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An article to understand the difference between BLDC and PMSM

Date:2022-06-09   Author:XINDA MOTOR

Modern motor and control technology divides permanent magnet brushless DC motors into two categories based on the difference in current drive mode:

1) Square wave drive motor, that is, brushless DC motor (BLDC);

2) Sine wave drive motor: that is, permanent magnet synchronous motor (PMSM).

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 placed 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 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. 

Although the basic structure of the permanent magnet synchronous motor and the brushless DC motor is the same, they are different in the way of driving, and yes, 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 stator winding distribution is different. PMSM adopts short-distance distributed winding, and sometimes fractional slot or sinusoidal winding is used to further reduce the ripple torque; while BLDC adopts full-pitch concentrated winding.

3) The operating current is different. In order to generate 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 PMSM permanent magnet is parabolic, and the magnetic density generated in the air gap is as sine wave distribution as possible; the shape of BLDC permanent magnet is tile-shaped, and the magnetic density generated in the air gap is trapezoidal wave distribution. .

5) Different operation modes, PMSM adopts three-phase work at the same time, and the current of each phase differs by 120° electrical angle, and a position sensor is required. BLDC uses windings that are turned on in pairs, each phase conducts 120° electrical angle, and commutates every 60° electrical angle, and 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:


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. However, in low-speed and direct-drive applications, torque ripple will seriously affect system performance, deteriorating system accuracy and repeatability. The vast majority of space precision electromechanical servo systems work in low-speed situations, so the problem of motor torque ripple is one of the key factors affecting system performance. Both PMSM and BLDCM suffer from torque ripple issues. Torque ripple is mainly caused by the following reasons: cogging and magnetic flux distortion, torque caused by current commutation and torque caused by machining.


power density

In applications with high performance indicators such as robots and space actuators, for a given output power, the smaller the motor weight, the better. Power density is limited by the ability of the motor to dissipate heat, which is the surface area of the motor stator. For permanent magnet motors, most of the power losses are generated in the stator, including copper losses, eddy current losses and hysteresis losses, while rotor losses are often neglected. So for a given frame size, the smaller the motor losses, the higher the allowable power density. Referring 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 also the same, the power density of BDLC can be increased by 15% compared to PMSM.


Torque inertia ratio

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



(1) Rotor position detection: In BLDC, only two-phase windings are turned on at each moment, and each phase is turned on by an electrical angle of 120°. As long as these commutation points are correctly detected, 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, and a continuous position sensor is required, the most common being an encoder with high accuracy.

(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 the cost.