Power Inverters: The Need-to-Know Essentials

Key Takeaways

• Learn basic inverter input and output schemes

• The variety of inverters and how they’re created

• Uses for consumer power inverters and additional applications

Power inverters are useful devices for converting DC to AC power.

Inverters are devices that transform direct current (DC) to alternating current (AC). They take power from the DC source and convert it to electrical power; they do not create any additional power and are therefore not generators. The input and output voltage and frequency are specific to each individual inverter and their designed task. Inverters used in applications with high currents and voltage are known as power inverters. Inverters used in applications with low currents and voltages are known as oscillators. Circuits that do the opposite–convert AC to DC–are called rectifiers. In this article, we’ll be doing an in-depth dive into inverters and their functions, design, and applications.

Inverter Functionality: Input and Output

Inverters have a DC input, a specific frequency, and AC voltage level–depending on their designed load.

Inverters use a stable DC power source as an input. Common input values range from low voltage to hundreds, depending on the inverter design. For example, 12 V DC input is common for consumer and commercial inverters that are powered from rechargeable lead acid batteries or other automotive electrical outlets. Home energy systems may use 24, 36, or 48 V DC, whereas inverters in photovoltaic solar panels may have 200 to 400 V inputs. Some inverters reach hundreds of thousands of volts in high-voltage direct current transmission systems. 

Inverters output an AC signal that is typically either a sine wave, square wave, or modified quasi-sine wave, depending on the application. Inverter signal outputs that aim to replicate mains power are commonly 50 or 60 Hz at 120 or 240 VAC to match standard power line frequencies and voltage. In cases where the output needs to be further processed or stepped up, the output frequency may be much higher for transformer efficiencies. 

Inverter power limits have their rating expressed in watts or kilowatts. This value describes the amount of power that the inverter can drive and the amount of power thus required by the DC source (without including inefficiencies). 

Inverter Loads

Inverters are used to power loads that require AC power. The two types of AC loads are single-phase and three-phase loads, so there are inverters designed for each type of load.  

Inverter Waveforms and Designs

Generally, inverters produce one of three waveforms: square, sine, or quasi-sine.

1. Square wave inverters are the least used but simplest to design. The square wave output can be used to power various home appliances or other equipment. Powering devices with square wave AC power can result in more inefficiency and loss. 
2. Often used in residential commercial applications, sine wave inverters are the most efficient type, but they are complex and costly to design. They generate a pure sine wave, similar to that used in grid power. Sine wave inverters have the least amount of losses. 
3. Modified sine wave or quasi-sine wave inverters generate a series of steps that resemble a sine wave but are not smooth. The most basic is a sum of two square waves delayed by a quarter period, resulting in a wave with three levels: peak negative, zero, and peak positive. Many inexpensive consumer power inverters create a modified sine wave rather than a pure one. These fall between square and sine wave inverters in terms of complexity and efficiency. 

Inverter Design 

Inverters are designed depending on the desired load and AC waveform of interest.

Square Waves

Structure of an H-bridge (highlighted in red). From Wikipedia.

For a single phase load, a half bridge inverter or full H-bridge design can be used in the inverter. With a half bridge configuration, two transistors are used on each side of the load. This works with an electronic toggle switch, such as a MOSFET, and is not able to switch the voltage polarity applied to the load.

For a full bridge configuration, as shown in the image above,  switches are on in pairs. S1 and S4 result in +V_IN across the load, whereas S3 and S2 -V_IN are powered across the load.

Inverter design used for three-phase loads. From Wikipedia.

Three phase inverters are generally used for variable-frequency drive applications such as motor driving. They consist of three-single phase inverter switches and each one is connected to one of the three load terminals, as shown in the image above.

Modified Square Wave and Quasi-Sine Wave

With a 3-phase inverter, there is a 6-step (23-2 states) switching sequence, resulting in 3 possible voltage levels across 2 arbitrary ports of the load.

Two six-step three-phase inverters connected in parallel will result in a higher current rating. Connecting them in series will result in a higher voltage rating. The output waveform shape is also improved, resembling more of a sine wave with increasing additional inverters. 

Pure Sine Wave 

Blue PWM wave used to create filtered sine wave (in red). From Wikipedia.

A good option for creating a sine wave is through a PWM and low pass filter. Switching must take place at a significantly faster rate than the desired frequency of the output wave, but the faster the switching, the more closely the harmonic content can be controlled. In this method, using a microcontroller capable of extremely fast switching and a feedback loop allows for a good approximation of sine waves.

Other methods of producing sine waves include using an oscillator architecture with filtering or an oscillating crystal with post-processing. 

Consumer Power Inverters

A power inverter that connects to the DC port of a vehicle.

Power inverters can be purchased as standalone devices for a variety of consumer needs.

Home Power Inverters

In case a building loses power, an inverter can help keep necessary appliances running. Smaller setups may involve a car battery attached to an inverter. The inverter converts power to AC that is then used by appliances, depleting the battery. In this case, keeping the car running will keep the battery charged, acting as a generator. Directly hooking up two 12V car batteries to an inverter can also be used in emergency cases. 

Portable Inverters for Larger Vehicles

Portable inverters can be used with larger vehicles such as trucks, boats, RVs, and more. A permanently-mounted inverter wired directly to a battery allows for users to run their computer and other larger appliances easily. 

Power Inverters for Industry

Inverters can also be used for off-grid job sites and other locations with limited access to power. These inverters run off batteries that are larger than a vehicle’s main battery. Power tools oftentimes require short periods of intense power during startup and use. In this case, using a suitable heavy duty inverter capable of providing output power to handle these surges is a good idea.

Inverters for Sensitive Electronics

Some devices such as ATMs, special computers, networking equipment, and specific audio and video devices may require microprocessor-controlled pure sine waves. This ensures the equipment runs cool and lasts longer without inefficiencies caused by quasi-sine waves. In this case, look for inverters that provide a pure sine wave output. 

Inverter Applications

Inverters are used in a variety of applications and are built into a lot of consumer devices. 

• As we’ve discussed, they can be used in a standalone fashion in conjunction with a DC power source (such as a battery) to act as an AC source to fuel AC equipment designed for mains operations. This can be used to create an uninterruptible power supply (UPS) if mains power shuts down. When the power is restored, a rectifier can then recharge the battery. 
• Inverters are also used for induction heating. AC mains power is first rectified to DC power, and then the inverter converts it to high frequency AC power used for induction cooking.
• Inverters are used in electroshock weapons to take energy from a 9V DC battery to hundreds or thousands of AC volts with a high frequency transformer. This is then rectified and stored in a capacitor until it is triggered when charge is dumped into a pulse transformer, stepping the final voltage up to 20 ~ 60 kV.
• Solar inverters convert the variable DC output of photovoltaic solar panel cells into AC that is then fed into a commercial electrical grid. Solar inverters are either stand-alone inverters, grid-tie inverters, battery backup inverters, or intelligent hybrid inverters. 
• Inverters are also used in motor speed controllers for motors that require AC power. Motor speed control includes industrial motor driven equipment, electric vehicles, rail transport systems, and other power tools.