# Research on Direct Drive Technology of the Permanent Magnet Synchronous Motor for Urban Rail Vehicles

Authors: Jianpu Wang; Chenglong Ren; Zhao Liu; Mingyue Mao
Journal: Mathematical Problems in Engineering
 (2022)

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## Abstract

Compared with the traditional gear reduction mechanism of the asynchronous motor, the permanent magnet synchronous motor (PMSM) direct drive technology has obvious advantages and has attracted increasing attention. We analyzed the advantages of the PMSM direct drive technology and put forward the hollow shaft structure of the coupling to reduce the impact force of the wheel and rail, additional noise, vibration, and loss. We deeply studied the topology and mathematical model of the PMSM. To keep the maximum running speed unchanged, it is necessary to conduct a flux weakening control above the rated speed to increase the speed regulation range of the PMSM. To prevent frequent switching jitter at the transition point and smooth the transition between different weak magnetic fields, we designed a new flux-weakening controller to control the current. Simulation results have been presented to verify the proposed approach efficiency. Taking the PMSM system train on Suzhou Metro line 3 as an example, the on-site energy consumption test was carried out. The energy consumption of the PMSM direct drive and asynchronous motor gear mechanism was compared. The energy consumption of the PMSM direct drive is increased by 16.50%. The flexible plate coupling can alleviate the adverse effects of vibration and improve the reliability of train operation. Finally, we put forward the technical difficulties and challenges of the PMSM direct drive technology. We need to optimize the mass distribution of the direct drive bogie to improve the dynamic performance of the bogie, the heat dissipation conditions, and the accurate control of the PMSM. It will have reference value for the development of the PMSM direct drive in the future.


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## Body

## 1. Introduction

Urban rail vehicles adopt electric traction. At present, most traction motors adopt AC asynchronous motors. When AC asynchronous motor is working, the rotor excitation current will produce rotor copper loss [1]. Permanent magnet traction motors adopt permanent magnet excitation. They have no excitation current, copper consumption of the rotor, and relatively small copper consumption of the stator. Compared with the asynchronous traction motors of the same specification, the rated efficiency can be increased from 3 percent to 5 percent [2–4], and the energy-saving effect of the permanent magnet motors is more obvious under all working conditions. The efficiency of the PMSM is better than that of an AC asynchronous motor under any operating condition. The energy consumption of a permanent magnet synchronous traction system is lower than that of asynchronous traction system under traction condition, and the regenerative energy of a permanent magnet synchronous traction system is more than that of asynchronous traction system under electric braking conditions [5, 6].The traditional gear reduction mechanism has the disadvantages of easy wear, lubricating oil leakage, high maintenance cost, and low overall efficiency of the system, which do not meet the requirements of economic development, energy conservation, and environmental protection [7]. Using a PMSM direct drive to replace the traditional gear reduction drive system has become a common understanding of scholars all around the world [8–10].In terms of the energy-saving effect of the PMSM, according to the test data of 200 000 km of 103 series tram of East Japan Railway, the test data of Toshiba urban rail vehicles, and the operation data of a line on Shenyang Metro, the energy-saving rate of permanent magnet synchronous traction system is about 10 percent [11–13]. Through the efficient simulation analysis of PMSM and asynchronous motor, 70 percent of PMSM works in a high-efficiency area between 95 percent and 97.8 percent. The efficiency is much higher than that of an asynchronous motor. The average efficiency under all working conditions is increased by 2.2 percent. The analysis results are shown in Table 1 [14].Table 1
Efficiency comparison between PMSM and asynchronous motor.
Efficiency proportional distributionPMSM (%)Asynchronous motor (%)95%≤η<97.8%70094%≤η<95%104590%≤η<94%1041.7η<90%1013.3Average efficiency94.792.5Due to the heat generated by the high-power switching loss in the urban rail vehicles control device, a control method that can reduce the switching loss is needed. A synchronous pulse width modulation control method based on variable switching frequency, a field weakening control method considering the maximum voltage modulation index, and an optimized over modulation method are proposed. The over modulation operation is one of the most important problems of PWM converter. The over modulation strategy can effectively improve the output fundamental voltage and current, shorten the dynamic response time of the system, and expand the steady-state working area [15, 16]. However, there are also some problems in the over modulation strategy, such as harmonic current, the complexity of the over modulation strategy, and the smooth transmission between linear and over modulation regions. These problems are also the main research problems of many scholars at present.Inverter is the main electrical equipment of urban rail vehicles. To reduce the switching loss of the inverter, a modulation model free predictive control with minimum switching loss is proposed [17]. Two adjacent current vectors are determined according to the predefined first-order cost function, and then, the current vector of the next control cycle is equal to the reference current vector by appropriately modulating the selected current vector. To maintain the optimal control performance, the optimal voltage vector is adjusted. The second-order cost function is designed to select the optimal voltage vector sequence, so that the switching will not occur when the phase current is maximum [18, 19].To solve a mismatch between the low rotating speed and high power density of the PMSM, the design of the PMSM with high power density is proposed to allow low-speed operation without the help of gear transmission [20]. The research focused on the winding design provides minimum power loss during the AC operation of the motor. The winding design is a compromise between low-speed and high-speed operating points with a given torque speed envelope. The winding design has a certain influence on the power output capacity of the constant-speed permanent magnet generator. To reduce the noise in the process of motor operation, the improved driving algorithm is applied to offset the noise component before data acquisition, the resonant controller is introduced, and it is set in parallel with the existing proportional integral controller to suppress the speed fluctuation [21]. However, the speed regulation range at maximum torque is narrow, which cannot ensure a smooth transition between different operation areas.Vector control (VC) applied to PMSM as well as direct torque control (DTC) is very traditional schemes. They needed parameters of the internal permanent magnet synchronous motor. Some parameters change with the change of temperature during the operation of the motor, which affect the control effect. The torque estimation method based on online differential inductance identification and data-driven finite control set (FCS) model predictive current control (MPCC) can realize open-loop torque control without knowing the accurate motor parameters, in addition to permanent magnet flux as data table parameters. In order to identify the parameters of the internal permanent magnet synchronous motor online, they injected high-frequency signals into the stator through the inverter [22]. However, on the one hand, high-frequency current will adversely affect the system performance (additional noise, vibration, loss, etc.), and on the other hand, the need for a high-frequency current controller will also increase the computational burden. The SPMSM torque estimation method using a single high-frequency signal was presented [23]. In addition, they used high-frequency voltage signals to avoid the use of high-frequency current controllers, so the implementation is simplified.VC may not be suitable for applications requiring a very high dynamic response. An improved MPC scheme is proposed, which combines a virtual vector expansion scheme and duty cycle control. This scheme improves steady-state performance while having a faster dynamic response than VC. In addition, a speed-free control scheme based on a finite position set phase-locked loop is proposed, which can achieve fast dynamic performance and ensure the accuracy of position estimation. DTC suffers from high-torque and flux ripple and variable switching frequency. In the current study, various advanced methods are investigated to replace and improve traditional schemes, such as model predictive control (MPC). The MPC compensation method based on virtual voltage vectors is proposed for the single-phase open-circuit fault of six-phase permanent magnet synchronous in-wheel motors [24–26]. This design method of the controller is more intuitive, which can realize the online simultaneous optimization of multidimensional control objectives. It can flexibly add various optimization objectives, such as the neutral point voltage balance [27] and switching frequency limit [28] of the multilevel inverter. MPC has the advantages of simple structure and fast dynamic response, but it still needs further research to reduce its computational load and improve its steady-state performance.When the asynchronous motor is designed at low rated speed, there are many poles, and the increase of the excitation current seriously reduces the power factor and efficiency. Therefore, the asynchronous motor is not suitable for low-speed and high-torque direct drive. The PMSM has the characteristics of small volume, light weight, fully enclosed structure design, high utilization rate of space and materials, high power density, low noise, reduced maintenance cost, large torque inertia, strong overload capacity, and low maintenance cost. Especially, it outputs large torque at low speed, which is suitable for low-speed starting of vehicles. Compared with AC asynchronous motor, the air gap magnetic field of the PMSM is excited by the permanent magnet, there is no excitation current, the pole pairs of the motor can be designed very high, and its torque density is high, so its volume and mass are greatly reduced, which makes it possible to realize direct drive. The PMSM can maintain good performance in a wide range of load variations and has obvious competitive advantages in the field of rail transit [29–31]. It will have a very broad application prospect in the field of rail transit.Through the comparison between permanent magnet direct drive and traditional asynchronous motor drive, we concluded that permanent magnet direct drive had an obvious energy-saving effect in urban rail vehicle drive. Through the comparison of common permanent magnet motor topologies, we pointed out that a W-shaped built-in permanent magnet motor was more suitable for rail transit electrical drive. To avoid the shortcoming that the PMSM caused the current oscillation in deep flux weakening, we designed a new field-weakening controller to control the current in the deep field weakening control area. The simulation waveform shows that the current control effect is better in the weak magnetic field region, and the smooth transition is realized. By comparing the energy data of the PMSM direct drive and asynchronous motor gear mechanism on Suzhou Metro line 3, the energy-saving rate of the permanent magnet motor direct drive system increased by 16.50 percent. The noise and vibration tests on the permanent magnet direct drive system results show that the vibration test data and the experimental data of noise are lower than the technical limit. Aiming at the existing problems of the direct drive of permanent magnet motor, we put forward the existing technical difficulties of the PMSM direct drive technology and point out the research direction.


