Three Phase Brushless Servo Motors

Scope

Brushless servo motors include a wide range of motor types including both permanent magnet brushless DC motors and permanent magnet brushless AC motors. They can also be designed for rotary or linear motion. Regardless of the different types, some things they have in common: they use permanent magnets and they are three phase motors.

Construction

Brushless servo motors replace the mechanical commutator found in brushed motors with electronic commutation. In brushless servo motors, it is the permanent magnet that acts as the rotor (rotating component) rather than the coils which are fixed. Rotation of the servo motor is achieved by individually altering the magnitude and direction of current into each set of coils, changing the direction of the magnetic fields generated by the stationary coils. There are typically three sets of coils in a brushless servo motor, each set of coils is individually energized in a particular order that creates a series of electromagnetic forces to keep the rotor in motion. This requires 3 power wires to supply current to each set of coils. Because the coils are fixed, direct electrical connections can be made to them easily, eliminating the need for mechanical commutators and brushes. The absence of brushes and commutators in the brushless motor makes this device very durable as the brushes and commutators in brushed motors are susceptible to wearing down as a result of continuous moving contact.

 

Because they do not have mechanical commutators, brushless servo motors require electronic commutation and feedback devices to control the speed of the motor. Electronic commutation has increased efficiency, high energy savings, and better control compared to mechanical commutation. However, with the lack of mechanical commutators most brushless servo motors require feedback devices to accurately control motor torque and position. There are different options when it comes to levels of feedback and commutation that depend on economic constraints and preferred motor precision. This allows the consumers to choose their preferred levels of feedback and type of electronic commutation to fit their application. Hall sensors and encoders are the feedback of choice for brushless servo motors, both devices can be used separately or in tandem to utilize the preferred type of commutation. Hall sensors alone are only capable of trapezoidal commutation while encoders alone are only capable of sinusoidal commutation. Using hall sensors and encoders together allows for the use of either sinusoidal or trapezoidal commutation. Trapezoidal commutation is standard in brushless servo motors because it is easily implemented and is great for delivering maximum torque. Although trapezoidal is preferred for most applications, sinusoidal commutation with brushless servo motors is helpful for applications where audible and mechanical noise must be extremely low. Sensorless feedback is a newer innovation in brushless DC servo motor technology, where motor position is determined from the back emf of the motor. Sensorless feedback can operate with either trapezoidal or sinusoidal commutation, however, sinusoidal commutation is preferred for more sensitive applications because it is less susceptible to torque ripple. Because sensorless feedback requires the motor to run based on its own internal feedback, sensorless feedback is most effective when the motor’s load is static. For example, drones have become a common application of sensorless feedback in brushless DC servo motors because the load does not change significantly while the drone is in motion and the lack of feedback devices allows it to be lighter.

History

Brushless DC motors came about at the start of the digital revolution in the 60s as we shifted from mechanical and analogue electronic technology to digital electronics. Brushed DC motors proved insufficient to withstand more intense application demands since brushes and commutators wore out so quickly, so the brushless DC motor was born. The availability of solid-state power semiconductors such as MOSFETs made the brushless DC motor possible as the first DC machine with solid-state commutation. The downside of early brushless DC motors was that they could not generate a great deal of power. When stronger permanent magnet materials became available in the 1980’s brushless motors were able to generate as much or even more power than their brushed counterparts.

Future Innovations

Today’s brushless motors overcome many of the limitations of brushed motors as they have a higher output power, smaller size, better efficiency, more durability, and very low electrical noise. These benefits also come with drawbacks, because brushless motors require feedback devices and a motor drive controller for electronic commutation, they tend to be more expensive than brushed motors. However, new advancements in brushless DC motor technology have enabled sensorless motor drives which will make these motors more affordable. In sensorless control technology, the position of the rotor is determined by sensing the back EMF (electromotive force) from one of the motor’s terminal voltages, eliminating the need for Hall sensors and encoders. Sensorless control will also allow brushless DC motors to be smaller, more reliable, and more durable because of the decreased number of components.

Another innovation that will soon become common in brushless DC motor design is the integration of brushless DC motors and drive electronics into a single package to create a simpler system. As the efficiency of electronic components increases power electronics are getting smaller and smaller, giving integrated brushless DC motor drives a key role in technological innovations

Uses/Applications

Because of their efficiency and longevity, brushless DC servo motors are widely used in devices that run continuously such as household appliances and other consumer electronics. For industrial applications, brushless DC motors are a popular choice in linear motors, servomotors, extruder drive motors, and feed drives for CNC machine tools because of their reliable and precise motion control capabilities. Brushless DC servo motors have also become the motor of choice for drones because they supply a lot of power while maintaining a low size and weight. It is expected that brushless DC servo motors’ applications will only continue to expand in the future. As these motors become more affordable it is likely that they will replace brushed DC servo motors entirely.

Benefits

  • High power density
  • Excellent heat dissipation
  • Less maintenance than brushed motors and longer lifespan
  • Electronic commutation allows for more accurate control and increased power savings
  • Reduced operational and mechanical noise as compared to brushed DC motors
  • Smaller and lighter than brushed DC motors

ADVANCED Motion Controls' Capabilities

  • Majority of off-the-shelf servo drives operate with brushless motors and include a wide variety of feedback and performance options
  • Brushless motors can be more difficult to configure but digital servo drives greatly simplify the process with features like AutoCommutation

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