The electric vehicle motor is evolving, aiming for higher performance for EVs

It has been 20 years since Hino Motors introduced a commercial hybrid bus in 1991. In the meantime, the Prius was unveiled in 1997, and the car became a hot seller in 2010, driven by CO2 emission reduction issues and subsidies and tax cuts for eco-friendly cars. Hybrid cars have gained "citizenship" in the car market. To consumers, hybrid cars are now like a highly fuel-efficient gasoline car. Against this backdrop, global automakers will continue to launch hybrid cars by 2015.

The three so-called "motor", "inverter" and "battery" are the supporting elements of the evolution of electric vehicles. Batteries have a significant impact on the price of vehicles and are a key technology that determines the basic performance of electric vehicles, that is, the continuous driving distance. In addition, battery technology has also made significant progress. Motors and inverters affect the driving performance and fuel efficiency (power consumption) of vehicles. This article will focus on motors and analyze their development status and current issues.

Evolution to IPM motors and PRM motors that utilize reluctance torque

Figure 1: Relationship between the type of motor using permanent magnets and torque

The motor used in hybrid vehicles must be able to be configured in the narrow space between the engine and the transmission. Therefore, the on-board motor must be small, high-power, and high-efficiency. Hino Motors' commercial hybrid buses use induction motors manufactured by Toshiba. The reason why induction motors are difficult to miniaturize is that strong magnets using neodymium were not available at the time, so it was difficult to manufacture high-power, high-efficiency motors required for hybrid vehicles. Later, strong magnets using neodymium were introduced, and the motors of hybrid vehicles began to develop from induction motors to permanent magnet motors. Currently, the mainstream motors used in hybrid vehicles are products using permanent magnets.

There are many types of motors that use permanent magnets, and their characteristics vary. (Figure 1) lists the relationship between the types of motors that use permanent magnets, torque, and output power. The SPM (Surface Permanent Magnetic) motor has magnets on the rotor surface. This motor can generate large torque in the low speed range. However, it is not suitable for driving at high speeds and fixed output power. This is because as the motor speed increases, the starter motor coil will generate a counter electromotive force with the rotation of the magnet, which offsets the power of the rotating rotor.

Figure 2: Example of PRM motor used in Toshiba's HEV and EV drive systems

The IPM (Interior Permanent Magnetic) motor and the PRM (Permanent Magnetic Reluctance) motor, a type of IPM motor, solve this problem of SPM. The IPM motor is a PM motor with permanent magnets embedded in the rotor. Unlike the SPM motor, the torque from the permanent magnets is weakened and the magnetic resistance torque of the rotor is used. This allows high efficiency even in the high speed range. The PRM motor is a motor that further strengthens this directionality. Compared with the torque from the permanent magnets, the magnetic resistance torque is mainly used. It can reduce the use of permanent magnets made of expensive materials such as neodymium, and expand the control range to the high speed range. By the way, the Prius uses the PRM motor.

(Figure 2) is an example of a PRM motor (used in HEV and EV hybrid systems developed by Toshiba). By arranging permanent magnets in a V-shape in the rotor core, a shape with strong magnetic anisotropy is achieved, which can generate large reluctance torque. At maximum torque, the ratio of reluctance torque to permanent magnet torque is 6:4. The efficiency is as high as 97%. The maximum speed is increased to 5 times the base speed.

Progress in the development of back-EMF suppression technology

Currently, motor development is advancing under the demand for further miniaturization and high efficiency. One of them is the technology to suppress the back electromotive force caused by the rotation of the magnet. This technology is expected to achieve high efficiency in the high speed range required by EVs, etc. There are roughly three types of back electromotive force suppression technology currently being developed. The first is the variable magnetic force method, which uses samarium cobalt magnets with variable magnetic force to change the magnetic force of the magnet in the rotating state. The second is the coil switching method, which is to divide the stator coil into two parts, and current flows through both coils at low speeds, and current flows through only one coil at high speeds. The third is the excitation coil method, which changes the current flowing through the coil to change the magnetic flux.

