An inverter is a converter that transforms direct current into alternating current, typically through controlled electronic switching.
So far we have seen the most common path in a power supply: starting from alternating current, rectifying it and filtering it to obtain direct current. Now we are going to take the opposite journey.
This is what we need in a UPS, in an off-grid solar installation, in a variable frequency drive, or in an electric car. We have a battery or a DC bus and we want to obtain an alternating voltage capable of powering a load.
A real inverter can work with dangerous voltages and currents. Here we are explaining the operating principle, not building equipment connected to the grid or 230V on the home workbench.
The Basic Idea
Direct current has a fixed polarity. One terminal is positive and the other negative. Alternating current, on the other hand, changes polarity periodically.
So the problem of an inverter is quite straightforward:
We need to make the load see one polarity first and then the opposite one, many times per second.
The simplest way to achieve this is to use electronic switches (MOSFET or IGBT transistors) to change the connection of the load.
The H-Bridge
The classic circuit for reversing polarity is the H-bridge. It is called this because, drawn on a schematic, it has a shape similar to an H.
It has four switches:
- Two connect the load to the positive terminal.
- Two connect the load to the negative terminal.
- Closing one diagonal, the current flows in one direction.
- Closing the opposite diagonal, the current flows in the opposite direction.
If we alternate these diagonals, the load sees a voltage that changes sign. We now have alternating current, although still quite rough.
The two switches on the same leg must never be closed at the same time. This connects positive and negative directly and causes a short circuit.
Square Wave
The simplest inverter generates a square wave. During half a cycle it applies +Vdc to the load, and during the other half cycle it applies -Vdc.
It works, it does. In fact, some very simple resistive loads can be powered this way. But a square wave has a major problem: it is full of harmonics.
These harmonics cause motors, transformers, and some power supplies to overheat, vibrate, or make noise. In short, the electrical grid doesn’t look like that by chance.
PWM to Approximate a Sine Wave
To generate something closer to a sine wave, we use PWM (Pulse Width Modulation).
The idea is to turn the transistors on and off very fast. If we want a high instantaneous voltage, we leave the pulse on for a longer time. If we want low voltage, we leave it on for less time.
The load does not see a perfect sine wave every microsecond. It sees a sequence of pulses. But if the switching frequency is high, and we filter well, the average result closely resembles a sine wave.
This is why many modern inverters are called “pure sine wave.” It doesn’t mean there is a mysterious sine wave stored inside a little box. It means they generate a very good approximation through switching and filtering.
The Output Filter
After the H-bridge, an LC filter usually appears, made up of inductors and capacitors.
Its job is to let the low-frequency component (50Hz or 60Hz) pass and attenuate the high-frequency switching of the PWM.
This achieves several things:
- Reduces ripple and high-frequency harmonics.
- Delivers a smoother voltage to the load.
- Decreases electromagnetic interference.
The filter is not just for show. In power electronics, transients, spikes, and interference are part of the real problem.
Single-Phase and Three-Phase Inverters
A single-phase inverter can use an H-bridge to generate a single alternating voltage.
A three-phase inverter uses three switching legs, one per phase. By controlling the PWM of each leg with a 120-degree phase shift, it can generate a complete three-phase system.
This is exactly what a variable frequency drive does: it starts from a DC bus and generates three phases with controlled frequency and voltage to regulate the speed of a motor.
The idea is this: an inverter does not “create” alternating energy from nothing. It rearranges the DC energy over time so that the load sees an alternating voltage.
Where They Are Used
Inverters appear in many modern systems:
- UPS/Battery Backup: power equipment when the grid fails.
- Photovoltaics: convert DC energy from panels or batteries into usable AC. If they work connected to the grid, they must synchronize with it and incorporate the required protections and certifications.
- Variable Frequency Drives: control industrial motors.
- Electric Vehicles: power AC motors from batteries.
- Off-Grid Systems: generate a local grid from accumulators.
In all cases, the principle is the same. The power, waveform quality, protections, and control change, but the foundation remains switching, modulating, and filtering.
