Scuola di Ingegneria Industriale e dell'Informazione
Corso di Laurea Magistrale in Ingegneria Elettrica
Design and Analysis of a 120kW High-speed Permanent Magnet
Motor for Centrifugal Compressor
Relatore: Prof. Antonino di Gerlando Correlatore: Prof. Hong Guo
Tesi di Laurea Magistrale di: Chongwei Duan Matricola: 876384
High-speed permanent magnet synchronous motor (HSPMSM) has many advantages, such as small size, high power density, and can be directly connected with high-speed loads, reducing the weight and volume of the system, etc., therefore, it has been widely used in many fields. Researching on high-speed permanent magnet synchronous motor technology would benefit the development of aerospace equipment manufacturing, automobile and shipboard industries, high-precision machining and advanced industrial equipment, which is of great significance for the industrial structure optimization.
In this thesis, a high-speed permanent magnet synchronous motor with a rated power of 120kW is studied. The design of this high-power high-speed permanent magnet synchronous motor is carried out. The main work contents are as follows:
(1) Based on the analysis of the motor operating conditions and the summarization of the design specifications, this thesis designed the motor by adopting the motor design routine which combines the magnetic circuit method and electromagnetic field finite element method. Through continuous optimization, the dimensional parameters of the motor are obtained.
(2) The electromagnetic properties of the motor were analyzed by electromagnetic field finite element analysis. The rationality and correctness of magnetic circuit design and finite element modeling are verified by steady-state static magnetic field simulation. The power angle characteristics, no-load performance and full-load performance of the motor are checked by transient field simulation analysis. The origins of the motor losses are analyzed, and the loss values at the rated condition are calculated to obtain the motor efficiency.
(3) Based on the specific working conditions of the compressor, a novel evaporative cooling configuration combining spiral channel and refrigerant cooling was designed. Through the reasonable simplification and assumption of the geometric model and boundary conditions, the steady temperature field module is used to analyze the temperature distribution of the motor, and the heat dissipation effect of the cooling structure is preliminarily verified. A water cooling
(4) The strength of the rotor structure under high-speed condition was checked. A rotor support scheme based on aerostatic bearings is used, and the overall mechanical structure of the motor assembly including the bearing is given.
1 Introduction ... 1
1.1 Background and significance ... 1
1.2 Research status analysis ... 3
1.2.1 The development trend of high-speed machines ... 3
1.2.2 Loss calculation and analysis of high speed machine ... 5
1.2.3 Calculation method of machine temperature rise ... 9
1.2.4 Cooling method of high-speed machine ... 12
1.2.5 Rotor support method for high-speed machine ... 16
1.2.6 Existing problems ... 18
1.3 Research content of the thesis ... 19
1.4 Chapter arrangement ... 20
2 Electromagnetic Design of a 120kW HSPMSM ... 22
2.1 Introduction ... 22
2.2 Design specifications ... 22
2.3 Design method ... 23
2.4 Magnetic circuit method design ... 24
2.4.1 Equivalent magnetic circuit of permanent magnet motor ... 24
2.4.2 Selection of permanent magnet materials ... 26
2.4.3 Stator structure design ... 29
2.4.4 Rotor structure design ... 29
2.4.5 Main dimensions ... 33
2.4.6 Air gap length selection ... 35
2.4.7 Permanent magnet size design ... 36
2.4.8 Slot shape ... 37
2.4.9 Winding parameters ... 39
2.4.10 Design summary ... 41
3.1 Introduction ... 44
3.2 Theoretical basis of finite element electromagnetic field analysis ... 44
3.2.1 Overview of electromagnetic field finite element method ... 44
3.2.2 Maxwell's equations ... 45
3.2.3 Differential equation ... 46
3.2.4 Boundary conditions ... 48
3.2.5 Introduction to commercial finite element software... 49
3.3 Establishment of motor finite element model ... 50
3.4 Static magnetic field analysis ... 52
3.5 Analysis of power angle characteristics ... 55
3.5.1 Setting the initial position of the rotor ... 56
3.5.2 Setting the initial phase of the voltage source ... 58
3.5.3 Relationship between power angle and internal power factor angle ... 59
3.6 No-load performance analysis ... 60
3.7 Full-load performance analysis ... 63
3.8 Loss analysis ... 64
3.8.1 Copper loss ... 64
3.8.2 Stator iron loss ... 65
3.8.3 Rotor eddy current loss ... 69
3.8.4 Mechanical loss ... 71
3.8.5 Motor efficiency ... 73
3.9 Chapter summary ... 73
4 Cooling method and temperature field analysis of 120kW HSPMSM ... 75
4.1 Introduction ... 75
4.2 Heat transfer inside the motor ... 75
4.3 Evaporative cooling structure design ... 77
4.3.3 Advantages of the novel cooling configuration ... 80
4.4 Establishment of temperature field analysis model ... 80
4.4.1 Analysis method ... 80
4.4.2 Modeling steps of thermal field finite element method ... 81
4.4.3 Slot area equivalence ... 82
4.4.4 Air gap equivalence ... 83
4.4.5 Heat source and boundary conditions ... 84
4.5 Temperature field analysis results ... 85
4.6 Water cooling design ... 86
4.6.1 Equivalent thermal path modeling ... 87
4.6.2 Simulation analysis ... 91
4.7 Chapter summary ... 95
5 Structural design of 120kW HSPMSM ... 96
5.1 Introduction ... 96
5.2 Analysis of rotor structural strength ... 96
5.2.1 Establishment of the rotor model ... 96
5.2.2 Structural strength analysis results ... 97
5.3 Air bearing configuration and overall mechanical structure of the motor ... 98
5.4 Chapter summary ... 99
Conclusion ... 101
1 Introduction
1.1 Background and significance
High-speed machines have been a research hotspot in the field of electrical machines since their appearance. In recent years, with the development and advancement of materials, manufacturing techniques, power devices, control systems and other related technologies, the highest speed that can be achieved by high-speed machines has gradually increased. Currently, high speed machines generally refer to machines with speeds above 10,000 r/min[1]. Compared
with traditional machines, high-speed machines have the advantage of high power density. At the same power level, the volume of high-speed machines is much smaller than that of conventional medium- and low-speed machines. Besides, the high-speed machine can be directly connected to the high-speed prime mover or load, which reduces the weight and volume of the system, reduces the cost, and increases the reliability[2]. Based on the above advantages,
high-speed machines have been widely used in many fields, such as more electric aircraft[3],
electric vehicles[4], marine power systems [5], electric spindles for CNC machine tools[6], and
centrifugal compression for refrigeration[7]. High-speed machines play the role of the core
components in these systems. Therefore, research and breakthroughs in high-speed machines related technologies are of great significance and will contribute to the development of aerospace equipment manufacturing, automobile and shipbuilding industry, high-precision machining and advanced industrial equipment, which could optimize industrial structure of modern society.
