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Principle and structure of permanent magnet motor

Permanent magnet motor uses permanent magnet to generate the magnetic field of the motor, without excitation coil or excitation current, with high efficiency and simple structure, it is a good energy-saving motor. With the advent of high-performance permanent magnet materials and the rapid development of control technology, the application of permanent magnet motor will become more extensive.

Types of permanent magnet motors

Permanent magnet DC motor

Permanent magnet DC motor uses permanent magnet to establish the magnetic field required by the motor, without using another power supply for excitation. In the motor courseware column, when introducing the principle of the motor, the magnetic field is generated by the permanent magnet. The principle model of the eight slot DC motor uses a U-shaped permanent magnet to generate the magnetic field. In the past, due to the poor magnetic properties of permanent magnet, weak magnetic force and easy demagnetization, it was only used in some motors with small output, and was mostly used in toys and teaching instruments. In recent ten years, permanent magnet motor has been developed rapidly. Thanks to the rapid development of permanent magnet, permanent magnet DC motor has also developed from toys, instruments, household appliances to large-scale motor such as vehicles.
The working principle and structure of permanent magnet DC motor are similar to ordinary DC motor, only permanent magnet pole is used to replace current excitation pole.
This is a four pole DC motor. The figure below shows the four permanent magnet poles of the motor arranged on the same circle.

20210504051706 96466 - Principle and structure of permanent magnet motor
Pole of permanent magnet
NdFeB is the best permanent magnet material at present, with very strong magnetic force and high coercivity. NdFeB is often used as the magnetic pole for permanent magnet motors with good performance. The magnetic pole is fixed in the casing, which is the yoke and provides the magnetic circuit for the motor.

Permanent magnet motor does not need excitation current to generate less heat, but the enclosure is mostly sealed to prevent iron powder pollution. Generally, it dissipates heat through multiple heat sinks around the enclosure; The end cover plate extends to the outside of the casing and serves as the machine foot. The whole stator frame of the motor is formed, and the motor is installed and fixed through the machine foot.

The rotor of permanent magnet DC motor is composed of rotor core, rotor winding, commutator and shaft, which is the same as the rotor of ordinary DC motor.

Permanent magnet DC motor can be divided into permanent magnet brushless DC motor and permanent magnet brushless DC motor.

Permanent magnet brushless DC motor

Permanent magnet brushless DC motor (PMBLDCM) is a DC motor with one or more permanent magnets to establish the magnetic field. Its performance is similar to that of a constant excitation current separately excited DC motor. It can be easily adjusted by changing the armature voltage. Compared with the separately excited DC motor, it has the advantages of small volume, high efficiency, simple structure and less copper consumption, so it is the main type of low power DC motor.

