China permanent magnet manufacturer: www.rizinia.com

What is a permanent magnet synchronous motor

What is a permanent magnet synchronous motor?

The permanent magnet synchronous motor provides excitation with permanent magnets, which makes the motor structure simpler, reduces processing and assembly costs, and eliminates the collector ring and brushes, which are prone to problems, improving the reliability of motor operation; and because there is no excitation current, there is no excitation loss, improving the efficiency and power density of the motor.
The permanent magnet synchronous motor is composed of stator, rotor and end cap. The stator is basically the same as the common induction motor, and the laminated structure is used to reduce the iron consumption when the motor is running. The rotor can be made solid or laminated. The armature winding can be made of centralized whole pitch winding, or distributed short pitch winding and unconventional winding.
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Structure of permanent magnet synchronous motor

A permanent magnet synchronous motor is mainly composed of stator, rotor and end cap. The stator is made of laminated sheets to reduce iron consumption during motor operation, and is equipped with a three-phase AC winding called armature. The stator is made of laminated sheets to reduce iron consumption during operation, and is equipped with a three-phase AC winding called the armature. The rotor can be made in solid form, or can be made of laminated sheets with permanent magnet material. Depending on the position of the permanent magnet material on the motor rotor, the permanent magnet synchronous motor can be divided into two structural forms: protruding and built-in, which are shown in Figure 1. The protruding rotor has a simple magnetic circuit structure and low manufacturing cost, but it cannot realize asynchronous starting because the starting winding cannot be installed on its surface.
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Magnetic circuit structure of three different forms of built-in rotors
The magnetic circuit structure of the built-in rotor is mainly radial, tangential and hybrid. The difference between them mainly lies in the relationship between the magnetization direction of the permanent magnet and the rotor rotation direction. Figure 2 shows the magnetic circuit structure of three different forms of built-in rotors. Since the permanent magnet is placed inside the rotor, the rotor surface can be made into a pole shoe, and the pole shoe can be placed into a copper bar or cast aluminum to play the role of starting and damping, and the steady-state and dynamic performance is better. The built-in rotor magnetic circuit is asymmetrical, which will produce reluctance torque in operation, helping to improve the power density and overload capacity of the motor itself, and such a structure is easier to achieve weak magnetic expansion speed.

Working principle of permanent magnet synchronous motor

When the three-phase current flows into the three symmetrical windings of the stator of a permanent magnet synchronous motor, the magnetic potential generated by the current is synthesized into a rotating magnetic potential of constant magnitude. Since its magnitude is constant, the trajectory of this rotating potential forms a circle, which is called circular rotating potential. Its size is exactly 1.5 times the maximum magnitude of the single-phase magnetic potential, i.e.
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In the formula, F is the circular rotational magnetic potential, (T.m); Fφl is the maximum magnitude of the single-phase magnetic potential, (T.m); k is the fundamental winding coefficient; p is the number of pairs of motor poles; N is the number of turns in series for each coil; I is the effective value of the current flowing in the coil, A because the speed of the permanent magnet synchronous motor is constant as the synchronous speed, so the rotor main magnetic field and the rotating magnetic field generated by the stator circular rotational magnetic potential remain relatively stationary. 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 rotor magnetic field to produce an electromagnetic torque Te that drives or hinders the rotation of the motor, i.e.
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In the formula, Te is the electromagnetic torque, (N・m); 0 is the power angle, rad; BR is the main rotor magnetic field, T; Bnet is the air-gap synthetic magnetic field, T. Due to the different relationship between the position of the air-gap synthetic magnetic field and the main rotor magnetic field, the permanent magnet synchronous motor can operate in both the motor and generator states. When the air gap synthesis magnetic field lags behind the rotor main magnetic field, the electromagnetic torque generated is opposite to the rotor rotation direction, and the motor is in the power generation state; on the contrary, when the air gap synthesis magnetic field is ahead of the rotor main magnetic field, the electromagnetic torque generated is the same as the rotor rotation direction, and the motor is in the electric state. The angle between the rotor main magnetic field and the air-gap synthetic magnetic field is called the power angle. 
A permanent magnet synchronous motor consists of two key components, a multipolarized permanent magnet rotor and a stator with appropriately designed windings. During operation, the rotating multipolarized permanent magnet rotor creates a time-varying magnetic flux in the air gap between the rotor and the stator. This flux generates an AC voltage at the stator winding terminals, thus forming the basis for power generation. The permanent magnet synchronous motor discussed here uses a ring-shaped permanent magnet mounted on a ferromagnetic core. Internal permanent magnet synchronous motors are not considered here. Since it is very difficult to embed a magnet into a plated ferromagnetic core, the air gap can be made very large (300 to 500 μm) without significant performance loss by using magnets of appropriate thickness (500 μm) and high performance magnetic materials in the rotor and stator cores, which allows the stator winding to occupy a certain amount of space in the air gap, thus greatly simplifying the manufacture of permanent magnet synchronous motors.

