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Permanent magnet brushless dc motor and permanent magnet synchronous motor compared and analyzed


The materials used for both are generally the same, mainly differing in design. In the general brushless DC motor design, the air gap magnetic field is a square wave (trapezoidal wave) and the flat top part is flatter and better, so in the pole pair The number selection generally selects integer slot concentrated windings such as 4 poles and 12 slots, and the magnets are generally concentric sector rings, radial magnetism, and are generally equipped with Hall sensors to detect the position and speed, and the driving method is generally a six-step square wave drive. For places where the location requirements are not very high;

The permanent magnet synchronous is a sine wave air gap. The better the sine wave, the better the fractional slot winding on the pole pair, such as 4 poles and 15 slots, 10 poles and 12 slots. Magnets are generally bread-shaped, parallel magnetizing, sensors are generally Incremental encoders, rotary transformers, absolute encoders, etc. The drive i mode is generally driven by a sine wave, such as the FOC algorithm. It is used in servo applications.

You can judge from the internal structure, sensors, actuators, and applications. This type of motor can also be used interchangeably, but it will degrade performance. For most of the air gap waveform between the two permanent magnet motor, the main way of looking at the drive .

The brushless DC motor usually uses the rotor magnetic pole tile type magnetic steel, through the magnetic circuit design, trapezoidal air gap magnetic density can be obtained, the stator windings use more concentrated winding, so the induced back EMF is also a trapezoidal wave. The control of the brushless DC motor requires position information feedback. There must be a position sensor or a position sensorless estimation technique to constitute an autonomous control speed control system. During the control, each phase current is also controlled as a square wave as much as possible, and the inverter output voltage can be controlled according to the brushed DC motor PWM method. In essence, the brushless DC motor is also a kind of permanent magnet synchronous motor, and speed regulation actually belongs to the category of variable voltage and variable frequency speed control.

Generally speaking, a permanent magnet synchronous motor has a stator three-phase distributed winding and a permanent magnet rotor. The induced electromotive force waveform is sinusoidal in the structure of the magnetic circuit and the distribution of the winding. The applied stator voltage and current should also be a sine wave. Generally, the permanent magnet synchronous motor converts pressure into alternating current. Frequency converter provided. Permanent-magnet synchronous motor control systems often use self-control, and also require position feedback information. They can use vector control (field-oriented control) or advanced torque control control strategies.

The difference between the two can be considered to be different from the design concept caused by square wave and sine wave control.

Finally correct a concept, 'DC converter' is actually AC frequency conversion, but the control object is usually called 'brushless DC motor'

In my understanding, it should be said that the difference between BLDC and PMSM is really hard to say and sometimes depends on the application. Traditionally speaking, their back-EMF is different. BLDC is close to square wave and PMSM is close to sine wave. In terms of control, BLDC generally uses a 6-tap square-wave drive to control the phase and turn-on time of a square wave. PMSM uses FOC. In terms of performance, the output power density of the BLDC will be greater because the torque of the BLDC makes full use of the harmonics, and therefore the harmonics of the BLDC will be severe.

1. Motor body of brushless DC motor:

The stator winding is a concentrated winding, and the permanent magnet rotor forms a square wave magnetic field; the permanent magnet synchronous motor of the motor body: the stator windings are distributed windings, and the permanent magnet rotor forms a positive magnetic field;

2, brushless DC motor position sensor:

Low-resolution, 60-degree resolution, Hall element, electromagnetic, photoelectric; position sensor of permanent magnet synchronous motor: high resolution, 1/256, 1/1024, rotary transformer, optical code disk;

3, control different:

Brushless DC motor: 120-degree square wave current with PWM control;

Permanent magnet synchronous motor: positive sinusoidal current, controlled by SPWM SVPWM.


Brushless DC Motors: Magnets are square wave magnets, the control voltage PWM is also square wave, and the current is also square wave. One electrical cycle has 6 space vectors. Simple control, low cost, general MCU can be achieved.

Permanent-magnet synchronous motors: Magnets are sinusoidal, magnets are back-EMFs are sine waves, and currents are sine waves. In general, vector control technology is used. Generally, there will be at least 18 vectors for an electrical cycle (more is better as a matter of course), which requires a high-performance MCU or DSP.

DC servo: This range is very wide. DC servo, under the control of the DC motor re-control system, operates according to the control command (rotation speed, position, angle, etc.), and is generally used to execute the mechanism.

First, the sensor's difference:

Brushless DC Motor (BLDC): Position sensor, such as Hall;

Permanent Magnet Synchronous Motors (PMSM): Speed and position sensors such as resolvers, optoelectronic encoders, etc.

Second, the back emf waveform is different:

BLDC: approximate trapezoidal wave (ideal state);

PMSM: Sine wave (ideal

Third, the three-phase current waveform is different:

BLDC: approximate square wave or trapezoidal wave (ideal state);

PMSM: Sine wave (ideal

Fourth, the difference between the control system:

BLDC: usually includes position controller, speed controller and current (torque) controller;

PMSM: Different control strategies will have different control systems;

Fifth, the difference between the design principles and methods:

BLDC: Try to broaden the width of the back-EMF waveform (to make it approximate to a staircase wave);

PMSM: make the back-EMF close to a sine wave;

Embodied in the design is mainly the difference between the stator windings, rotor structure (such as pole arc coefficient).