The types of machines currently successfully used in high-speed fields mainly include induction machines, switched reluctance machines, and permanent magnet machines. Compared with other types of machines, high-speed permanent magnet synchronous machines (HSPMSMs) have the advantages of high efficiency, high power density, high power factor, high reliability, good control characteristics, etc., as a result, HSPMSMs are favored in the field of high-speed machines, and are applied to all power levels. At present, most of the high-power high-speed permanent magnet synchronous machines are generators[8, 9], and there are few
studies on high-power high-speed permanent magnet synchronous motors. Therefore, this thesis intends to perform a design and analysis of a high-power high-speed permanent magnet synchronous motor, study some key issues to achieve high efficiency and high reliability design of the motor, and provide reference for the development of similar motors.
The high efficiency and high reliability of high-power high-speed permanent magnet synchronous motor is based on reasonable electromagnetic design and calculation, accurate analysis of motor loss and temperature rise, and selection of reliable heat dissipation method and rotor bearings.
Compared with conventional motors of the same power level, high-speed permanent magnet synchronous motors have smaller volume and higher power supply frequency, so the power loss per unit volume, namely the loss density, is larger. Loss will generate heat inside the motor, and the small size of the high-speed permanent magnet synchronous motor means that it is difficult to dissipate heat. Therefore, the increase in loss density could easily cause an increase in the internal temperature rise of the motor. If the internal temperature rise of the motor exceeds the thermal limit of the winding insulation material and the permanent magnet, it will lead to insulation failure and irreversible demagnetization of the permanent magnet, which will reduce the reliability of the motor. As the increase in output power, the loss density and temperature rise would inevitably rise, which could lead to a further increase in the above problems. Therefore, loss analysis and temperature rise calculation are crucial for the design of high-power high-speed permanent magnet synchronous motors for compressors. Accurate analysis and calculation of loss and temperature rise, and the selection of appropriate heat dissipation methods, have great significance for improving the reliability of the motor.
The high-speed permanent magnet synchronous motor for compressor is in long-term working state, and its rotor support technology directly determines whether the motor can operate reliably for a long term. Reasonable selection of the rotor support method could help to reduce rotor friction loss, improve motor running stability, and reduce maintenance costs.
To sum up, the design of high-speed permanent magnet synchronous motor combines electromagnetic field, temperature field and structural design and analysis, which is quite
complicated. This thesis provides a reference for the design and manufacture of high-power high-speed permanent magnet synchronous motor by carrying out a multi-field design and analysis of 120kW high-speed permanent magnet synchronous motor. It has certain theoretical and practical significance for the development of high-speed motor.
1.2 Research status analysis
This section introduces several research branches of high-speed machines, including the development trend of high-speed machines, the calculation and analysis of machine losses, the calculation method of machine temperature rise, the cooling method and the rotor support method.
1.2.1 The development trend of high-speed machines
High-speed machines are usually referred to as electrical machines with a speed of more than 10,000 rpm or a difficulty value of more than 105[1]. The difficulty value is a criterion for
evaluating the performance of high-speed machines, calculated by multiplying the rotational speed (rpm) by the square root of power ( √ kW)[10]. The types of machines currently
successfully used in high-speed fields mainly include induction machines, switched reluctance machines, and permanent magnet machines. The advantage of the induction machine is that it has a simple rotor structure, high temperature resistance and low cost, but its rotor has large loss, low efficiency and low power factor, which limits its application to a certain extent. At present, the highest-power high-speed induction machine is a 15MW, 20000r/min machine developed by ABB[11]. Switched reluctance machines have some advantages of induction
machines, such as simple structure, high temperature resistance, low cost, etc. Moreover, its winding ends are shorter, and the efficiency is slightly higher than that of induction machines. Many institutions have carried out certain research on switched reluctance machines[12-14].
However, the switched reluctance machine has the disadvantages of large torque fluctuation, large rotor mechanical vibration, large noise and complicated control. Therefore, it is not suitable for applications where vibration and noise are strictly restricted.
In recent years, permanent magnet machines have been favored in the field of high-speed machines due to their high efficiency, high power density, high reliability, and good control
characteristics. High-speed permanent magnet machines are widely used in various power levels, as small as 1.1mW micro-power permanent magnet motors[15], as large as
megawatt-class turbo generators[16]. Permanent magnet machines, with their broad application prospects
and clear technical advantages, have gained wide attention from scholars all over the world and have great application potential. In the field of high-speed machines, the inner rotor permanent magnet machine has a small rotor radius and high reliability, so it has an advantage in high-speed applications. Currently the internal rotor high-high-speed permanent magnet machine has a maximum power of 8 MW. It is a high-speed steam turbine generator developed by Direct Drive Systems of California, USA. Its rotor is protected by a carbon fiber bundling and supported by a magnetic suspension bearing and its maximum speed can reach 18000 rpm. The company performed a dynamic analysis of the rotor, the results show that the maximum speed of the rotor is 20% lower than the first-order critical speed, which proves that the bearing configuration is reliable. The cooling system combined three types of cooling methods: axial air cooling, end cover water cooling and spiral water jacket, which meets the heat dissipation requirements and has been verified by experiments[16]. Professor Seok-Myeong Jang of Chungnam University in
South Korea developed a 50kW, 70,000 rpm permanent magnet brushless DC motor for use in centrifugal compressor applications[7]. The electromagnetic field, back EMF and loss of the
motor are analyzed analytically, and the analysis results are verified by finite element method and prototype experiment.
Professor Binder of Darmstadt University of Technology in Germany recorded and fitted the rated power and speed of some high-speed machines[17]. The range covers 0.1kW-80MW,
3800rpm-500,000rpm. Dr. Dong Jianning from Southeast University made similar statistics for high-speed permanent magnet machines[18], and the results are shown in Figure 1(a) and (b),
+ Permanent magnet synchronous machines Cage induction machines homo-polar machines
(a) (b)
Figure 1 High-speed machine power versus speed statistics
As can be seen from the above figure, as the speed increases, the rated power of the high-speed machine decreases. According to the statistics in Figure 1(b), permanent magnet machines with powers ranging from tens to hundreds of kilowatts account for a large portion. High-speed machines of this power class are mainly used in industrial fields such as compressors and micro gas turbines. With the rapid development of industry, high-speed machines, especially high-speed permanent magnet machines, will have broader application prospects in the above industrial fields.
1.2.2 Loss calculation and analysis of high speed machine
The loss of a high-power permanent magnet synchronous machine will inevitably increase as the output power increases. Moreover, the iron loss of the high speed permanent magnet synchronous machine will increase as the frequency of the power supply increases. Therefore, for high-speed permanent magnet synchronous machines, the calculation and analysis of losses is crucial. The losses of high-speed permanent magnet synchronous machines mainly include stator iron loss, stator copper loss, eddy current loss on the rotor surface, and friction loss.
