BLDC：Brushless Direct Current
PMSM：Permanent-Magnet Synchronous Motor
On the surface, the basic structure of BLDC and PMSM is the same:
1) Their motors are all permanent magnet motors, the rotor is composed of permanent magnets, and the stator is equipped with multi-phase AC windings;
2) The torque of the motor is generated by the interaction of the alternating current of the permanent magnet rotor and the stator;
3) The stator current in the winding must be synchronized with the rotor position feedback;
4) The rotor position feedback signal can come from the rotor position sensor, or it can be obtained by detecting the back electromotive force of the phase winding of the side motor as in some sensorless control methods.
Although the basic structure of the permanent magnet synchronous motor and the brushless DC motor are the same, they are different in the driving mode, and there are obvious differences in the design and control details.
1) The back EMF is different, PMSM has a sine wave back EMF, while BLDC has a trapezoidal wave back EMF;
2) The distribution of stator windings is different. PMSM uses short-pitch distributed windings, and sometimes fractional slots or sinusoidal windings to further reduce ripple torque; while BLDC uses full-pitch concentrated windings.
3) The operating current is different. In order to generate a constant electromagnetic torque, PMSM is a sine wave stator current; BLDC is a rectangular wave current.
4) The shape of the permanent magnet is different. The shape of the PMSM permanent magnet is parabolic, and the magnetic density generated in the air gap is distributed as a sine wave as much as possible; the shape of the BLDC permanent magnet is tile-shaped, and the magnetic density generated in the air gap is distributed in a trapezoidal wave. .
5) The operation mode is different. PMSM uses three phases to work at the same time, and the current difference of each phase is 120° electrical angle, requiring a position sensor. BLDC uses windings to conduct in twos, each phase is conducted at an electrical angle of 120°, and the phase is commutated every 60° electrical angle, only the position detection of the commutation point is required. It is these differences that make the control methods, control strategies and control circuits of PMSM and BLDCM very different.
Due to the difference in design and control, the characteristics of PMSM and BLDC are also different. The performance comparison is as follows:
1 torque ripple
Torque ripple is the biggest problem of electromechanical servo system, it directly affects precise position control and high performance speed control is difficult. At high speeds, the rotor inertia can filter out torque ripple. But in low speed and direct drive applications, the torque ripple will seriously affect the system performance, which will deteriorate the accuracy and repeatability of the system. Most space precision electromechanical servo systems work at low speeds, so the motor torque ripple problem is one of the key factors affecting system performance. Both PMSM and BLDCM have torque ripple problem. Torque ripple is mainly caused by the following reasons: cogging effect and flux distortion, torque caused by current commutation and torque caused by machining.
2 power density
In applications with high performance indicators such as robots and space actuators, for a given output power, it is required that the weight of the motor be as small as possible. Power density is limited by the motor's ability to dissipate heat, i.e. the surface area of the motor's stator. For permanent magnet motors, most of the power loss occurs in the stator, including copper loss, eddy current loss and hysteresis loss, while the rotor loss is often ignored. So for a given structure size, the smaller the motor loss, the higher the allowable power density. Refer to "Permanent Magnet Brushless DC Motor Technology", it is known that under the same size, BDLC can provide 15% more power output than PMSM. If the iron loss is the same, the power density of BDLC can be increased by 15% compared with PMSM.
3 Torque to inertia ratio
The torque inertia ratio refers to the maximum acceleration that the motor itself can provide. Because BDLC can provide 15% more output power than PMSM, it can obtain 15% more electromagnetic torque than PMSM. If the BDLC and PMSM have the same speed and their rotors have the same moment of inertia, then the torque-to-inertia ratio of the BDLC is 15% larger than that of the PMSM.
4 sensor aspect
(1) Rotor position detection: In BLDC, only two-phase windings are turned on at each moment, and each phase is turned on at an electrical angle of 120°. As long as these commutation points are detected correctly, the normal operation of the motor can be guaranteed. Usually, 3 Halls are used sensor. In PMSM, a sine wave current is required, all three phase windings are turned on at the same time when the motor is working, a continuous position sensor is required, and the most common is a high-precision encoder.
(2) Current detection: For a three-phase motor, in order to control the winding current, it is necessary to obtain three-phase current information. Usually two current sensors are used because the sum of the three-phase currents is zero. For some simple brushless DC motor control system clocks, only one current sensor can be used to detect the current of the bus to reduce costs.