Tech Tips

What kind of technology is an inverter?

What kind of technology is an inverter?

Power Inverter circuit and power inverter device

The term "inverter" basically refers to a circuit that converts DC current into AC current (power inverter circuit), but it can also refer to a power inverter device, especially in home appliances such as air conditioners and washing machines.

Home appliances are not the only ones that benefit from power inverter devices. The reason why elevators and conveyors do not accelerate or stop suddenly is because the acceleration of the motor is well regulated. Power inverter devices help to adjust the speed of the motor.

What kind of technology is a power inverter device? Let's take an air conditioner as an example. An air conditioner without an inverter can only suspend operation when it gets too cold and resume operation when it gets too hot. This is very inefficient, as the indoor temperature is not stable and power consumption is high.

On the other hand, an air conditioner equipped with an inverter turns the motor at high speed to rotate the fan when it starts cooling, and when the temperature approaches the setpoint, the fan is slowed down to continue operation with a gradual change. The result is less wasted motion and more energy-efficient operation than an air conditioner that only turns on and off.

This is why the word "inverter" is often used in the field of home appliances. In recent years, inverters have also played an active role in the increasing number of induction cookers, which use AC currents with extremely high frequencies ranging from 20 kHz to 90 kHz. Induction cookers use AC currents of extremely high frequency, ranging from 20kHz to 90kHz, which can be used by changing the frequency with a power inverter device.

The ability to flexibly change the rotation speed and acceleration of the motor to suit the application is very useful. So why does a power inverter device allow us to flexibly change the rotation speed of a motor?

Frequency and Rotation Speed

Power inverter devices are often used to change the AC current from an outlet to a desired frequency or voltage.

The voltage and frequency supplied from the outlet is determined as 100V, 50Hz for eastern Japan, and 100V, 60Hz for western Japan, and the rotation speed of the motor is determined by the frequency.

The motor speed is determined by the frequency.

Power Inverter device configuration

How a power inverter works

How does an inverter change the frequency? An inverter consists of three elements: a converter circuit that converts AC current into DC current, a capacitor, and a power inverter circuit.

First, the converter circuit converts the AC into DC, and then repeatedly charges and discharges the capacitor to create stable DC. Next, the power inverter circuit converts the DC into AC at a desired frequency and voltage for output.

Principles of power inverter circuits

In this section, we will explain the principle of power inverter circuit operation based on the circuit diagram with four switches as shown in the figure below. A power inverter circuit converts direct current into alternating current.

When a DC power supply is connected to the circuit and switches (1) and (4) are turned on, switches (2) and (3) are turned off, and when switches (1) and (4) are turned off, switches (2) and (3) are turned on, repeating in pairs in a fixed cycle, the direction of the current flowing to the load is switched and output in positive and negative voltages, creating an AC current. This technology is called "switching technology.

A circuit using switching technology can also convert AC into DC. If switches (1) and (4) are closed when the voltage is in the positive direction, and switches (2) and (3) are closed when the voltage is in the negative direction, the current will always flow in the same direction to the load.

Principle of power inverter circuit
Figure 1: Principle of an power inverter circuit

Power inverter Circuits and Smart Grids

With the development of IT and the widespread use of IoT, the concept of a "smart grid" has been proposed to supply and consume electricity more efficiently. The smart grid is a concept that uses IT to understand real-time power consumption and concentrate power transmission near it.

In order to make smart grids possible, the latest technologies such as electric vehicles and solar cells are needed. Here too, inverter circuits are expected to make further contributions.

Electric cars run on electricity from the batteries installed in the car to drive the motor. However, since the electricity obtained from the battery is DC current, it needs to be converted to AC current in order to run the motor efficiently. This is where inverters come into play.

The same applies to mega solar power plants. Since the electricity produced by solar cells is DC current, it is converted to the same frequency and voltage as that of commonly used electricity and sent out.

Mega Solar

Principle of switching power supply

The principle of inverters, which can output power at will by operating and controlling switches at high speed, is also used to make converters more energy efficient and compact. Converters that use switches to convert power are called "switching power supplies".

Electronic devices require a stabilized direct current with little voltage fluctuation. In the past, a type of "AC adapter" called a linear power supply was used to convert household AC power to DC at a lower voltage. The adapters for game consoles and laptops are probably the most obvious examples.

A linear power supply first reduces the voltage of incoming electricity through a transformer. Then, through a circuit of silicon diodes, the alternating current was rectified to one side, smoothed by a capacitor, and on its way to the circuit of the electronic device.

Because of the large iron core used for this transformer, conventional AC adapters were large and heavy.

Conventional AC adapter

Converters that use switching circuits have changed this significantly. In a converter using a switching circuit, the voltage is passed through the switching circuit instead of being reduced at the beginning.

The direction of the AC current is adjusted by the switching circuit, and then the voltage conversion is done. However, in general, rectified electricity cannot be used for voltage conversion by a transformer.

Therefore, a semiconductor element is passed through to convert it into a pulse wave with a constant voltage flowing intermittently. This creates a pseudo alternating current, which can be converted to a voltage by a transformer. Since the frequency at this time is high, ranging from several tens of kHz to several hundred kHz, it has the advantage of reducing the size of the transformer. This results in a compact and light converter.

The latest technology in power inverters

The latest trend in inverters focuses on power devices. Power devices are a category of circuit elements made of semiconductors and can supply power as semiconductors in inverters and converters. Power devices capable of switching include power transistors and thyristors.

Power devices are characterized by high current capacity and voltage tolerance, low heat generation, and good heat dissipation, but they also suffer from power loss, such as part of the power escaping as heat when power is applied. Although recent improvements have reduced the loss, it is said that physical improvements are unlikely.

In response to this, the development of power devices using new materials that conduct electricity more easily and cause less power loss than silicon, the material currently used for semiconductors, has been promoted, and silicon carbide (silicon carbide) and gallium nitride (gallium nitride) is attracting attention.

Although there are still many problems, such as high manufacturing costs, the range of applications for power inverters will continue to expand as more complex and sophisticated circuits are built.