(1) Stator iron loss
High-speed permanent magnet synchronous machine has a higher power supply frequency, so the alternating frequency of the magnetic field in the stator core is correspondingly higher, resulting in a large proportion of the stator iron loss in the total loss. Therefore, studying stator iron loss is of great significance for reducing the total loss of the machine and improving the efficiency of the machine. Currently, the most commonly used stator core loss calculation models could be mainly divided into two categories, namely the classical iron loss separation
model considering only alternating magnetization and the elliptical rotation model as well as orthogonal decomposition model considering both rotational magnetization and alternating magnetization.
The classical iron loss separation model was proposed by Italian Professor Bertotti Giorgio[19]. According to the cause of iron loss, the model divides the core loss into three parts:
hysteresis loss, eddy current loss and additional loss. The coefficients are calculated separately for the three-part loss. The theory was initially proposed to consider only the loss calculations under sinusoidal flux. Subsequently, Fausto Fiorillo et al. explored the iron loss harmonic analysis method under non-sinusoidal alternating magnetization conditions[20]. Ignoring the
small hysteresis loop inside the ferromagnetic material and the anisotropy of the material, the iron loss generated by any magnetic field waveform in the ferromagnetic material is equal to the sum of the iron loss generated by the fundamental wave of the magnetic field and the iron loss generated by the harmonics of different order acting on the ferromagnetic material respectively. Through the harmonic analysis method, the classical iron loss separation model has been improved.
The classical iron loss separation model only considers the influence of alternating magnetization, and in the actual permanent magnet synchronous machine, the stator core is often in a rotating magnetic field. Therefore, in addition to alternating magnetization, rotational magnetization will also cause iron loss in the stator core. Studies by scholars have shown that in some cases, the calculated value of loss after ignoring rotational magnetization has a 20% error compared to the actual measured value[21]. The elliptical rotation model comprehensively
considers the rotating magnetization and the alternating magnetization, that is, the magnitude and magnetization direction of the rotating electric machine are changed from time to time. The characteristics of the rotating hysteresis loss are based on the experimentally summarized rules, and the mechanism is not yet clear. Zhu Jianguo from the University of Technology of Sydney proposed a calculation method for elliptical rotation model with high calculation accuracy. However, the premise of the method is to obtain the loss coefficient of ferromagnetic material under elliptical rotating magnetic field by using two-dimensional iron loss test equipment[22].
The device is currently owned by only a few research institutions, limiting the use of this method. Researchers at Southeastern University have proposed a method for calculating the loss of an elliptical rotating magnetic field by using two equivalent orthogonal alternating magnetic fields, namely the orthogonal decomposition model[23]. The model considers both the
effects of elliptical rotating magnetization and the loss caused by harmonic magnetic fields. Moreover, the method does not require a special magnetization characteristic test instrument, and only requires a loss coefficient under alternating magnetization to achieve a more accurate iron loss calculation, so the method has been widely applied.
(2) Stator copper loss
The copper loss produced by the stator windings of the machine has a close relationship with the current frequency flowing through the windings. When the current frequency is high, the current in the conductor will tend to be distributed over the surface. The high-frequency current in the stator conductor will also induce a magnetic field with a high alternating frequency, resulting in nonuniform current distribution inside the adjacent conductor. The above two phenomena, called the skin effect and the proximity effect, respectively, cause the current inside the conductor to be nonuniformly distributed along the cross section, thereby reducing the effective current carrying area of the conductor. As the current frequency in the stator winding conductor increases, the high-frequency equivalent resistance of the conductor will also increase significantly under the influence of skin effect and proximity effect[24, 25], which
will cause the increase of stator copper loss. At present, researchers often combines finite element simulation and experiment to model and analyze the copper loss inside the conductor. The advantage of this method is that the factors which could influence the copper loss, such as the wire diameter, the number of windings, the current frequency and the current harmonics, can be explored in depth. [26, 27].
(3) Rotor eddy current loss
The rotor speed of a permanent magnet synchronous machine rotates at a synchronous speed with the rotating magnetic field of the stator, so the rotor iron loss is often ignored in calculating the machine loss. However, due to the presence of time and space harmonic
components in the air gap magnetic field, eddy current losses are generated on the rotor permanent magnet, the core, and the conductive protecting sleeve. Specifically, the eddy current loss of the rotor may be caused by stator slotting, non-sinusoidal distribution of stator magnetomotive force, and stator non-sinusoidal phase current[28]. Due to the high speed of
HSPMSM, the harmonic frequency of the air gap magnetic field is relatively high, and the eddy current loss induced on the rotor is large, and this loss will cause the temperature rise of the rotor to be increased. At the same time, due to poor heat dissipation conditions of the rotor, higher temperature rise may cause irreversible demagnetization of NdFeB permanent magnets. Therefore, for high-speed permanent magnet synchronous machines, accurate calculation of rotor eddy current loss is important for ensuring the performance and reliability of the machine. The calculation method of rotor eddy current loss is divided into analytical method and finite element method. The analytical method is mostly used in a surface-mount permanent magnet synchronous machine with a relatively simple structure. On the premise of neglecting the influence of stator slotting and end effect, Professor Zhu Ziqiang proposed an accurate and improved eddy current loss analytical calculation model for surface-mount permanent magnet synchronous machine. This model considered eddy current response, based on the calculation results of the two-dimensional electromagnetic field in the air gap and magnetic pole region, the rotor eddy current loss is analytically calculated under the polar coordinate system. The method takes both the space and the time harmonics of magnetomotive force into consideration, and can calculate the eddy current loss on the rotor magnetic pole and the protective sleeve, and is suitable for calculating the eddy current loss of the overlapping and non-overlapping winding machines and special load conditions[28]. The eddy current loss caused by the slotting of the
stator needs to be analytically calculated by the equivalent current sheet method[29].
The analytical method has the advantages of being relatively intuitive and fast in calculation. However, the use of analytical methods requires many simplifying assumptions about the actual machine, which will introduce large errors in the calculation of eddy current losses. The finite element method can consider the factors such as the segmentation of the permanent magnet and the end effect, and calculate the eddy current loss of the rotor accurately.
With the continuous improvement of computer computing performance, the application of finite element method in eddy current loss calculation will be more extensive. In order to reduce the computational complexity of the three-dimensional finite element model and improve the computational efficiency, Japanese scholars proposed a two-dimensional and three-dimensional hybrid finite element method. Firstly, the two-dimensional time domain finite element analysis method is used to solve the harmonics of magnetic flux density on the rotor, and the three-dimensional frequency domain analysis method is used to obtain the rotor eddy current loss at each harmonic frequency. Under the premise of ensuring the accuracy of calculation, the method shortens the calculation time to less than one percent of the full three-dimensional model[30].