## 2. Topology and Structural Design

### 2.1. Topology

According to the shape and position of the permanent magnet, PMSM can be divided into four topologies, namely, surface type, segmented built-in type, V-shaped built-in, and W-shaped built-in [32], as shown in Figure 1.Figure 1
PMSM four rotor cross sections. (a) Surface type. (b) Segmented built-in. (c) V-shaped built-in. (d) W-shaped built-in.
(a)(b)(c)(d)Through the finite element analysis of four topological structures of the PMSM rotor, under the condition of the basic parameters of the motor, the position and shape of permanent magnet, the total weight of the permanent magnet per pole, the total distortion rate of voltage harmonic, salient rate and magnetic weakening rate were compared. The analysis result is shown in Table2.Table 2
Comparison of PMSM four topology structures performance parameters.
Position and shape of permanent magnetTotal weight of permanent magnet per pole (g)Total distortion rate of voltage harmonic(%)Salient rateMagnetic weakening rateSurface type103.26752.1Minimum0.1022Segmented built-in90.50185.2Maximum0.1700V-shaped built-in64.181312.5More0.2102W-shaped built-in79.07634.1Less0.6288It can be seen that the W-shaped built-in permanent magnet motor has a good magnetic weakening rate (0.6288), whose speed regulation ability is strong and the wide constant power operation range is wide. Its efficiency also meets the requirements. Therefore, W-shaped built-in PMSM is more suitable for rail transit electrical drive.


### 2.2. Structural Design

The torque of the PMSM with a direct drive structure is doubled, and the quality of the motor is relatively large [33, 34]. According to the different suspension modes of permanent magnet direct drive motor, it can be divided into rigid suspension axle holding type and elastic suspension axle holding type. Rigid suspension axle holding is adopted. Most of the PMSM mass belongs to the unsprung mass, which will aggravate the vibration between the wheel and rail when the train runs at high speed, makes the working environment of the motor band, and reduces the stability of high-speed operation of the train.To solve the unsprung mass issue, we adopt the coupling transmission mode. The mass of the PMSM does not directly act on the wheel set, and the PMSM is indirectly connected to the axle or wheel through the vibration-damping element. The PMSM is suspended, and the mass of the PMSM becomes the sprung mass, which is conducive to reducing the force of wheel rail impact action. In the traction system of urban rail vehicles, the torque output by the PMSM is transmitted to the wheel set through the hollow coupling. The PMSM is not directly connected to the axle. The flexible plate coupling alleviates the adverse impact of vibration and improves the operation reliability. The high-power direct drive is realized through the hollow shaft structure of the coupling. A group of flexible plates is set at both ends of the hollow shaft of the coupling, one end of the hollow shaft is connected to the driving pin through the coupling, and the force transmission plate is installed on the driving wheel. The other end of the hollow shaft is connected with the output end of the PMSM through the end-tooth structure of the coupling. The torque of the PMSM is directly transmitted to the wheel through the coupling, as shown in Figure2.Figure 2
Structure diagram of the PMSM direct drive system.The PMSM configurations allow smaller pole pitch, especially as a wide CPSR is required, and reducing further PMSM weight is crucial, the design of which did not use high pole numbers. The more the pole number of the PMSM, the more complex the structure of the PMSM and the polar number are related to the speed of the vehicle.


## 2.1. Topology

According to the shape and position of the permanent magnet, PMSM can be divided into four topologies, namely, surface type, segmented built-in type, V-shaped built-in, and W-shaped built-in [32], as shown in Figure 1.Figure 1
PMSM four rotor cross sections. (a) Surface type. (b) Segmented built-in. (c) V-shaped built-in. (d) W-shaped built-in.
(a)(b)(c)(d)Through the finite element analysis of four topological structures of the PMSM rotor, under the condition of the basic parameters of the motor, the position and shape of permanent magnet, the total weight of the permanent magnet per pole, the total distortion rate of voltage harmonic, salient rate and magnetic weakening rate were compared. The analysis result is shown in Table2.Table 2
Comparison of PMSM four topology structures performance parameters.
Position and shape of permanent magnetTotal weight of permanent magnet per pole (g)Total distortion rate of voltage harmonic(%)Salient rateMagnetic weakening rateSurface type103.26752.1Minimum0.1022Segmented built-in90.50185.2Maximum0.1700V-shaped built-in64.181312.5More0.2102W-shaped built-in79.07634.1Less0.6288It can be seen that the W-shaped built-in permanent magnet motor has a good magnetic weakening rate (0.6288), whose speed regulation ability is strong and the wide constant power operation range is wide. Its efficiency also meets the requirements. Therefore, W-shaped built-in PMSM is more suitable for rail transit electrical drive.


