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Working Principle and Optimization Scheme of Electric Vehicle Motor Control

The three electric vehicles of new energy vehicles refer to: Power Battery, Drive Motor, and Vehicle Electronic Control.

Sanden International, USA, Inc. is the core of new energy vehicles, and in the development of power battery technology, new technologies and new hot spots appear from time to time. In the field of electronic control, our development has been in a relatively early stage.

The improvement of electronic control efficiency can significantly improve the vehicle economy of pure electric vehicles.

Electronic control, in a broad sense, electronic control has vehicle controller, motor controller and battery management system.

This article describes the working principle and optimization scheme of motor control.

01. Motor controller

 Motor controller in electric vehicles

The motor controller is the nerve center connecting the motor and the battery, which is used to adjust the performance of the whole vehicle, and the intelligent electronic control can not only ensure the basic safety and precise control of the vehicle, but also allow the battery and motor to exert sufficient strength.

02. The working process of the motor controller

The working process of the motor controller in electric vehicles

The core of the motor controller unit is the control of the drive motor. The provider of the power unit, the power battery, provides direct current, and the drive motor requires three alternating currents. Therefore, what the electronic control unit wants to achieve is a process called inverter in power electronics technology, that is, the direct current at the power battery end is converted into alternating current on the input side of the motor.

In order to realize the inverter process, the electronic control unit needs DC link capacitors, IGBTs and other components to work together. When the current is output from the power battery end, it first needs to pass through the DC bus capacitor to eliminate the harmonic component, and then, through the control of the IGBT switch and other control units, the direct current is finally inverted into alternating current, and finally used as the input current of the power motor. As mentioned above, by controlling the frequency of the three input currents of the power motor and the feedback value of the speed sensor and temperature sensor on the power motor, the electronic control unit finally realizes the control of the motor.

The different working principles of the motor directly affect the complexity and accuracy of the control process.

According to the control from easy to difficult arrangement, they are DC brushless motor, permanent magnet synchronous motor, switched reluctance motor, asynchronous motor.

The difficulty of electronic control includes not only the scale and cost of hardware system design, but also the control accuracy achieved by software algorithms and the robustness of the strategies and methods used to achieve this accuracy.

What people expect is a control system with simple hardware structure, simple software algorithm, high control accuracy and good system stability.

03. Motor controller main circuit selection

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Selection basis: As a specific function of the inverter, the motor controller uses the voltage regulation and frequency regulation technology in power electronics technology to adjust the direct current stored in the power battery into rectangular wave or positive wave alternating current required to control the motor, change the voltage, current amplitude or frequency of the output power, and then change the motor speed and torque to achieve the purpose of controlling the speed and acceleration of the whole vehicle.

Power electronic circuit design, according to different speed regulation needs, to make different degrees of complexity, different costs of design.

For example, control of DC motors. If a single-tube chopper circuit is used, only the speed can be regulated in one direction, and the current cannot be commutated; If a double-tube chopper circuit is used, the energy feedback action can be realized, but the DC motor cannot be commutated; If the H-bridge chopper circuit is used, the DC motor speed can be adjusted, energy can be restored, and the excitation current can be reversed.

But the above three choices, one is more complex than one, and one is more expensive than one. Designers need to choose between performance and cost, the most expensive is not necessarily the best, the most suitable is good.

04. Integrated control of distributed drive electric vehicle

04. Integrated control of distributed drive electric vehicle

Distributed drive electric vehicles have always been the focus of research and development with the advantages of good controllability, short transmission chain, compact structure, and high space utilization in the car. Moreover, the drive motor of each wheel can be controlled independently, and through the reasonable distribution of motor torque, the high efficiency range of the motor is fully utilized, and combined with the feedback braking strategy, the economy of the vehicle can be improved.

In order to improve the adaptability of the control system to vehicle parameters, status and vehicle driving environment, it is necessary to design a state estimation and parameter identification algorithm that meets the control requirements, and at the same time ensure the stability of the control-estimation system, and the distributed drive provides greater possibilities for the vehicle state estimation algorithm.

