PhD Oral Exam - Mohanraj Muthusamy, Electrical and Computer Engineering

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

The requirement of a clean energy environment has created the need for electric vehicles in the automotive industry. Electrical machines are the most vital component in traction applications as they provide electromechanical energy conversions. Traction applications require motor drive systems that can operate with high torque and high-power densities with reduced maintenance costs and losses. Since the 20th century, induction machines (IMs) have been a popular choice owing to ruggedness and reduced cost (absence of commutators and slip rings). But it is less efficient when compared to the other kinds of AC machines such as the permanent magnet synchronous machines (PMSM). In the IM, a part of the rotor flux is created by the currents flowing in the rotor conductors, made up of either aluminium or copper bars. But in PMSMs, the rotor flux is created by the permanent magnets (PMs) mounted on the rotor structure. The absence of the rotor cage reduces the mass of the PMSMs, resulting in lower inertia and faster torque response when compared to IMs. Also, the lack of rotor currents eliminates the rotor losses leading to higher efficiency.

In PMSM machines, a realistic value of the copper fill factor ranges between 35% to 40% with the laminated stator for air cooling. A higher current rating is required for a low voltage machine to meet the torque and power specifications. Higher current rating machines usually utilize parallel windings to reduce the number of strands. With the parallel winding pattern, the turns per coil will be multiplied with the number of parallel paths reducing the wire diameter and increasing the copper fill factor. The high copper fill factor increases the contact between the wires, and this allows for better heat transfer from the conductors. Therefore, the temperature of the windings is reduced, the resistance decreases and also the copper losses.

This thesis compares the performance of the SMC and laminated steel-based design for a radial flux outer rotor permanent magnet synchronous machine (PMSM) and its impact on the core loss at higher frequencies. Also, this thesis examines two different winding patterns (distributed and fractional) for field weakening operation with a surface mount outer rotor PMSM.

This thesis presents a cold spray additive manufacturing technique to spray the NdFeB magnet on to the rotor laminations. With this technique, magnet shaping is more feasible. The magnets are sprayed in a cobra shape to obtain better electromagnetic performances. This cold spray technique is developed at NRC Canada which makes the magnet shaping inexpensive.

This thesis presents a modified spoke type rotor that significantly reduces the torque ripple, back electromagnetic force (EMF) harmonics, and cogging torque from conventional spoke type designs. The effectiveness of the modified spoke type rotor is proved with state-of-the-art designs such as Toyota Prius and Honda Accord motors. Three different SMC materials are compared for the same machine specification. The tooth body length of SMC stator is varied from 36 mm to 56 mm to analyze the performance of the motor.

Along with the modified spoke type rotor focus is on a segmented soft magnetic composite (SMC) stator core to improve the copper fill factor and improve torque density by reducing the tooth length to allow additional space for the end windings within the stack length of the motor. A comparison between the laminated steel stator and SMC stator is presented. The laminated motor is capable of producing higher torque than the SMC motor when both machines are designed to operate with the same temperature, though the torque density in terms of volume of SMC motor is higher than the laminated motor. In all cases, the machines are designed to fit into the same frame. A cost comparison is presented between the SMC and laminated motors, which shows that SMC motors can be manufactured at lower cost than laminated motors. Also, the SMC stator includes axial magnet segmentation, flux weakening capability, thermal analysis, stator stress analysis due to electromagnetic forces, and a rotor mechanical stress analysis.

Finally, this thesis aims to present the development of SMC teeth considering manufacturing and assembling challenges. It briefly presents the SMC tooth manufacturing including the heat treatment. Also, it presents the rotor manufacturing with prototype pictures. Finally, it presents the experimental results for the SMC motor, along with the simulation results.