## 2.2. Structural Design

The torque of the PMSM with a direct drive structure is doubled, and the quality of the motor is relatively large [33, 34]. According to the different suspension modes of permanent magnet direct drive motor, it can be divided into rigid suspension axle holding type and elastic suspension axle holding type. Rigid suspension axle holding is adopted. Most of the PMSM mass belongs to the unsprung mass, which will aggravate the vibration between the wheel and rail when the train runs at high speed, makes the working environment of the motor band, and reduces the stability of high-speed operation of the train.To solve the unsprung mass issue, we adopt the coupling transmission mode. The mass of the PMSM does not directly act on the wheel set, and the PMSM is indirectly connected to the axle or wheel through the vibration-damping element. The PMSM is suspended, and the mass of the PMSM becomes the sprung mass, which is conducive to reducing the force of wheel rail impact action. In the traction system of urban rail vehicles, the torque output by the PMSM is transmitted to the wheel set through the hollow coupling. The PMSM is not directly connected to the axle. The flexible plate coupling alleviates the adverse impact of vibration and improves the operation reliability. The high-power direct drive is realized through the hollow shaft structure of the coupling. A group of flexible plates is set at both ends of the hollow shaft of the coupling, one end of the hollow shaft is connected to the driving pin through the coupling, and the force transmission plate is installed on the driving wheel. The other end of the hollow shaft is connected with the output end of the PMSM through the end-tooth structure of the coupling. The torque of the PMSM is directly transmitted to the wheel through the coupling, as shown in Figure2.Figure 2
Structure diagram of the PMSM direct drive system.The PMSM configurations allow smaller pole pitch, especially as a wide CPSR is required, and reducing further PMSM weight is crucial, the design of which did not use high pole numbers. The more the pole number of the PMSM, the more complex the structure of the PMSM and the polar number are related to the speed of the vehicle.


## 3. Mathematical Model and Control Strategy Optimization

### 3.1. Mathematical Model

In the precise control of the PMSM, it is necessary to obtain its accurate rotor position. The rotor initial position identification process should identify the part of the rotor axis and then determine the magnetic polarity of the rotor.In the synchronous rotatingd-q coordinate system, the field linkage equation and voltage equation can be expressed in the following equations [35]:(1)Ψd=Ldid+Ψf,(2)Ψq=Lqiq,(3)uduq=Rs+pLⅆ−ωrLqωrLⅆRs+pLqidiq+0ωrΨf.The electromagnetic torque can be expressed in the following equation:(4)Tc=32pnΨdiq−Ψqid=32pnLdiq+Ψfiq−Lqiqid=32pnΨfiq+Ld−Lqidiq,where ψf is the maximum value of the field linkage generated by the permanent magnet, Ld is d-axis inductance, Lq is q-axis inductance, id is d-axis current, iq is q-axis current, pn is polar pairs, and ωr is rotor angular velocity.Due to the asymmetry of the AC-DC axis magnetic circuit of the built-in PMSM, it can be seen from (4) that there are two components in the electromagnetic torque. One component is the permanent magnet torque generated by the synchronous operation of the stator-rotating magnetic field and the permanent magnet magnetic field. Another component is the reluctance torque caused by the rotor magnetic circuit asymmetry because of the salient pole of the iron core. For urban rail vehicles with high requirements for weak magnetic speed regulation capacity, the counter electromotive force of the motor should be appropriately reduced. At the same time, to increase the load capacity in the constant power speed regulation stage, the salient pole rate should be increased, and the reluctance torque should be fully utilized. As for the magnetic circuit structure design of the built-in W-type rotor, the embedded depth of the permanent magnet can be appropriately reduced.Stator currentis is affected by the maximum working current that the inverter can withstand ilim limitations, which can be expressed in the following equation:(5)is2=id2+iq2≤ilim2.As in (5), the maximum current trajectory is centered on the origin in the d-q coordinate system, and ilim is a circle with a fixed radius, which is called the current limit circle.Stator voltageus is affected by the maximum working voltage output by the inverter ulim limitations, which can be expressed in the following equation:(6)us2=ud2+uq2≤ulim2.By ignoring the voltage drop on the resistance, substitute (3) into (6) to obtain the stator current. The voltage limit equation of the coordinate axis can be expressed in the following equation:(7)Ldid+Ψf2+Lqiq2≤ulimω2.(7) forms the voltage limit ellipse, and its two-axis length and speed ω are in inverse proportion, and with the increase of motor speed, a cluster of gradually smaller ellipses is formed [36]. The current and voltage trajectories of the built-in PMSM operation are shown in Figure 3.Figure 3
Current and voltage trajectories of built-in PMSM operation.