Basic principle of permanent magnet brushless DC motor

Permanent magnet brushless DC motor is a typical mechatronics product, which is composed of motor body and driver.
1. The stator winding of motor is mostly made into three-phase symmetrical star connection, which is very similar to three-phase asynchronous motor. The rotor of the motor is attached with a magnetized permanent magnet. In order to detect the polarity of the rotor, a position sensor is installed in the motor. The driver is composed of power electronic devices and integrated circuits. Its function is to receive the starting, stopping and braking signals of the motor to control the starting, stopping and braking of the motor; Receiving position sensor signal and forward and reverse signal, it is used to control the on-off of each power transistor of inverter bridge and generate continuous torque; Receive speed command and speed feedback signal to control and adjust speed; Provide protection and display, etc.
The main circuit is a typical voltage source AC-DC-AC circuit. The inverter provides symmetrical alternating rectangular wave with equal amplitude and equal frequency 5-26khz modulation wave.
The permanent magnet N-S alternates to make the phase difference of position sensor 120 ° U, V, w square wave, combined with forward / reverse signal to generate effective six state coding signal: 101, 100, 110, 010, 011, 001, through logic component processing to generate T1-T4 conduction, T1-T6 conduction, T3-T6 conduction, t3-t2 conduction, t5-t2 conduction, t5-t4 conduction, that is to say, the DC bus voltage is successively applied to a + B -, a + C -, B + C -, B + a -, C + a -, C + B -, In this way, each time the rotor turns a pair of N-S poles, the T1-T6 power transistor turns on in turn according to the fixed combination of six states. In each state, only two-phase windings are electrified, changing one state in turn, and the magnetic field axis generated by stator winding rotates 60 degrees in space ° Electric angle, the rotor follows the stator magnetic field rotation is equivalent to 60 ° When the rotor is in a new position, the position sensors u, V and W generate a new set of codes according to the agreement. The new codes change the conduction combination of the power tube, and make the magnetic field axis generated by the stator winding advance 60 times ° In this way, the permanent magnet DC motor will produce continuous torque and drag the load for continuous rotation. Because the commutation of permanent magnet DC motor is produced by itself, not forced by inverter, it is also called self-control synchronous motor.
2. The position sensor coding of permanent magnet brushless DC motor makes the position of the magnetic field axis of the rotor ahead of the position of the magnetic field axis of the rotor, so no matter where the starting position of the rotor is, the motor will produce enough starting torque at the moment of starting, so there is no need to set another starting winding on the rotor.
Because the axis of the stator magnetic field can be regarded as perpendicular to the axis of the rotor, the average electromagnetic torque is proportional to the winding current when the core is not saturated, which is the same as the current torque characteristics of the separately excited DC motor.
The torque of the motor is proportional to the average current of the winding:

  • Tm=KtIav (N·m)

The difference of back EMF between two phase windings is proportional to the angular velocity of the motor

  • ELL=Keω (V)

Therefore, the average current in the motor winding is:

  • Iav=(Vm-ELL)/2Ra (A)

Where VM= δ· VDC is the average voltage applied between motor lines, and VDC is the DC bus voltage, δ Is the duty cycle of the modulation wave, RA is the winding resistance of each phase. Thus, the electromagnetic torque of DC motor can be obtained

  • Tm=δ·(VDC·Kt/2Ra)-Kt·(Keω/2Ra)

KT and Ke are the structure constants of the motor, ω Is the angular velocity of the motor (rad / s), therefore, in a certain range ω Change the duty cycle δ, Then the electromagnetic torque of the motor can be changed linearly, and the same control and mechanical characteristics as the armature voltage control of separately excited DC motor can be obtained.
The speed setting of permanent magnet brushless DC motor depends on the speed command VC. If the maximum speed command is + 5V, the corresponding maximum speed is VC (max) ó N max, then, any level below + 5V corresponds to the corresponding speed n, which realizes the variable speed setting.
When VC is set, no matter load change, power supply voltage change or ambient temperature change, when the speed is lower than the command speed, the feedback voltage VFB becomes smaller and the duty cycle of the modulation wave becomes smaller δ It will increase the armature current, increase the electromagnetic torque generated by the motor and generate acceleration until the actual speed of the motor is equal to the command speed; On the contrary, if the actual speed of the motor is higher than the command speed, δ The deceleration occurs until the actual speed is equal to the command speed. It can be said that the difference between the steady speed and the command speed of permanent magnet DC motor is about 1% within the allowable power grid fluctuation range and under the allowable overload capacity, and the permanent magnet DC motor can realize constant torque operation within the speed range.
Because the excitation of permanent magnet brushless DC motor comes from permanent magnet, it does not need to draw excitation current from power grid like asynchronous motor; Because there is no alternating flux in the rotor, there is no copper loss and no iron loss on the rotor, so the efficiency is about 10% higher than that of asynchronous motor with the same capacity. Generally speaking, the force and energy of permanent magnet DC motor is the pointer( η cos θ) 12% – 20% higher than the same capacity three-phase asynchronous motor.
3. Because the permanent magnet brushless DC motor is operated in self-control mode, it will not add another starting winding on the rotor like the synchronous motor with heavy load starting under variable frequency speed regulation, and it will not produce oscillation and out of step when the load changes suddenly.
The permanent magnet of medium and small capacity permanent magnet brushless DC motor mostly uses Nd-Fe-B material with high magnetic energy product. Therefore, the volume of rare earth permanent magnet brushless motor is one frame number smaller than that of three-phase asynchronous motor with the same capacity.
In the past 30 years, the research on variable frequency speed regulation of induction motor is to find a way to control the torque of induction motor in the final analysis. The current of permanent magnet DC motor or the terminal voltage of armature is the physical quantity to directly control the torque of motor. In the past, due to the high price of rare earth permanent magnet, the application field of rare earth permanent magnet DC motor was limited. However, with the continuous innovation of technology, the price of rare earth permanent magnet DC motor has dropped rapidly. For example, the price of a BS series permanent magnet DC motor is almost the same as that of asynchronous motor and common inverter. Rare earth permanent magnet DC motor will show its advantages in the field of speed regulation because of its wide speed regulation, small volume, high efficiency and small steady-state speed error.
According to the different permanent magnet materials, permanent magnet brushless DC motor is divided into alnico permanent magnet DC motor, ferrite permanent magnet DC motor and rare earth permanent magnet DC motor. Alnico permanent magnet brushless DC motor needs to consume a lot of precious metals and has high price, but it has good adaptability to high temperature and is used in the occasions with high ambient temperature or high temperature stability requirements. Ferrite permanent magnet brushless DC motor is widely used in household appliances, automobiles, toys, power tools and other fields because of its low cost and good performance. The rare earth permanent magnet brushless DC motor made of rare earth permanent magnet material has small volume and better performance, but it is expensive. It is mainly used in aerospace, computer, downhole instruments and so on. However, in recent years, a new generation of rare earth permanent magnet DC motor, NdFeB permanent magnet brushless DC motor, has emerged. Because China has more than 80% of the world’s neodymium ore resources, it has a unique advantage in price. The cost performance of high-performance NdFeB permanent magnet materials has been greatly improved, which makes the high-quality and low-cost The cheap Nd-Fe-B permanent magnet DC motor has been widely used in industrial production, and it also promotes the rapid development of the performance and structure of permanent magnet brushless DC motor.

