Power Inverters: What Are They & How Do They Work?

What is an Inverter?

An inverter, or power inverter, is a power electronics converter that changes direct-current electrical power into alternating-current electrical power at the required voltage and frequency. It is used when a DC source must supply an AC load or exchange power with an AC grid.

A solid-state inverter transfers and controls power supplied by another source. It does not create energy, so it is a converter rather than a generator. Its output power is lower than its input power because switching, conduction, magnetic and filter losses produce heat.

A standalone inverter can supply AC loads from a battery or an isolated solar power system. A UPS can use its inverter to draw energy from a battery during a mains failure. A grid-connected solar inverter instead synchronises its AC output with the grid and disconnects or changes operating mode when required by its protection scheme.

HVDC converter stations use high-power valves to convert AC to DC or DC to AC at the ends of HVDC transmission lines. The DC-to-AC operating role is called inversion, but an HVDC station is not normally called a grid-tie inverter; that term usually describes a distributed-energy inverter that operates in synchronism with an AC grid.

How Does an Inverter Work?

To understand how an inverter works, imagine a bulb connected to a battery, creating a closed circuit that allows current to flow through the bulb.

Call the bulb terminals A and B. Connect the battery’s positive terminal to A and its negative terminal to B. Current then flows through the resistive filament in one direction.

Reverse the battery connections and the bulb still glows, but the voltage polarity and current direction through the filament are reversed.

An AC current changes direction periodically. An inverter creates that reversal electronically rather than by physically turning a battery.

Frequency counts complete cycles per second. A 50 Hz output completes 50 positive-and-negative cycles each second, and a 60 Hz output completes 60. A rotating-contact analogy would need 3,000 or 3,600 revolutions per minute if one revolution produced one electrical cycle.

This analogy explains polarity reversal only. A modern solid-state inverter has no rotating switching mechanism.

Modern inverters use controlled semiconductor switches such as IGBTs and MOSFETs. Gate-drive circuits and a controller support the power stage. Current and voltage sensors provide feedback, protection circuits respond to faults, and many designs also include an output filter.

The single-phase full-bridge circuit below shows the basic switching principle.

Four switches connect the two load terminals alternately to the positive and negative DC rails. The controller must prevent the upper and lower switches in the same bridge leg from conducting together.

With S1 and S2 on and S3 and S4 off in the labels used by this diagram, the bridge applies one DC polarity to the load. This produces the positive output state.

With S3 and S4 on and S1 and S2 off, the bridge applies the opposite polarity. For a resistive load, the current also reverses and this produces the negative output state. An inductive load can keep current flowing through a switch or its anti-parallel diode during a commutation interval.

The switching sequence sets the fundamental output frequency. Simple bipolar switching produces a square voltage waveform. Most sine-wave inverters use pulse-width modulation at a higher switching frequency and an LC filter to reduce switching harmonics. Gate drivers insert dead time between complementary switches to prevent a DC-bus short circuit.

Types of Inverter

According to the Output Waveform

There are three types of inverters.

• Square wave inverter
• Modified sine wave inverter
• Sine wave inverter

Square Wave Inverter

A square-wave inverter is a simple topology whose output alternates abruptly between positive and negative levels. Many appliances are designed for a sine wave, so compatibility must be checked before using a square-wave supply.

The square wave is an AC waveform because its polarity reverses periodically, but it has substantial harmonic content. A basic square-wave inverter can use fewer control and filter components than a sine-wave design.

Its harmonics can cause extra heating, audible noise, poor motor torque or incorrect operation in some loads. A manufacturer’s compatibility guidance and the load’s starting current must be checked before connection.

A filter can attenuate switching harmonics, but practical power outputs use passive inductors and capacitors rather than small-signal active low pass filters. Pulse-width modulation makes the required filter more practical than filtering a low-frequency square wave alone.

Modified Sine Wave Inverter

A modified sine-wave inverter, also called a modified square-wave or stepped-wave inverter, adds zero-voltage intervals between the positive and negative output levels. Its waveform is still different from a sine wave.

The stepped waveform can reduce some low-order harmonics compared with a square wave, but it still contains enough harmonic energy to affect motors, transformers, timing circuits and some electronic power supplies.

Its control is generally more involved than simple square-wave switching and less involved than a tightly filtered PWM sine-wave design. Cost and complexity depend on the power rating, protection and output-quality requirements.

Sine Wave Inverter

A sine-wave inverter controls and filters its switched output to approximate a sinusoid at the rated voltage and frequency. Output quality is stated through limits such as total harmonic distortion, regulation and transient response.

Most modern designs generate a high-frequency PWM waveform whose average follows a sinusoidal reference, then use a passive output filter and feedback control to reduce switching ripple.

A sine-wave output does not by itself guarantee the highest efficiency. Efficiency depends on semiconductor losses, switching frequency, magnetic components, load level and cooling. Sine-wave inverters are widely used where equipment compatibility or grid connection requires controlled AC quality.

