Class A Commutation simple explanation. By Yuvi - Study Point

Hey everyone, so today we are going to study about Class A commutation in brief. I will make sure that this topic becomes easy or simple to understand.

So, without wasting much time lets get started.

INTRODUCTION & WORKING

Firstly, we are going to start with a question. What is commutation?

• Commutation is a process by the help of which we can turn off the SCR (Silicon controlled rectifier) or thyristor. 

• So, we have source voltage (Vs) in every circuit and the current passing through it is known as source current (Is). 

• And (Ig) is the gate pulse which we apply to the thyristor or SCR so that the thyristor comes into conduction mode. 

• Or we can say that to come into conduction mode we need to apply a triggering pulse to the thyristor gate with an alpha angle. 

• And alpha is the firing angle. So that the thyristor comes into conduction mode.  

• The voltage drop across thyristor is considered as Vt.  

• And when the thyristor is in conduction mode after applying the triggering pulse. The current starts properly flowing in the circuit. 

• The Class A commutation circuit consists of inductor, capacitor & resistor. The capacitor & resistor can be in series or in parallel with the inductor. 

• And that circuit is known as an underdamped circuit. 

• When the current starts flowing in the circuit then the capacitor starts charging gradually and this gradual charging is due to the inductor. Because inductor opposes the flow of current. 

• And when the voltage of capacitor means (Vc) becomes equal to the maximum voltage which means the supply voltage (Vs) { Vc = Vs }. 

• Then the inductor changes its polarity and reverses current starts flowing through the circuit which turns off the thyristor. The inductor is turning of the SCR or thyristor. An inductor is a load. 

OTHER NAMES OF CLASS A COMMUTATION AND REASON 

• The other names of Class A commutation are Self Commutation & load commutation. And why we called it as Self and load commutation. I will also explain that to you in this blog. 

• An inductor is a load which is turning off the thyristor by reversing its polarity when the capacitor voltage becomes equal to the maximum voltage or supply or source voltage {Vc = Vs}.  

• So, it means the load is turning off the thyristor. So, This is the reason why we call Class A commutation as Load commutation. 

• Class A commutation is also known as Self commutation because SCR is getting turned off by self turning off process.

Now let's talk about the waveforms

• When our thyristor is in induction mode which means (Ton) then the capacitor starts increasing gradually because of the inductor. As I have told you that inductor opposes the flow of current and capacitor gradually charges because of the inductor.

• And because of the flow of current, our output current also increases which means (Io).

• And when the current is flowing properly in the circuit then our voltage drop (Vt) across the thyristor remains 0.

• When Vs becomes equal to Vc {Vs=Vc}. Which means when the capacitor voltage reaches its maximum value or we can say that when the capacitor voltage becomes equal to the source or supply voltage.

• The capacitor current still increases after when it reaches the maximum voltage it still increases because of the decrement in output current (Io).

• As the decrease amount of output current charge or charge of (Io) also store in the capacitor which will make the increment in capacitor charging after it reaches the maximum or source voltage.

• So, the output current (Io) decreases and the voltage drop (Vt) still remains 0.

• But when out output current (Io) becomes totally 0 then the voltage of the capacitor also decreases gradually due to this.

• Our output current (Io) first goes to negative and then comes back to 0.

• And at this time when our output current (Io) becomes 0 then the voltage drop across the thyristor (Vt) starts increases and it reaches to the maximum value which means Vt becomes equal to the Vs.

• Or we can say that the voltage drop (Vt) becomes equal to the source voltage (Vs).

• And when voltage drop (Vt) reaches the maximum position {Vt=Vs}. Then again the same steps will flow.

The same procedure will start again.

• Which means again the thyristor gets turned on and due to the flow of current capacitor gradually starts charging due to the inductor it is gradually charging because inductor opposes the flow of current.    

• And again the same procedure will start. The output current (Io) starts to increase.

• The voltage of the capacitor reaches its maximum value which means {Vs=Vc} and still, it gradually increases.

• And this is due to the output current decrement because its decrement charge also stores in the capacitor so this is the reason why capacitor charge still increases after it reaches the maximum value.  