(Figure 3) shows an example of a variable magnetic motor using samarium cobalt magnets. Toshiba uses this motor in a washing machine, achieving high torque at low speeds during washing and high efficiency at high speeds during dehydration. Toshiba has reduced power consumption by 16%. If this technology is introduced into the motor of an electric vehicle, it can achieve higher torque in the low speed range than before and higher efficiency in the high speed range. It can be said that this is an indispensable technology for EVs that run solely on motors.

In a hybrid vehicle, the motor has three main functions: assisting the engine torque, motor driving, and utilizing regenerative energy. Hybrid vehicles use the motor to assist the engine when starting, driving at low speeds, and accelerating when the engine efficiency is poor, which helps improve fuel efficiency. Since the motor driving uses the power stored in the battery to drive, no fuel is consumed. In terms of utilizing regenerative energy, when the engine brakes without pressing the accelerator pedal, and when the brake pedal is pressed to decelerate manually, the motor is used as a generator and the generated power is stored in the battery.

The relationship between vehicle weight and driving torque is 3.3N•m/kg

Table 1: Motor and engine characteristics of electric vehicles from various manufacturers

Theoretically, the motor generates maximum torque when stationary (Figure 4). On the contrary, the torque decreases in the high speed range. The engine generates maximum torque in the high speed range above a certain fixed value. For example, the third-generation Prius generates a maximum torque of 142N•m at 5200 rpm. Hybrid vehicles achieve maximum fuel efficiency by compensating for this difference in torque characteristics between the motor and the engine. Specifically, it uses a motor that can generate high torque at low speed when starting, and uses the driving force of the engine when driving at high speed.

(Table 1) lists the characteristics of the motors and engines used in electric vehicles from Toyota, Nissan, Honda, and Mitsubishi. These manufacturers all use IPM motors that can achieve high efficiency from low to high speed ranges, but the performance of the motors they use varies greatly. Of course, these differences are caused by the different vehicle weights and hybrid design methods. From the motors in the table, the maximum output power and maximum torque have a wide distribution range, with the former ranging from 10kW to 165kW and the latter ranging from 78N•m to 400N•m. Among them, the maximum output power and maximum torque of the motors used in Honda INSIGHT and Fit are lower than those of other models. The reason lies in the design concept of Honda's hybrid system "IMA (Integrated Motor Assist System)". The IMA used by INSIHGT and Fit was developed with the purpose of using the engine as the main engine and the motor as the auxiliary engine, and achieving it at a low cost.

The acceleration performance of a hybrid vehicle when starting is determined by the motor torque and the vehicle weight. (Figure 5) shows the relationship between the two. Compared with the Lexus 600h, which weighs 2,230 kg, the second-generation Prius uses a high-torque motor with a vehicle weight of only 1,290 kg. At first glance, there seems to be no relationship between vehicle weight and motor torque. This is because the torque of the motor itself is not expressed as torque that directly drives the tires. There are various gears between the mechanism that drives the tires and the motor. These gears can increase the motor torque and transmit it to the tires. Considering the speed ratio and reduction ratio of these gears, the relationship between vehicle weight and driving torque can be shown. (Figure 6) shows the relationship between torque and vehicle weight under the conditions of considering the speed ratio, reduction ratio, and reduction gear ratio used by Prius. The coordinate points of all models are almost arranged in a straight line. The inclination of the straight line is about 3.3 N•m/kg.

Car manufacturers use motors with the best performance after considering the speed ratio, reduction ratio and reduction gear ratio. For example, when the Prius transitioned from the second generation to the third generation, a reduction gear was set. A reduction mechanism with a gear ratio of 2.936 was set between the final reducer of the drive tire and the power split mechanism. Through this measure, the motor torque was reduced from 400N•m to 207N•m. The weight of the motor is basically proportional to the torque. Toyota has made the motor of the third-generation Prius small and light by setting a reduction gear.