BLDC: Brushless DC

  PMSM: Permanent Magnet Synchronous Motor

  On the surface, the basic structure of brushless DC motors and permanent magnet synchronous motors is the same:

  1) Their motors are all permanent magnet motors, with rotors composed of permanent magnets and stators equipped with multi-phase AC windings;

  2) The torque of the motor is generated by the interaction between the permanent magnet rotor and the alternating current of the stator;

  3) The stator current in the winding must be synchronized with the rotor position feedback;

  4) The rotor position feedback signal can either originate from a rotor position sensor or be acquired by detecting the back electromotive force of the phase windings of the side motor, akin to certain sensorless control methods.

  Although the basic structure of permanent magnet synchronous motors and brushless DC motors is similar, their driving methods differ, and there are also significant differences in design and control details.

  1) The back electromotive force (EMF) differs between permanent magnet synchronous motors and brushless DC motors. The back EMF of a permanent magnet synchronous motor is a sine wave, whereas the back EMF of a brushless DC motor is a trapezoidal wave;

  2) The distribution of stator windings is different. Permanent magnet synchronous motors employ short-pitch distributed windings, and sometimes fractional slot or sine wave windings, to further reduce torque ripple; whereas brushless DC motors utilize full-pitch concentrated windings.

  3) Different operating currents. To generate a constant electromagnetic torque, permanent magnet synchronous motors use sine-wave stator currents, while brushless DC motors use square-wave currents.

  4) The shapes of permanent magnets differ. The shape of PMSM permanent magnets is parabolic, and the magnetic density distribution generated in the air gap is as close to a sine wave as possible; the shape of BLDC permanent magnets is tile-like, and the magnetic density distribution generated in the air gap exhibits a trapezoidal waveform

  5) Different operation modes. Permanent magnet synchronous motors (PMSMs) operate with three phases simultaneously, with each phase current differing by 120° in electrical angle, thus requiring position sensors. In contrast, brushless DC motors (BLDCMs) operate with two windings conducting at a time, with each phase conducting at an electrical angle of 120° and switching phases every 60° in electrical angle, requiring only the detection of commutation points. These differences result in significant disparities in the control methods, control strategies, and control circuits of PMSMs and BLDCMs.

  Due to differences in design and control methods, the characteristics of permanent magnet synchronous motors and brushless DC motors also differ. The performance comparison is as follows:

  1.Torque ripple

  Torque ripple is the biggest problem faced by electromechanical servo systems, as it directly affects precise position control and makes it difficult to achieve high-performance speed control. At high speeds, the inertia of the rotor can filter out torque ripple. However, in low-speed and direct drive applications, torque ripple can severely impair system performance, leading to reduced system accuracy and repeatability. Most space precision electromechanical servo systems operate at low speeds, making motor torque ripple one of the key factors affecting system performance. Both permanent magnet synchronous motors (PMSMs) and brushless direct current motors (BLDCMs) suffer from torque ripple issues. Torque ripple is mainly caused by the following reasons: cogging effect and magnetic flux distortion, torque caused by current commutation, and torque caused by manufacturing processes.

  2.Power density

  High-performance applications such as robotics and space actuators, it is required that the motor weight be minimized for a given output power. Power density is limited by the heat dissipation capacity of the motor, namely, the surface area of the motor stator. For permanent magnet motors, most power losses occur on the stator, including copper loss, eddy current loss, and hysteresis loss, while rotor losses are usually neglected. Therefore, given a certain structural size, the lower the motor losses, the higher the allowable power density.       According to "Permanent Magnet Brushless DC Motor Technology", it can be seen that under the same size conditions, the power output of BDLC can be 15% higher than that of PMSM. If the iron loss is the same, the power density of BDLC can be 15% higher than that of PMSM.

  3.Torque-to-inertia ratio

  The torque-inertia ratio refers to the maximum acceleration that a motor can provide on its own. Since the output power of BDLC is 15% higher than that of PMSM, its electromagnetic torque is also 15% higher than that of PMSM. If the rotational speeds of BDLC and PMSM are the same, and their rotors have the same moment of inertia, then the torque-inertia ratio of BDLC will be 15% higher than that of PMSM.

  4. Sensor aspect

  (1) Rotor position detection: In a brushless DC motor, only two phase windings are energized at any given time, with each phase being energized at an angle of 120°. As long as these commutation points can be accurately detected, the normal operation of the motor can be ensured. Typically, three Hall sensors are used for motor control. In a permanent magnet synchronous motor, sine wave current is required, so all three phase windings are energized simultaneously when the motor is operating. This necessitates the use of continuous position sensors, the most common of which is a high-precision encoder.

  (2) Current detection: For three-phase motors, to control the winding current, it is necessary to obtain information on the three-phase currents. Typically, two current sensors are used, as the sum of the three-phase currents is zero. For some simple brushless DC motor control systems, only one current sensor is required to detect the bus current, reducing costs.