In order to ensure the good driving performance of distributed drive electric vehicles under complex working conditions, solve the coordination problems of multi-control targets, multi-control functions, multi-actuators and multi-dimensional motion, integrated control has become the current research focus of distributed drive electric vehicle dynamic control.

Traditional stand-alone controllers have their own clear control objectives. However, there is a certain degree of functional overlap and interference between various systems, so the action distribution of multiple execution systems and the coordination of multiple control objectives are the key to the system integration control strategy.

05. Electronic control system efficiency optimization technology

The efficiency of the electronic control system is increased by 1%, which is very advantageous for the economy and weight of the whole vehicle, and the efficiency optimization technology includes carrier frequency dynamic adjustment, DPWM wave technology, overmodulation technology, and wide-area high-efficiency HSM motor.

  • Carrier frequency dynamic adjustment technology
Carrier frequency dynamic adjustment technology

The main source of loss of the electronic control system is the inverter part, and 70% of the inverter loss comes from the switching part.

From the perspective of switching loss reduction, the dynamic adjustment technology of carrier frequency was studied. Through simulation tests, it is found that after adjusting the switching frequency, the efficiency of the controller can be increased by up to about 2%, the use of dynamic carrier frequency technology, especially at low speed, when the carrier frequency requirements are not so high, adjusting the carrier frequency can effectively reduce the loss of the controller, provide the efficiency of the controller, the preliminary estimate is that every 100 kilometers can provide about 1.5 kilometers, the carrier frequency can not be adjusted indefinitely, but also need to consider the vehicle noise and motor control needs.

  • DPWM emitter technology application
DPWM emitter technology application

The application of discontinuous wave technology, the use of DPWM technology is 1/3 less than COWM technology, which can significantly reduce the number of switching times and achieve the purpose of reducing switching losses.

When the modulation ratio M>0.816, the harmonics under CPWM and DPWM modulation are approximately the same. DPWM technology can be used in this area to reduce device losses.

  • Application of overmodulation techniques
Application of overmodulation techniques

Controller losses include switching losses and conducting losses. The conduction loss has a lot to do with the output current, and when the output power is fixed, the output voltage corresponding to the reduction of the output current needs to be increased accordingly.

By adding over-modulation, the output power and output torque in the weak magnetic area can be effectively improved, the output voltage can be increased by 4%, and the peak power can be increased by about 4%, so as to improve the power performance of the vehicle at high speed;

By adding overmodulation, the output of the same power, the current will be significantly reduced, which can reduce the system heating, improve the overload capacity of the controller, and improve the power performance of the whole vehicle;

By adding over-modulation, the fundamental voltage can be effectively improved, compared with no over-modulation, the motor efficiency can be effectively improved, the motor current can be significantly reduced (0~8%), and the efficiency improvement can effectively extend the cruising range.

  • Wide area high efficiency HSM motor
IPM and HSM motors

In addition to the improvement of electronic control efficiency, it also includes the improvement of motor efficiency.

HSM motor hybrid synchronous motor, compared with IPM motor, can balance low-speed zone efficiency and high-speed zone efficiency. HSM has more obvious efficiency advantages, especially in medium and high-speed constant power operation areas. The test found that in the low-speed area and high-speed area, the HSM efficiency is higher than that of conventional IPM motors, and the motor efficiency can be improved after using HSM technology.

In the working conditions of bus and group vehicles, IPM and HSM motors are compared, and HSM motors are dominant.

Considering the comprehensive energy efficiency directional optimization technology of the whole vehicle working conditions, the directional optimization of efficiency is realized by adjusting the proportion of each loss component of the motor, and the motor with higher comprehensive energy efficiency is customized to improve the cruising range by combining the road condition information of the specific model.

06. Development trend of electric vehicle motor controller technology

Development trend of electric vehicle motor controller technology

High security, this is a basic requirement. With more and more integration functions, the higher the security requirements.

  • High power density. The shape and volume develop with the sub-packaging to miniaturization.
  • High pressure is a fundamental trend. The direction of GBT is 650V IGBT design towards higher 750V and 1200V.
  • EMC ratings are getting higher and higher. The next step is to achieve the CLASS5 level.

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