### 3.2. Control Strategy Optimization

A frequency converter must power a PMSM, and the closed-loop control system of rotor position detection can operate normally [37]. The control methods of permanent magnet direct drive motor used in urban rail vehicles are mainly vector control and direct torque control.Hysteresis controller is adopted in direct torque control. The selection of the stator voltage vector is rough, which will produce large torque ripple and current harmonic distortion in the control process. Based on the space coordinate transformation theory, VC realizes the decoupling of armature current and excitation current. It can control the field and torque of the motor, respectively. It can obtain better speed regulation and current control effects and has been widely used in urban rail transit traction systems.In the vector control of the PMSM, it is necessary to obtain the accurate position of the permanent magnet to realize the decoupling of field linkage and torque current [38]. The bogie on direct drive transmission puts forward new technical requirements for the characteristics and structural design of the motor. To meet the need for direct drive transmission, the PMSM must have the characteristics of low speed and high torque. When the motor’s output power is constant, increasing the low-speed torque will reduce the rated speed in the constant torque speed regulation stage below the rated speed. To keep the maximum speed unchanged, field weakening control is required above the rated speed, and the speed regulation range of motor field weakening is increased.The PMSM operates in the constant torque area below the base speed. To make full use of the stator current, the linear maximum torque per ampere (MTPA) control is adopted, which is shown in curve OA in Figure3. MTPA’s working point is to pursue the maximum torque output when the stator current is certain and to minimize the value of the stator current when the PMSM operates in constant torque mode. MTPA current closed-loop control method is adopted to control the excitation current of motor stator current id and torque current iq and to perform decoupling control, respectively, and their equations can be expressed as follows:(8)id=ψf2Ld−Lq−ψf24Ld−Lq2+iq2,(9)iq=8Teψf/3pn2−4ψf2−4Ld−Lq24Te/3pn2−ψf22ψf2−4Ld−Lq2+8Teψf6pnψf2−4Ld−Lq2.When the motor voltage reaches the limit value of the inverter output voltageulim, adjusting the maximum value of field linkage generated by the permanent magnet cannot make the motor speed continue to increase. Only by adjusting the stator current, the direct axis demagnetization current component (−id) is increased to make the speed increase. At the same time, iq limited by the current limit cycle should be reduced. To ensure that the armature current does not exceed the limit value ilim, the motor will run along the constant torque curve between MTPA and the maximum torque per voltage (MTPV). The curves DE and FG are show in Figure 3. This region is called weak magnetic region I.(10)id=−ψfLd+ulimωLd2−LqiqLd2,iq=ilim2−id2.With the further increase of speed, the motor will run along the MTPV curve, which is called field-weakening region II. MTPV is the minimum stator voltage required to reach the maximum speed with the same output torque. The MTPV curve is the line connecting the tangent point of the voltage limit circle and the torque hyperbola. The equation can be expressed as follows [39]:(11)∂Te∂id∂us∂iq−∂Te∂iq∂us∂id=0.In high-speed steady-state operation, the voltage equation can be expressed as follows:(12)us2=−ωLqiq2+ωLdid+ωψf2.By (10)–(13), the trajectory equation of MTPV can be obtained, which can be expressed in the following equation:(13)id=−ψfLd+−Lqψf+Lq2ψf2+4Lq2Ld−Lq2iq22LdLd−Lq.The gradient descent method of voltage limit ellipse is used for magnetic weakening [40]. The field-weakening region is determined by the angle θ between the direction of the constant torque curve and the decreasing direction of the voltage limit ellipse. The decreasing direction information of the voltage limit ellipse is calculated by the gradient descent method. As the speed of the PMSM increases, the cosθ value decreases when it is reduced to zero. The operation state of the motor is switched from field weakening control area I to field weakening area II, and the PMSM will operate along the MTPV curve, as shown in curve BC in Figure 3.The expression of the constant torque curve direction can be expressed in the following equation:(14)−3pLd−Lqid+ψf23pLd−Lqiq2=TdTq.The permanent magnet synchronous motor operates at a high speed in the weak magnetic field area, and the voltage drop of the stator winding is ignored. The voltage limit elliptical falling direction can be expressed in the following equation:(15)−Ldωr2Ldiq+ψf−Lq2ωr2id=uduq.WhenTs=Td2+Tq2, us=ud2+uq2, θ can be expressed in the following equation:(16)θ=arccosTd,Tqud,uqTsus.In the process of field weakening, the key is how to determine the reference value of the current. The reference value of the required current can be determined by using the direction information of voltage, torque variation, and voltage difference. When a given reference torque isTe2, the running track of the motor starts from point O to point F along the PATA curve, passes through the constant torque curve to point G, and finally reaches point C.In order to improve the stability of the system and increase a wider speed regulation range when the maximum torque and to ensure a smooth transition between different operation areas, we designed a new field-weakening controller to control the current in the deep field weakening control area. The trajectory no longer follows the current limit circle, which eliminates the factors causing current instability. To smooth the transition between different weak magnetic fields and prevent frequent switching at the transition point of the jitter, the first-order low-pass filter is used to suppress the switching function. Combined with the filter, the formula of smooth switching of current with speed can be obtained, which can be expressed in the following equation:(17)id∗=sgnω^−ωid2∗id3∗ωcs+ωc,where ω^ is a given speed and ωc is cut-off frequency.Using (17), the smooth switching of different weak magnetic intervals can be realized according to the speed. The control algorithm block diagram of the PMSM is shown in Figure 4.Figure 4
The control algorithm block diagram of the PMSM.According to the relationship between the phase margin and the parameters of the low-pass filter, the phase margin is used as a parameter to design the low-pass filter. The method considers the system’s stability. The cut-off frequency determines the reactive power compensation characteristics. The maximum cut-off frequency can be determined according to the relationship between the cut-off frequency and the phase margin under different switching frequencies. The cut-off frequency can be expressed in the following equation [41]:(18)ωc=2fswcotγ,where fsw is the switching frequency and γ is the phase margin.


### 3.3. The Specific Steps of Field Weakening Control

The flow chart of the field weakening control is shown in Figure5.Figure 5
Flow chart of flux weakening control.Step 1.
The PMSM operates below the base speed. To make full use of the stator current, the linear maximum torque per ampere (MTPA) control is adopted, which is in the constant torque region. At this stage, the PMSM voltage reaches the limit value of the inverter output voltage, i.e.,us<ulim.Step 2.
When the PMSM voltage reaches the limit value of the inverter output, the voltage will be controlled with a constant torque.Step 3.
Whenθ=arccosTd,Tqud,uq/Tsus≥90°, the PMSM will run along the MTPV curve. The stator voltage required to reach the maximum speed is the smallest under the condition of the same output torque.


## 3.1. Mathematical Model

In the precise control of the PMSM, it is necessary to obtain its accurate rotor position. The rotor initial position identification process should identify the part of the rotor axis and then determine the magnetic polarity of the rotor.In the synchronous rotatingd-q coordinate system, the field linkage equation and voltage equation can be expressed in the following equations [35]:(1)Ψd=Ldid+Ψf,(2)Ψq=Lqiq,(3)uduq=Rs+pLⅆ−ωrLqωrLⅆRs+pLqidiq+0ωrΨf.The electromagnetic torque can be expressed in the following equation:(4)Tc=32pnΨdiq−Ψqid=32pnLdiq+Ψfiq−Lqiqid=32pnΨfiq+Ld−Lqidiq,where ψf is the maximum value of the field linkage generated by the permanent magnet, Ld is d-axis inductance, Lq is q-axis inductance, id is d-axis current, iq is q-axis current, pn is polar pairs, and ωr is rotor angular velocity.Due to the asymmetry of the AC-DC axis magnetic circuit of the built-in PMSM, it can be seen from (4) that there are two components in the electromagnetic torque. One component is the permanent magnet torque generated by the synchronous operation of the stator-rotating magnetic field and the permanent magnet magnetic field. Another component is the reluctance torque caused by the rotor magnetic circuit asymmetry because of the salient pole of the iron core. For urban rail vehicles with high requirements for weak magnetic speed regulation capacity, the counter electromotive force of the motor should be appropriately reduced. At the same time, to increase the load capacity in the constant power speed regulation stage, the salient pole rate should be increased, and the reluctance torque should be fully utilized. As for the magnetic circuit structure design of the built-in W-type rotor, the embedded depth of the permanent magnet can be appropriately reduced.Stator currentis is affected by the maximum working current that the inverter can withstand ilim limitations, which can be expressed in the following equation:(5)is2=id2+iq2≤ilim2.As in (5), the maximum current trajectory is centered on the origin in the d-q coordinate system, and ilim is a circle with a fixed radius, which is called the current limit circle.Stator voltageus is affected by the maximum working voltage output by the inverter ulim limitations, which can be expressed in the following equation:(6)us2=ud2+uq2≤ulim2.By ignoring the voltage drop on the resistance, substitute (3) into (6) to obtain the stator current. The voltage limit equation of the coordinate axis can be expressed in the following equation:(7)Ldid+Ψf2+Lqiq2≤ulimω2.(7) forms the voltage limit ellipse, and its two-axis length and speed ω are in inverse proportion, and with the increase of motor speed, a cluster of gradually smaller ellipses is formed [36]. The current and voltage trajectories of the built-in PMSM operation are shown in Figure 3.Figure 3
Current and voltage trajectories of built-in PMSM operation.


