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How does an inverter work?

How and what does an inverter take control of? A brief explanation to grasp the basic structure.

Starting off from the converter circuit and inverter circuit to have a proper understanding of the inverter device

We'll start the introduction by explaining the inverter device's mechanism in detail. The inverter device's role is to control the voltage and frequency of the power supply and seamlessly change the rotation speed of motors used in home appliances and industrial machineries.

The first thing to keep in mind when it comes to enriching your understanding of the internal structure of an inverter device, is that the converter circuit converts alternating current (AC) coming from the power source into direct current (DC), and the inverter circuit changes the converted direct current (DC) back into alternating current (AC). They work as a set. The diagram below shows the role they both play and the way they work.

Firstly, the converter circuit used in the front part constantly converts alternating current to direct current. This process is called rectification. The wave’s direction and magnitude changes periodically over time since alternating current is a sine wave. Therefore a diode, which is a semiconductor device, is used so as to pass electricity in a forward direction to convert it into direct current, but not in the reverse direction.

When direct current goes through the diode, only the forward direction passes electricity and a positive peak appears. However, the other half of the cycle will be wasted because it does not pass the peak in the negative direction. The reason why the diode's structure is shaped like a bridge is so that it can pass the negative peak in a forward direction. This is called full-wave rectification due to the fact that it transforms both the forward and negative wave peaks.

However, full-wave rectification by itself cannot produce a smooth waveform as traces of the alternating current and rippled voltage fluctuations will remain. Therefore, in order to clean these up, the capacitor is repeatedly charged and discharged, gently smoothing and changing the waveform close to that of direct current.

The inverter circuit then outputs alternating current with varying voltage and frequency. The DC/AC conversion mechanism switches power transistors such as "IGBT (Insulated Gate Bipolar Transistor)" and changes the ON/OFF intervals to create pulse waves with different widths. It then combines them into a pseudo sine wave. This is called “Pulse Width Modulation (PWM)”.

The computer controls the pulse width automatically. Some of the dedicated one-chip computers that control the motor include a product with the PWM function pre-installed. This makes it possible to create pseudo sine waves of various frequencies and control the rotation speed of the motor simply by specifying desired parameters.

Categorizing use cases of inverter devices and circuits by voltage and frequency

Inverter circuits and devices are used in various electrical products such as household air conditioners, refrigerators, IH (induction heating) cookers, fluorescent lights, computer power supplies (including UPS), industrial fans, pumps, elevators, and cranes. They are widely used and have become an integral part of our lives.

Type Elements to change Inverter usage VVVF Voltage/frequency Industrial motors, pumps, air conditioners, refrigerators, etc. CVVF Frequency only Electromagnetic cooker, rice cooker, fluorescent lights, etc. CVCF Constant voltage and frequency Computer power supply, UPS (uninterruptible power supply), etc.

As mentioned in the beginning, inverter circuits and devices are used in household air conditioners, refrigerators, industrial pumps, elevators, etc. to adjust the motor's rotation speed. In this case, the inverter is used to change both voltage and frequency, this is called "VVVF (Variable Voltage Variable Frequency)".

There are no built-in motors in IH cookers or fluorescent lamps, but changing the frequency with the inverter circuit lets you finely adjust heat and brightness. For example, an IH cooker uses high frequency in its coil that heats the pot, utilizing the inverter circuit. Fluorescent lamps also use alternating current in high frequency to increase the lighting speed in order to maintain brightness and suppress flickering with low power consumption. At this time, the inverter circuit changes only the frequency, so it is called "CVVF (Constant Voltage Variable Frequency)".

Last but not least, the inverter circuit also works in computer power supply units. It may seem meaningless because it is used to output a constant AC voltage or frequency from a constant AC (or DC) voltage or frequency. However, it can be used as a stable power supply when the frequency of the AC commercial power supply fluctuates or a power failure occurs. Since it maintains a constant voltage and constant frequency, it is called "CVCF (Constant Voltage Constant Frequency)".

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Related column

How D.C. to A.C. Inverters Work 

April 22nd, 2014

By

Steven McFadyen

on

Kehong Product Page

Traditionally generation of electricity has involved rotating machines to produce alternating sinusoidal voltage and current (a.c. systems). With the development of power electronics during the last several decades, semi-conductor devices are now frequently used to convert direct current (d.c.) to a.c. to power conventional alternating current systems.  

Devices used to convert d.c. to a.c. are called inverters.  These are used numerous applications, including PV systems, battery storage systems, traction drives,  variable speed drives and others.  While they are said to convert d.c. to a.c., this is a simplification, and as we will see, the output of any inverter is built up by pulsed d.c. voltages.

