What Is an Inverter? DC to AC for Air Conditioners and EVs
An inverter is a device that turns direct current (DC) into alternating current (AC) at a frequency and voltage you choose. Inside, semiconductor switches turn on and off about a thousand to tens of thousands of times a second. By changing how long each switch stays on, the inverter shapes the wave. Air conditioners and electric cars use this to change how fast their motors turn.
What you'll learn
- What an inverter converts, and into what
- Its job in air conditioners, electric cars, solar power and backup power supplies
- How four switches flip the direction of current, and how PWM shapes the wave
- Why changing speed saves energy, and when it does not
- How it differs from converters and rectifiers, plus notes on noise, heat and safety
Prerequisites: the basic difference between DC and AC ("AC vs DC Explained"). It helps to know that a power semiconductor is "a switch that turns large currents on and off fast" ("What Is a Power Semiconductor?").
In everyday life: from air conditioners to backup power
In appliance stores, you may see an "inverter air conditioner (variable-speed)" or an "inverter refrigerator." According to a Daikin spokesperson, 100% of home air conditioners in Japan, the EU and Australia have an inverter. In the United States and some other places, only some do (Weathernews interview, August 2024). In Japan, for example, almost every air conditioner has an inverter, even if the buyer never looks for one.
Inverters also work outside home appliances.
- Electric and hybrid cars: they turn the battery's DC into AC to drive the motor that moves the car
- Solar power: it turns the DC made by the panels into AC that a home can use. This device is called a solar inverter, or in Japan a "power conditioner"
- Uninterruptible power supply (UPS): during a power outage, it turns a battery's DC into AC and keeps computers and servers running
- Trains: they make AC suited to the motors from the power taken from the overhead wire
Some UPS designs always turn the incoming power into DC first, then back into AC with an inverter. With this design, the power does not break even during an outage. Designs that switch to the battery only after an outage break for about one hundredth of a second (explainers by ITmedia and ASCII.jp).
In every example, either only DC is available but AC is needed, or the speed of the AC needs to change.
What an inverter does: from DC to "AC made to order"
Direct current (DC) always flows in the same direction, like the power from batteries and solar panels. Alternating current (AC) changes direction back and forth at a steady rhythm. The number of back-and-forth cycles in one second is called the frequency. In Japan, for example, wall outlets use 50 or 60 cycles a second.
An inverter takes in DC and makes AC. Its key feature is that it can choose the frequency and the size of the voltage to suit the device it powers. A wall outlet has a fixed frequency. An inverter can make 10 cycles a second, or more than 100.
Turning AC into DC is sometimes called forward conversion. Turning DC into AC is the reverse, so it is called inversion. "Inverter" is the name for a device that does inversion (Panasonic's glossary). This site's glossary also explains the inverter in more technical detail.
How it works, part 1: four switches flip the direction of current
The basic idea behind turning DC into AC is very simple. You just flip the connection between a battery and a motor, fast. Instead of turning the battery around by hand, you use four switches controlled by electrical signals.
As in the figure below, the four switches are arranged in the shape of the letter H, with the motor in the crossbar. This layout is called an "H-bridge" or "full bridge."
Turn on S1 at the top left and S4 at the bottom right, and current flows through the motor to the right. Next, turn on S2 at the bottom left and S3 at the top right, and it flows to the left. Repeat this over and over, and you get electricity that goes back and forth: AC. Change the pace of the flipping, and you change the frequency too.
The switches are power semiconductors that can handle large currents and high voltages. Examples include the IGBT, the power MOSFET and the SiC (silicon carbide) MOSFET. "What Is a Power Semiconductor?" and "IGBT vs Power MOSFET" explain how they differ. Here, just think of them as "switches that an electrical signal turns on and off very fast."
There is one state that must be avoided at all costs. The top and bottom switches in the same column, such as S1 and S2, must never be on together. That would connect the plus and minus sides of the power source through the switches alone. A large current would flow and destroy the parts. A switch takes a moment to turn fully off. So at each changeover, both the top and bottom switches stay off for a very short wait. This wait is called dead time.
A motor cannot stop its current suddenly. So each switch has a part beside it that lets current pass in one direction only: a diode. The diode gives the current an escape path the instant the switch turns off ("What Is a Diode?").
How it works, part 2: chop finely to get close to a smooth wave
Flipping the direction alone makes a square wave. The voltage jumps between "full plus" and "full minus." The AC from a wall outlet is a sine wave that rises and falls smoothly. To run a motor quietly and without waste, a shape close to a sine wave works better.