Selection principle of permanent magnet brushless DC motor

(1) Selection of types
Ferrite permanent magnet DC motor with high efficiency, low price and low temperature rise should be preferred. Aluminum nickel cobalt permanent magnet DC motor or rare earth permanent magnet DC motor should be selected only when the performance requirements are strict, the volume is small and the ambient temperature is high.
(2) Reasonable selection of motor power
The maximum output power of the motor is limited. If the power of the motor is too small and the load exceeds the rated output power of the motor, the motor will be overloaded. When the load is overloaded, the motor will generate heat, vibration, speed drop, abnormal sound and other phenomena. When the load is serious, the motor will be burned. If the power is too large, it will cause economic waste. Therefore, it is very important to choose the power of motor reasonably.
(3) Specification selection
Often because the actual production of product specifications are not many, to choose products more difficult. When selecting the product specifications, we can consider: in the case of adjustable power supply voltage, we can select the specifications whose torque and speed are close to the corresponding rated value of the product according to the actual needs, and get the required speed by changing the voltage; When the power supply voltage is fixed, if there is no product of appropriate specification to choose from, the appropriate specification can be selected according to the torque, and the voltage and speed of the product can be adjusted appropriately.
In the application of permanent magnet brushless DC motor, attention should be paid to:
(1) In general (such as alnico permanent magnet DC motor or ferrite permanent magnet DC motor), permanent magnet DC motor is not allowed to operate in reverse braking mode under rated voltage if the product is not specified, otherwise permanent magnet demagnetization will be caused; If it is necessary to operate in this way, a current limiting resistor should be added to limit the excessive current.
(2) Carry out preliminary inspection on the motor according to the following steps:
First, check the appearance of the motor: there should be no scratch, bruise and coating falling off; Then rotate the shaft, which should be able to rotate flexibly without obvious jamming. Check whether the wiring of the motor is firm and power on. There should be no friction in the process of motor rotation, and the most prominent one is bearing friction. After the bearing is worn, it will make abnormal sound, and local overheating and temperature rise will appear.
(3) Pay attention to the demagnetization of permanent magnet caused by excessive current, temperature change and open circuit of magnetic circuit during disassembly and assembly. Especially for alnico permanent magnet motor, the magnetic short circuit protection of permanent magnet circuit shall be carried out during disassembly and assembly, otherwise additional magnetization shall be conducted after demagnetization.

