The process of turning off of a conducting SCR is known as "commutation. Once the SCR is fired (turned on), the gate loses control over it.

Depending on the nature of the source (ac or de) the commutation can be natural or forced. Different forced commutation methods have  been discussed in this post. 

The classification of commutation techniques:

A)Commutation techniques:
a) Natural commutasion(AC source).
b) Forced commutasion (DC source).

  1. Current commutasion. 
  2. Voltage commutasion. 

Commutation   techniques   (SCR turn off method) :

As seen from figure. The commutation techniques are broadly categorized into two categories namely natural commutation and forced commutation depending on whether the source voltage is AC or DC

The forced commutasion circuit are further classified into two categories namely current commutasion and voltage commutasion. These are the two ways of turning off a conducting SCR. 

1) Current commutation:
if the SCR is turned off by reducing its anod current below the holding current value, then the commutasion is called as current commutasion. 

2) Voltage commutation:
if the conducting SCR is turned off by applying a large reverse voltage across it then the commutasion is called as voltage commutasion

A) Natural commutation:
When the SCR is turned off, due to its forward current going below the holding current, naturally, it is said to be naturally commutated. The natural commutasion usually takes place when AC supply is used at the input of the Thyristories circuit.
Natural commutation
in which the source voltage is AC and the load is resistive. Therefore the load voltage and current will have the same shape and they will be in phase with which other. 

The current flowing through the Thyristor T1 is same as that flowing through R. As shown figure. The thyristor current passes through a neutral zero and a reverse voltage appear across the thyristor there after. 

The conducting thyristor is then turned off due to its anod current going to zero naturally. Hence it is known as neutral commutation

Neutral commutasion is used in ac- voltage controllers and phase controlled rectifiers. 

Benefit of commutation:

  1. It does not require any external commutation components. 
  2. It is reliable and simple. 

B) Forced commutation:
When the SCRs operate on a puro DC input voltage, their forward current cannot be reduced below holding current naturally .

therefore the thyristors must be commutated forcibly by using additional commutasion circuit. 

 This external commutation circuit will turn off the thyristor by means of either current commutasion or voltage commutation. 

in the voltage commutasion, the conducting SCR is turned off by applying a reverse voltage across it whereas in current commutasion, an externally produced reverse current is forced to flow through the conducting SCR to reduce its net forward current below the holding current value. 

Normally the forced commutasion will be used in the thyristorised inverters or chopper circuits as these circuits operate on DC power supplies.

a) Conditions for Successful Commutation: 
Opera F In order to achieve the commutasion of SCR successfully the following conditions must be satisfied:

1) Forward current of the thyristor must be reduced to zero. 

2) Reverse voltage must be applied across the thyristor for a duration more than the turn off time of the device 

3) Critical rate of rise of Voltage of the device should never be exceeded, to avoid Retriggering of the thyristor (dv / dt of the reapplied voltage should not be too large.)

b) Classification of forced commutation Methods:
There are six different classes by which the energy is switched across the SCR which is to be turned off. They are as follows:

1) Class  A  :  self commutated by resonating the load. 

2) Class B  :  self commutated by a LC circuit. 

3) Class C  :  complementary commutation. 

4) Class D  :  Auxiliary commutation. 

5) Class E  : An External pulse source for commutation. 

6) Class F :  AC line commutation. 

c) Self commutation by Resonating the load (Class A) :
Circuit diagram : 
Figure shows the SCR circuit that uses a class A commutasion .

The class A commutasion is also called as self commutation In Figure L and C are the commutating components and is the load resistance .

The commutasion components L and C form an underdamped resonant circuit . Various voltage and current waveforms and equivalent circuits are shown in Figure.
Self commutation by Resonating the load (Class A)
Operation of the circuit:
Refer to Figure . At t = t zero the SCR , is tumed on the de supply voltage is applied to the resonant circuit . Due to the presence of underdamped LC resonant circuit , the anode current of SCR1 is sinusoidal in nature ( see Figure ) . This current will charge the capacitor C with its upper plate. 

At t = t one the voltage on capacitor C reaches its maximum and the anode current goes to zero . The capacitor will then attempt to force a reverse current through SCR1 which turns off due to net anode current going to zero . In this way the class A is current commutation.
As soon as the SCRone is turned off , the capacitor starts discharging through R. The voltage across SCR1 at the instant tone is negative as the voltage on the capacitor is higher than the input supply voltage Vs The voltage across SCRone then gradually increases towards zero as the capacitor discharges and finally reaches the positive supply voltage level + Vs when C is completely discharged . 

The voltage across SCR one is nearly zero from t0 to t1 when it is conducting . 

The time period tc show by shaded portion in figure is called as circuit turn off time. 

For successful commutation, tc should be greater than or equal to the SCR turn off tq.
Self commutation by Resonating the load (Class A)

Satish jagtap

Hello iam Satish jagtap Electrical engineer. From: Aurangabad, Maharashtra. Edu: MIT college of Engineering.

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