Transistors｜Evaluation

The Importance of the Reverse Recovery Characteristics of Switching Elements in Inverter Circuits Types of Inverter Circuits and Energization Methods

2023.11.22

Points of this article

・Inverter circuits can be classified into two main types, single-phase inverter circuits and 3-phase inverter circuits.

・For motor driving, 3-phase inverters are used because of the stable torque obtained and the ability to suppress vibrations and noise.

・Energization methods that can be used when driving a motor with a 3-phase inverter include square-wave driving (120° commutation), sinusoidal driving (3-phase modulation, 2-phase modulation), and others, each with its own advantages and drawbacks.

・Here sinusoidal driving (3-phase modulation), which is often used for motor driving, is taken as an example.

This is the first article “Types of Inverter Circuits and Energization Methods”.

■Types of inverter circuits and energization methods
■Basic operation of 3-phase modulation inverter circuits
■Comparison of losses in a PrestoMOS™ MOSFET and a standard SJ MOSFET using double-pulse tests (actual measurement results)
■Comparison of efficiency of a PrestoMOS™ MOSFET and a standard SJ MOSFET in a 3-phase modulation inverter circuit (simulations)

Types of Inverter Circuits and Energization Methods

Inverter circuits can mainly be classified into two types, single-phase inverter circuits and 3-phase inverter circuits. Fig. 1 and 2 show the circuit diagram and an approximate waveform of the output current for a single-phase inverter circuit. The single-phase inverter circuit converts a direct current into a single-phase alternating current, and so is used in power conditioners and uninterruptible power supplies (UPSes) which assume commercial power supplies in general households.

Next, Fig. 3 to 5 show the circuit diagram and approximate waveforms of the output current for a 3-phase inverter circuit. Fig. 4 is the current waveform for sinusoidal driving (180° commutation), and Fig. 5 is the current waveform for square-wave driving (120° commutation). The 3-phase inverter circuit is configured to convert direct current into 3-phase alternating current; such circuits are used to drive motors in air conditioner compressors, in electric automobiles, and in other applications.

When driving a motor, either a single-phase or a 3-phase inverter circuit can be used. However, due to its design, a single-phase inverter circuit has intervals in which the output current always goes to zero (see Fig. 2). As a result, the motor torque fluctuates considerably, resulting in increased motor vibrations and driving noise. On the other hand, a 3-phase inverter circuit employs a control method that ensures that current is always flowing in one of the phases (see Figs. 4, 5), so that output current fluctuations are small compared with a single-phase inverter circuit, and the motor torque is stable, so that vibrations and noise are suppressed. For these reasons, in general 3-phase inverter circuits are used for motor driving.

Energization methods or modes used by a 3-phase inverter to drive a motor include square-wave driving (120° commutation) and sinusoidal driving (180° commutation/3-phase modulation, 2-phase modulation), among others.

In 120° commutation like that seen in the current waveform of Fig. 5, over a 180° half-wave interval, switching occurs only for 120°, so that compared with sinusoidal driving, switching losses can be reduced. However, because phase currents have a square-wave shape, high harmonics are increased, and so there is the disadvantage that motor efficiency is worsened.

In the sinusoidal driving (180° commutation) of Fig. 4, because phase currents approximate the fundamental frequency, high harmonics can be reduced, with the advantage that motor efficiency can be improved. However, switching is performed over the entire 180° half-wave interval, so that switching losses are increased compared with square-wave driving.

Table 1. Energization methods and their features

	Square-wave driving（120° commutation）	Sinusoidal driving （3-phase modulation）	Sinusoidal driving （2-phase modulation）
Switching loss	Low	High	Medium
High harmonics in output AC	Large	Small	Small
Motor efficiency	Low	High	High
Control	Easy	Somewhat difficult	Difficult

In this section, the explanations will use as an example sinusoidal driving (3-phase modulation), which is widely employed in motor driving.

Transistors

Basic

Evaluation