Permanent magnet brushless DC motor

Basic principle of permanent magnet brushless DC motor

The stator of the brush motor is provided with a fixed main magnetic pole and an electric brush, and the rotor is provided with an armature winding and a commutator. The electric energy of the DC power supply enters the armature winding through the brush and commutator to generate the armature current. The magnetic field generated by the armature current interacts with the main magnetic field to generate the electromagnetic torque, which makes the motor rotate to drive the load. Due to the existence of brush and commutator, brush motor has complex structure, poor reliability, many faults, heavy maintenance workload, short service life, commutation spark is easy to produce electromagnetic interference.
There is a magnet in the fixed part of the brushless DC motor, which is called the main magnetic pole here; The fixed part also has brushes. The rotating part has an annular core and a winding wound on the annular core.
The fixed part (stator) of the two pole brushless DC motor is equipped with a pair of static main magnetic poles N and s excited by DC, and the armature core is installed on the rotating part (rotor). There is an air gap between stator and rotor. An armature coil connected by two conductors A and X is placed on the armature iron core. The head end and end end of the coil are respectively connected to two arc-shaped copper pieces, which are called commutating pieces. The commutators are insulated from each other, and the whole composed of commutators is called commutator. The commutator is fixed on the shaft, and the commutator and the shaft are insulated from each other. A pair of fixed brushes B1 and B2 are placed on the commutator. When the armature rotates, the armature coil is connected with the external circuit through the commutator and the brush.

Types of permanent magnet brushless DC motor

  • ① Motor with brush disc winding. The brush disc winding motor is bonded on a cylinder body with rare earth material, and the disc winding made of enamelled copper wire is placed in the cylinder body to form a rotor. The phase of motor is adjusted by mechanical phase changer. The mechanical inverter adjusts the voltage phase by friction between the fixed carbon brush and the rotating copper commutation surface. The brush of this kind of motor is always worn in use, and its service life is difficult to exceed 2000h. At the same time, due to the high speed of the motor, two-stage gear reduction must be adopted, which brings two problems. One is the large noise, the other is the large efficiency loss. After the reduction, the rated efficiency of the motor can only reach 68% ~ 72%. The capacity of the battery used in the electric bicycle is limited, which is generally 36V / 12Ah. If the motor efficiency is not high, the power consumption will increase and the driving mileage will be affected.
  • ② Printed motor with brush winding. The printed copper foil is used as the winding of the motor with brush printed winding, which reduces the weight of the motor. Because this kind of motor is all produced on the automatic production line, and the process is reliable, the service life of the motor is increased to 3000h, the noise is greatly reduced, and the efficiency is increased to 72% ~ 76%. But this kind of motor has “buzzing” high frequency noise, the efficiency is still not ideal after gear reduction, and the service life of the motor can not be further improved due to the use of brush commutator.
  • ③ Motor with brush pressed winding. By pressing the wound copper wire into a new type of winding, the efficiency of the motor can be increased to 74% ~ 78%. This kind of motor is still adopted by many electric bicycle manufacturers, but its efficiency, noise and life defects still need to be improved.

