**1.2. Current research situation and new challenges for electric drives in AFVs**

AFV electric drive systems include the energy storage systems (battery and/or ultracapacitor), electronic controller, power converters, motors, and so on. The electronic controller is divided into three functional units: sensors, circuits, and processors. In driving and regenerative processes, the energy flow between the energy source and the motor is adjusted by the power converter.

The electric drive system itself as a whole is closely linked with the energy subsystem, which is the core technology of electric vehicles. The main function of electric drive system is to transfer energy from storage system to wheels, which is used for overcoming rolling resistance, air resistance, acceleration resistance, and climbing resistance. When braking, the mechanical energy is converted to electrical energy back to the energy storage system. AFVs can choose single or dual motor drives. AFVs can also employ in-wheel motors. Now, low-power electric vehicles on the market almost all employ single-motor power systems with simple structure. However, high-power electric vehicles such as electric buses generally use dual motor powered system. AC drives can be divided into induction motor drive system and synchro‐ nous motor drive system. Synchronous motor drive systems can be classified as permanent magnet synchronous motor drive system (PMSM Drives) and switched reluctance motor (SRM) drive system. Currently, the commercial application of power systems for electric vehicles is divided into three categories: the DC motor drive system, induction motor drive system, and permanent magnet synchronous motor drive system. Representative products include "Peugeot 106 EV" (DC motor), "General EVI" (induction motor), and "Honda EV Plus" (permanent magnet synchronous motor), respectively. With the development of power electronics, micro-electronics, and control technology, the digital AC drive systems (IM Drives and PMSM Drives) are widely applied in commercial AFVs.

A 3-phase 4-pole induction motor and an electronics module with 72 IGBTs are adopted by the famous electric car Tesla Model S. The efficiency of the battery pack, power electronics modules, and motor system can reach 85–95%. It weighs only 70 pounds, can generate large torque at very low speed, and the max motor speed could be 13,000 rpm, which cannot be done by the internal combustion engine.

At present, there are still disadvantages in the electric drive system. First, the long charging times, which usually exceed 30 minutes [4] for a full charge, as well as the range limitation of EVs due to the available battery technologies, are still challenging problems.

Short driving distance, greatly limits the application and development of electric vehicles. In electric vehicles, the power supply did no progress a lot. Effective use of the vehicle power supply energy is an urgent problem. Torque of the drive motor, speed control accuracy, and efficiency optimization requires improvement over the entire range.

The motor runs under bad conditions, e.g., the vehicle engine vibration and high-temperature environment. Therefore, the motor must be adapted to environmental conditions: reliable, stable, and safe operation. Due to space limitations, high-density, compact, and lightweight motors are in need.