Classification of permanent magnet synchronous motors

Classification by excitation current supply method

A permanent magnet synchronous motor is a synchronous motor that uses permanent magnets to create an excitation field, with a rotating magnetic field generated by the stator and a rotor made of permanent magnet material. The synchronous motor requires a DC magnetic field to achieve energy conversion, and the DC current that generates this field is called the motor’s excitation current.
Other-excited motor: A motor that receives its excitation current from another power source.
Self-excited motor: A motor that receives its excitation current from the motor itself.

Classification by frequency of power supply

Permanent magnet brushless motors include two types of permanent magnet brushless DC motors and permanent magnet brushless AC motors, both of which require variable frequency power supply when operating as motors. The former requires only square wave inverter power supply, while the latter requires sine wave inverter power supply.

Classification according to the distribution of air gap magnetic field

Sine wave permanent magnet synchronous motor: The magnetic poles are made of permanent magnet material, and the air gap magnetic field is distributed according to the sinusoidal law when the three-phase sinusoidal current is input, which is referred to as permanent magnet synchronous motor.
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 DC motor. Self-controlled inverter synchronous motors composed of trapezoidal wave permanent magnet synchronous motors are also called brushless DC motors.

Control method of permanent magnet synchronous motor

Constant voltage to frequency ratio control method of permanent magnet synchronous motor

The constant voltage to frequency control method of permanent magnet synchronous motor is similar to the constant voltage to frequency control method of AC induction motor, in which the amplitude and frequency of the input voltage of the motor are changed at the same time, so that the flux of the motor is constant.
Without the feedback of physical signals such as current, voltage or position, a certain control accuracy can still be achieved, which is the biggest advantage of the constant voltage to frequency ratio control method. The control algorithm of constant voltage to frequency ratio control method is simple and the hardware cost is low, so it is widely used in the field of general-purpose inverter. The disadvantages of constant-voltage frequency ratio control method are also obvious. Since there is no feedback of speed, position or any other signal in the control process, it is almost impossible to obtain the information of motor running state, and it is even more impossible to precisely control the speed or electromagnetic torque. Obviously, this control method cannot control the torque and excitation current separately, and it is easy to have a large excitation current in the control process, which affects the efficiency of the motor. Therefore, this control method is often used in general-purpose inverters with low performance requirements, such as air conditioning, conveyor belt drive control of assembly lines, energy-saving operation of pumps and fans, etc.

Direct torque control technology for permanent magnet synchronous motors

Direct Self-Control (DSC) models the magnetic chain and electromagnetic torque in the stator stationary coordinate system and applies different voltage vectors to control the electromagnetic torque and stator chain. The direct torque control method has the advantages of simple algorithm and good torque response, therefore, it is widely used in applications requiring high transient torque response.
Due to the inherent disadvantages of the control, the direct torque control method has low control frequency and high torque pulsation at low speeds. Therefore, the reduction of torque pulsation at low speeds has become a hot topic of research in direct torque control methods, and Sun Xiaohui et al. reduced the torque pulsation at low speeds by optimizing the voltage vector action time with good results.