## 3.2. Control Strategy Optimization

A frequency converter must power a PMSM, and the closed-loop control system of rotor position detection can operate normally [37]. The control methods of permanent magnet direct drive motor used in urban rail vehicles are mainly vector control and direct torque control.Hysteresis controller is adopted in direct torque control. The selection of the stator voltage vector is rough, which will produce large torque ripple and current harmonic distortion in the control process. Based on the space coordinate transformation theory, VC realizes the decoupling of armature current and excitation current. It can control the field and torque of the motor, respectively. It can obtain better speed regulation and current control effects and has been widely used in urban rail transit traction systems.In the vector control of the PMSM, it is necessary to obtain the accurate position of the permanent magnet to realize the decoupling of field linkage and torque current [38]. The bogie on direct drive transmission puts forward new technical requirements for the characteristics and structural design of the motor. To meet the need for direct drive transmission, the PMSM must have the characteristics of low speed and high torque. When the motor’s output power is constant, increasing the low-speed torque will reduce the rated speed in the constant torque speed regulation stage below the rated speed. To keep the maximum speed unchanged, field weakening control is required above the rated speed, and the speed regulation range of motor field weakening is increased.The PMSM operates in the constant torque area below the base speed. To make full use of the stator current, the linear maximum torque per ampere (MTPA) control is adopted, which is shown in curve OA in Figure3. MTPA’s working point is to pursue the maximum torque output when the stator current is certain and to minimize the value of the stator current when the PMSM operates in constant torque mode. MTPA current closed-loop control method is adopted to control the excitation current of motor stator current id and torque current iq and to perform decoupling control, respectively, and their equations can be expressed as follows:(8)id=ψf2Ld−Lq−ψf24Ld−Lq2+iq2,(9)iq=8Teψf/3pn2−4ψf2−4Ld−Lq24Te/3pn2−ψf22ψf2−4Ld−Lq2+8Teψf6pnψf2−4Ld−Lq2.When the motor voltage reaches the limit value of the inverter output voltageulim, adjusting the maximum value of field linkage generated by the permanent magnet cannot make the motor speed continue to increase. Only by adjusting the stator current, the direct axis demagnetization current component (−id) is increased to make the speed increase. At the same time, iq limited by the current limit cycle should be reduced. To ensure that the armature current does not exceed the limit value ilim, the motor will run along the constant torque curve between MTPA and the maximum torque per voltage (MTPV). The curves DE and FG are show in Figure 3. This region is called weak magnetic region I.(10)id=−ψfLd+ulimωLd2−LqiqLd2,iq=ilim2−id2.With the further increase of speed, the motor will run along the MTPV curve, which is called field-weakening region II. MTPV is the minimum stator voltage required to reach the maximum speed with the same output torque. The MTPV curve is the line connecting the tangent point of the voltage limit circle and the torque hyperbola. The equation can be expressed as follows [39]:(11)∂Te∂id∂us∂iq−∂Te∂iq∂us∂id=0.In high-speed steady-state operation, the voltage equation can be expressed as follows:(12)us2=−ωLqiq2+ωLdid+ωψf2.By (10)–(13), the trajectory equation of MTPV can be obtained, which can be expressed in the following equation:(13)id=−ψfLd+−Lqψf+Lq2ψf2+4Lq2Ld−Lq2iq22LdLd−Lq.The gradient descent method of voltage limit ellipse is used for magnetic weakening [40]. The field-weakening region is determined by the angle θ between the direction of the constant torque curve and the decreasing direction of the voltage limit ellipse. The decreasing direction information of the voltage limit ellipse is calculated by the gradient descent method. As the speed of the PMSM increases, the cosθ value decreases when it is reduced to zero. The operation state of the motor is switched from field weakening control area I to field weakening area II, and the PMSM will operate along the MTPV curve, as shown in curve BC in Figure 3.The expression of the constant torque curve direction can be expressed in the following equation:(14)−3pLd−Lqid+ψf23pLd−Lqiq2=TdTq.The permanent magnet synchronous motor operates at a high speed in the weak magnetic field area, and the voltage drop of the stator winding is ignored. The voltage limit elliptical falling direction can be expressed in the following equation:(15)−Ldωr2Ldiq+ψf−Lq2ωr2id=uduq.WhenTs=Td2+Tq2, us=ud2+uq2, θ can be expressed in the following equation:(16)θ=arccosTd,Tqud,uqTsus.In the process of field weakening, the key is how to determine the reference value of the current. The reference value of the required current can be determined by using the direction information of voltage, torque variation, and voltage difference. When a given reference torque isTe2, the running track of the motor starts from point O to point F along the PATA curve, passes through the constant torque curve to point G, and finally reaches point C.In order to improve the stability of the system and increase a wider speed regulation range when the maximum torque and to ensure a smooth transition between different operation areas, we designed a new field-weakening controller to control the current in the deep field weakening control area. The trajectory no longer follows the current limit circle, which eliminates the factors causing current instability. To smooth the transition between different weak magnetic fields and prevent frequent switching at the transition point of the jitter, the first-order low-pass filter is used to suppress the switching function. Combined with the filter, the formula of smooth switching of current with speed can be obtained, which can be expressed in the following equation:(17)id∗=sgnω^−ωid2∗id3∗ωcs+ωc,where ω^ is a given speed and ωc is cut-off frequency.Using (17), the smooth switching of different weak magnetic intervals can be realized according to the speed. The control algorithm block diagram of the PMSM is shown in Figure 4.Figure 4
The control algorithm block diagram of the PMSM.According to the relationship between the phase margin and the parameters of the low-pass filter, the phase margin is used as a parameter to design the low-pass filter. The method considers the system’s stability. The cut-off frequency determines the reactive power compensation characteristics. The maximum cut-off frequency can be determined according to the relationship between the cut-off frequency and the phase margin under different switching frequencies. The cut-off frequency can be expressed in the following equation [41]:(18)ωc=2fswcotγ,where fsw is the switching frequency and γ is the phase margin.


## 3.3. The Specific Steps of Field Weakening Control

The flow chart of the field weakening control is shown in Figure5.Figure 5
Flow chart of flux weakening control.Step 1.
The PMSM operates below the base speed. To make full use of the stator current, the linear maximum torque per ampere (MTPA) control is adopted, which is in the constant torque region. At this stage, the PMSM voltage reaches the limit value of the inverter output voltage, i.e.,us<ulim.Step 2.
When the PMSM voltage reaches the limit value of the inverter output, the voltage will be controlled with a constant torque.Step 3.
Whenθ=arccosTd,Tqud,uq/Tsus≥90°, the PMSM will run along the MTPV curve. The stator voltage required to reach the maximum speed is the smallest under the condition of the same output torque.


## 4. Simulation and Test Results Analysis

### 4.1. Simulation Analysis

To verify the control strategy for the PMSM proposed in this paper, Matlab/Simulink is used for simulation. The parameters of the simulation PMSM are shown in Table3.Table 3
PMSM main parameters.
ParametersNumerical valuesRated power/kW1.5Rated voltage/V311Stator resistance/Ω0.025Straight shaft inductors/mH0.2Cross-axis inductors/mH0.47Pole pairs12Permanent magnet chains/Wb0.062The speed waveform of the simulation results with the field weakening control strategy is shown in Figure6, id waveform is shown in Figure 7, and iq waveform is shown in Figure 8.Figure 6
Simulation diagram of PMSM speed waveform.Figure 7
Simulation diagram of PMSMid waveform.Figure 8
Simulation diagram of PMSMiq waveform.The speed increases from zero to 1500 r/min, and the current remains unchanged at 0.1 s during the simulation. From 0.1 s to 0.15 s, the PMSM is in the field weakening stage. The simulation waveforms of the speed,id and iq, show that the control effect is very good in the deep magnetic field weakening area, without any oscillation, and a smooth transition can be achieved.