(4) Friction loss
The friction loss mainly includes the loss caused by the friction of the rotor and the air gap during the rotation of the rotor and the bearing loss. In a high-speed permanent magnet synchronous machine, the friction loss of the rotor will increase as the rotational speed increases. Traditional friction loss calculations generally use empirical formulas based on surface friction coefficients. The researchers conducted a large number of experiments and summarized many empirical formulas for calculating the surface friction coefficient[31]. Since the air flow rate
inside the high-speed machine is fast and the state is complicated, the calculation of friction loss by the empirical formula method may face the problem of insufficient precision. Computational fluid dynamics has a complete theoretical basis. By modeling the internal airflow of the machine, the friction loss of the machine can be directly determined and its accuracy is guaranteed[32], but the calculation speed and difficulty of the method are far less
than the empirical formula method.
1.2.3 Calculation method of machine temperature rise
Compared with other machines of the same power level, high-speed machines are small in size and high in power density, so the loss per unit volume, namely the loss density, is relatively large. At the same time, the small size means that the heat dissipation space is insufficient, so the internal temperature rise of the high speed machine tends to be high. In order to ensure the
reliable operation of the machine, it is necessary to study the temperature distribution of the machine. There are mainly three commonly used machine temperature rise calculation methods: simplified formula method, equivalent heat path method and temperature field numerical solution.
(1) Simplified formula method
The simplified formula method is usually used by the machine manufacturer in the preliminary design of the machine. This method assumes that all of the heat generated inside the machine is dissipated from the surface of the stator or rotor cylinder and there is no heat exchange between the windings. The Newton heat dissipation formula allows us to estimate the average temperature rise of the machine core and the windings. This method is simple to calculate and has a fast calculation speed, but has the disadvantage of poor calculation accuracy, so it is only suitable for the primary stage of the machine design.
(2) Equivalent heat path method
The core idea of the equivalent heat path method is to compare the heat path with the electrical circuit, discretize the whole machine into many temperature nodes and establish a thermal network model similar to the circuit network. The loss generated by various components inside the machine is regarded as a concentrated heat source, and heat is emitted from the heat source and transmitted to each temperature node through different heat transfer modes. The heat transfer characteristics between the temperature nodes can be expressed by the thermal resistance. The value of the thermal resistance depends on the heat transfer mode between the temperature nodes, the characteristics of the heat transfer material, and the geometric size, etc. Thermal resistance connects the various temperature nodes together, creating a thermal network for the entire machine. The heat source is analogous to the current source, the thermal resistance is compared to the resistance, and the temperature rise of the machine is analogized to the voltage drop in the circuit network. The temperature rise of each temperature node in the machine can be obtained by the solution method of the circuit network. The equivalent thermal path method can adjust the scale of the machine thermal network according to the accuracy requirements to obtain satisfactory results.
The equivalent heat path method has the advantages of fast calculation speed and convenient modeling, and is widely used in thermal analysis of machines. At present, the commercial software based on the equivalent thermal path method, MotorCAD, is often used in the calculation of machine temperature. It has been proved that the analysis accuracy can meet the requirements in some occasions[33, 34].
(3) Temperature field numerical solution
During the operation of the machine, some parts may have excessive local heat generation. It is difficult to find the local hot spot by the average temperature rise calculated by the equivalent heat path method. The temperature field numerical solution can provide an intuitive and detailed internal temperature distribution of the machine, but its modeling and calculation takes a long time, so it can only be used in the later scheme verification and optimization.
The numerical solution of temperature field is mainly divided into two methods: Finite Element Method (FEM) and Computational Fluid Dynamics (CFD). The finite element method takes the two-dimensional or three-dimensional model of the machine as the solution area, discretizes it into many solving units, and loads the corresponding input conditions to solve the problem, and finally obtains the visual and detailed distribution of the temperature field. The input conditions of the finite element method include the corresponding loss of each part of the machine and the constraints on each boundary surface. For machines with convective heat transfer, if we want to obtain the convective heat transfer coefficient accurately, we need to analyze the fluid field inside the machine, that is, adopt the CFD method. The CFD method can analyze the fluid flow state inside the machine and solve the heat transfer coefficient on the boundary surface. Through the modeling and analysis of the machine entity and the fluid, the temperature field analysis inside the machine can be obtained, without the help of the empirical formula, so the result is more accurate.
In recent years, researchers have combined several temperature field analysis methods, and using the advantages of each method, some temperature field analysis methods with both computational efficiency and accuracy have been proposed. Zeng Yingyu and others from Tsinghua University proposed a method for analyzing the temperature rise of permanent
magnets combined with the lumped parameter thermal path, temperature field and fluid field. Firstly, the finite element method is used to solve the stator temperature field and the equivalent thermal path parameters of the stator are obtained. Then the two-dimensional CFD method is used to analyze and the equivalent thermal resistance of the air gap and the end. Finally, the equivalent thermal network model is established to analyze the rotor temperature. This method can control the error of rotor temperature rise under PWM voltage excitation to within 4%[35].
Kolondzovski Z presented a combination of CFD and FEM, First analyzed the fluid distribution by two-dimensional CFD, obtained the convective heat transfer coefficient of each boundary surface, and then loaded it into the three-dimensional FEM temperature field analysis model to obtain the machine temperature distribution[36].
1.2.4 Cooling method of high-speed machine
High-speed permanent magnet synchronous machine has a small volume and a high internal temperature rise. In order to avoid the high temperature affecting the reliable operation of the machine, how to properly and effectively configure the cooling system and improve the heat dissipation capability of the machine has become one of the key issues in the design of high-speed permanent magnet synchronous machines. The cooling methods commonly used in current machines mainly include air cooling, liquid cooling, and evaporative cooling.
(1) Air cooling
Air cooling uses air as the cooling medium, the air flows through the heating surface of the machine to remove heat. It is mainly divided into natural convection and forced convection. Natural convection means that no cooling device is added inside the machine, and heat is dissipated by the conduction, convection and heat radiation between the heating surface of the machine and the environment. Natural convection has a low cooling capacity, so it is often used in small-capacity, low-heat machines. For forced convection, a cooling fan is installed at the end of the machine, and the air is forced into the machine through the fan to perform convection heat exchange. As the machine power level increases, the internal loss of the machine will inevitably increase. In order to increase the cooling capacity, it is necessary to continuously increase the air volume delivered by the cooling fan. The increase of ventilation would easily
bring noise, dust and other problems. In response to this drawback, Japanese researchers have proposed a fully enclosed traction machine cooling structure, which effectively reduces noise and saves maintenance costs such as the cost of cleaning internal fouling, and machine efficiency is also improved[37]. The hydrogen has low density, excellent thermal conductivity
and good heat dissipation capacity, therefore it has some applications in the cooling structure of turbo generators[38]. However, the characteristics of hydrogen, such as flammable, explosive
and difficult to store, restrict the development of hydrogen cooling. (2) Liquid cooling
The thermal conductivity of liquids is much bigger than gases, so liquid cooling provides a better choice for researchers when air cooling does not meet the cooling requirements of the machine. The commonly used cooling medium mainly include water and oil. For water-cooling structures, it mainly includes axial waterways, circumferential waterways, and parallel waterways, as shown in Figure 2.