The wheel hub type brush DC motor with gear drive is composed of two parts: the disc armature brush motor and the gear reduction and transmission system. Disk armature is a rotor that rotates at high speed. The structure of the wheel type gear driven brushless DC motor is shown in Figure 2-2. The torque of the motor is transmitted to the first stage gear through the shaft, and the wheel hub shell is driven to rotate by the gear reduction.
The disc armature of brush toothed hub motor is thin sheet, small in volume, light in weight and easy to install. After the winding is made up, the resin and glass fiber are put into the mold for hot pressing. In operation, due to the friction between the brush and the commutator, and the meshing speed reduction of the gear, the running sound of the brush motor is greater than that of the brushless motor.
In order to adapt to the hub structure, the brushless motor is designed as a low-speed motor with armature on the outside as the rotor, magnet steel in the motor as the stator, multiple magnets with multiple windings, and the design speed is about 180r / min.
The end sleeve of the stator shaft of the brush motor is provided with a nut, which is used to prevent the thread on the shaft from being damaged during processing.
Due to the improvement in the design of the brush motor, it can achieve low noise and low cost without gear reduction. This kind of motor is widely used in many low-cost electric bicycles. However, this kind of motor has the advantages of small torque, small load, poor climbing ability and high power consumption. The mechanical brush commutator is still used and the problem of motor life has not been solved. Therefore, this kind of motor is not used in middle and high grade electric bicycles.

Application of permanent magnet brushless DC motor

In the field of stage lighting, permanent magnet DC motor, especially small permanent magnet dc gear motor, is widely used. In the computer industry, printers, scanners, hard disk drives, CD-ROM drives, recorders, cooling fans and so on all need to use a large number of permanent magnet DC motors.
In the automotive industry, all kinds of fans, wipers, water pumps, flameouts, mirrors, air pumps are used in a variety of permanent magnet DC motors. In hotels, permanent magnet DC motors are used in automatic doors, automatic locks, automatic curtains, automatic water supply systems, soft tissue machines, etc. in weapons and equipment, permanent magnet DC motors are widely used in missiles, artillery, artificial satellites, spacecraft, ships, aircraft, tanks, rockets, radars, combat vehicles and other occasions.
In industry and agriculture, permanent magnet DC motor is also widely used in electrical and automation control and instrumentation. In the medical field, permanent magnet DC motor is more useful, such as various medical instruments, surgical tools, such as the electric bone saw in brain surgery, especially in the field surgery. Permanent magnet DC motor is used in the disabled products, such as manipulator, disabled car and so on. In life, more useful, even toothbrush is also made of permanent magnet DC motor electric toothbrush. The application of permanent magnet DC motor is numerous, can be said to be everywhere.
With the development of the times, the application of permanent magnet DC motor will be more and more. Especially after the appearance of permanent magnet brushless motor, the production quantity of permanent magnet DC motor is increasing. There are billions of permanent magnet DC motors produced in China every year, and there are countless manufacturers of permanent magnet DC motors.

Disc permanent magnet DC motor

The axial dimension of disc permanent magnet motor is small and its shape is like a disc. Generally, the direction of air gap magnetic field between stator and rotor is meridional, while that of disk motor is axial. In addition to the different magnetic field direction, the working principle is the same as ordinary motor.
The following describes the structure of a disc permanent magnet DC motor, which is a four pole DC motor. Four fan-shaped permanent magnet poles are placed in a plane, as shown in the figure below:

20210504052157 71135 - Principle and structure of permanent magnet motor
Sector permanent magnet pole
Four fan-shaped permanent magnet poles are installed on the main end cover, which is stator yoke.
The main end cover and the upper end cover are made of steel with good magnetic conductivity. After the two end covers are closed, a good magnetic circuit is formed.
Disc type coreless permanent magnet motor is very suitable for thin motor because of its coreless rotor, small inductance and small commutation spark.

Permanent magnet synchronous motor (PMSM)

Permanent magnet synchronous motor uses permanent magnet to provide excitation, which makes the structure of the motor simple, reduces the processing and assembly costs, saves the collector ring and brush which are easy to cause problems, and improves the reliability of the motor operation; Because there is no excitation current and no excitation loss, the efficiency and power density of the motor are improved.
Permanent magnet synchronous motor is composed of stator, rotor and end cover. The stator is basically the same as the common induction motor, and the lamination structure is adopted to reduce the iron consumption of the motor. The rotor can be made solid or laminated. The armature winding can be centralized full pitch winding, distributed short pitch winding and unconventional winding.

