Scope of Research

The research presented in this thesis is mainly concerned with the following two topics:  Development of advanced IMs for EV/HEV applications.

 Investigation of non-sinusoidal bar current in IMs.

To be able to overcome one of the major problems in the HEV applications, which is the reduction of the size of the traction machine without sacrificing the torque density, output power, and efficiency for mounting the traction machine into very limited place, an advanced topology with non-overlapping winding technique, whose basic schematic is shown in Fig. 1.41, has been developed and then globally optimised in order to meet the desired drive specifications. Furthermore, the second topic is to investigate the non-sinusoidal bar current phenomenon in IMs by considering the influence of design and operating parameters, such as slip, electric loading, magnetic saturation, air-gap length, rotor slot number, stator slot and pole number combinations, rotor slot geometry parameters, rotor skewing, and coil pitches.

Structure of straded winding slot

Structure of bar winding slot

Fig. 1.41 Schematic of the proposed improvement methods for IMs.

The research scope and the contributions of this thesis are briefly illustrated in Fig. 1.42.

Fig. 1.42 Demonstration of research scope and contributions.

The contents of subsequent chapters are summarised as follows:

The research on the development of advanced IMs is presented in Chapters 2-4 and mainly divided into the following steps:

ISDW with

Short-Pitch FSCW

• Low MMF harmonics • Overlapping winding • Long end-winding • High stator copper loss

• High MMF harmonics • Non-overlapping • Short end-winding • High rotor copper loss  MMF harmonic cancellation  Non-overlapping

 Short end-winding  Low copper loss

(24S/8P-2L with )

 Reduce the Size  Improve the Efficiency  Increase the Torque Density

Combination (Non-overlapping) (12S/8P-2L with ) (24S/8P-2L with Non-overlapping) MMF harmonics in IMs Ch. 2 The MMF harmonic reduction method, development of novel winding configuration, and

influence of design parameters

Ch. 3

Comprehensive performance comparison of conventional and novel IMs and performance of novel

IM with different rotor topologies

Ch. 4

Novel Squirrel-Cage IMs

Influence of magnetic saturation Ch. 5 Influence of design parameters Ch. 6 Influence of operating parameters Ch. 7

Non-Sinusoidal Bar Current Phenomenon in IMs

Step 1: Chapter 2. Investigation of different conventional winding topologies to reveal the relationship between the end-winding length and the performance characteristics.

Step 2: Chapter 2. Determination of suitable winding configurations for IMs for EV/HEV applications.

Step 3: Chapter 3. Development of a new winding topology with very short end-windings but high quality MMF waveform.

Step 4: Chapter 3. Investigation of the key design parameters on electromagnetic performance and comparison of performance characteristics of IMs having adapted windings designed by different specifications including the rotor, slot, and pole number combinations, stack length, number of turns, geometrical parameters, etc. in order to determine the best candidate meeting the desired drive specifications.

Step 5: Chapter 4. Comprehensive performance comparison between conventional and advanced IMs.

Step 6: Chapter 4. Performance of advanced IM with different rotor topologies.

The research on the investigation of non-sinusoidal bar current in IMs is presented in Chapters 5-7 and mainly divided into the following steps:

Step 1: Chapter 5. Investigation of rotor bar current and verification of non-sinusoidal bar current waveforms under various operating conditions.

Step 2: Chapter 5. Investigation of the reasons behind the non-sinusoidal bar current waveform. The levels of iron saturation in different parts, including stator and rotor back iron, tooth body and tooth tips etc., are examined and their influences are then investigated, while the dominant part which causes the non-sinusoidal rotor bar current waveform is identified. Step 3: Chapter 6. Investigation of the influence of design parameters on the non-sinusoidal bar

current waveform.

Step 4: Chapter 7. Investigation of the influence of operating parameters on the non-sinusoidal bar current waveform.

Step 5: Chapter 7. Verification of non-sinusoidal bar current waveforms by numerical locked-rotor analyses.

