Executive Summary : | Electric vehicles (EVs) demand high torque density motors due to stringent space constraints specially for two wheelers. Peak power rating (PPR) of high performance vehicles with top speed above 45 kmph varies from 1.5 kW to 2.8 kW. PPR of two wheeler: Hero Electron Photon Li, Okinawa Praise and Ather 450 is 1.5, 2.5 and 2.8 kW respectively. Moreover most of these vehicles have gear mechanism to coordinate motor and wheel speed. Transverse flux machines (TFM) are suitable for EVs in this class of PPR. TFM offers high torque density and low base speed operation (direct drive or geared drive with low gear ratio) [1-3].
In a four wheeler, space constraints for driving motor are moderately relaxed. PPR for Renault ZOE Z.E.40 and Nissan LEAF is 80 kW and 110 kW respectively. For this, permanent magnet synchronous motor (PMSM) is suitable. PMSM have better torque density with fractional slot concentrated winding (FSCW) compared to distributed winding [4] and is suggested for EV [5,6]. It has high phase inductance and low mutual inductance which are desired characteristics for achieving tolerance to short-circuit winding faults [7].
Switched reluctance motor (SRM) is suitable for high speed applications such as electric locomotive. SRM has simplest rotor construction without conductors and permanent magnets.
However these motors have certain limitations. Selected limitations that will addressed in this project and their impact are mentioned below:
a) Limitations of TFM: high leakage inductance, eddy current losses, reduced utilization of magnets and manufacturing difficulties [8-12]. If utilization of magnets in TFM can be increased, cost of EV can be reduced besides meeting space constraint and achieving top speed and acceleration similar to IC engine vehicles.
b) Limitations of PMSM with FSCW: low saliency, requirement of converter with more than three legs for fault tolerance [13-16]. By increasing saliency, fault tolerance with regard to position sensor can be achieved. Further, to reduce the cost of EV, fault tolerance with regard to open and short circuits (in windings) need to be achieved on three phase full bridge converter.
c) Limitations of SRM: low power factor, high torque ripple and higher noise
For all control technique based noise and torque ripple reduction solutions, there is a penalty associated such as higher current requirement for same average torque. Even the design improvements that exist are mostly preventive techniques such as using supporting ribs between stator poles. Torque ripple and noise reduction through only active design changes (that reduce the cause of noise and torque ripple and don't mitigate noise once the design is made such) is the most appropriate solution. Reducing torque ripple and noise will also improve performance parameters such as efficiency and torque density.
Commercial feasibility of EVs will increase by addressing the selected limitations facilitating widespread adoption. |