Structure of permanent magnet synchronous motor

Permanent magnet synchronous motor is mainly composed of stator, rotor and end cover. The stator is made of lamination to reduce the iron loss when the motor is running. It is equipped with three-phase AC winding, which is called armature. The rotor can be made into solid form or pressed by lamination, which is equipped with permanent magnet material. According to the position of the permanent magnet material on the rotor of the motor, the permanent magnet synchronous motor can be divided into two structural forms: protruding type and built-in type. The corresponding schematic diagram is given in Figure 1. The magnetic circuit structure of the protruding rotor is simple and the manufacturing cost is low, but the asynchronous starting can not be realized because the starting winding can not be installed on its surface.

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Fig. 1 magnetic circuit structure of two kinds of surface rotor

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Fig. 2 magnetic circuit structure of three different types of built-in rotors
There are three kinds of magnetic circuit structure of built-in rotor: radial type, tangential type and hybrid type. The main difference between them is the relationship between the magnetization direction of permanent magnet and the rotation direction of rotor. Figure 2 shows the magnetic circuit structure of three different types of built-in rotor. Because the permanent magnet is placed inside the rotor, the rotor surface can be made into pole shoes, and the pole shoes can play the role of starting and damping by inserting copper bars or cast aluminum. The steady-state and dynamic performance are better. Because of the asymmetry of the magnetic circuit of the built-in rotor, the reluctance torque will be generated in operation, which helps to improve the power density and overload capacity of the motor itself, and this structure is easier to realize the field weakening speed expansion.

Working principle of permanent magnet synchronous motor

When the three-phase current flows into the three-phase symmetrical windings of the permanent magnet synchronous motor stator, the magnetomotive force generated by the current generates a rotating magnetomotive force with constant amplitude. Because of its constant amplitude, the trajectory of the rotating magnetomotive force forms a circle, which is called the circular rotating magnetomotive force. Its magnitude is just 1.5 times of the maximum amplitude of single-phase magnetomotive force:

20210504060054 12916 - Principle and structure of permanent magnet motor

In the formula, f is the circular rotating magnetomotive force (t · m); Fφl is the maximum amplitude of single-phase magnetomotive force (t · m); K is the fundamental winding coefficient; P is the number of motor poles; N is the number of series turns of each coil; I is the effective value of the current flowing through the coil. A because the speed of permanent magnet synchronous motor is constant synchronous speed, the rotating magnetic field generated by the rotor main magnetic field and the stator circular rotating magnetomotive force remains relatively static. The two magnetic fields interact to form a synthetic magnetic field in the air gap between the stator and the rotor, which interacts with the main magnetic field of the rotor and produces an electromagnetic torque Te that pushes or hinders the rotation of the motor:

20210504060156 21190 - Principle and structure of permanent magnet motor

In the formula, Te is the electromagnetic torque, (n · m); 0 is the power angle, rad; BR is the main magnetic field of rotor, t; Bnet is the synthetic magnetic field of air gap, t. Due to the different position relationship between the air gap synthetic magnetic field and the main magnetic field of the rotor, the permanent magnet synchronous motor can run in either the motor state or the generator state. The three operation states of the permanent magnet synchronous motor are shown in Figure 3. When the resultant magnetic field of air gap lags behind the main magnetic field of rotor, the electromagnetic torque generated is opposite to the rotation direction of rotor, and the motor is in power generation state; On the contrary, when the air gap synthetic magnetic field is ahead of the main magnetic field of the rotor, the electromagnetic torque generated is the same as the rotation direction of the rotor, and the motor is in the electric state. The angle between the main magnetic field of the rotor and the resultant magnetic field of the air gap is called the power angle.

Permanent magnet synchronous motor (PMSM) consists of two key components, namely a multipolar permanent magnet rotor and a stator with properly designed windings. During the operation, the rotating multipolar permanent magnet rotor forms a time-varying flux in the air gap between rotor and stator. This flux generates AC voltage at the stator winding terminals, which forms the basis for power generation. The PMSM discussed here uses an annular permanent magnet mounted on a ferromagnetic core. Internal permanent magnet synchronous motors are not considered here. Because it is very difficult to embed the magnet into an electroplated ferromagnetic core, the magnet with appropriate thickness (500 mm) is used μ m) As well as in the rotor and stator core of high-performance magnetic materials, the air gap can be made very large (300 ~ 500) μ m) There is no obvious performance loss, which makes the stator winding occupy a certain space in the air gap, thus greatly simplifying the manufacturing of permanent magnet synchronous motor.

