low line voltage conditions. And, it’s simple to predict outcome with temperature variations.

For example, Figure 3 presents a speed-torque characteristic curve. The motor has a load of T1 and voltage V1 is applied; speed will be S1. If the load would be increased to T2, then motor speed will drop to S2. The question now becomes whether this is acceptable in the application. This analysis creates awareness on the designer’s part and, if it creates consequences for the application, then plans can be made to handle or correct the situation. It’s easy to determine the operating point on this linear speed-torque characteristic curve. Thus, the machine designer knows what performance will be delivered.

An inherent design feature of brushless motors is the fact that they deliver more speed, torque, and power. There are two ways in which brushless motor specifications will be reviewed: the first by comparing motors of similar torque and the second comparing motors of the same diameter.

Table 1 – Speed and power of DC PM and brushless (BL) motors with similar stall torques.

Specs for motors with the same continuous stall torque are presented in Table 1, for brush motors (graphite brushes) and brushless motors. For this comparison, it can be seen that brushless motors provide a higher top speed capability improvement in excess of 5 times. However, in general top speed improvements of 3 to 5+ times may be expected. Power output capability shows an improvement of more than 3 times.

Higher speed and power capability reflects directly to the potential of increased productivity rates, that is, producing more parts per hour. Applications have seen 80 to 90 percent productivity improvement with brushless motors. This directly reflects to the bottom line of businesses.

Table 2 – Peak torques for DC PM and brushless motors with similar stall torques.

Additionally for motors with similar continuous stall torques, brushless technology will provide improved peak torque capability for many applications (See Table 2). In general, a DC PM motor can provide 200 to 500 percent more peak torque (ratio of peak vs. continuous capability), and brushless can provide 400 to 1,000 percent. This peak torque capability aids in providing for high bandwidth that results in fast compensation for any machine load disturbances and, therefore, makes corrections faster for any errors that may occur.

As an example, velocity loop bandwidths for motors with a feedback device can be on the order of 50 Hertz; brushless will have bandwidths of 100 Hertz and higher. The bottom line is that higher torques allow for accuracy and precision in making corrections and parts.

Machines utilizing brushless have improved positioning and accuracy/precision which has reduced reject rates by up to 75 percent. If the distance to be moved is long or short, at slow or fast speeds, the brushless servomotor will provide precise and accurate positioning.

Table 3 – Comparison of DC PM and brushless motors in same package size.

Specs for motors in the same package size are presented in Table 3. In this comparison, it is noted that brushless motors present a higher top speed capability— indicating potential for increased productivity. Brushless delivers more continuous stall torque, usually 20% or much more—indicating potential for getting into position faster with more precision and accuracy. Additional brushless motors provide more peak torque capability. In other words for brushless motors, torque per volume is maximized. (See Table 3)

Using brushless motors in new designs or upgrading existing equipment will improve the machine’s response, precision, and productivity. For example, in an application of brushless servos used in labeling equipment, a builder boasted profits of $6,000/day had increased to $11,250/day with the improvement in cycle time alone.

Conclusion

Automation is one of the best investments a manufacturer can make in order to maintain a competitive edge. Brushless motors with their inherent advantages and benefits are designed to provide applications with high speed and continuous duty. The primary difference between brushless and other motor types is that brushless are electronically commutated. Brushless technology provides high power, high torque (upon demand), high speed (when needed), and long life (all the time).

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this article was contributed by Motor Consultants, LLC, with contribution by MICROMO Engineering, Clearwater, FL. For more information, Click Here .