Vector control technique for permanent magnet synchronous motor

Vector control technology was born in the early 1970s. The vector control system of permanent magnet synchronous motor is based on the control strategy of DC motor, using coordinate transformation to decompose the collected vectors of motor three-phase stator current and magnetic chain into two components according to the direction of rotor magnetic chain, which is a rotating vector, one is along the direction of rotor magnetic chain, called straight-axis excitation current; the other is orthogonal to the direction of rotor magnetic chain, called cross-axis torque current. The excitation and torque currents are adjusted according to different control objectives to achieve precise control of speed and torque, so that the control system can obtain good steady-state and dynamic response characteristics.
According to different control objectives, the vector control algorithms of permanent magnet synchronous motor can be divided into the following categories: id=0 control, maximum torque/current control, weak magnetic control, etc. All these performance targets can be achieved by independent control of the straight-axis excitation current and the cross-axis torque current.

Advantages of permanent magnet synchronous motor

Permanent magnet synchronous motor can install the motor on the axle as a whole to form an integral direct drive system, that is, an axle is a drive unit, eliminating a gearbox. The advantages of permanent magnet synchronous motor are as follows:

  • Permanent magnet synchronous motor itself has high power efficiency and high power factor;
  • Permanent magnet synchronous motor has small heating, so the motor cooling system has the advantages of simple structure, small volume and low noise;
  • The system adopts fully enclosed structure, without transmission gear wear, transmission gear noise, lubricating oil and maintenance;
  • Permanent magnet synchronous motor allows large overload current and significantly improves its 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 large;
  • Since there is no gearbox, the bogie system can be designed arbitrarily, such as flexible bogie and single axle bogie, which greatly improves the dynamic performance of the train.
  • Due to the use of permanent magnetic poles, especially rare earth permanent magnets (such as neodymium iron boron, etc.), the magnetic energy product is high and high 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, iron loss, friction loss of collector ring and brush, and has high operation efficiency.
  • Small moment of inertia, large allowable pulse torque, high acceleration, good dynamic performance, compact structure and reliable operation.

Research hotspots of permanent magnet synchronous motor

Torque characteristics of motors

In order to improve the torque characteristics of motors, many scholars and research institutions have made bold attempts and innovations in the structural design of permanent magnet synchronous motors, and many new advances have been made. In order to solve the contradiction between slot width and tooth width, transverse flux machine technology has been developed, in which the armature coil and slot structure are perpendicular in space and the main flux flows along the axial direction of the motor, thus increasing the power density of the motor; the double-layer permanent magnet arrangement is used to increase the cross-axis conductance of the motor, thus increasing the output torque and maximum power of the motor. Changing the stator tooth shape and pole shape to reduce the torque pulsation of the motor, etc.

Speed expansion capability of weak magnetism

With weak magnetic control, the operating characteristics of PM synchronous motors are better suited to the drive requirements of electric vehicles. With the same power requirements, the inverter capacity is reduced and the efficiency of the drive system is improved. Therefore, it is common for permanent magnet synchronous motors for electric vehicle drive to adopt weak magnetic expansion speed. For this reason, research institutes at home and abroad have proposed various solutions, such as adopting a double-set stator structure and using different windings at different speeds to maximize the use of the permanent magnet magnetic field; adopting a composite rotor structure and increasing the reluctance section of the rotor to control the reactance parameters of the motor’s straight and cross shafts to increase the motor’s speed expansion capability; and adopting a deep slot in the stator to increase the straight shaft leakage resistance to expand the motor’s speed range.

Motor control theory

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

Causes and preventive measures of demagnetization of permanent magnet motor

Once the permanent magnet motor loses excitation, it can only choose to replace the motor, and the maintenance cost is a lot. How to judge the loss of excitation of the permanent magnet motor? Let’s go on.
1. When the machine starts running, the current is normal. After a period of time, the current becomes larger. After a long time, it will report that the frequency converter is overloaded.
Firstly, it is necessary to make sure that the type selection of the frequency converter of the air compressor manufacturer is correct, and then confirm whether the parameters in the frequency converter have been changed.
If there is no problem with both, it is necessary to judge through the back electromotive force, disconnect the head from the motor, carry out no-load identification, and operate at no-load to the rated frequency. At this time, the output voltage is the back electromotive force. If it is more than 50V lower than the back electromotive force on the motor nameplate, the demagnetization of the motor can be determined.
2. After demagnetization, the operating current of permanent magnet motor will generally exceed the rated value
Those cases that report overload only at low speed or high speed or occasionally report overload are generally not caused by demagnetization.
3. Demagnetization of permanent magnet motor takes a certain time, some months or even a year or two
If the manufacturer’s selection error leads to current overload, it does not belong to motor demagnetization.

