Thiagarajar College of Engineering
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SCR Commutation - Introduction

Commutation is defined as the process of turning-off a SCR.
Once an SCR starts conducting, gate loses control over the device, therefore, external means may have to be adopted to commutate the SCR.
An SCR is said to be turned OFF if there is no flow of forward current and even if the SCR is once again forward biased, the SCR will not conduct without gate pulse.
There are majorly two types of SCR commutation:
1. Natural Commutation
2. Forced Commutation

Conditions to properly turn-off an SCR

The forward current of SCR must be reduced below the holding current.
A sufficient reverse voltage must be applied across the SCR to regain its forward blocking state.

When the SCR is turned OFF by reducing forward current to zero, excess charge carriers exists in different layers. To regain the forward blocking state of an SCR, these excess carriers must be recombined. Therefore, in order to accelerate this recombination process, a reverse voltage is applied across the SCR.

Natural Commutation

Natural Commutation occurs only in AC circuits and it is named so because it does not require any external circuit.
If the SCR is connected to an AC supply, at every end of the positive half cycle, the anode current naturally becomes zero (due to the alternating nature of the AC Supply).
As the current in the circuit goes through the natural zero, a reverse voltage is applied immediately across the SCR (due to the negative half cycle).
This method of commutation is also called as Source Commutation or AC Line Commutation or Class F Commutation.
This commutation is possible with line commutated inverters, controlled rectifiers, cyclo converters and AC voltage regulators.

Forced Commutation

Forced Commutation is used only in DC circuits since there is no natural zero crossing in DC Circuits.
It is also called as DC commutation
It requires commutating elements like inductors and capacitors to forcefully reduce the anode current of the SCR below the holding current value. This is why it is called as Forced Commutation.
Forced commutation is used in Chopper and Inverters circuits.
Forced commutation is further divided into five categories:
1. Class A: Self or Load Commutation
2. Class B: Resonant Pulse Commutation
3. Class C: Complementary Commutation
4. Class D: Impulse Commutation
5. Class E: External Pulse Commutation

Class-B Commutation: Resonant Pulse Commutation

In class-B commutation, the LC circuit is connected in parallel to the main SCR.

Additional Theory

Load Commutation

Integral cycle control consists of switching on the supply to load for an integral number of cycles and then switching off the supply for a further number of cycles
This control technique is used in applications where mechanical time constant or thermal time constant is of the order of several seconds.
For example, mechanical time constant for many of the speed-control drives or thermal time constant for many of the heating load is usually quite high. For such applications, almost no variation in speed or temperature will be noticed if control is achieved by connecting the load to source for some on-cycles and then disconnecting the load for some off-cycles
The equation of output RMS voltage is given below.

where, n indicates the number of on-cycles, m indicates the number of off-cycles and V_s is the RMS value of supply voltage.

Other possible configurations of AC voltage controllers

Note: The below are the other configurations of phase controlled full wave AC voltage controller.

ACVR with Triac

This configuration is suitable for low-power applications where the load is resistive and has only a small inductance.

ACVR with only one SCR

This configuration uses 4 power diodes and 1 SCR.
In the positive half cycle, diodes D1, D3 and SCR T1 will conduct. During negative half cycle, diodes D2, D4 and SCR T1 will conduct.
This configuration is cheaper as only one SCR is used (only one firing pulse circuitry is required)
The disadvantage of this configuration is that, at any time, 3 devices (2 diodes and 1 SCR) will be conducting. This causes more voltage drop.