Inverter Basics : Building Blocks of Modern Power Electronics, as we know it!

Hi folks,

The purpose of inverters is to convert AC power into DC power. They are used in several applications including Uninterruptible Power Supplies, Control of Electrical Machines, Active Power filtering and Grid integration of renewable. Before we start with how inverter functions, we will have a look at fundamentals of AC and DC power. On screen, you can see six waveform. Now try to identify yourself, which one is AC and which one is DC. This might be very easy for waveform A and B, as they are familiar. How about other four? Are they AC or DC? (Please forgive me for my drawing skills and I expect you to think before you scroll down for the answer.)

The key concept here is any periodic waveform with average value over a cycle equal to zero is an AC waveform. Any waveform with non-zero average value over a cycle, may be positive or negative, is a DC waveform. Now you can easily identify that waveform C and D are AC waveform whereas waveform E and F are DC waveform.

Now that we have a clear concept of AC and DC waveform, Let us see how inverter works. We have a DC source and we need to generate an AC waveform. To do this, we are supposed to generate a waveform that has zero average value over a cycle. The simplest way to do this is to have a mechanism, which can connect DC source directly to the AC load for some time, reverse connect the DC source directly to the AC load for some time and should be able to do this repeatedly. We all know that the frequency of AC supply available at our home is 50 Hz. So this means, we need to change the connection between DC and AC side for every 10 milliseconds which is not possible manually or by using mechanical switches. So here, we take help from semiconductor devices which can turn on and off a million times per second.

Now below we can see a simplest circuit that can satisfy our requirements. This is a half bridge inverter. When we turn on switch S1 and turn off S2, the voltage difference between point A and Point B is Vdc/2. Then we turn off S1 and turn on S2 and voltage difference between point A and Point B is -Vdc/2. If we do this process repeatedly, we will generate AC power from DC power. The important drawback of this circuit is: even though we have total DC voltage of Vdc, we can apply only half of it at a time across AC load. Can you think of a circuit that can apply the complete DC voltage across the AC load and also able to change the polarity of it?

This circuit is called full bridge inverter or single phase inverter. More popularly, it is known as H bridge structure. When we turn on switch S1 and S4, the voltage difference between point A and Point B is Vdc. Then we turn off S1 and turn on S2 and voltage difference between point A and Point B is -Vdc. So now we have a circuit that can give us complete utilization of available DC voltage. This circuit operation also has a drawback. Many applications, especially control of electrical machines, require variable AC voltage. In this circuit the only way to vary the output AC voltage is to change the value of Vdc. To change the value of Vdc, additional DC-DC converter will be required that will lead to additional circuitry, losses and extra cost.

Can we operate the same structure in such a way that we can vary the output AC voltage while keeping the DC voltage constant? This can be done using various modulation strategies. Let us discuss this in our upcoming articles. 

Thank you for your interest. Please let me know about the topics you want to read, I will try to post accordingly. Please leave comments if you have any doubt or suggestions.