Causes of motor demagnetization

  • The cooling fan of the motor is abnormal, resulting in high temperature of the motor.
  • The motor is not equipped with temperature protection device.
  • The ambient temperature is too high.
  • Unreasonable motor design.

How to prevent demagnetization of permanent magnet motor?

Correctly select the power of permanent magnet motor

Demagnetization is related to the power selection of permanent magnet motor. Choosing the power of permanent magnet motor correctly can prevent or delay demagnetization.
The main reason for demagnetization of permanent magnet synchronous motor is too high temperature, and overload is the main reason for too high temperature.
Therefore, a certain margin shall be reserved when selecting the power of permanent magnet motor. Generally, about 20% is appropriate according to the actual situation of load.

Avoid heavy starting and frequent starting

Cage asynchronous starting synchronous permanent magnet motor shall avoid heavy load direct starting or frequent starting as far as possible.
In the process of asynchronous starting, the starting torque oscillates. In the valley of starting torque, the stator magnetic field demagnetizes the rotor magnetic pole.
Therefore, heavy load and frequent starting of asynchronous permanent magnet synchronous motor shall be avoided as far as possible.

Improving the design of permanent magnet motor

1. Properly increase the thickness of permanent magnet
From the point of view of design and manufacture of permanent magnet synchronous motor, the relationship among armature reaction, electromagnetic torque and demagnetization of permanent magnet should be considered.
Under the combined action of the magnetic flux generated by the torque winding current and the magnetic flux generated by the radial force winding, the permanent magnet on the rotor surface is easy to cause demagnetization.
When the air gap of the motor remains unchanged, the most effective way to ensure that the permanent magnet does not demagnetize is to appropriately increase the thickness of the permanent magnet.
2. There is a ventilation slot circuit inside the rotor to reduce the temperature rise of the rotor
Demagnetization of permanent magnet is an important factor affecting the reliability of permanent magnet motor. If the rotor temperature rises too high, the permanent magnet will produce irreversible loss of excitation.
In the structural design, the internal ventilation circuit of the rotor can be designed to directly cool the magnetic steel. It not only reduces the temperature of magnetic steel, but also improves the efficiency.

Permanent magnet motor design – demagnetization treatment

The development of permanent magnet motor has a history of more than 100 years. Its permanent magnet materials (ferrite, NdFeB, etc.) are the core of the motor. As the most excellent material at present, NdFeB has been widely used. In its use, the design of demagnetization treatment has to be avoided. Take n42sh material as an example.
1. Demagnetization curve
The following are n42sh related parameters:

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The parameters involved, such as BR and HCB, should be understood by everyone. The key to demagnetization is the demagnetization curve in the figure above, including demagnetization curve and another intrinsic demagnetization curve.
Demagnetization curve: it represents the B-H relationship when the permanent magnet material is completely magnetized without external excitation. Permanent magnet materials have no external excitation in general applications, so demagnetization curve is the main characteristic curve representing the characteristics of permanent magnet materials.
Intrinsic demagnetization curve: it has nothing to do with the motor design itself and mainly affects the stability of magnet materials. The internal magnetic induction generated by permanent magnet materials magnetized under the action of external magnetic field is called intrinsic magnetic induction Bi, also known as magnetic polarization J.

