Quickly understand the difference between BLDC and PMSM

Modern motor and control technology divides permanent magnet brushless DC motors into two categories based on different current drive modes:

1) Square wave drive motor, also known as brushless DC motor (BLDC);

2) Sine wave drive motor, also known as permanent magnet synchronous motor (PMSM).

Similarities

On the surface, the basic structures of BLDC and PMSM are 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 AC current between 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 be obtained from the rotor position sensor, or by detecting the back electromotive force of the motor phase winding as in some sensorless control methods.

Differences

Although the basic architecture of permanent magnet synchronous motors and brushless DC motors is the same, there are obvious differences in their design and control details due to their different driving methods.

1) Back electromotive forces: PMSM has a sine wave back electromotive force, while BLDC has a trapezoidal wave back electromotive force;

2) Stator winding distribution: PMSM uses short-pitch distributed windings, and sometimes fractional slots or sinusoidal windings are used to further reduce ripple torque; while BLDC uses full-pitch concentrated windings.

3) Operating current: In order to generate constant electromagnetic torque, PMSM is a sine wave stator current; BLDC is a rectangular wave current.

4) Shapes of permanent magnets: PMSM permanent magnets are in the shape of a parabola, and the magnetic density generated in the air gap is distributed as a sinusoidal wave as much as possible; BLDC permanent magnets are in the shape of a tile, and the magnetic density generated in the air gap is distributed in a trapezoidal wave. .

5) Operating modes: PMSM uses three phases to work simultaneously, and the current of each phase differs by 120° electrical angle, requiring a position sensor. BLDC uses windings to conduct in pairs, each phase conducts at an electrical angle of 120°, and commutates every 60° electrical angle. It only needs to detect the position of the commutation point. 

It is these differences that make the control methods, control strategies and control circuits of PMSM and BLDCM very different.

Due to differences 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 systems, it directly affects precise position control and high-performance speed control is difficult. At high speeds, the rotor inertia can filter out torque ripples. However, in low-speed and direct drive applications, torque ripple will seriously affect system performance and worsen system accuracy and repeatability. Most space precision electromechanical servo systems work in low-speed situations, so the motor torque pulsation problem is one of the key factors affecting system performance. 

Both PMSM and BLDC have torque ripple problems. Torque ripple is mainly caused by the following reasons: cogging effect and magnetic flux distortion, torque caused by current commutation and torque caused by mechanical processing and manufacturing.

2.Power density

In high-performance index applications such as robots and space actuators, for a given output power, the smaller the weight of the motor, the better. Power density is limited by the motor's heat dissipation capacity, which is the surface area of the motor's stator. For permanent magnet motors, most power losses occur in the stator, including copper losses, eddy current losses and hysteresis losses, while rotor losses are often ignored. So for a given structural size, the smaller the motor losses, the higher the permissible power density. 

Referring to "Permanent Magnet Brushless DC Motor Technology", we know 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

Torque to 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 BDLC and PMSM have the same speed and their rotor moments of inertia are also the same, then the torque-inertia ratio of BDLC is 15% larger than that of PMSM.

4.Sensor aspects

1)Rotor position detection: 

In BLDC, only two phase windings are conductive at each moment, and each phase is conductive at an electrical angle of 120°. As long as these commutation points are correctly detected, the normal operation of the motor can be ensured. Three Hall sensors are usually used. 

In PMSM, a sine wave current is required, all three-phase windings are turned on at the same time when the motor is working, and a continuous position sensor is required, most commonly a high-precision encoder.

2) Current detection: For three-phase motors, in order to control the winding current, three-phase current information needs to be obtained. Usually two current sensors are used because the sum of the three phase currents is 0. For some simple brushless DC motor control systems, only one current sensor can be used to detect the bus current to reduce costs.