The main components of the HEV system: motor and inverter

The main components of the HEV system include motors, inverters, power supply systems, high-voltage auxiliary systems, etc. Most vehicle-mounted motors use AC motors, which have strict requirements on small size, light weight, high output power, and high speed. The inverter is composed of power components, capacitors, and control circuits. This article will explain the characteristics and performance requirements of the two.

The driving heart of the hybrid electric vehicle (HEV) is the motor. First, let's look at the characteristics of the motor.

The motor is an important component that provides driving force for HEVs and electric vehicles (EVs). The motors used in passenger cars generally have an output power of about 10k to 60kW. Although EVs and small EVs modified from commercially available vehicles use DC motors, AC motors are still the mainstream (Table 1).

Permanent magnet motors are widely used

The performance requirements for HEV motors include high output power, high speed, high voltage, high efficiency for fuel efficiency, and maintenance-free driving systems. Table 2 lists the advantages and disadvantages of DC motors and AC motors in terms of requirements.

AC motors have advantages in supporting high voltage and maintenance, and small and efficient permanent magnet motors are widely used. Permanent magnets have achieved significant miniaturization and high output power by using rare earth magnets such as neodymium.

Figure 1: Rotor structure of SPM and IPM
According to the configuration of magnets, it can be divided into surface magnet type (SPM) and internal magnet type (IPM).

Figure 2: Utilization of reluctance torque
Most HEVs use IPMs that can utilize reluctance torque.

Permanent magnet motors can be divided into surface magnet type (SPM) and internal magnet type (IPM) according to the different magnet installation positions (Figure 1). Most HEVs use IPM (Figure 2) motors that are easy to achieve high speed and can utilize magnetic resistance torque. SPM motors are mainly used in electric power steering wheels and other aspects with vibration noise problems. However, when permanent magnet motors use a large number of rare earth magnets, there are issues in terms of cost and stable supply.

Figure 3: HEV motor
uses an IPM rotor.

Figure 3 is an example of a motor for HEV. Although it is not possible to tell from the photo, this motor uses an IPM rotor.

Overall structure and function of the inverter

The role of the inverter is to convert DC power into three-phase AC power using six power elements bridged with the DC power supply of the main battery to supply power to the motor (Figure 4). Here, the power element refers to a combination of IGBT (insulated gate bipolar transistor) and diode (flying diode). Its control principle is as follows.

First, the HEV and ECU calculate the required drive torque based on the accelerator opening command that shows the amount of accelerator pedal operation by the driver, and issue a drive signal to the IGBT. At this time, the torque obtained based on the relationship between the voltage phase and the rotor position is not fixed, so it is necessary to determine the timing of power on based on the premise that the maximum torque can be obtained by detecting the rotor position. The drive of

the IGBT uses PWM (pulse width modulation) control, and the working method is to output a three-phase AC with a variable voltage sine wave from the power element to control the drive torque.

Principle of generating 3-phase

AC The principle of generating a 3-phase AC with a variable voltage sine wave is described below. Comparing the sine wave voltage command and the triangular wave with a phase difference of 120 degrees in Figure 5, the on/off signals of the two IGBTs located in each phase of U/V/W are formed as shown in Figure 4, and the phase voltage VU/VV/VW will be converted into a sinusoidal pulse voltage with a phase difference of 120 degrees (the average voltage change of each pulse is sinusoidal).

Due to the change in the amplitude of the voltage command, the on/off ratio of the pulse will change, and the voltage value will change accordingly. So far, the current only flows through the motor when it is on, and flows through the parallel diode when it is off. In this way, a continuous sinusoidal current can be passed to the motor. In addition, although increasing the frequency of the triangular wave can suppress the electromagnetic noise and current ripple of the motor, it will increase the loss of the power component, so the frequency is usually set to 5k~10kHz.

Figure 4: Inverter structure
It consists of 6 power components (IGBT and diode) and capacitors.

Figure 5: Principle of three-phase AC generation
First, the sine wave voltage command and the triangle wave with a phase difference of 120 degrees are compared to generate the on/off signal of the IGBT.