(a) axial (b) circumferential (c) parallel
Figure 2 Water-cooling structures
The axial waterway has more corners, so the water resistance is larger, and the pressure required by the water supply pump also increases. In order to solve the problem of excessive water resistance, researchers at Xiangtan University optimized the axial waterway, reduced bending and water resistance[39]. The circumferential waterway is spiral, the curve is smooth,
the water resistance is small, the contact area with the machine is large, and the heat dissipation effect is good, but it would easily cause the temperature gradient difference between the axial ends. In order to solve the problem of axial temperature difference of the machine, multiple annular waterways are used in parallel waterway, but this waterway would bring the problem
of nonuniform water flow distribution. Many researchers has adjusted and optimized the waterway structure based on the flow characteristics of the cooling medium[40]. When using
water cooling, it is also necessary to pay attention to the safety and maintainability of the cooling system to avoid leakage.
Because oil has good insulation performance and excellent heat dissipation efficiency, oil-cooled machines have also been widely used. The cooling structure of the oil-oil-cooled machine generally includes installing an oil passage structure inside the machine and directly spraying the oil to the end of the winding for cooling. Some scholars use a fiberglass sheath to separate the rotor from the cooling oil. The stator end windings are immersed in the cooling oil and the end windings are sealed to prevent oil leakage. The experimental results show that compared with the water-cooling, the cooling method combines end oil cooling and water cooling has reduced the average temperature rise of the machine by 31.6%[41]. Dong H L et al. optimized
the oil spray cooling structure of the hub machine, effectively reduced the temperature rise, the cooling structure is shown in Figure 3[42]. The inlet of the cooling oil is placed on the inner bore
of the machine shaft and flows through the bearing, coil, stator core and reducer at a high flow rate to cooling the inside of the machine while also lubricate the mechanical transmission.
(a) (b)
Figure 3 Oil spray cooling structure
(3) Evaporative cooling
The liquid cooling method has high requirements on the liquid supply device, such as a pump or a compressor, and the pressure is so large that it could cause a liquid leakage problem
and it could be difficult to maintain. In order to achieve a safer and more efficient cooling method, many scholars have studied the evaporative cooling structure of large machines. Evaporative cooling utilizes the huge latent heat of vaporization of the cooling medium to remove the heat generated inside the machine during the evaporation process of the cooling medium, thereby achieving an effective control of the temperature rise of the machine. As early as the 1950s, the United States, Japan and other countries have successively launched applications for phase-change cooling machines, but most of the results are still in the experimental research stage[43]. In 1983, China applied the normal temperature self-circulating
evaporative cooling technology for the first time in the 10MW hydropower generation equipment of Yunnan Dazhai Power Plant[44]. The team led by Academician Gu Guobiao of the
Institute of Electrical Engineering of the Chinese Academy of Sciences carried out some in-depth research on the evaporative cooling technology of large-scale machines. At present, the technology is relatively mature and is applied to megawatt generator sets of large-scale projects such as the Three Gorges Power Station. Evaporative cooling mainly includes two types: immersed self-circulation and forced internal circulation. Figure 4 shows the cooling structure of a 330MW immersed self-circulating turbo generator[45]. The structure is mostly used for a
horizontal machine, and an isolation sleeve is placed at the inner circumference of the stator. The sleeve is sealed between the end of the stator core and the end cover, and a sealed space is formed around the stator core. The space is filled with a low boiling point cooling medium. The top of the machine is equipped with a condenser. When the machine is running, the heat generated inside is absorbed by the cooling medium. After reaching the boiling point of the medium, the medium evaporates and cools, further taking away a large amount of heat, and the evaporated gas rises to the condenser to transfer heat to the condensed water, which is liquefied again to form a circulation. The principle of forced internal cooling machine is shown in Figure 5. The inner winding of the machine slot contains a wire rod made of hollow strands, and the cooling medium is pressurized by the pump and flows through the hollow wire rod to absorb the heat generated inside the machine to evaporate, and then enters the condenser to condense and liquefied again to form a circulation. Throughout the process, the pump is the core to
overcome the coolant flow resistance and the power source of the forced circulation.
Figure 4 Immersed self-circulating evaporative turbo generator
Figure 5 Forced internal evaporative cooling machine
Wen Zhiwei et al. proposed an evaporative cooling turbine generator combined with immersed cooling and forced internal cooling. The temperature field distribution of the new evaporative cooling machine was obtained by numerical simulation and experimental comparison[46].
1.2.5 Rotor support method for high-speed machine
The high-speed machine has a high speed and the friction loss between the rotor and the bearing is large. Therefore, the rotor support method is an important factor determining whether the high-speed machine can operate reliably. How to reasonably choose the support method of the rotor, in order to ensure the reliable operation of the machine, reduce the friction loss of the bearing as much as possible, and prolong the service life of the machine, is one of the problems that need to be considered in the design of high-speed machines. At present, the bearings used in high-speed machines mainly include contact type and non-contact type. The former mainly
refers to high-speed ball bearings, and the latter includes air bearings and magnetic suspension bearings.
The advantages of ball bearings are small size, stability and simple structure. However, due to factors such as temperature rise and bearing life, the application of ball bearings in high-speed machines still needs to be further explored. Ball bearings can now be used in high high-speed applications by increasing the number of balls, reducing ball size, optimizing balls and lubricating materials. Swiss scholars have used high-speed ball bearings to achieve ultra-high-speed permanent magnet machines of 1 million rpm[47].
The working principle of the air bearing is to form a high-pressure gas film between the bearing and the shaft, which uses the elastic potential of the air to support the rotor. According to the principle of film formation, air bearings can be divided into aerodynamic bearings and aerostatic bearings. For an aerodynamic bearing, when the rotor rotates at a high speed, a relative motion occurs between the rotor and the foil or the bearing chamber. The suction air forms a gas film and generates pressure. When the rotor is stationary or rotates at a low speed, the rotor and the foil or the bearing chamber contact, so its stability is not high. Researchers at Beihang University achieved the ultra-high-speed permanent magnet synchronous machine for micro gas turbine power generation system with foil air bearing. In order to ensure the reliable operation of the machine, the unbalanced magnetic pull force caused by magnetization was calculated and analyzed[48]. The aerostatic bearing uses a pressure applied by an external device
to form a high-pressure gas film, which could suspend the rotor at static states and has high stability. Air bearings require high precision machining of the shaft, so high control accuracy can be achieved. The air bearing has no contact with the rotor during the operation of the machine, and has great advantages in reducing friction loss and improving the reliability of the machine operation.
