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Permanent magnet synchronous and brushless dc motor drives

The permanent magnet DC synchronous motor is different from the brush motor structure we learned in the textbook. It uses the coil winding as the stator and the permanent magnet as the rotor. The permanent magnet is mainly made of neodymium iron boron magnetic material, and since it contains rare earth, the cost is very high. Fortunately, the Chinese style is a country with a very high rare earth content in the world, so vigorously developing electric vehicles will not endanger national security. 钕Magnetism may be familiar to many friends who play audio. If the speaker is made of neodymium, its magnetic properties will be very high, which means that a small volume can make a loud sound and requires high power. The bass that can be pushed can be shocking. Therefore, using the neodymium magnet as a permanent magnet in the motor will also greatly increase the power density of the motor, reducing the volume and weight.

The stator of a permanent magnet DC synchronous motor is composed of three-phase windings. Therefore, the rotor is not energized and the current is switched on by the stator. A rotating magnetic field is required to make the motor rotate. Since the rotor is already a permanent magnet and its magnetic level is fixed, the rotating magnetic field can only be generated by the stator windings.

Permanent magnet synchronous and brushless dc motor drives

Performance advantages of permanent magnet DC synchronous motor

Since the battery pack for the vehicle outputs high-voltage DC power, the permanent magnet DC synchronous motor does not require a high-power inverter to convert the DC power into a sinusoidal AC power compared to the AC asynchronous motor. After all, this conversion process is Cause a certain degree of electrical energy loss. Therefore, in this respect, the permanent magnet DC synchronous motor improves the efficiency of using the battery.

The rotor adopts a permanent magnet structure, so the rotor itself has a magnetic field, and does not need to generate a magnetic field by an additional induced current like an AC asynchronous motor. That is to say, the rotor does not need electricity to generate magnetism, so the energy consumption is lower than that of the AC asynchronous motor.

After using rare earth as a high magnetic material, the weight of the rotor is reduced and the power density of the motor is improved. Therefore, in the same power situation, the permanent magnet DC synchronous motor is lighter in weight and smaller in size, and the response speed of the rotor is faster.

The permanent magnet synchronous motor can integrally mount the motor on the axle to form an integral direct drive system, that is, one axle is a drive unit, eliminating one gearbox. The characteristics of permanent magnet synchronous motors are mainly as follows:
(1) PMSM itself has high power efficiency and high power factor;
(2) PMSM has low heat generation, so the motor cooling system has a simple structure, small volume and low noise;
(3) The system adopts fully enclosed structure, no transmission gear wear, no transmission gear noise, no lubrication, no maintenance;
(4) The overload current allowed by PMSM is large, and the reliability is significantly improved;
(5) The whole transmission system is light in weight, and the unsprung weight is lighter than that of the conventional axle transmission, and the power per unit weight is large;
(6) Since there is no gear box, the bogie system can be freely designed: such as a soft bogie and a single-axle bogie, the train's dynamic performance is greatly improved.

In changing the excitation current of the generator, it is generally not directly carried out in its rotor circuit, because the current in the circuit is large and it is not convenient to perform direct adjustment. The commonly used method is to change the excitation current of the exciter to achieve the regulation of the generator. The purpose of the rotor current. Common methods include changing the resistance of the excitation circuit of the exciter, changing the additional excitation current of the exciter, changing the conduction angle of the thyristor, etc.

Permanent magnet synchronous and brushless dc motor drives

What is the relationship between DC brushless motors and permanent magnet synchronous motors?
In brushless DC motors, the rotor poles are usually made of tile-type magnetic steel. Through the magnetic circuit design, the air gap magnetic density of trapezoidal waves can be obtained. The stator windings are mostly concentrated and integrated, so the induced back electromotive force is trapezoidal. The control of the brushless DC motor requires position information feedback. It must have a position sensor or a position sensorless estimation technique to form a self-controlled speed control system. When controlling, the phase currents are also controlled as square waves 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 the speed regulation actually belongs to the category of variable voltage variable frequency speed regulation.

Generally speaking, a permanent magnet synchronous motor has a stator three-phase distributed winding and a permanent magnet rotor, and the induced electromotive force waveform is sinusoidal in the magnetic circuit structure and the winding distribution, and the applied stator voltage and current should also be sinusoidal waves, generally relying on AC voltage transformation. The inverter provides. The permanent magnet synchronous motor control system often adopts self-control type and also needs position feedback information. It can adopt vector control (field direction control) or advanced control strategy of direct torque control.
The difference between the two can be considered as the design concept caused by square wave and sine wave control.