According to the Type of Load

Inverters can also be classified by the number of AC phases they supply. Two common categories are:

• Single-phase inverter
• Three-phase inverter

Single-phase Inverter

A single-phase inverter supplies one AC phase. Two basic voltage-source topologies are:

• Half-bridge inverter
• Full-bridge inverter

Single-phase Half-bridge Inverter

The historical half-bridge circuit shown below uses two thyristors , S1 and S2, with two feedback diodes , D1 and D2. Modern implementations commonly use transistors with anti-parallel diodes.

Two equal DC-link sections split the supply voltage, allowing the switching node to apply positive or negative half of the DC-link voltage to the resistive load used in this example.

Mode-1

Thyristor S1 is ON and S2 is OFF during this mode. The current flowing path is V/2-S1-B-RL-A-V/2.

For the reference direction in the diagram, load current flows from B to A while a positive V/2 appears across the load, producing the positive output state.

Mode-2

Thyristor S2 is ON and S1 is OFF during this mode. The current flowing path is V/2-A-RL-B-S2-V/2.

For the resistive example, load current flows from A to B while a negative V/2 appears across the load, producing the negative output state.

Single-phase Full-bridge Inverter

A full bridge uses four controlled switches and typically four anti-parallel feedback diodes across one DC source.

In this simplified switching scheme, one half-bridge switch or one diagonal pair of full-bridge switches conducts at a time. Real PWM operation also includes freewheeling and dead-time intervals.

Mode-1

Thyristor S1 and S2 are ON and thyristors S3 and S4 are OFF during this mode. The current flowing path is V-S1-A-RL-B-S2-V.

Current through the resistive load flows from A to B, producing the positive output state.

Mode-2

Thyristor S3 and S4 are ON and thyristor S1 and S2 are OFF. The current flowing path is V-S3-B-RL-A-S4-V.

Current through the resistive load flows from B to A, producing the negative output state.

Three-phase Inverter

A three-phase inverter supplies three output phases with controlled phase displacement. It is common in motor drives, renewable-energy systems and industrial power conversion.

A basic two-level three-phase bridge has six controlled switches, normally with anti-parallel diodes. Classical six-step operation can be described by two conduction schemes:

• 120-degree mode of operation
• 180-degree mode of operation

120-Degree Mode of Operation

Two switches conduct during each 60-degree switching interval. Each switch is commanded to conduct for 120 electrical degrees and remains off for the other 240 degrees of the cycle.

For a balanced star-connected resistive load, the phase voltage is a six-step waveform with an unpowered 60-degree interval, while the line voltage changes among positive, zero and negative DC-link levels.

180-Degree Mode of Operation

Three switches conduct during each 60-degree interval, with one device from each bridge leg. Each switch is commanded to conduct for 180 electrical degrees.

The line-to-line voltages form six-step quasi-square waveforms. With a floating star point, each phase-to-neutral voltage has multiple steps whose levels depend on the switching state and load connection.

Complementary switches in one bridge leg must never conduct together because that would short-circuit the DC source. The controller inserts dead time between one switch turning off and its complement turning on. This requirement applies to both 120-degree and 180-degree schemes.

The 120-degree scheme includes intervals in which one phase is disconnected, but gate-drive interlocking and dead time are still required to handle switching delays and prevent shoot-through.

Applications of Inverter

Some of the applications of an inverter include:

• When mains power is unavailable, an uninterruptible power supply (UPS) can use a battery and inverter to support its protected load.
• A power inverter function at an HVDC transmission line terminal converts DC to AC. Back-to-back HVDC stations can connect asynchronous AC systems.
• A solar inverter converts a photovoltaic array’s DC output to AC and, in a grid-connected system, synchronises and controls that output to meet grid requirements.
• A motor drive uses feedback and control in a close-loop inverter system to vary output frequency and voltage. This controls motors in compressors, rail transport, induction motor speed control systems and electric vehicles.
• An AC-DC front end followed by an inverter can produce the controlled high-frequency AC used for induction heating.

Who Invented the Inverter?

Before the inverter was invented, a motor-generator set and rotary converter were used to convert DC power into AC power.

David C. Prince probably coined the engineering term inverter. His article titled “The Inverter” appeared in the General Electric Review in 1925 and is the earliest known open publication to use the term for a circuit performing the inverse function of a rectifier.

The term rectifier had already been in use for more than two decades. Before reliable electronic power switches, motor-generator sets and rotary converters performed many conversion tasks. Rotary converters configured for DC-to-AC service were sometimes termed “inverted rotaries”.

Vacuum tubes, mercury-arc devices, thyristors and later power transistors expanded the practical voltage, power, frequency and control range of inverters. Modern MOSFET, IGBT and wide-bandgap devices support applications from small UPS units to motor drives, renewable generation and high-voltage grid converters.