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It is simply understood that when the magnet is in the external magnetic field in the demagnetization state, the characterization value is br HC, but the magnetic field of the actual magnet is bi-h. That is, the magnetic field intensity is reduced by u0h due to external influence.
2. Causes and manifestations of demagnetization
Causes: high frequency vibration, extreme temperature, large current, corrosion and magnetization, and the combined effect of several causes.
Manifestations: motor working point drift, motor torque and performance decline, sharp increase of vibration and noise, unstable operation, abnormal temperature rise, etc.
Detection means: rotor magnetic flux and magnetic field intensity distribution on rotor surface.
3. Optimal design of demagnetization

  • Optimize the air gap magnetic conductivity: increase the air gap, increase the number of stator slots, reduce the width of stator slot, and adopt magnetic slot wedge or closed slot;
  • Reduce motor harmonic: adopt low harmonic winding (double-layer short distance distribution, single and double-layer winding), improve the carrier frequency of the frequency converter or add a filter at the output side of the frequency converter;
  • Surface insulation treatment of permanent magnet: replace galvanized or nickel copper nickel with “nickel copper + epoxy” coating, or dip paint and dry with magnetic rotor;
  • The permanent magnet is designed in blocks and sections;
  • Select high intrinsic coercivity permanent magnet, such as eh instead of uh;
  • Increase the magnetization direction size of permanent magnet;
  • Adopt multipole motor design scheme;
  • Aging treatment of permanent magnet;
  • Deep buried permanent magnet, that is, the permanent magnet shall be arranged as far away from the armature winding as possible;
  • The parts where permanent magnets are easy to demagnetize are often edges and corners, so permanent magnets can be chamfered;
  • By increasing the magnetic leakage of the magnetic circuit, the purpose of demagnetizing the armature reaction and shunting the magnetic flux is achieved. For example, the built-in type is used to replace the surface paste type scheme, increase the thickness of the magnetic bridge, reduce the length of the magnetic bridge, etc.

4. Simulation design of demagnetization
Case description
Simulation design of N42SH magnet of IPM motor under different working environments.
Product background: 12V low voltage, high torque motor, 18 SLOT 6-pole motor design scheme, mainly used in parking air conditioning and automatic fast constant temperature system.

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(1) Demagnetization effect caused by high temperature
Comparison of back EMF waveform (demagnetization occurs when it is set at 0.015s)
After demagnetization, the main flux of rotor magnetic field deviates, resulting in the change of motor working point and the decrease of back EMF.
The cogging torque of the motor will also be affected, although the cogging torque is reduced.

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As the temperature increases, the magnetic properties of the magnet decrease. When it rises to a certain temperature, the magnetization disappears. This temperature is called the Curie temperature of the magnet. Generally, the Curie temperature of n42sh is 150 ℃.
Load state
When the temperature causes the magnet to demagnetize, the power and torque of the motor will decrease.
Note: when the temperature rises, the motor is always in a high-temperature environment, and the phenomena such as torque and power drop are normal, because it can be clearly seen in the figure below that BR decreases due to the increase of temperature, resulting in the decrease of magnetic field strength and magnetic flux.

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(2) Demagnetization effect caused by starting current or instantaneous large current
In order to illustrate the demagnetization effect, we take the ambient temperature at 120 ℃ as an example and select point3 as the monitoring point
The current is 3 times of the rated current as the starting current, and 1.2 and 1.5 times of the starting current are tested respectively.

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Rated current

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Demagnetization current

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1.2 times demagnetization current

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1.5 times demagnetization current
It can be seen from the figure that when the demagnetization current increases, the permeability of the magnet decreases rapidly and demagnetizes.
Personally, I think the comparison of permeability is the most convenient and fast means. Of course, there are also methods such as the operation range of magnet B-H demagnetization curve and the comparison of demagnetization rate.

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After high temperature demagnetization, there is a magnitude reduction in motor counter potential, and the effect is not very obvious because the design of low voltage motor counter potential is relatively low.
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load condition, irreversible demagnetization occurs after high temperature demagnetization, and there is a significant reduction in torque after restoring the normal operating temperature.
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The cloud diagram of the demagnetization rate of the magnetic field after demagnetization is shown above. The demagnetization of the magnetic field after demagnetization is shown above. And the demagnetization rate to have the highest 20%.

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

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