The principle of DC brushless motor is the same as that of DC motor with carbon brush. DC can think of square wave as the combination of two direct currents with different directions (not superimposed), one will be positive, one will be negative, only this way The current can make the motor armature continue to rotate. In fact, if the current of the armature in the brushed DC motor is the same as this current

Related characteristics
1, voltage regulation
The automatic adjustment of the excitation system can be seen as a negative feedback control system with voltage as the amount to be adjusted. The reactive load current is the main cause of the voltage drop at the generator terminal. When the excitation current is constant, the terminal voltage of the generator will decrease as the reactive current increases. However, in order to meet the user's requirements for power quality, the terminal voltage of the generator should remain basically the same. The way to achieve this requirement is to adjust the excitation current of the generator with the change of reactive current.
2. Adjustment of reactive power:
When the generator and the system are operated in parallel, it can be considered to operate with the busbar of the infinite large-capacity power supply. The excitation current of the generator must be changed, and the induced potential and the stator current also change. At this time, the reactive current of the generator also changes. When the generator is operated in parallel with an infinite capacity system, in order to change the reactive power of the generator, the excitation current of the generator must be adjusted. The generator excitation current that is changed at this time is not so-called "regulation", but merely changes the reactive power that is sent to the system.

3. Distribution of reactive load:
The generators operating in parallel are proportionally distributed with reactive current according to their respective rated capacities. Large-capacity generators should bear more reactive load, while smaller ones provide less reactive load. In order to realize the automatic distribution of reactive load, the excitation current of the automatic high-voltage regulation can be used to change the excitation current of the generator to maintain the terminal voltage constant, and the inclination of the generator voltage regulation characteristic can be adjusted to realize the parallel operation of the generator. Reasonable distribution of reactive load.

Permanent magnet synchronous and brushless dc motor drives

The difference between permanent magnet synchronous motor and brushless DC motor
Generally, when the brushless DC motor is designed, the air gap magnetic field is square wave (trapezoidal wave) and the flat top portion is as flat as possible. Therefore, in the pole logarithm selection, an integer slot concentrated winding such as a 4-pole 12 slot is generally selected, and The magnetic steel is generally a concentric fan-shaped ring, which is radially magnetized. It is generally equipped with a Hall sensor to detect the position and speed. The driving method is generally a six-step square wave drive for occasions where the position requirement is not very high;

The permanent magnet synchronization is a sinusoidal air gap, the better the sinusoidal, so the fractional slot winding is selected on the pole logarithm, such as 4-pole 15 slot, 10 pole 12 slot, etc. The magnetic steel is generally bread-shaped, parallel magnetization, and the sensor is generally Configure incremental encoder, resolver, absolute encoder, etc. Drive i mode is generally driven by sine wave, such as FOC algorithm. For servo applications.

You can distinguish between internal structures, sensors, drivers, and applications. This type of motor can also be used interchangeably, but it will degrade performance. For most air gap waveforms, there is a permanent magnet motor between the two, mainly depending on the drive mode. .
The speed of the permanent magnet brushless DC motor can be changed. Permanent magnet synchronous motors require special drives to shift speeds, such as the three-crystal S3000B servo drive.

According to the requirements of different industrial and agricultural production machinery, motor drive is divided into three types: fixed speed drive, speed control drive and precision control drive.
1, fixed speed drive
There are a large number of production machinery in industrial and agricultural production that require continuous operation in a single direction at roughly constant speeds, such as fans, pumps, compressors, and general machine tools. In the past, most of these machines were driven by three-phase or single-phase asynchronous motors. Asynchronous motors are low in cost, simple in structure and easy to maintain, and are very suitable for driving such machines. However, the asynchronous motor has low efficiency, low power factor and large loss, and this type of motor has a large surface area, so a large amount of electric energy is wasted in use. Secondly, the large number of fans and pumps used in industry and agriculture often need to adjust their flow rate, usually by adjusting the damper and valve, which wastes a lot of electric energy. Since the 1970s, people used inverters to adjust the speed of asynchronous motors in fans and pumps to adjust their flow rate, and achieved considerable energy savings. However, the cost of the inverter limits its use, and the low efficiency of the asynchronous motor itself still exists.

For example, household air-conditioning compressors originally used single-phase asynchronous motors, and their operation was controlled by switching, and the noise and high temperature variation range were insufficient. In the early 1990s, Toshiba Corporation of Japan first adopted the variable frequency speed regulation of asynchronous motor in the compressor control. The advantages of frequency conversion speed regulation promoted the development of inverter air conditioner. In recent years, Japan's Hitachi, Sanyo and other companies have begun to use permanent magnet brushless motors instead of asynchronous motor frequency control, significantly improving efficiency, achieving better energy savings and further reducing noise at the same rated power and rated speed. Next, the volume and weight of the single-phase asynchronous motor are 100%, and the volume of the permanent magnet brushless DC motor is 38.6%, the weight is 34.8%, the amount of copper is 20.9%, and the amount of iron is 36.5%. More than 10%, and the speed is convenient, the price is equivalent to the asynchronous motor frequency control. The application of permanent magnet brushless DC motor in air conditioner promotes the upgrading of air conditioner.

