Design and application of PMSM system stall for pure electric vehicles

Abstract: This paper introduces a stall control of a permanent magnet synchronous motor system for pure electric vehicles, including the analysis of principles, regulatory requirements, test methods and strategic control applications. By transforming the basic working principle of the permanent magnet synchronous motor into the working principle of the stall and the causes of the problems caused, the temperature protection strategy of the drive motor and IGBT is implemented by establishing a stall strategy and simulation calculation, and the selection and design of the drive motor and IGBT are established. The design is checked through simulation and the design goals are verified through bench and vehicle tests. The system performance is verified, and the safety is high. While the vehicle goals are checked through design, over-development is prevented and the system development cost is reduced.

0 Introduction

New energy electric vehicles, due to their characteristics of low speed, high torque, high speed and high power, bring powerful instantaneous explosive power at startup, but also bring a series of driving conditions problems, which are manifested in performance and reliability, such as the impact of temperature rise on magnetic steel, and the impact of temperature rise on the performance of the drive motor controller power devices such as insulated gate bipolar transistors (IGBT) and MOS tubes, which will reduce power and current output. In addition, there is a problem of NVH noise and jitter, which affects the subjective driving comfort and safety of the drive system. At the same time, when the rotor is blocked, it corresponds to the drive PWM The influence of wave generation is relatively large, and the random carrier frequency and segmented frequency also need to be calibrated. Through the reasonable application scenario environment matching wave generation strategy, it is suitable for the application needs of vehicles with different load types such as manned and cargo. Current national regulations such as mandatory inspection: Technical conditions and test methods for drive motor systems for electric vehicles also define general requirements for range characteristics such as stalling, but the detailed parameter-level constraints on stalling are not specific. Enterprises need to conduct adaptive development and in-depth research on their own product characteristics to decide, especially under full load conditions, stalling and other severe working conditions will occur, posing challenges to the safety of vehicles and personnel. For example, in the field of urban logistics vehicles, the load may be more flexible. If the stalling performance and safety cannot match, it will directly affect the vehicle's backward slide and bring risks.

From the perspective of product characteristics and applications, due to the different power device architecture solutions of the drive motor controller, there are IGBT, MOS single tube and IGBT module solutions, among which the modules are divided into half-bridge and full-bridge. Different electrical architectures and layouts bring different du/dt and di/dt, which brings challenges to the control of power device stall performance differences. In addition, the application of products such as SiC, flat wire motors and oil cooling systems has greatly improved the ability to balance temperature rise and improve stalling ability. This paper is based on the design of a city logistics vehicle with a peak power of 60 kw, a rated power of 30 kw, a stall target torque of 200 nm not less than 5 s, and a peak torque of 220 N·m. The system design, through the power requirements of the whole vehicle and Matlab/simulink to establish a simulation model implementation strategy and through bench and vehicle testing and analysis and verification of the working condition application.

1. Working principle of stalling

The stator winding of a permanent magnet synchronous motor (PMSM) is usually composed of round wire or flat wire. The rotor uses magnetic steel made of aluminum iron boron and other materials to achieve rotor excitation. The rotating magnetic field of the rotor generates a back electromotive force EMF in the stator winding excitation magnetic field. According to the PMSM theory, the control equivalent circuit has a voltage balance equation:

If the rotor is blocked, from the perspective of power balance, the originally running stator and rotor magnetic fields are generated separately, and the rotor induced current magnetic field suppresses the increase of current through the back electromotive force in the stator winding and becomes a rotor-free current induced magnetic field. The resistance and self-inductance in the stator magnetic field, including leakage inductance and excitation inductance, are directly added to the voltage after the blockage. The three-phase drive before the blockage is a sine wave. Once the blockage occurs and zero speed occurs, the position of the rotor is locked in a fixed position. This is achieved through a fixed locking mechanism. At this time, the phase current changes from a sine wave to a DC waveform.

Then the above formula becomes:

It is obvious from the formula that when the voltage Ui remains unchanged, Is will show a sharp rise, and all voltages are applied to the stator winding resistance. Moreover, since the rotor position is directly related to the current, with the arrival of the maximum position, the maximum current will appear. Since the thermal capacity of the power device IGBT is relatively small compared to the drive motor winding, the temperature will generally rise faster, leading the NTC temperature of the drive motor winding to rise. If the system temperature rise and balance cannot be controlled in time, the IGBT and drive motor NTC temperatures will exceed the threshold and reach the alarm, resulting in power reduction. In order to cope with severe working conditions, the method generally adopted to reduce power is to linearly reduce torque until it drops to zero. Under the intervention of the cooling system, when the temperature reaches below the threshold, it will automatically recover. In addition, due to the drive motor PWM Wave generation is adjusted by matching the load coefficient to achieve random carrier. According to the principle of vector control, the switching frequency is proportional to the energy generated per unit time. Therefore, once the stall is triggered, the carrier frequency is reduced. Currently, after the stall is at a fixed position, a fixed-point frequency is used to achieve wave generation. The higher the wave generation frequency, the higher the temperature rise. The lower the frequency, the fewer the switching times and the slower the temperature rise. However, too low a frequency will bring NVH noise and vehicle jitter problems, so reasonable calibration and matching verification are required.

According to the test requirements of GBT 18488.2-2015, the drive motor rotor is blocked on a specific tooling. The drive motor system includes the motor and controller. It works in the actual cold state. The drive motor controller applies the required blocking torque to the drive motor, records the blocking torque and blocking time, and changes the equivalent position of the drive motor stator and rotor. Take 5 blocking points in equal parts along the circumferential direction, and repeat the above test in a distributed manner. Before each test, restore the drive motor system to the actual cold state, and the blocking time should be the same. It can be seen from this that the national standard requires the drive system to have the ability to block, but the specific blocking torque point and range, blocking duration and step length are not defined, and are defined by product technology. During the development process, the torque can be set in sections in combination with the national standard test requirements, and the blocking torque point and corresponding duration can be specified.

2 Design Goals

At different voltages and when the rotor is in different positions, the motor controller power device still needs to respond to the torque demand and output a larger current to meet the stall request torque, especially when stalling at the highest operating voltage. The IGBT current output capability is extremely demanding, and the target parameters are as shown in Table 1:

In order to achieve the most stringent design, the simulation and test conditions are set under the following conditions: the maximum working voltage is 420 V, the input water temperature of the cooling system is not higher than 60 ℃, the flow rate is 10 L/min, and the process data is recorded through the message and oscilloscope waveform. Based on the above analysis, relevant temperature monitoring is carried out, and the main target parameters are confirmed, and the following specific strategy analysis is carried out according to the target parameters.

2.1 Key Parameters

2.2 Zero point position

First, define the stall current. The stall current obtained when and only when the rotor is at the maximum position angle is called the maximum current. If the corresponding torque at this time is the maximum stall torque, but this torque meets the target stall torque, it can meet the design requirements. As shown in Figure 2 below, the three-phase output current sine wave waveforms of the drive motor are U, V, and W phases, respectively, and the colors are green, red, and blue. If the current phase at the first position during stall is in the vertical position of the black line in Figure 1, it can be seen that the U phase stall current is the maximum value at this time, which is equal to the sum of the output current values ​​of V and W phases, and the direction is opposite to the U phase current. The U phase is the maximum value and a positive value, indicating that the stall is at the U phase upper bridge at this time, and the current direction is out of the controller.