### 4.2. Test Results Analysis

The permanent magnet direct drive vehicles on Suzhou Metro line 3 adopt a permanent magnet direct drive bogie and a running air-cooled permanent magnet direct drive motor, which has a leading position in the world. It is the world’s first all-SiC permanent magnet direct drive urban rail train. The use of SiC converters is a genuine gesture of technology promotion. The economic analysis has shown that the cost of the power stage of the hard-switching SiC inverter is about 1.5 times that of the hard-switching Si inverter. Although the cost of the SiC inverter is high, the bill of materials (BOMs) of the two design methods are compared, and the result is that the BOM of the SiC inverter solution is relatively reduced, which can provide a more cost-competitive solution with an efficiency of 99% [42].The PMSM is a twelve-pole synchronous motor with permanent magnets and self-cooling. The PMSM is intended for the individual drive of the subway wheel set. The PMSM is placed directly on the vehicle axle. The stator’s torque responses are absorbed by the foot of the reaction rod fitted with silent blocks. The reaction rod is located on the stator’s side. The PMSM’s rotor is permanently connected to the vehicle axle. The PMSM stator’s perimeter is fitted with aluminium radiators. The PMSM is cooled down by emission radiation. The PMSM’s stator is supported by two roller bearings with ceramic elements. The permanent magnet direct drive motor on Suzhou Metro line 3 is shown in Figure9.Figure 9
Permanent magnet direct drive motor.We took a permanent magnet synchronous traction system train on Suzhou Metro line 3 as an example to illustrate the on-site energy consumption test. The PMSM parameters on Suzhou Metro line 3 are shown in Table4.Table 4
The PMSM parameters on Suzhou Metro line 3.
ParametersNumerical valuesNominal line voltage∗3 × 898 VRated power150 kWRated current∗119 ARated frequency32.5 HzRated speed325 rpmMax speed713 rpmPole pairs12Torque4 407 NmMotor weight863 kg ± 5%∗denotes values of 1 harmonic based on the type test results.In June and July 2021, we tested five experiments about energy-saving verification of the permanent magnet direct drive electric traction vehicles on Suzhou Metro line 3. The PMSM direct drive energy consumption test data on Suzhou Metro line 3 are shown in Table5.Table 5
The PMSM direct drive energy consumption test data on Suzhou Metro line 3.
DateTraction energy (kwh)Braking energy (kwh)Mileage (kw)Traction energy(kwh/km)Braking energy(kwh/km)Total energy(kwh/km)June 18th, 20215879.83180.054610.76885.82424.9446June 19th, 20215988.73225.154610.96835.90685.0615June 20th, 20215019.72702.445611.00815.92635.0818July 19th, 20214910.72680.745710.74565.86594.8797July 20th, 20214886.92672.245610.71685.86014.8567To further illustrate the advantages of the permanent magnet direct drive electric traction system, we compare the energy consumption of the permanent magnet direct drive electric traction system with that of the asynchronous motor gear mechanism. Comparison of energy data of PMSM direct drive and asynchronous motor gear mechanism on Suzhou Metro line 3 is shown in Table6.Table 6
Comparison of energy data of PMSM direct drive and asynchronous motor gear mechanism on Suzhou Metro line 3 (kWh/km).
Energy consumptionPMSM direct driveAsynchronous motor gear mechanismTraction energy10.8411.55Braking energy5.875.61Total energy4.965.94According to the test data, the energy regeneration rate of the asynchronous motor gear mechanism is 48.57 percent. Energy regeneration rate of permanent magnet motor direct drive is 54.15 percent. Total energy consumption of the asynchronous motor gear mechanism is 5.94 kWh. Total energy consumption of the permanent magnet motor direct drive is 4.96 kWh. Compared with the asynchronous motor gear mechanism, the energy-saving rate of the permanent magnet motor direct drive system is increased by 16.50 percent.Through the noise and vibration tests on the permanent magnet direct drive system, the test results show that the vibration test data are 1.5 mm/s at the rated speed (325 rpm) and 3.1 mm/s even at the highest speed (713 rpm), which is also lower than the technical limit of 3.5 mm/s. The experimental data of noise are 86.3 dB (A), which is lower than the technical limit of 103 dB (A). The flexible plate coupling can alleviate the adverse effects of vibration and improve the reliability of train operation.


## 4.1. Simulation Analysis

To verify the control strategy for the PMSM proposed in this paper, Matlab/Simulink is used for simulation. The parameters of the simulation PMSM are shown in Table3.Table 3
PMSM main parameters.
ParametersNumerical valuesRated power/kW1.5Rated voltage/V311Stator resistance/Ω0.025Straight shaft inductors/mH0.2Cross-axis inductors/mH0.47Pole pairs12Permanent magnet chains/Wb0.062The speed waveform of the simulation results with the field weakening control strategy is shown in Figure6, id waveform is shown in Figure 7, and iq waveform is shown in Figure 8.Figure 6
Simulation diagram of PMSM speed waveform.Figure 7
Simulation diagram of PMSMid waveform.Figure 8
Simulation diagram of PMSMiq waveform.The speed increases from zero to 1500 r/min, and the current remains unchanged at 0.1 s during the simulation. From 0.1 s to 0.15 s, the PMSM is in the field weakening stage. The simulation waveforms of the speed,id and iq, show that the control effect is very good in the deep magnetic field weakening area, without any oscillation, and a smooth transition can be achieved.