So the switches chop the power much more finely. During one cycle of the AC, they turn on and off tens to hundreds of times. Near the peak of the wave, each on-time is long. Near zero, it is short. The averaged voltage then rises and falls, tracing the shape of a sine wave.
Setting the average voltage by the length of the on-time, or pulse width, is called PWM (pulse-width modulation). In an inverter, this width keeps changing over time to follow the shape of the wave.
There are several ways to choose the widths. A typical one compares the target sine wave with a much faster triangle wave. While the sine wave is higher, the plus-side switch is on. While it is lower, the minus-side switch is on. This method is called sinusoidal PWM (we only give the name here).
The actual voltage is still a row of on-off pulses. Even so, the current through the motor becomes smooth. The motor's wound wire, the coil, resists fast changes in current more than slow ones (see "AC vs DC Explained"). The fine jagged parts are smoothed out, and the slow sine-wave change remains in the current.
This is also how the frequency and voltage are chosen. Speed up the rhythm of widening and narrowing, and the frequency goes up. Make the widths wider overall, and the voltage goes up; make them narrower, and it goes down. A control chip, such as a microcontroller, works out which switch to flip and when ("What Is a Microcontroller?").
How it works, part 3: six switches make three-phase AC
Many motors, such as those in electric cars and air conditioner compressors, take power through three wires. A compressor is the machine that squeezes the cooling gas. The three wires carry the same AC wave, each shifted by one third of a cycle. This way of supplying power is called three-phase AC. The three waves grow strong in turn, which creates a rotating magnetic pull inside the motor.
A three-phase inverter has three columns, each with a top and a bottom switch: six switches in all. Each column uses PWM to make the voltage on one wire. Shift the timing of the three columns' waves by one third of a cycle each, and you get three-phase AC.
The speed of the rotating magnetic pull is set by the frequency of the AC. Halve the frequency, and the motor turns at roughly half the speed. The induction motor, a type of motor common in factory pumps and fans, behaves this way too. However, lowering only the frequency lets too much current flow into the motor. So the basic rule is to lower the voltage along with the frequency (this is simplified).
In an electric car, the inverter changes the frequency and voltage moment by moment to match how far the accelerator is pressed. The flow of power back into the battery during braking is shown in a figure in "What Is a Power Semiconductor?"
Why changing speed saves energy
An air conditioner without an inverter ran its compressor at one fixed speed. When the room got too cold, it stopped. When the room warmed up, it ran at full power again, over and over. With an inverter, it can run fast at first to cool the room quickly, then keep running slowly. Toshiba was the first in the world to mass-produce this type of air conditioner: for businesses in 1980 and for homes in 1981. The IEEE, the international professional society for electrical engineering, recognizes it as a historic achievement.
Stopping and starting wastes energy. According to Daikin, an air conditioner uses a lot of power right after it starts (2016 announcement). That is when the room is far from the set temperature. If the speed can be finely adjusted, the unit runs at full power less often. The temperature also swings less. In 2023, Daikin showed a high-efficiency model at COP28, the UN climate change conference. The company says it can cut power use by 50% or more compared with models without an inverter.
For fans and pumps, slowing down has an even bigger effect. For these, the power needed is roughly proportional to the cube of the speed (the speed multiplied by itself three times). A U.S. Department of Energy document says that cutting speed by 20% cuts the power needed by about half. Multiplying 0.8 three times gives about 0.51, which matches (our own calculation).
Here is why it is a cube, greatly simplified. When a fan turns more slowly, the amount of air it moves each second drops in proportion to the speed. At the same time, the pressure pushing the air out drops with the square of the speed. The power needed is "air flow × pressure," so together it drops with the cube of the speed.
Air conditioner compressors, however, do not save as much as fans. The same document says that for compressors, which squeeze air, power is roughly proportional to speed. One major reason air conditioners save energy is the one we saw earlier: less waste from "stopping, then running at full power."
Heat and losses: the switches get a little warm too
An inverter cannot pass on all the power it takes in. The lost part becomes heat. In 2017, Hitachi announced a solar inverter (power conditioner) with a conversion efficiency of 96.5%. This means it sends out 96.5% of the power it takes in. If it sends out its rated 5.5 kilowatts at this efficiency, about 0.2 kilowatts becomes heat (our own calculation).
The first kind of loss comes from a small resistance that remains in a switch even when it is on. Heat is made the whole time current flows. This resistance is called on-resistance, and the loss it causes is called conduction loss.
The second kind is switching loss. It happens in the brief moment while a switch changes between on and off. During the change, the switch has both voltage across it and current through it.
