Modern motor and control technology divide the permanent magnet brushless DC motor into two categories based on the difference of current drive mode: 1) square wave drive motor, also known as brushless DC motor (BLDC); 2) Sine wave drive motor: permanent magnet synchronous motor (PMSM).

BLDCM is the English abbreviation of Brushless Direct Current Motor; PMSM is called permanent magnet synchronous motor, that is, permanent magnet synchronous motor.

On the surface, the basic structure of BLDC and PMSM is the same: 1) their motors are permanent magnet motors, the rotor is composed of permanent magnets, and the stator is placed with polyphase AC windings; 2) are generated by the interaction of AC current between permanent magnet rotor and stator motor torque; 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 side motor phase winding as in some sensorless control modes.

Although the basic architecture of permanent magnet synchronous motor and brushless DC motor is the same, there are obvious differences in the design and control details due to their different driving modes. 1) The backpotential is different. PMSM has sine-wave backpotential, while BLDC has trapezoidal wave backpotential. 2) The stator windings are distributed differently. PMSM uses short-distance distributed windings and sometimes fractional slot or sinusoidal windings to further reduce ripple torque; BLDC uses integral distance concentrative winding. 3) Different running currents, PMSM is sine-wave stator current in order to generate constant electromagnetic torque; BLDC is rectangular wave current. 4) The shape of permanent magnets is different. The shape of PMSM permanent magnets 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 shape, and the magnetic density generated in the air gap is trapezoidal wave distribution. 5) Different operation modes, PMSM uses three phases working at the same time, each phase current difference of 120° electrical Angle, requires a position sensor. BLDC adopts two winding conduction, each phase conduction 120° electrical Angle, every 60° electrical Angle commutation, only need phase commutation point position detection. 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, PMSM and BLDC have different characteristics. The performance comparison is as follows:

1. Torque fluctuation

Torque ripple is the biggest problem of electromechanical servo system, it directly affects accurate position control and high performance speed control is difficult. At high speed, the rotor inertia can filter the torque fluctuation. However, in low speed and direct drive applications, torque fluctuations will seriously affect the system performance, which will deteriorate the accuracy and repeatability of the system. Most of the space precision electromechanical servo systems work at low speed, so the problem of motor torque ripple is one of the key factors affecting the system performance. Both PMSM and BLDCM have torque ripple problems. Torque pulsation is mainly caused by the following reasons: tooth groove effect and flux distortion, the torque caused by current commutation and the torque caused by machining and manufacturing.

2. Power density

In high-performance applications such as robots and space actuators, for a given output power, the weight of the motor should be as small as possible. The power density is limited by the heat dissipation capacity of the motor, i.e. the surface area of the motor stator. For permanent magnet motor, most of the power loss is generated in stator, including copper loss, eddy current loss and hysteresis loss, while rotor loss is often ignored. So for a given structure size, the smaller the motor loss, the higher the allowable power density. Referring to "Permanent Magnet Brushless DC Motor Technology", it is known that BDLC can provide 15% more power output than PMSM under the same size. If the iron consumption is the same, the power density of BDLC is 15% higher than that of PMSM.

3. Torque 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 BDLC and PMSM have the same velocity and their rotor inertia is the same, then the torque inertia ratio of BDLC is 15% larger than that of PMSM.

4. Sensor

(1) Rotor position detection: in BLDC, only two phase windings are on at every moment, and each phase is on at 120° electrical Angle. As long as these phase-changing points are correctly detected, the normal operation of the motor can be guaranteed. Usually, three Hall sensors are used. In PMSM, sine wave current is required, all three phase windings are switched on simultaneously when the motor is operating, and continuous position sensors are required, most commonly high precision encoders.

(2) Current detection: For three-phase motors, in order to control winding current, three-phase current information needs to be obtained. Two current sensors are usually used because the sum of the three phases of current is 0. For some simple brushless DC motor control system clocks, only a current sensor can be used to detect the current of the bus to reduce the cost.

Brushless direct flow magnetoelectric motor (BLDCM) and automatic permanent magnet synchronous motor (PMSM) have different characteristics and different application fields in addition to their linear mechanical properties.

Motor characteristics and applications

● Brushless DC permanent magnet motor (BLDCM) features and application fields

1) Insufficient use of armature winding;

2) The magnetic pole of the rotor permanent magnet is expected to generate a magnetic field close to rectangular or trapezoidal wave in the working air gap:

3) Hall device is usually used as the rotor position sensor of the motor;

4) The control circuit is simple;

5) the torque pulsation is relatively large;

6) Low control accuracy;

7) The price is cheaper.

Brushless DC permanent magnet motor (BLDCM) is suitable for single speed and steady speed operation with power class below 300W, for example, Computer floppy disk drives (FDD), hard disk drives (HDD), audio-visual equipment SPINDLE drives, laser printers, electric bicycles, automobile motors, forklifts, lifting mechanisms, electric sewing machines, fans, and pumps.

The characteristics and application fields of automatic permanent magnet synchronous motor (PMSM)

1) Make good use of armature winding;

2) It is hoped that the pole of the rotor permanent magnet can generate a magnetic field with nearly sinusoidal waveform in the working air gap;

3) Non-contact rotary transformer is usually used as the rotor position sensor of the motor;

4) The control circuit is complicated;

5) Small torque pulsation;

6) High control precision and good dynamic performance;

7) The price is expensive.

Automatic permanent magnet synchronous motor (PMSM) is suitable for precision servo control system with power class above 500W, such as numerical control machine tools, textile machinery, paper machinery, precision positioning system, robot, etc.