## 4.2. Test Results Analysis

The permanent magnet direct drive vehicles on Suzhou Metro line 3 adopt a permanent magnet direct drive bogie and a running air-cooled permanent magnet direct drive motor, which has a leading position in the world. It is the world’s first all-SiC permanent magnet direct drive urban rail train. The use of SiC converters is a genuine gesture of technology promotion. The economic analysis has shown that the cost of the power stage of the hard-switching SiC inverter is about 1.5 times that of the hard-switching Si inverter. Although the cost of the SiC inverter is high, the bill of materials (BOMs) of the two design methods are compared, and the result is that the BOM of the SiC inverter solution is relatively reduced, which can provide a more cost-competitive solution with an efficiency of 99% [42].The PMSM is a twelve-pole synchronous motor with permanent magnets and self-cooling. The PMSM is intended for the individual drive of the subway wheel set. The PMSM is placed directly on the vehicle axle. The stator’s torque responses are absorbed by the foot of the reaction rod fitted with silent blocks. The reaction rod is located on the stator’s side. The PMSM’s rotor is permanently connected to the vehicle axle. The PMSM stator’s perimeter is fitted with aluminium radiators. The PMSM is cooled down by emission radiation. The PMSM’s stator is supported by two roller bearings with ceramic elements. The permanent magnet direct drive motor on Suzhou Metro line 3 is shown in Figure9.Figure 9
Permanent magnet direct drive motor.We took a permanent magnet synchronous traction system train on Suzhou Metro line 3 as an example to illustrate the on-site energy consumption test. The PMSM parameters on Suzhou Metro line 3 are shown in Table4.Table 4
The PMSM parameters on Suzhou Metro line 3.
ParametersNumerical valuesNominal line voltage∗3 × 898 VRated power150 kWRated current∗119 ARated frequency32.5 HzRated speed325 rpmMax speed713 rpmPole pairs12Torque4 407 NmMotor weight863 kg ± 5%∗denotes values of 1 harmonic based on the type test results.In June and July 2021, we tested five experiments about energy-saving verification of the permanent magnet direct drive electric traction vehicles on Suzhou Metro line 3. The PMSM direct drive energy consumption test data on Suzhou Metro line 3 are shown in Table5.Table 5
The PMSM direct drive energy consumption test data on Suzhou Metro line 3.
DateTraction energy (kwh)Braking energy (kwh)Mileage (kw)Traction energy(kwh/km)Braking energy(kwh/km)Total energy(kwh/km)June 18th, 20215879.83180.054610.76885.82424.9446June 19th, 20215988.73225.154610.96835.90685.0615June 20th, 20215019.72702.445611.00815.92635.0818July 19th, 20214910.72680.745710.74565.86594.8797July 20th, 20214886.92672.245610.71685.86014.8567To further illustrate the advantages of the permanent magnet direct drive electric traction system, we compare the energy consumption of the permanent magnet direct drive electric traction system with that of the asynchronous motor gear mechanism. Comparison of energy data of PMSM direct drive and asynchronous motor gear mechanism on Suzhou Metro line 3 is shown in Table6.Table 6
Comparison of energy data of PMSM direct drive and asynchronous motor gear mechanism on Suzhou Metro line 3 (kWh/km).
Energy consumptionPMSM direct driveAsynchronous motor gear mechanismTraction energy10.8411.55Braking energy5.875.61Total energy4.965.94According to the test data, the energy regeneration rate of the asynchronous motor gear mechanism is 48.57 percent. Energy regeneration rate of permanent magnet motor direct drive is 54.15 percent. Total energy consumption of the asynchronous motor gear mechanism is 5.94 kWh. Total energy consumption of the permanent magnet motor direct drive is 4.96 kWh. Compared with the asynchronous motor gear mechanism, the energy-saving rate of the permanent magnet motor direct drive system is increased by 16.50 percent.Through the noise and vibration tests on the permanent magnet direct drive system, the test results show that the vibration test data are 1.5 mm/s at the rated speed (325 rpm) and 3.1 mm/s even at the highest speed (713 rpm), which is also lower than the technical limit of 3.5 mm/s. The experimental data of noise are 86.3 dB (A), which is lower than the technical limit of 103 dB (A). The flexible plate coupling can alleviate the adverse effects of vibration and improve the reliability of train operation.


## 5. Technical Difficulties

Compared with the asynchronous motor traction system, the PMSM direct drive traction has an obvious energy-saving effect, but further research is needed to realize the batch application of rail transit vehicles.


### 5.1. Study on the Dynamic Performance

The structure of the PMSM direct drive system is compact, but the PMSM mass belongs to the unsprung mass of the train, and the PMSM is greatly affected by wheel rail vibration [43, 44]. Therefore, the direct drive structure is only suitable for rail vehicles with small power at present [45].To adapt to high-power rail vehicles, it is necessary to optimize the direct drive transmission structure and analyze the influence of primary and secondary suspension parameters of the PMSM direct drive bogie, bogie frame structure, antihunting damper, and antiroll torsion bar on train driving safety [46]. The elastic suspension mode of the PMSM and the optimal matching parameters with the direct drive bogie are studied to optimize the mass distribution of the direct drive bogie and improve the dynamic performance of the bogie.


### 5.2. Research on the PMSM

To make the railway vehicle run smoothly, the PMSM is required to have higher torque density and smaller torque ripple. When the speed of the permanent magnet direct drive motor is low, the value of the output torque is large, the amplitude of the actual torque ripple is large [47], and the PMSM may have obvious bumping during operation, causing serious vibration and noise. This will affect the control accuracy of the direct drive system and shorten the service life of the drive system.Low-speed and high-torque permanent magnet direct drive motor usually has a large number of pole slots. When the rated frequency is low, the air gap magnetic field distortion is serious. When the rail vehicles are in the electric working condition, the output harmonic component of the variable frequency controller will increase at low frequency, and the distortion of the PMSM counter electromotive force and armature current will become serious, resulting in the increase of ripple torque. How to select the pole slots fit to keep the electromagnetic vibration and torque ripple at a low level? It is the content that needs further research for ultralow-speed permanent magnet direct drive motors.The PMSM is the key component of the PMSM directly driving electric locomotives. The reasonable cooling structure can alleviate the temperature rise effect of the PMSM [48]. The PMSM adopts a fully sealed structure such as water cooling or shell surface cooling. It is difficult to dissipate heat at the end of its stator winding [49].The highest temperature of the stator winding appears at its end and at the center. The high-temperature rise will affect the electromagnetic performance and service life of the PMSM. To ensure the safe and efficient operation, the online temperature monitoring is essential. At present, although the parameters of the PMSM meet the design requirements and stable operation, there is still a lot of space to be optimized, such as the quality and counter-electromotive force of the PMSM [50]. By improving the temperature rise algorithm of the PMSM, constructing a more accurate three-dimensional temperature field model, and exploring new methods to improve the heat dissipation conditions, these still need further research.The loss of excitation risk of a high-power permanent magnet direct drive motor is also one of the technical risks. All potential risks shall be fully considered in the design of the PMSM, and a sufficient safety margins shall be reserved. Through the use of high insulation grade materials, coating treatment on the surface of the permanent magnet, and simulation analysis of short-circuit current under the short-circuit state of the motor, the permanent magnet loss of the permanent magnet can be prevented to ensure the normal operation of PMSM. The MMF distribution saturation issue and the leakage flux density still need further research.


### 5.3. Research on Control Technology, Fault Protection, and Operation

In order to increase the reliability and stability of the system, it is crucial for realizing the high-frequency control of the system and reducing the equipment quality and harmonic current. Compared with an asynchronous motor, the PMSM drive system has many particularities in the process of control, fault protection, and operation [51]. The unique control technologies of the PMSM drive system include belt speed heavy head technology, counter electromotive force control technology, rotor magnetic field compensation technology, and interphase short-circuit fault protection [52]. A high-order sliding mode (HOSM) control strategy has been developed to enhance system performances, ensure the maximum power point tracking (MPPT), and track the generator reference speed [53].In the process of precise control of the PMSM, it is necessary to obtain its accurate rotor position [54]. The rotary encoder will directly affect the compactness of the PMSM, which is installed on the permanent magnet direct drive bogie to obtain the rotor position signal. Affected by the environment, the reliability of the encoder is difficult to be guaranteed [55–57].The permanent magnet direct drive traction motor adopts sensorless control technology. Sensorless control technology can achieve better position estimation by using various types of observer technology at medium and high speeds. The sensorless control applied to the MG-PGS improves the performance of the global control. The PMSM mechanical speed, stator resistance, and inductance could be estimated precisely, and the dynamical property of the system is obviously improved [58]. Due to the low counter electromotive force of the PMSM, the high-frequency signal injection method is used to achieve a more accurate position estimation effect at zero speed and low speed. However, on the one hand, the high-frequency signal injection method will reduce the voltage utilization and additional loss of the inverter and reduce the steady-state control performance of the system. On the other hand, it will also cause some noise pollution. These factors restrict the application of sensorless control technology to a certain extent [59]. Therefore, a series of adverse effects caused by high-frequency signal injection need to be further studied.


