Optimised space vector modulation for variable speed drives

The dissertation documents research work carried out on Pulse Width Modulation (PWM) strategies for hard switched Voltage Source Inverters (VSI) for variable speed electric drives. This research is aimed at Hybrid Electric Vehicles (HEV). PWM is at the heart of all variable speed electric drives; they have a huge influence on the overall performance of the system and may also help eventually give us an extra degree of freedom in the possibility to rethink the inverter design including the re-dimensioning of the inverter components.HEVs tend to cost more than conventional internal combustion engine (ICE) vehicles as they have to incorporate two traction systems, which is the major discouraging factor for consumers and in turn for manufacturers. The two traction system increases the maintenance cost of the car as well. In addition the electric drives not only cost extra money but space too, which is already scarce with an ICE under the hood. An all-electric car is not yet a viable idea as the batteries have very low energy density compared with petrol or diesel and take considerable time to charge. One solution could be to use bigger battery packs but these add substantially to the price and weight of the vehicle and are not economically viable. To avoid raising the cost of such vehicles to unreasonably high amounts, autonomy has to be compromised. However hybrid vehicles are an important step forward in the transition toward all-electric cars while research on better batteries evolves. The objective of this research is to make electric drives suitable for HEVs i.e. lighter, more compact and more efficient -- requiring less maintenance and eventually at lower cost so that the advantages, such as low emissions and better fuel efficiency, would out-weigh a little extra cost for these cars. The electrical energy source in a vehicle is a battery, a DC Voltage source, and the traction motor is generally an AC motor owing to the various advantages it offers over a DC motor. Hence the need for a VSI, which is used to transform the DC voltage into AC voltage of desired amplitude and frequency. Pulse width modulation techniques are used to control VSI to ensure that the required/calculated voltage is fed to the machine, to produce the desired torque/speed. PWM techniques are essentially open loop systems where no feedback is used and the instantaneous values differ from the required voltage, however the same average values are obtained. Pulse width modulated techniques produce a low frequency signal (desired average value of the switched voltage) also called the fundamental component, along with unwanted high frequency harmonics linked to the carrier signal frequency or the PWM period. In modern cars we see more and more mechanical loads driven by electricity through digital processors. It is very important to eliminate the risk of electromagnetic interference between these systems to avoid failure or malfunction. Hence these unwanted harmonics have to be filtered so that they do not affect the electronic control unit or other susceptible components placed in the vicinity. Randomised modulation techniques (RPWM) are used to dither these harmonics at the switching frequency and its multiple. In this thesis a random modulator based on space vector modulation is presented which has additional advantages of SVM. Another EMI problem linked to PWM techniques is that they produce common mode voltages in the load. For electric machines, common mode voltage produces shaft voltage which in turn provokes dielectric stress on the motor bearings, its lubricant and hence the possibility of generating bearing currents in the machine that can be fatal for the machine. To reduce the common mode voltage a space vector modulation strategy is developed based on intelligent placement of zero vectors. (.)