Projects:2019s1-183 Development of Motor Drive System for Electric Vehicles

Large companies such as Tesla, Porsche and BMW are all releasing electric vehicles into today’s car market, why? Electric motors have the potential to be more powerful, durable and most importantly can utilise energy produced by the world’s ever growing renewable resources sector. The problem is ensuring our motor control is responsive, accurate and robust to a range of terrain while still performing as the driver would expect. The aim for this project was to understand the theory and mechanics behind motor control and apply this to a Mazda MX-5 electric conversion. After applying our knowledge and testing, we have developed software to process feedback from a range of sensors, in real time, to control the motor in in the Mazda. We present a system which is capable of reducing emissions on our roads by taking advantage of cleaner energy from renewable sources.

Project Overview

Aims

The goal of this segment of the project was to implement the motor control technique of Field Oriented Control (FOC) by programming the onboard microprocessor of the Infineon board with integrated inverter.

Project students

• James Licciardi
• Brady Martin

Supervisor

• Nesimi Ertugrul

Background

Electric Vehicles

An electric vehicle (EV) is a vehicle which is powered by electricity unlike traditional petroleum fueled vehicles. The main components of an electric vehicle are shown in Figure 1 below.

• Power System: This consists of a three-phase electric motor and an inverter which takes DC power from the battery system and supplies three phase AC power to the motor.
• Control System: This consists of a resolver to provide rotor position, a current sensor for each phase to the motor and a control board with an embedded processor.
• Driver Input: The control system takes a speed reference from the accelerator to determine motor speed, a signal from the brake to activate regenerative braking and a direction signal to determine forward or reverse drive of the motor.
• Battery System: This involves a battery bank and a DC/DC step up converter to provide power to the inverter.

Figure 1: High Level EV Block Diagram

Figure 2: Mazda MX-5 EV Project

This general topology for an EV is applicable for buses, trucks and bikes as well as cars, providing the capability to power most of the transport industry with electricity. Advantages of EVs:

• Zero Local Emissions: There are no greenhouse gas emissions from operating the car and can be completely green if the car is charged by renewable energy sources
• High Efficiency: Electric motors are very efficient compared to internal combustion engines
• Vehicle to Grid Capability: When plugged in to an outlet the car has the same functionality as a personal electric battery system for the home
• Reduced Fuel and Maintenance Costs

The range of applications of EVs and their social, environmental and economic benefits have led cities such as Madrid, Paris and Athens devise strategies to cease the importation and production of diesel cars and vans by 2025 and Denmark aiming for the end of 2020!

Previous Work

This project is an on-going mission to design and build an EV for city usage. It utilises a 1997 Mazda MX-5 (Figure 2), retrofitted with a three-phase brushless permanent magnet (BPM) motor from a Toyota Prius, a high voltage battery system (~500V enabling high power with reduced current losses), an Infineon Inverter with a built in micro processing unit, a DC/DC step up converter and a cooling system for the converter (Figure 3).

Figure 3: Block Diagram of the EV System within the MX-5

Field Oriented Control

FOC is a technique of motor control which utilises feedback from rotor position and current sensors to calculate the voltage/current component corresponding to the desired levels of system torque and flux. Primarily, phase current waveforms are measured and fed back to the system, from which, they may be transformed into a rotating reference frame, by the Clarke and Park mathematical transformations (Figure 4).

Figure 4: Clarke and Park Reference Frame Transformations

Once the appropriate current has been transformed by the system, the next set of phase voltages may be calculated based on the system Torque and Flux equations. These voltages, currently in the rotating reference frame, are then converted back to the 3 phase voltages required to drive the system. A proportional Integral (PI) controller is utlised to stabilise the difference between the desired phase torque current (iq) and phase flux current (id) with their respective measured counterparts (after transformation), ensuring torque and flux control is as accurate as possible. The above transformations and control can be seen in the closed loop system block diagram (Figure 5).

Figure 5: FOC Operation Flow Block Diagram

Test Setup

This project is tested in two phases, firstly using a small brushless permanent magnet (BPM) motor in a benchtop setup (Figures 6 & 7) and secondly using the system within the Mazda MX-5 (Figures 2 & 3 ). Both phases use the Infineon Hybrid Kit 1 Pin Fin Inverter as the controller and inverter for the motor for a smooth transition between the two phases.

First Test Phase

• The current sensors were tested to ensure that the current from each phase going to the motor is detectable by the sensors and readable by the controller.
• The motor has a built-in resolver, the output pins were tested to identify the function of each pin and to ensure the resolver functions correctly.
• The Infineon Mini-Wiggler debugging connector was found and purchased, it is used to physically connect the Infineon control board to the computer for coding as well as sending debugging and trace information to the computer.

This benchtop test phase includes the use of an additional motor coupled to the BPM motor to simulate the real-world effects of braking and uphill and downhill travel to observe the change in current required by the BPM motor in a reproducible manner.

Figure 6: Block Diagram of Test Setup

Figure 7: Photo of Test Setup

Second Test Phase

• Testing existing components of the MX-5 to ensure they are still functioning, eg battery system, Infineon board, 3ph motor, accelerator input etc.
• Identifying parameters to change in the control code
• Implementation of control code onto the Infineon board in the MX-5
• Fine tuning of parameters and code to achieve high system efficiency

Conclusion

Outcomes

The age and lack of current support for the Infineon board limited progress for the year with multiple different attempts and strategies to failing to successfully connect the Infineon board to the computer. The benchtop test setup (Figures 6 & 7) is completed, all components tested and ready for use by continuing project groups in the future.

Future Work

An additional outcome of the MX-5 project this year will be to identify a replacement for the Infineon Hybrid Kit which can connect directly to the computer, does not require an external debugging connector and meets all the requirements for the MX-5 equipment. This will allow future groups to immediately begin development of motor control software, utilise the test setup to fine tune the code and finally be able to control the motor using FOC.