How widely are brushless DC motors used?

Brushless DC motors (BLDCMs) are also known as electronically commutated motors which have trapezoidal back electromotive force (EMF) and are fed with rectangular stator currents. Owing to their advantages of smaller size, lighter weight, higher efficiency, simpler structure, lower maintenance, greater longevity, easier control, and higher performance, BLDCMs have been widely used in many fields from aerospace (to drive flywheels, gyroscopes, and solar panels of spacecrafts or rudders of missiles and aircrafts), military (actuators of gun turrets, radars, inertial stabilized platforms, and tank traction systems), industrial (in electrical vehicles, CNC machines, computer peripherals, robots) and domestic applications such as powering energy-saving reciprocating compressors in air conditioners and refrigerators.

Why is sensorless control necessary?

To realise high-performance control of BLDCMs, the rotor position sensors are often mounted in the motors to obtain the information of rotor position and velocity. However, this not only increases the size, weight and cost, but also decreases the reliability. Even if the simple switch-mode Hall position sensor is used, the reliability can not be guaranteed in high temperature, strong vibration and strong corrosion environments. Sensorless control is a necessary solution to lower the size, weight, and cost, while improving the reliability in harsh environments.

What are the limitations of the existing sensorless control solutions?

Among the various sensorless control strategies, the zero-crossing point of the back EMF has been widely used to provide the rotor position information. However, information on the rotor position is difficult to derive from the weak back EMF when the motor is at low speed or still. A simple starting method for BLDCMs is ‘align and go’ which does not need the information of the rotor position, but large instantaneous peak currents, even starting failure, are probably incurred due to improper commutation. Although the level of phase currents can be used to correct commutation points, three-phase currents in AC link need to be measured. The terminal voltage at the commutation point is also used to improve commutation positions, but it is sensitive to the measurement noise.

Are there any applications in particular that inspired you to come up with a new approach?

BLDCMs are used to drive high-speed rotors of control moment gyroscopes which are used for attitude control in the spacecrafts. Sensorless control is preferred, but the open-loop starting method without rotor position information does not start the motor successfully every time, due to the large inertia and heavy load of control moment gyroscopes. To improve the reliability of the starting procedure without additional sensors, we developed our terminal-voltage-based starting strategy.

Can you describe the main features of the approach reported in your Electronics Letters paper?

The starting failure of BLDCMs often arose in our experiments, until we found an interesting relationship in which the waveform of the filtered terminal voltages were affected by commutation angles. Only when the commutation instant coincided with the proper one would the waveform during the former and the latter conduction interval of 60° be symmetric. Otherwise, the waveform during a conduction interval of 120° would rise or fall when the commutation instant was advanced or retarded respectively. How could we explain this relationship and how could we use this relationship to start BLDCMs?

The approach presented in our Letter is the first to use the waveform of average terminal voltages to start BLDCMs. It can be regarded as a quasi closed-loop starting strategy, because it can correct the commutation frequency automatically, depending on the information of commutation angles derived from the waveform of average terminal voltages. Additional sensors are not necessary: it only needs the detection of the filtered terminal voltage. It has better robustness to measure noise because of its dependence on the waveform index, instead of the zero-crossing detection or the instantaneous voltage. Owing to its high performance and ease of implementation, it should attract much more attention in this field, and we think that this approach might make its way into real-world use in a year.

How will your group continue this work?

The areas of interest of our group mainly include sensorless control of BLDCMs, advanced control of servo motors and attitude control of spacecrafts. In the future, we will study the realisation of closed-loop speed regulation based on this work, and extend the results to space applications with high-reliability requirements, such as rate gyroscopes, scanning-type infrared earth sensors, momentum wheels and control moment gyroscopes. We believe that, over the next decade, sensorless BLDCMs will achieve as good a control performance as conventional motors with position sensors and, with their advantages, they will be used much more widely.

Further reading

This interview is based on the letter 'Terminal-voltage-based starting strategy for brushless DC motors without position sensors' (new window)

A PDF version (new window) of this feature is also available