Chapter 2

In this chapter, the influence of different winding configurations on the electromagnetic performance of squirrel-cage IM is investigated. These different winding topologies, namely integer-slot distributed winding (ISDW) with short- and long-slot pitches, integer-slot concentrated winding (ISCW), and fractional-slot concentrated winding (FSCW), are studied considering the number of winding layers

and slot/pole number combinations. In order to reveal the benefits and drawbacks of each winding topology, squirrel-cage IMs designed with one of these winding topologies are compared in terms of winding factor, winding factor harmonics, total axial length, average torque, torque ripple, machine losses, efficiency, etc. It has been revealed that because of very high MMF harmonics induced in the bar current, the bar copper loss of the IM designed with FSCW topology is significant. Therefore, it has been revealed that although the compactness is a vital issue for the electrical machines designed for Hybrid Electrical Vehicle (HEV) applications, it is not appropriate to use FSCW IMs because of the insufficient performance and unwanted high torque ripple and parasitic effect issues. In addition, it is also revealed that the parasitic effects of double-layer winding configuration is much lower than the single-layer winding configuration

Chapter 3

This chapter investigates the design and analysis of an advanced squirrel-cage IM with non- overlapping windings. The aim of this study is to improve a new winding and a stator topology for IMs which lead to reduce the total axial length without sacrificing the torque, power, and efficiency. In order to reduce the MMF harmonics, phase shifting method is employed for 2x9S/6P integer-slot distributed non-overlapping winding with 2 slot-pitch IM. By adopting this method, ~43% of the MMF harmonics have been reduced. The results show that according to conventional counterpart of the IM designed with developed windings, it is possible to shorten the total axial length by ~25% without sacrificing the torque, output power and efficiency. Moreover, the influences of some major design parameters, such as stator slot/pole number combinations, rotor slot number, stack length, number of turns, slot geometric parameters, etc., on the electromagnetic and flux-weakening performance characteristics of the advanced IM have also been investigated in this chapter.

Chapter 4

This chapter presents a comprehensive comparison on the performance characteristics of the advanced IMs designed with various stator slot/rotor slot/pole numbers, stack lengths, and number of turns. The various advanced IMs having different pole numbers with different stack lengths and number of turns, have been quantitatively compared with their conventional counterpart. Among the various advanced IMs, the best candidates for the EV/HEV applications are determined. Moreover, influence of various rotor types, such as insert-bar, open-slot cast-rotor, closed-slot cast-rotor with straight bridge, and closed-slot cast-rotor with u-shaped bridge on the electromagnetic and flux-weakening characteristics of the advanced IM have also investigated and then the obtained results have been compared.

Chapter 5

This chapter investigates the influence of magnetic saturation on the rotor bar current waveform and performance characteristics of a conventional IM. The levels of iron saturation in different parts are

examined and their influences on the electromagnetic performance characteristics and bar current waveform are investigated, whilst the dominant part which causes the non-sinusoidal rotor bar current waveform is identified. It has been revealed that the magnetic saturation, particularly in the rotor tooth, has a significant effect on the bar current waveform.

Chapter 6

In this chapter, the influences of a large number of design parameters on the electromagnetic performance characteristics of conventional IMs are examined in detail with particular reference to the bar current waveform. It has been revealed that each considered parameter has a significant effect on the bar current waveform and the key performance characteristics, such as torque, torque ripple, power losses, efficiency, etc. The conditions when the non-sinusoidal rotor bar current waveform occurs and the reasons behind this phenomenon are investigated by FEA.

Chapter 7

In this chapter, the influences of operating parameters on the electromagnetic performance characteristics and rotor bar current waveform of squirrel-cage IMs are investigated in detail. It is observed that after exceeding the certain slip rate and electric loading level, the rotor bar current waveform becomes non-sinusoidal even if the stator windings are fed with a symmetrical and balanced sinusoidal source. The conditions for such non-sinusoidal rotor bar current to occur and the reasons behind this phenomenon are investigated with particular reference to the numerical verification of the obtained non-sinusoidal bar current waveforms by locked-rotor analyses.

Chapter 8

This chapter contains the general conclusions of the thesis and potential future works in the development of IMs and non-sinusoidal bar current phenomenon areas.