Magnetic suspension bearings are divided into two categories: active magnetic bearings and passive magnetic bearings. The current-carrying coil inside the active magnetic bearing forms a magnetic field to support the rotor. The control system can actively control the coil magnetic field by detecting the position information of the rotor to adjust the attitude of the
rotor. Passive magnetic bearings have internal magnetic fields that are formed by permanent magnets and cannot be actively controlled. The magnetic suspension bearing has no contact with the rotor and has the advantages of low maintenance cost. Literature [49] designed and analyzed the multi-physical field of a high-speed permanent magnet machine with magnetic bearings. In the rotor support scheme using magnetic suspension bearings, to achieve stable operation of the machine, at least one degree of freedom needs to be controlled by the active magnetic bearing. Therefore, the configuration scheme of the magnetic suspension bearing includes a complicated bearing body and a control system, etc., it has high development difficulty.
Binder A has a statistical analysis of the rotor support method for high-speed machines[17],
as shown in Figure 6. It can be seen from the statistical results that in the case of high speed and high power, there are many machines using magnetic suspension bearings, and in the case of slightly smaller power, mechanical ball bearings have more applications. However, some systems have high requirements for the electromagnetic environment, in this case, magnetic suspension bearings may not be suitable. Air bearings do not generate electromagnetic interference, so they can be used in such applications. At present, there are few high-speed machines realized by air bearings, further research and practice are needed.
• Active magnetic bearings ▲ Mechanical bearings
■ Air bearings
Figure 6 High-speed machine bearing statistics
1.2.6 Existing problems
(1) The current research on the application of high-speed permanent magnet synchronous machines is mainly concentrated in low-power applications. There are few studies on the design and application of high-power high-speed permanent magnet synchronous motors.
(2) The loss of high-power high-speed permanent magnet synchronous machine has great influence on the performance index such as efficiency and reliability of the machine. Because the current frequency and the alternating frequency of the magnetic field of the high-speed machine is relatively high, the mechanism of its influence on the loss is more complicated. In-depth study of the electromagnetic performance and loss of high-power high-speed permanent magnet synchronous machine are needed.
(3) The high-power high-speed permanent magnet synchronous motor for compressors has a high temperature rise. Many common cooling methods have different drawbacks. Therefore, it is necessary to propose a reasonable and efficient cooling method.
(4) There are few studies on high-power high-speed motors using air bearings for rotor support. Therefore, the configuration and reliability of air bearings need further research.
1.3 Research content of the thesis
On the basis of comprehensive review and detailed analysis of the development trend, key technologies and current problems of speed machines, this paper takes power high-speed permanent magnet synchronous motor as the research object, and carries out the following researches:
(1) Research on electromagnetic design of high-power high-speed permanent magnet synchronous motor. According to the given specifications, the electromagnetic circuit method and electromagnetic field finite element simulation are combined to realize the electromagnetic design and optimization of 120kW high-speed permanent magnet synchronous motor by ANSYS Maxwell software, and the structural parameters satisfying the output performance of the motor are given.
(2) Electromagnetic performance analysis and loss study of high-power high-speed permanent magnet synchronous motor. The finite element electromagnetic field simulation is carried out for the designed motor, and its various operating performances are analyzed. In addition, this paper will also analyze the causes of different losses of high-speed permanent magnet synchronous motor under high power, and explore the influence of high speed characteristics of the motor on various losses of the motor. The magnetic field distribution of
the motor is obtained by electromagnetic field finite element analysis, the loss of the motor is calculated, and it is verified whether the efficiency index of the motor meets the requirements. (3) Research on temperature field calculation and heat dissipation method of high-power high-speed permanent magnet synchronous motor. According to the specific working conditions of high-speed permanent magnet synchronous motor for centrifugal compressor, an evaporative cooling heat dissipation structure based on spiral cooling channel and refrigerant is designed to solve the problem of heat dissipation of high-power high-speed permanent magnet synchronous motor. The loss value obtained by electromagnetic field analysis is used as the input condition of temperature field analysis. The numerical calculation method is used to analyze the temperature field distribution of the motor, and it is preliminarily verified whether the temperature rise of the motor is within the allowable limit of the material. A water cooling configuration is also designed and validated.
(4) Research on rotor strength and support method of high-power high-speed permanent magnet synchronous motor. Check the strength of the rotor structure of the high-speed motor to ensure the safe operation of the motor. In addition, specific bearing arrangements are applied for high speed situations and the final motor mechanical diagram is given.
1.4 Chapter arrangement
According to the above research content, this thesis will be divided into five chapters, the specific arrangements are as follows:
Chapter 1: Introduction. Elaborate the research background and significance of this paper, summarize the research status in relevant fields in detail, point out the problems existing in the current research, and further propose the research content and chapter arrangement of this paper. Chapter 2: Electromagnetic design of 120kW HSPMSM. Firstly, the operating conditions of the motor are analyzed, and the design specifications of the permanent magnet synchronous motor for the compressor are summarized. The overall work flow of the electromagnetic design of the motor is introduced. Then the magnetic circuit method is used to design and optimize the motor and to determine the motor structure form and size parameters that meet the electromagnetic performance requirements of the motor.
Chapter 3: Finite element analysis of electromagnetic field of 120kW HSPMSM. Firstly, the finite element simulation analysis is carried out for the motor designed by magnetic circuit method to verify whether the running performance of the motor meets the specifications. Since the loss of high-power high-speed permanent magnet synchronous motor has an important influence on the performance of the motor, it is necessary to analyze the loss of the high-speed permanent magnet synchronous motor, and obtain the impact of high speed characteristics on the losses, such as copper loss, iron loss, rotor eddy current loss and friction loss, and the mechanism of the impact. The magnetization characteristics of the stator core of the motor are studied, and the loss of the motor is calculated by the finite element simulation method. The efficiency under the rated state of the motor is verified to meet the specification.
Chapter 4: Cooling method and temperature field analysis of 120kW HSPMSM. Firstly, the heat transfer mode inside the motor is introduced. According to the specific working conditions of the high-speed permanent magnet synchronous motor for centrifugal compressor, an evaporative cooling heat dissipation structure based on spiral cooling channel and refrigerant is designed. After making reasonable assumptions and simplifying the model, the loss value calculated by electromagnetic field analysis is imported as the input condition of temperature field analysis. The finite element method is used to calculate the temperature field distribution of the motor, and to check whether the temperature rise of the motor is within the limit of the material. A water cooling design is also designed in the case where the evaporative cooling is not suitable, and its effect is checked by equivalent thermal path method.