## 5.1. Study on the Dynamic Performance

The structure of the PMSM direct drive system is compact, but the PMSM mass belongs to the unsprung mass of the train, and the PMSM is greatly affected by wheel rail vibration [43, 44]. Therefore, the direct drive structure is only suitable for rail vehicles with small power at present [45].To adapt to high-power rail vehicles, it is necessary to optimize the direct drive transmission structure and analyze the influence of primary and secondary suspension parameters of the PMSM direct drive bogie, bogie frame structure, antihunting damper, and antiroll torsion bar on train driving safety [46]. The elastic suspension mode of the PMSM and the optimal matching parameters with the direct drive bogie are studied to optimize the mass distribution of the direct drive bogie and improve the dynamic performance of the bogie.


## 5.2. Research on the PMSM

To make the railway vehicle run smoothly, the PMSM is required to have higher torque density and smaller torque ripple. When the speed of the permanent magnet direct drive motor is low, the value of the output torque is large, the amplitude of the actual torque ripple is large [47], and the PMSM may have obvious bumping during operation, causing serious vibration and noise. This will affect the control accuracy of the direct drive system and shorten the service life of the drive system.Low-speed and high-torque permanent magnet direct drive motor usually has a large number of pole slots. When the rated frequency is low, the air gap magnetic field distortion is serious. When the rail vehicles are in the electric working condition, the output harmonic component of the variable frequency controller will increase at low frequency, and the distortion of the PMSM counter electromotive force and armature current will become serious, resulting in the increase of ripple torque. How to select the pole slots fit to keep the electromagnetic vibration and torque ripple at a low level? It is the content that needs further research for ultralow-speed permanent magnet direct drive motors.The PMSM is the key component of the PMSM directly driving electric locomotives. The reasonable cooling structure can alleviate the temperature rise effect of the PMSM [48]. The PMSM adopts a fully sealed structure such as water cooling or shell surface cooling. It is difficult to dissipate heat at the end of its stator winding [49].The highest temperature of the stator winding appears at its end and at the center. The high-temperature rise will affect the electromagnetic performance and service life of the PMSM. To ensure the safe and efficient operation, the online temperature monitoring is essential. At present, although the parameters of the PMSM meet the design requirements and stable operation, there is still a lot of space to be optimized, such as the quality and counter-electromotive force of the PMSM [50]. By improving the temperature rise algorithm of the PMSM, constructing a more accurate three-dimensional temperature field model, and exploring new methods to improve the heat dissipation conditions, these still need further research.The loss of excitation risk of a high-power permanent magnet direct drive motor is also one of the technical risks. All potential risks shall be fully considered in the design of the PMSM, and a sufficient safety margins shall be reserved. Through the use of high insulation grade materials, coating treatment on the surface of the permanent magnet, and simulation analysis of short-circuit current under the short-circuit state of the motor, the permanent magnet loss of the permanent magnet can be prevented to ensure the normal operation of PMSM. The MMF distribution saturation issue and the leakage flux density still need further research.


## 5.3. Research on Control Technology, Fault Protection, and Operation

In order to increase the reliability and stability of the system, it is crucial for realizing the high-frequency control of the system and reducing the equipment quality and harmonic current. Compared with an asynchronous motor, the PMSM drive system has many particularities in the process of control, fault protection, and operation [51]. The unique control technologies of the PMSM drive system include belt speed heavy head technology, counter electromotive force control technology, rotor magnetic field compensation technology, and interphase short-circuit fault protection [52]. A high-order sliding mode (HOSM) control strategy has been developed to enhance system performances, ensure the maximum power point tracking (MPPT), and track the generator reference speed [53].In the process of precise control of the PMSM, it is necessary to obtain its accurate rotor position [54]. The rotary encoder will directly affect the compactness of the PMSM, which is installed on the permanent magnet direct drive bogie to obtain the rotor position signal. Affected by the environment, the reliability of the encoder is difficult to be guaranteed [55–57].The permanent magnet direct drive traction motor adopts sensorless control technology. Sensorless control technology can achieve better position estimation by using various types of observer technology at medium and high speeds. The sensorless control applied to the MG-PGS improves the performance of the global control. The PMSM mechanical speed, stator resistance, and inductance could be estimated precisely, and the dynamical property of the system is obviously improved [58]. Due to the low counter electromotive force of the PMSM, the high-frequency signal injection method is used to achieve a more accurate position estimation effect at zero speed and low speed. However, on the one hand, the high-frequency signal injection method will reduce the voltage utilization and additional loss of the inverter and reduce the steady-state control performance of the system. On the other hand, it will also cause some noise pollution. These factors restrict the application of sensorless control technology to a certain extent [59]. Therefore, a series of adverse effects caused by high-frequency signal injection need to be further studied.


## 6. Conclusion

The permanent magnet direct drive system has the advantages of high efficiency, light weight, low noise, and good dynamic performance. It has a broad application prospect in urban rail vehicle traction systems. This paper mainly studied the PMSM direct drive technology from the two aspects of permanent magnet direct drive bogie motor structure and control strategy. The permanent magnet motor direct drive technology cancels the traditional gear transmission device and adopts flexible plate coupling to realize the direct drive between the permanent magnet motor and wheel set, which can alleviate the adverse effects of vibration and improve the reliability of train operation. The efficiency, energy saving, and environmental protection of electric locomotives using direct drive technology will be greatly improved. The average efficiency under all working conditions is increased by 2.2 percent. The PMSM direct drive system adopts an independent shaft control converter and a PMSM. The converter adopts maximum torque current ratio and field weakening control technology, with high-torque control accuracy and low harmonic content. The PMSM direct drive technology has been successfully applied to the field of medium and low-speed rail transit. Compared with the asynchronous motor gear mechanism, the energy-saving rate of the PMSM direct drive system is increased by 16.50 percent. Through the noise and vibration tests on the permanent magnet direct drive system on Suzhou Metro line 3, the test results show that the vibration test data and noise data are far below their technical limit value.In the field of rail transit, the working environment of the PMSM is usually worse. In the future, the PMSM will also face many challenges, such as the loss of excitation risk of permanent magnets, many particularities in the process of control, fault protection, and the complexity of the permanent magnet synchronous traction system. The maturity of any specific technology requires a certain process. The permanent magnet synchronous traction system is related to mechanical, electromagnetic design, thermal, control, materials, and other disciplines, which determines that this technology needs to accumulate experience in practice.



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*Source: 8312121-2022-12-20.xml*