How Does an Inverter Work: Schematic and Operation

solar windHow Does an Inverter Work

How Does an Inverter Work

Welcome to this informative page. In this page we are going to explain what an inverter is, what is its function, what it is made of, what its principle of operation and what are the main types of inverters used in the most common situations and needs. Thanks to the help of simple illustrated diagrams, you will be guided in understanding this important electronic device that is indispensable in many fields of application. You will also find the wiring diagram to build a square wave inverter and many practical tips so you can make a safe, conscious and lasting purchase.

What is an Inverter and why is it so important?

An Inverter is an electronic device capable of transforming a DC (DC) current into an alternating current (AC) at a given voltage and frequency. For example, if we have to supply a household appliance that operates in alternating current 230V (50Hz frequency) but we do not have the AC power available, we can still power it by using an inverter such as a 12V (DC) . It is therefore indispensable to use it to power by DC, electrical devices that work in AC. Inverters are used in stand alone photovoltaic systems for powering electrical devices of isolated houses, mountain huts, camper vans and boats, and are also used in grid-connected photovoltaic systems to enter the current produced by the plant directly into the power grid distribution (photovoltaic inverters).

inverter waveforms

The inverters are also used in many other applications, ranging from UPS to the speed controllers of electric motors, from switching power supply to lighting. The term "inverter" also refers to a "rectifier-inverter" group, powered by alternating current and used to vary the voltage and frequency of the output alternating current in function of the input voltage (for example, for power supply of particulars operating machines). The most popular inverters used to supply AC power are three types: square wave inverters (suitable for purely resistive loads), modified sinusoidal wave inverter (suitable for resistive and capacitive loads, with inductive loads can produce noise ) And pure sinusoidal wave inverters (suitable for all types of loads because they faithfully reproduce a sinusoidal wave equal to that of our domestic power grid).

How does an Inverter work?

Well, let's start explaining this interesting energy transformation phenomenon. We said that an inverter can get an alternating current starting from a direct current. To understand this phenomenon it is good to start with the explanation of an alternator. The alternator is a rotating electric machine that transforms mechanical energy into electricity as an alternating current through the natural phenomenon of electromagnetic induction (an example is the bicycle dynamo). In its simplest form, it consists of a wire coil with a rotating magnet near it. As soon as a magnet pole approaches the coil, it will create a induced current in the coil and this will flow in the opposite direction to the magnet rotation. An alternating current is then produced.

Alternator Operation

Now let's see how a transformer works. A transformer also produces an alternating current induced in the coil, but this time, the variable magnetic field is produced not by a magnet but by another coil (called the primary coil) having an alternating current flowing therein. Each coil crossed by an alternating electric current behaves like a magnet and produces a magnetic field. If the direction of the current changes, the polarity of the magnetic field changes.

Trasformator Operation

The usefulness of a transformer is that the voltage produced in the secondary coil is not necessarily the same as that applied to the primary coil. If the secondary coil consists of a double winding (twice the speed) with respect to the primary coil, the secondary voltage will be twice the voltage applied to the primary coil. We can actually produce any voltage we want by varying the size of the coils.

If in the primary coil, instead of the alternating current, we run the continuous current of a battery, no induced current is generated in the secondary coil, as the magnetic field does not vary. But if we change direction to current continuously and quickly, then we have already realized a very simple and functional inverter. This inverter outputs a square wave, whose frequency depends on the time when we change the direction of the continuous current circulating in the primary coil.

Inverter Operation

How to make these continuous and rapid changes possible automatically? By using a transistor circuit, or even better by using MOSFET, or thyristor or IGBT, which are more efficient. Below you will find the diagram for making a simple square wave inverter using an astable multivibrator circuit for piloting the primary coil. Of course, this kind of inverter is rich in harmonics and therefore is not suitable for powering neither capacitive nor inductive loads. Only purely resistive loads, such as filament lamps or electric heaters, can be fed.

Schematic of a square wave inverter

Schematic diagram of a square wave inverter

Transistors Q1 and Q2, as well as transformer T1, determine how much power the inverter can deliver. Q1 and Q2 are transistors 2N3055 and T1 is a transformer with maximum current 15A; In this case the inverter can deliver approximately 300 watts. Remember that if you operate with high currents, this inverter will absorb considerable amounts of current from the battery and in a short time you may find the battery seriously damaged. It is therefore best to set up a sensor to automatically stop the inverter operation as soon as the battery "drops" below a certain voltage threshold. It is also good to insert a protective fuse before the circuit is started up.

Sine wave pure inverter

To obtain an alternating sinusoidal current at the output of our transformer, we must apply a sinusoidal current at the input. To produce a sinusoidal wave at the input of the primary coil we need an oscillator. One of the simplest oscillators we can accomplish is definitely the Wien Bridge with FET transistor. The output is stable thanks to feedback.

How does a pure wave inverter work

In most oscillating circuits, the output current will be low intensity or in any case not enough to drive the main coil. This current will then need to be amplified from what will be more or less equivalent to a powerful audio amplifier so as to produce a high current for the primary transformer coil. The transformer, though very useful, does nothing at all. As voltage increases, the current decreases, and the power (current x voltage) remains the same (ignoring internal losses to the transformer). In other words, to get 1Kw output in alternating current, we need to supply 1Kw DC input.

The best and most expensive inverters are managed by a microcontroller and rely on pulse width modulation (PWM). The system can be retracted to provide a stable output voltage at the input input variable. For both types of modulation, the signal quality is determined by the number of bits used. It ranges from a minimum of 3 bits to a maximum of 12 bits, capable of describing the sinusoid with very good approximation.

Grid Tie Inverter

A grid-tie-type inverter has a different function than the inverter described here. It not only transforms a DC current into alternating current but is able to put this current into the national power grid. To do this, the grid tie inverter must do network voltage sampling and synchronization for the transfer.

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