Chapter 5: Structural design of 120kW HSPMSM. The ANSYS finite element analysis software is used to check the rotor structural strength of high-speed motor in order to ensure safe and reliable operation of the motor. In addition, the rotor support scheme is studied for high speed conditions, a specific bearing arrangement based on aerostatic bearing is adopted, and the overall mechanical structure of the motor is given.
Finally, the thesis is summarized in the conclusion section, and the main work of this thesis is concluded.
2 Electromagnetic Design of a 120kW HSPMSM
2.1 Introduction
As the core component of the centrifugal compressor, the high-speed permanent magnet synchronous motor will directly affect the running performance of the compressor. Electromagnetic design is the basis of high-speed permanent magnet synchronous motor design, and its design results play a decisive role in motor performance. Therefore, this chapter will elaborate on the electromagnetic design process of high-speed permanent magnet synchronous motor. Firstly, the operating conditions of the motor are analyzed, and the design specification of the motor is given. Then, the design work flow is introduced. After that, according to the summarized specifications, the detailed electromagnetic circuit design of the motor is carried out, including the main size selection of the motor, permanent magnet design, stator and rotor design and air gap design. All the design results are summarized at the end.
2.2 Design specifications
The 120kW HSPMSM is used to drive the centrifugal compressor, and it needs to meet the working requirements of the compressor as the core component of the marine environmental control system. Therefore, the power, speed, torque and voltage of the motor are determined according to the requirements of the compressor. Since the compressor needs to work continuously for a long time, the S1 continuous duty is required for the motor. As the core driving component of the compressor, the efficiency of the motor plays a vital role in the efficiency of the compressor. Therefore, in order to improve the energy efficiency of the compressor and the entire environmental control system, the motor needs to meet high efficiency requirements. In combination with the above analysis, the design specifications of the motor are as shown in TABLE I.
TABLE I Design specifications
Specification Unit Value
Rated power kW 120.1
Rated torque Nm 64.5
Rated voltage V 380
Quota / S1 (continuous duty)
Efficiency / ≥95%
2.3 Design method
The design methods commonly used in the electromagnetic design of permanent magnet synchronous motor are magnetic circuit design and finite element method. The magnetic circuit design has the advantages of easy operation and fast calculation speed. At the same time, the calculation of magnetic circuit method relies on empirical formula. It may generate error during the calculation, so it has a disadvantage of low calculation accuracy. The finite element analysis method has enough calculation accuracy, but it takes a long time. The high-power high-speed permanent magnet synchronous motor in this paper is designed by the combination of magnetic circuit method and finite element simulation. The specific design process is shown in Figure 7.
Firstly, analyze the operating conditions of the motor and determine the design specifications of the motor. Then use the magnetic circuit method to calculate the basic structure and size parameters of the motor. After that, use the magnetic circuit method analysis module RMxprt in Maxwell, the electromagnetic field simulation software, to obtain the performance of the motor under no-load and full-load working conditions. The results are compared with the motor performance required by the specifications, and the structural parameters are adjusted according to the insufficiency in performance. Taking the advantage of fast calculation speed, the magnetic circuit method is used to iteratively calculate the performance of the motor until it meets the requirements. And the last structural parameters are imported into the electromagnetic field finite element module for analysis. Since the finite element software can fully consider the nonlinear factors inside the motor, its calculation accuracy has great advantages compared with the magnetic circuit method. Therefore, at the end of the electromagnetic design process of the motor, the finite element method is used to verify whether the output performance of the motor meets the requirements. If the requirements are not met, iteration is required until the design performance index is met. In this chapter, the thesis mainly gives the specific design process of the magnetic circuit method, and gives the motor structural parameters. The finite element analysis and performance check of the electromagnetic field of the motor will be introduced in the third chapter.
2.4 Magnetic circuit method design
The magnetic circuit method combines the magnetic circuit model and the empirical formula to transform the complex calculation of magnetic field inside the motor to the simple magnetic circuit calculation. It has the advantages of fast calculation speed and short required time. It is widely adopted in the initial design stage of the motor. Therefore, this paper first uses the magnetic circuit method to determine the basic structure and size of the motor.
2.4.1 Equivalent magnetic circuit of permanent magnet motor
The magnetic circuit inside the permanent magnet synchronous motor is complicated. The excitation magnetic field is generated by the permanent magnet, and the performance of the permanent magnet is affected by various factors such as manufacturing process, shape, size,
magnetization method, etc. Besides, diverse magnetic circuit structures, complex magnetic flux leakage paths, and nonlinear factors of ferromagnetic materials have brought many difficulties to the calculation of magnetic circuits for permanent magnet synchronous motors. Therefore, in order to simplify the analysis and calculation, the magnetic circuit method is adopted in the primary stage of the motor design, and the complex electromagnetic field calculation is converted into a magnetic circuit calculation similar to the electrical circuit, which can shorten the calculation time and improve the efficiency of the design process. The equivalent magnetic circuit of the motor designed in this thesis is shown in Figure 8[50].
Figure 8 Equivalent magnetic circuit of a loaded machine
In the figure, Fc represents the equivalent magnetomotive force of the permanent magnet,
0
represents the internal magnetic permeability of the permanent magnet, represents the m total magnetic flux provided by the permanent magnet to the outer magnetic circuit, represents the main magnetic flux of the air gap, indicates the main magnetic permeability, that is, the magnetic permeability of the core material, Fa indicates the equivalent
magnetomotive force generated by the armature winding, indicates the leakage flux， indicates the leakage flux, i.e., the air gap permeability. Let Fa 0，the no-load equivalent magnetic circuit of the permanent magnet synchronous motor can be obtained.
Figure 8 is the basis for motor design using the magnetic circuit method. Firstly, assuming the working point of the permanent magnet, then the calculation method similar to the electrical
circuit is used to obtain each magnetic permeability and the magnitude of the magnetic flux in the magnetic circuit. Compared with the assumption, if the error goes beyond the requirement, iteration is needed to achieve the desired precision.
2.4.2 Selection of permanent magnet materials
The rotor of the high-power HSPMSM designed in this thesis consists of permanent magnets, a rotor core and a rotating shaft. The permanent magnet and the rotor core constitute the rotor magnetic circuit, and the permanent magnet acts as a magnetic source of the motor, generating a sufficient magnetic flux density in the air gap of the motor via the magnetic circuit, which interacting with a circular rotating magnetic field formed by the three-phase alternating current in order to realize the conversion of electromechanical energy, generate corresponding electromagnetic torque and push the rotor to rotate. Therefore, the performance of permanent magnet materials is of vital importance in determining the performance and operating characteristics of the motor. The performance of different permanent magnet materials varies greatly, so the choice of permanent magnet materials is crucial. At present, the more commonly used permanent magnet materials mainly include aluminum-nickel-cobalt, ferrite, rare-earth samarium-cobalt material and neodymium-iron-boron[50].