Classification of permanent magnet synchronous motor

Classification according to the supply mode of excitation current

Permanent magnet synchronous motor (PMSM) is a kind of synchronous motor which uses permanent magnet to establish excitation magnetic field. Its stator generates rotating magnetic field and its rotor is made of permanent magnet material. The synchronous motor needs a DC magnetic field to realize energy conversion, and the DC current that generates the magnetic field is called the excitation current of the motor.
Separately excited motor: the motor that obtains excitation current from other power sources.
Self excited motor: a motor that obtains excitation current from the motor itself.

Classification by power supply frequency

Permanent magnet brushless motor includes permanent magnet brushless DC motor and permanent magnet brushless AC motor. The former only needs square wave inverter, while the latter needs sine wave inverter[ 6] 

Classification by air gap magnetic field distribution

Sine wave permanent magnet synchronous motor: the magnetic pole is made of permanent magnet material. When the three-phase sine wave current is input, the air gap magnetic field is distributed according to the sine law, which is called permanent magnet synchronous motor for short.
Trapezoidal wave permanent magnet synchronous motor: the magnetic pole is still permanent magnet material, but the input square wave current, the air gap magnetic field is trapezoidal wave distribution, the performance is closer to that of DC motor. The self-control frequency conversion synchronous motor composed of trapezoidal wave permanent magnet synchronous motor is also called brushless DC motor.

Control mode of permanent magnet synchronous motor

Constant voltage frequency ratio control method of permanent magnet synchronous motor

The constant voltage frequency ratio control method of permanent magnet synchronous motor is similar to that of AC induction motor, which controls the amplitude and frequency of motor input voltage to change at the same time, so as to make the motor flux constant. The constant voltage frequency ratio control method can adapt to the requirements of wide range speed regulation system.
Without feedback of current, voltage, position and other physical signals, the control accuracy can still be achieved, which is the biggest advantage of constant voltage frequency ratio control method. Constant voltage frequency ratio control method has been widely used in the field of general inverter because of its simple control algorithm and low hardware cost. The disadvantages of constant voltage frequency ratio control method are also obvious. Because there is no feedback speed, position or any other signal in the control process, it is almost impossible to obtain the running state information of the motor, let alone accurately control the speed or electromagnetic torque. The system performance is general and the dynamic response is poor, especially when the given target speed changes or the load changes suddenly, It is easy to cause out of step and oscillation. Obviously, this control method can not control the torque and the excitation current separately, and there is a large excitation current in the control process, which affects the efficiency of the motor. Therefore, this control method is often used in general inverter with low performance demand, such as air conditioning, conveyor drive control of pipeline, energy-saving operation of water pump and fan, etc.

Direct torque control technology of permanent magnet synchronous motor

Direct self control (DSC) constructs the flux linkage and electromagnetic torque model in the stator static coordinate system, and realizes the control of electromagnetic torque and stator flux linkage by applying different voltage vectors. Direct torque control method has the advantages of simple algorithm and good torque response. Therefore, this method has been widely used in the occasions requiring high transient torque response.
Due to the inherent shortcomings of the control, the direct torque control method has low control frequency and large torque ripple at low speed. Therefore, reducing the torque ripple at low speed has become a research hotspot in the direct torque control method. Sun Xiaohui and others reduce the torque ripple at low speed by optimizing the voltage vector action time, and the effect is better. D. Casadei and others applied the direct torque control method to the control of AC induction motor based on the discrete space vector modulation technology to reduce the torque ripple.