2, speed control drive
There are quite a lot of working machines, and their running speed needs to be arbitrarily set and adjusted, but the speed control accuracy requirements are not very high. Such drive systems have a large number of applications in packaging machinery, food machinery, printing machinery, material handling machinery, textile machinery and transportation vehicles. The most used in this kind of speed regulation application field is the DC motor speed control system. After the development of power electronics technology and control technology in the 1970s, the variable frequency speed regulation of asynchronous motor quickly penetrated into the application field of the original DC speed control system. . This is because on the one hand, the performance price of the asynchronous motor variable frequency speed control system is comparable to that of the DC speed control system. On the other hand, the asynchronous motor has a simple manufacturing process, high efficiency, and less copper for the same power motor than the DC motor. The advantages of convenient maintenance and so on. Therefore, the asynchronous motor frequency conversion speed regulation has quickly replaced the DC speed regulation system in many occasions.

3, precision control drive
1 High precision servo control system
Servo motors play an important role in the operation control of industrial automation. The application performance requirements of servo motors are also different. In practical applications, servo motors have various control methods, such as torque control/current control, speed control, position control, and the like. The servo motor system has also experienced DC servo system, AC servo system, stepper motor drive system, and until recently, the most attractive permanent magnet motor AC servo system. Most of the imported automation equipment, automatic processing equipment and robots imported in recent years have adopted the AC servo system of permanent magnet synchronous motor.

2 Permanent magnet synchronous motor in information technology
Nowadays, information technology is highly developed, and various computer peripherals and office automation equipment are also highly developed. The demand for micro-motors with key components is high, and the accuracy and performance requirements are getting higher and higher. The requirements for such micromotors are miniaturization, thinning, high speed, long life, high reliability, low noise and low vibration, and the accuracy requirements are particularly high.

Permanent magnet synchronous and brushless dc motor drives

The permanent magnet synchronous motor is a synchronous motor that generates a synchronous rotating magnetic field by permanent magnet excitation. The permanent magnet acts as a rotor to generate a rotating magnetic field. The three-phase stator winding passes through the armature reaction under the action of a rotating magnetic field to induce a three-phase symmetrical current.
At this time, the kinetic energy of the rotor is converted into electric energy, and the permanent magnet synchronous motor is used as a generator. In addition, when the stator side is connected to the three-phase symmetrical current, since the three-phase stator differs by 120 in the spatial position, the three-phase stator current is in space. The rotating magnetic field is generated, and the rotating magnetic field of the rotor is subjected to the action of the electromagnetic force. At this time, the electric energy is converted into kinetic energy, and the permanent magnet synchronous motor is used as a motor.

Way of working:
1. Several ways for the generator to obtain the excitation current
1) Excitation mode of DC generator power supply
This type of excitation generator has a dedicated DC generator. This special DC generator is called a DC exciter. The exciter is generally coaxial with the generator. The excitation winding of the generator passes through a slip ring mounted on the large shaft. And the fixed brush receives DC current from the exciter. This excitation mode has the advantages of independent excitation current, reliable operation and reduced consumption of self-use electricity. It is the main excitation mode of generators in the past few decades and has mature operation experience. The disadvantage is that the excitation adjustment speed is slow and the maintenance workload is large, so it is rarely used in units above 10MW.

2) Excitation mode of AC exciter power supply
Some modern large-capacity generators use an exciter to provide excitation current. The AC exciter is also mounted on the generator's large shaft. The AC current output is rectified and supplied to the generator rotor for excitation. At this time, the excitation mode of the generator belongs to the excitation mode, and because of the static rectification device, it is also called For the excitation of the static excitation, the AC secondary exciter provides the excitation current. The AC secondary exciter may be a permanent magnet measuring device or an alternator having a self-exciting constant voltage device. In order to improve the excitation regulation speed, the AC exciter usually uses a medium frequency generator of 100-200 Hz, while the AC auxiliary exciter uses an intermediate frequency generator of 400-500 Hz. The DC excitation winding and the three-phase AC winding of the generator are wound in the stator slot. The rotor has only teeth and slots and no windings, like a gear. Therefore, it has no rotating parts such as brushes and slip rings, and has reliable operation. The utility model has the advantages of simple structure, convenient manufacturing process and the like. The disadvantage is that the noise is large and the harmonic component of the AC potential is also large.

3) Excitation mode of the exciter
In the excitation mode, a special exciter is not provided, and the excitation power is obtained from the generator itself, and then rectified and then supplied to the generator itself for excitation, which is called self-excited static excitation. Self-excited static excitation can be divided into self-excitation and self-re-excitation. Self-excitation mode It obtains the excitation current through the rectifier transformer connected to the generator outlet, and supplies it to the generator for excitation after rectification. This excitation mode has the advantages of simple structure, less equipment, less investment and less maintenance. In addition to the rectification and transformation, the self-re-excitation mode also has a high-power current transformer connected in series to the stator circuit of the generator. The function of this transformer is to provide a large excitation current to the generator in the event of a short circuit to compensate for the shortage of the rectifier transformer output. This excitation method has two types of excitation power sources, a voltage source obtained by a rectifier transformer and a current source obtained by a series transformer.

Permanent magnet synchronous and brushless dc motor drives

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