(1) Aluminum-nickel-cobalt (AlNiCo) material
The AlNiCo permanent magnet material was successfully developed in the 1930s. It has the characteristics of small temperature coefficient and high remanence. The remanence can reach up to 1.35T. However, its coercive force Hc is very low, usually less than 160 kA/m . The AlNiCo permanent magnet material has the characteristic that the demagnetization curve exhibits nonlinear variation, and its return line does not coincide with the demagnetization curve. This makes the designer pay additional attention to its particularity when designing the magnetic circuit. It must be subjected to a magnetization treatment before use. Aluminium-nickel-cobalt materials are hard and brittle, resulting in poor machinability, only a small amount of grinding or EDM are allowed, and they cannot meet the processing needs of complex special shapes[51].
The ferrite materials commonly used in motors are barium ferrite (BaO6Fe2O3) and
strontium ferrite (SrOFe2O3). The main advantages of ferrite permanent magnet materials are
low price, the manufacturing process is relatively simple; the coercive force is relatively large,
Hc is 128-320 kA/m , so the anti-demagnetization ability is strong; the density is small; the
demagnetization curve is nearly a straight line, the return line basically coincides with the straight line portion of the demagnetization curve, and does not need to be subjected to the magnetization treatment, therefore it is widely used in the motor. However, ferrite materials also have their shortcomings, its remanence is low, only 0.2~0.44T, and the maximum magnetic energy product(BH)max is small, only 6.4~40kJ/m3. It should be pointed out that the magnetic properties of ferrite permanent magnet materials are greatly affected by the ambient temperature, and the maximum demagnetization operating point at the lowest ambient temperature needs to be checked before use. In addition, similar to the AlNiCo material, the ferrite material is also hard and brittle, and it is difficult to process. Usually, a soft grinding wheel is used for a small amount of grinding[50].
(3) Rare-earth samarium-cobalt material
Rare earth cobalt permanent magnet materials emerged in the mid-1960s, and they have excellent magnetic characteristics. The remanence, coercivity and maximum magnetic energy product are high. The demagnetization curve of the rare earth cobalt permanent magnet material is basically a straight line, and the return line basically coincides with it, the anti-demagnetization ability is also strong. In addition, the remanence temperature coefficient of the rare earth cobalt material is lower than that of the ferrite material, and the Curie temperature is relatively high, generally 710 to 880°C, so the magnetic stability of the material is the best. The main disadvantage of this material is that it is expensive and leads to higher motor costs. In addition, the rare earth cobalt material has poor mechanical properties, low tensile strength and bending strength, it has a tensile strength of only 25 to 35 MPa.
(4) Neodymium-iron-boron (NdFeB) material
NdFeB permanent magnet material is a high-performance permanent magnet material that was introduced in 1983. Its magnetic properties are the highest among many permanent magnet
materials, and it is called “Magnetic King”. Its advantages are its outstanding magnetic properties: remanence Br at room temperature is up to 1.47T, coercivity Hc up to 992 kA/m ,
maximum magnetic energy product higher than 397.9 3
kJ/m . The mechanical properties of NdFeB permanent magnet materials are superior to those of the rare earth samarium-cobalt permanent magnet materials, the tensile strength is about 80~140MPa, and it has the advantage of easy processing. The main disadvantages are the poor thermal stability, high temperature coefficient, low Curie temperature and maximum working temperature is about 180°C.
For the high-power high-speed permanent magnet synchronous motor in this paper, the motor speed is high and the rotor surface speed is correspondingly high. In order to ensure the reliable operation of the motor, the permanent magnet material with relatively high tensile strength should be selected and a protecting sleeve is added to the periphery of the rotor. In addition, materials with better magnetic properties, that is, magnets with higher remanence, higher coercive force and larger magnetic energy product, can reduce the volume and weight of the motor to a certain extent, save material usage, and improve the power to weight ratio of the motor. Since the motor uses evaporative cooling in actual work, it can effectively limit the temperature of the motor to 150 degrees Celsius. At the same time, the power of the motor is relatively large, and the amount of permanent magnets is relatively large. In this case, cost becomes one of the factors that must be considered. Therefore, after comprehensively considering the above factors, the NdFeB permanent magnet material with large remanence, coercive force and maximum magnetic energy product is preliminarily determined as the permanent magnet material of the motor. NdFeB permanent magnet material N33UH has a reasonable price, its maximum working temperature can reach 180°C, and its magnetic properties and strength can meet the requirements. Therefore, the pre-selected permanent magnet is N33UH, and its main magnetic properties are as follows:
Remanence Br 1.14T Coercivity Hc820kA/m
Maximum magnetic energy product 3 max
2.4.3 Stator structure design
The stator core of the permanent magnet synchronous motor can adopt structures such as no-slot type, less-slot type, and more-slot type. The no-slot structure does not generate high frequency tooth harmonic magnetic fields, which is beneficial to reduce eddy current loss on the rotor. However, such a stator structure could cause large air gap, and the magnetic field generated by the permanent magnet in the air gap would be weak, resulting in a low utilization rate of the permanent magnet material. The air gap magnetic flux density of the less-slot structure is the largest of the three structures, and the utilization of the permanent magnet material is the best, but the tooth harmonic magnetic field with a large amplitude would be caused, so that the rotor eddy current loss will increase. The more-slot structure can achieve higher air gap flux density, improve the utilization of permanent magnet materials, and does not cause excessive eddy current losses on the rotor. Therefore, the motor designed in this paper adopts a more-slot stator core structure, and the number of stator slots is 24 slots.
HSPMSM has the characteristics of high speed and high fundamental frequency, so its harmonic electromotive force and magnetomotive force frequency are also relatively high. Harmonic electromotive force and harmonic magnetomotive force will produce harmonic current in stator three-phase winding., and cause various problems such as additional loss, vibration and noise, which would affect the performance of the motor. Therefore, in the design stage of HSPMSM, corresponding measures should be taken to reduce the high-order harmonics in the electromotive force and the magnetomotive force. The use of distributed windings and short-distance windings is an important way to weaken high order harmonics. Therefore, the high-speed permanent magnet synchronous motor in this paper adopts a double-layer short-distance distributed winding scheme. In addition, in order to eliminate the high-order harmonic components of the back EMF, a skew is adopted on the stator, and the angle of the skew is 1 slot, that is, 15 degrees.
2.4.4 Rotor structure design
The rotor structure of the permanent magnet synchronous motor determines the magnetic circuit of the rotor, and different rotor magnetic circuits have different effects on the