• The voltage drop (Vt) still remains 0. But when output current (Io) reaches 0 from going to negative for some time.

• At that time when output current (Io) reaches 0 then the voltage drop starts increases to the maximum value which means {Vt=Vs}.

• After reaching the maximum value the thyristor or SCR again turn on and the same procedure will start following.  

So, this is the proper working of the Class A commutation, its waveform operation and also I have explained to you about commutation that What is commutation? or why we need this process?

Now let's study more about it. 

There are two types of commutations exactly. One is forced commutation and another is natural commutation.

• In forced commutation, there are a total of five classes ~

1. Class A commutation
2. Class B commutation
3. Class C commutation
4. Class D commutation
5. Class E commutation

• And in natural commutation there is only one commutation named as Class F commutation.

• And now I am going to give you some information about natural and forced commutation. So that you will get the difference between forced and natural commutation.
• And you guys do not get confused.  

In this blog, you have studied about class A commutation which is a forced commutation technique.

• In class A commutation the commutating elements are inductor and capacitor.
• By the help inductors, inverse polarity at a certain time period the SCR forcefully turns off.

FORCED COMMUTATION

• This force commutation is also known as DC commutation because it is used in DC circuits.

• It is not like natural commutation because there is no natural zero current in DC circuits.

• Already I have explained to you about what does force commutation means.

• Commutating elements it requires, for example, ~ inductor and capacitor.

• And what this commutating element actually do? They basically reduce the value of anode current of the thyristor below the value of holding current and due to this forced commutation occurs.

• I have already told you about the working of the inductor. As it reverses the polarity to turn off the SCR or thyristor. 

APPLICATION OF FORCED COMMUTATION

• Forced commutation is used in inverters circuits, Choppers, etc.

After learning about force commutation what we understood?

1. In force commutation external components are required.
2. In force commutation due to voltage and current SCR turns off.
3. In force commutation power loss takes place during commutation.
4. Its cost is significant.
5. It is used in choppers, inverters, etc.
6. And the main information as it requires only DC voltage at the input.

NATURAL COMMUTATION

• Natural commutation is just opposite of Forced commutation as it requires AC voltage at the input.

• It only requires AC circuits and it does not require any external circuits. It is just opposite of forced commutation.

• Natural commutation occurs in controllers, AC voltage, phase controlled rectifiers, etc.

• And when a positive cycle reaches zero and the anode current also reaches to zero then an immediate reverse voltage of negative cycle is applied across the SCR or thyristor.

• And which causes the thyristor to turn off. 

After learning and knowing about natural commutation what we understand?

1. Unlike the forced commutation, a natural commutation does not require any external components.
2. In natural commutation, SCR turns off due to negative voltage supply.
3. It does not have a significant cost but it h
4. It has no cost or we can say it is of zero cost.
5. There is no power loss during commutation.
6. And as I have told you natural commutation requires AC voltage at input unlike forced commutation because it requires DC voltage at the input.

So, let's recap what is the thyristor commutation technique?

• By applying a triggering pulse to the gate terminal we can turn on the thyristor.

• And there are various triggering methods.

• Similarly, there are various techniques to turn off the thyristor.
• And this can be done by bringing the thyristor back to its forward blocking state from the forward conduction state.

• And to do this forward current value is reduced or we need to decrease this forward current value below the holding current value.

• So, this is the thyristor commutation technique which we studied.

SIMPLY

Simply commutation is a process of turning off the thyristor or SCR.

During the turn off process, a reverse voltage is applied across the thyristor or SCR.

The main operation is to get the forward blocking state from the forward conducting state.

{ And if you guys want a more simple and brief explanation with diagrams. Then just go and check out this video whose link is also given below and also check out my channel. }

Video & link of Class A commutation ~

https://youtu.be/AM6PvucOqH0

Channel link ~

https://www.youtube.com/channel/UCN-PzGYqhxvRlKfkg9gu6ww

{PLEASE GO & CHECK OUT THIS VIDEO AND MY CHANNEL FOR SIMPLE LEARNING WITH BRIEF EXPLANATION OF DIFFERENT IMPORTANT TOPICS.}

Thank you!