Vector control technology of permanent magnet synchronous motor

Vector control technology was born in the early 1970s. The vector control system of permanent magnet synchronous motor (PMSM) refers to the control strategy of DC motor. By using coordinate transformation, the collected three-phase stator current, flux and other vectors of motor are decomposed into two components according to the direction of rotor flux. One is along the direction of rotor flux, which is called direct axis excitation current; The other is orthogonal to the rotor flux direction, which is called quadrature axis torque current. According to different control objectives, the excitation current and torque current are adjusted to achieve accurate control of speed and torque, so that the control system can obtain good steady-state and dynamic response characteristics.
According to different control objectives, PMSM vector control algorithm can be divided into the following: id = 0 control, maximum torque / current control, flux weakening control and so on. These performance indexes can be achieved by independent control of direct axis excitation current and quadrature axis torque current.

Advantages of permanent magnet synchronous motor

Permanent magnet synchronous motor can be installed on the axle as a whole to form a whole direct drive system, that is, an axle is a driving unit, which saves a gearbox. The advantages of PMSM are as follows:

  • Permanent magnet synchronous motor has high power efficiency and power factor;
  • Permanent magnet synchronous motor (PMSM) has the advantages of simple structure, small volume and low noise;
  • The system adopts fully enclosed structure, no transmission gear wear, no transmission gear noise, no lubricating oil, no maintenance;
  • The permanent magnet synchronous motor (PMSM) has high overload current and high reliability;
  • The weight of the whole transmission system is light, the unsprung weight is lighter than that of the traditional axle transmission, and the power per unit weight is larger;
  • Since there is no gearbox, the bogie system can be designed at will, such as flexible bogie and single axle bogie, which greatly improves the dynamic performance of the train.

Due to the use of permanent magnet material pole, especially the use of rare earth metal permanent magnet (such as NdFeB, etc.), its magnetic energy product is high, and higher air gap flux density can be obtained. Therefore, when the capacity is the same, the motor has small volume and light weight.
The rotor has no copper loss and iron loss, and no friction loss of collector ring and brush, so the operation efficiency is high.
Small moment of inertia, large allowable pulse torque, high acceleration, good dynamic performance, compact structure and reliable operation.

Research hotspot of permanent magnet synchronous motor

Torque characteristics of motor

In order to improve the torque characteristics of the motor, many scholars and research institutions have made bold attempts and innovations in the structural design of permanent magnet synchronous motor, and made many new progress. In order to solve the contradiction between the slot width and the tooth width, the transverse flux machine technology is developed. The armature coil and the tooth slot structure are vertical in space, and the main flux flows along the axial direction of the motor, which improves the power density of the motor; The double-layer permanent magnet arrangement improves the quadrature axis conductance of the motor, thus increasing the output torque and maximum power of the motor; In order to reduce the torque ripple of the motor, the stator tooth shape and pole shape are changed.

The expanding ability of weak magnetic field

After using flux weakening control, the operation characteristics of PMSM are more suitable for the driving requirements of electric vehicles. Under the condition of the same power requirement, the capacity of the inverter is reduced and the efficiency of the drive system is improved. Therefore, flux weakening speed expansion is widely used in PMSM for electric vehicle drive. For this reason, research institutions at home and abroad have put forward a variety of schemes, such as using double stator structure, using different windings at different speeds to maximize the use of permanent magnet magnetic field; The composite rotor structure is adopted, and the reluctance section is added to the rotor to control the reactance parameters of the direct axis and the quadrature axis of the motor, so as to increase the speed expansion capacity of the motor; Deep slots are used in the stator to increase the leakage reactance of the direct shaft to expand the speed range of the motor.

Control theory of motor

Due to the nonlinear and multivariable characteristics of permanent magnet synchronous motor, its control is difficult and the control algorithm is complex. The traditional vector control method often can not meet the requirements. Therefore, some advanced control methods are applied in PMSM speed control system, including adaptive observer, model reference adaptive, high frequency signal injection, fuzzy control, genetic algorithm and other intelligent control methods. These control methods do not depend on the mathematical model of the control object, have good adaptability and robustness, and have unique advantages for the strong nonlinear system such as permanent magnet synchronous motor.

Source: China Permanent Magnet Manufacturer – www.rizinia.com

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