This creates a design dilemma. The more finely you chop, the smoother the current and the quieter the motor. But every switching causes some loss, so more switching means more heat. SiC and GaN parts have low switching loss, so they ease this dilemma and are getting attention ("What Is GaN (Gallium Nitride)?").
Sorting out the terms: converters and rectifiers
| Conversion | Common names | Examples |
|---|---|---|
| AC → DC | Rectifier, converter (forward conversion) | Chargers, the input stage of an air conditioner |
| DC → AC | Inverter (inversion) | Driving motors, solar power |
| DC → DC at a different voltage | DC-DC converter | Inside computers, electric cars |
"Converter" is a broad word for "a device that changes the form of electricity." Turning AC into a one-way flow has its own name: rectification.
An inverter air conditioner actually contains both. First, a converter turns the AC from the outlet into DC. Then an inverter rebuilds it as AC at any frequency to drive the compressor motor. So when an air conditioner or refrigerator is called an "inverter" model, it is a variable-speed product: its motor speed can be changed this way. An "inverter" light is different. It raises the frequency of the power sent to the lamp, for a different purpose.
"Converter vs Inverter" sums up the differences in a table. A logic circuit that flips 0 and 1 is also called an inverter, but it is a different thing that just shares the name ("What Is a Logic Gate?").
Sound, electrical noise and safety
A motor driven by an inverter sometimes gives off a high-pitched whine. The iron parts of the motor vibrate slightly in time with the switching rhythm. In Japan, for example, some Keikyu railway trains made this sound like a musical scale when starting off. People called them "singing trains." The sound came from Siemens equipment on trains such as the 2100 series, introduced in 1998. It could no longer be heard after summer 2021.
Switching more often than people can hear, above about 20,000 times a second, makes the sound quieter. However, it increases the electrical noise that affects nearby equipment (Johnson Controls installation guide). More switching also means more loss. To prevent problems such as static on radios, inverters have built-in parts that suppress noise, called filters.
For safety, take care even after the power is off. An inverter contains a large capacitor that stores DC ("What Is a Capacitor?"). After the power is turned off, a high voltage may remain for a while. The manual from German maker SEW-EURODRIVE says to wait at least 10 minutes after turning off the power. Each product's manual states how long to wait. Do not open home appliances and touch the inside; leave repairs to professionals.
Common myths
Myth: An appliance with an inverter always saves energy, however you use it
In fact, it saves energy when it can run at reduced speed for long stretches. A machine that always runs at full power gains nothing from changing speed. Its power use may even rise slightly, by the inverter's own loss of a few percent. With air conditioners, too, the way you use them matters. In Daikin's 2016 test, for a daytime outing of about 30 minutes, leaving it on cost less than turning it off. But Daikin says that leaving it on all the time backfires. Ads stress "inverter = energy saving," so people tend to assume it always pays off.
Myth: The AC an inverter makes is "cleaner" than the AC from a wall outlet
In fact, an inverter puts out a voltage made of on-off pulses in a row. In a motor drive, it is the current that is smooth, after the coil averages it. Some devices have proper parts to smooth the pulses and offer "sine wave output." But some cheap devices put out square waves or stepped waves. Schneider Electric warns that some equipment cannot work with a UPS that puts out square waves. "Electricity made by electronics" sounds high-tech, so people tend to think it beats the original.
By the numbers
| Item | Number | Note |
|---|---|---|
| Share of home air conditioners with an inverter | Japan 100% | Same in the EU and Australia (Daikin spokesperson, 2024) |
| Power when a fan runs 20% slower | About half | Theoretical guide (U.S. Department of Energy) |
| Power when a fan runs at half speed | About 1/8 | 0.5 multiplied three times (our own calculation) |
| Conversion efficiency of a solar inverter (power conditioner) | 96.5% | Hitachi, product announced in 2017 |
| Heat when it sends out 5.5 kW | About 0.2 kW | Our own calculation |
Inverters are widely used in fans and pumps because slowing down just a little cuts power use a lot.
Learn more
- Compare in a table: Converter vs Inverter, IGBT vs Power MOSFET, SiC vs GaN
- Terms for the next step: inverter (detailed explanation), sinusoidal PWM, dead time, gate driver (the circuit that operates the switches), three-phase AC, induction motor
- Try the numbers: the DC-DC converter calculator (share of on-time and average voltage), the junction temperature calculator (loss and part temperature)
- Related articles: "What Is a Power Semiconductor?", "How Do Solar Cells Work?", "How Lithium-Ion Batteries Work"