If you are not familiar with sinusoidal a.c. voltage and current, you can refer to the following notes for some revision:

 

Type of Inverter

Converting a d.c. voltage to a sine wave is not a straight forward process.  The general approach is to chop (pulse) the d.c. voltage so that it approximately resembles a sine wave.  This waveform can then be filtered to bring it closer to that of a sine wave.    The level (and associated costliness) to which these techniques are applied determine the final quality of any sine wave produced. 

When considering inverters, the quality of their output is often classified into general categories:

• Square Wave Inverter
• Modified Sine Wave Inverter
• Pure Sine Wave Inverter

Comparison of various waveform types

A square wave is very simple, with the d.c. supply switched between positive and negative.  Depending on the circuitry, the simple square wave can be adapted to give a modified sine wave as shown.  By utilising Pulse Width Modulation (see below) and filtering techniques, the waveform can be refined until it closely resemble that of a pure sine wave. 

There is no exact cut off which defines a pure sine wave and various manufacturers will have differing specifications. It is generally argued that when the total harmonic distortion of the voltage waveform is less than 3%, for all practical purposes this can be considered as a true sine wave. 

A lot of equipment will work well on modified sine wave inverters, including motors, household appliances and other items.  Some types of loads they can be problematic and do require a pure sine wave converter. A well know example are loads requiring a pure sine wave are devices that include crystal oscillator electronic timing circuits which rely on a zero crossover of the sine wave for the functioning.

Pure sine wave inverters are more complex and cost more.  It is best to select the type of inverter to match the application for which it will be used. 

Pulse Width Modulation

Most inverters use a technique called Pulse Width Modulation (PWM) to turn the d.c. voltage on and off.  The width of each pulse is varied, so that the overall electrical effect is similar to that of a sine wave.  This technique is often applied when powering  a..c motors.  For a more detailed explanation of how PWM works, please see our Variable Frequency Drive note).

Pulse Width Modulation - the width of pulses is varied to simulate a sine wave

Pulse Width Modulation - the width of pulses is varied to simulate a sine wave

It should be understood that the output of this type of inverter is not a pure sine wave.  It is a series of d.c. pulses.  This can make it unsuitable for certain types of equipment.

The diagram below illustrates a PWM waveform for a standard inverter - where a single d.c. voltage is with switched on or of to generate the required output.  In this instance, more input d.c. voltage levels are used to create an output waveform which more closely resembles a sine wave.  Multilevel type inventors are more complex and costly to produce. 

Multi-level (3 levels) Wave Form

Harmonics - with the PWM waveforms not being sinusoidal, harmonics will be generated.  For multilevel inverters, the more levels employed, the closer the output will approximate a sine wave and the harmonic content will be lower.

Inverters frequently employ the use of transformers, capacitors and inductors to filter the PWM waveform and reduce the harmonic content.

H-Bridge Circuit

At its simplest, an inverter consists of what is known as a H-Bridge arrangement.   The circuit below illustrates the implementation of a single phase H-Bridge circuit using Insulated Gate Bipolar Transistors (IGBT).

H-Bridge using IGBT

The operation of the bridge is straight forward.  The IGBT act as a switch (when a signal is applied to the gate, they turn on and then turn off when the signal is removed).  By closing Q1 and Q4, a positive d.c. supply is applied to the load.  Q2 and Q3 will result in a negative d.c. supply across the load. Control circuits are used to generate the necessary gate signals to produce the required PWM waveform.

To avoid short circuits (closing both Q1 and Q2 at the same time for example); when changing polarity it is necessary to turn off one set of IGBT before turning on the next.  During the transition, all the IGBT on off.   Diodes provide a necessary path for inductive current in order to limit potential voltage build up during the transition period.

The capacitor provides smoothing to even out any variation in the d.c. supply. 

By utilising six IGBT, it is possible to use the bridge arrangement to supply three phase loads.

H-Bridge arraignments are also commonly implemented using Bipolar Junction Transistors ((BJT) or  Metal Oxide Semiconductor Field Effect Transistors (MOSFET)

Putting it together

Inverter Block Diagram

The block diagram illustrates the key components of a d.c. to a.c. inverter. 

• Input Filter - the input filter removes any ripple or frequency disturbances on the d.c. supply, to provide a clean voltage to the inverter circuit.
• Inverter - this is the main power circuit.  It is here that the d.c. is converted into a multilevel PWM waveform.
• Output Filter - the output filter removes the high frequency components of the PWM wave, to produce a near sinusoidal output.

Note: if you are familiar with Fourier Analysis, it will be seen that the periodic PWM waveform consists of a main fundamental frequency component and higher order (but lower magnitude) harmonic components.  It is these higher order harmonics which the output filter is eliminating.

Inverters are complex devices, but they are able to convert d.c. to a.c. for general power supply use.  With advances in power electronics and microprocessors, the application of inverters in many fields is increasing.  Overall, inverters allow us to tap into the simplicity of d.c. systems and utilise equipment designed these to work in a conventional a.c environment.

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