Industrial Electronics – I (Thyristor)

 Commutation Circuits of SCR – Natural and Forced Commutation (Class A to E): -

Silicon Controlled Rectifiers (SCRs) are one of the most widely used power semiconductor devices in power electronics. They are preferred for their high efficiency, robustness, and ability to handle large power levels. However, one of the most important aspects of using SCRs is commutation—the process of turning off the device once it has been triggered into conduction. Unlike transistors, SCRs cannot be turned off simply by removing the gate signal. Once the SCR is conducting, it continues to conduct until the current through it naturally drops below the holding current or an external commutation circuit forces it to turn off.

In this blog, we will dive into the concept of commutation, the difference between natural commutation and forced commutation, and explore the different classes of forced commutation circuits (Class A to Class E).

What is commutation?

Commutation refers to the process of switching off a conducting SCR. This is essential because, in most applications, SCRs need to operate in controlled ON and OFF states to regulate power flow.

There are two broad categories of communication:

1. Natural Commutation
2. Forced Commutation

Natural Commutation

Natural commutation occurs when the SCR turns off automatically as the current through it drops to zero during the negative half cycle of the AC supply. In other words, the alternating nature of AC ensures that the current naturally passes through zero, allowing the SCR to turn off.

• This method is simple and does not require extra circuitry.
• It is commonly used in AC applications, such as rectifiers.

Forced Commutation

In DC circuits, the current does not naturally fall to zero. Therefore, additional circuits are required to force the current through the SCR to zero for a sufficient period so that it regains its blocking state. This method is called forced commutation.

Different arrangements of capacitors, inductors, and auxiliary switches are used to achieve forced commutation. The types are classified as Class A, B, C, D, and E commutation circuits. Let’s explore them one by one.

Class A – Load Commutation

In Class A commutation, the load itself provides the mechanism for turning off the SCR. This occurs in resonant load circuits, where a capacitor and an inductor are connected in series with the SCR.

When the SCR is triggered, the resonant LC circuit causes the current to rise and then naturally fall to zero, turning off the SCR.

• Commonly used in inverters.
• Suitable for circuits where the load has an oscillatory current.

Class B – Resonant Commutation

Class B commutation is also known as resonant pulse commutation. In this method, a pre-charged capacitor is connected across the SCR through an inductor. When triggered, the capacitor discharges in the opposite direction of the SCR current, forcing the current through zero and turning the SCR off.

• Used in choppers and inverters.
• Provides controlled commutation by timing the capacitor discharge.

Class C – Complementary Commutation

Class C uses two SCRs in a complementary manner. When one SCR is conducting, the other is triggered to turn it off by providing reverse current. The commutating capacitor is charged in such a way that firing one SCR forces the other to turn off.

• Used in inverter circuits with paired SCRs.
• Provides reliable commutation where two SCRs work alternately.

Class D – Auxiliary Commutation

In Class D commutation, an auxiliary SCR is used along with a commutating capacitor. When the main SCR needs to be turned off, the auxiliary SCR is triggered. This discharges the capacitor across the main SCR in the reverse direction, forcing it to turn off.

• Widely used in chopper circuits.
• Offers controlled and quick commutation.

Class E – External Pulse Commutation

In Class E commutation, an external pulse source is used to apply a reverse voltage across the SCR. This external source can be another circuit or a pulse transformer that generates the reverse current needed to turn off the SCR.

• Suitable for high-frequency applications.
• Provides precise commutation but requires an additional pulse circuit.

Comparison of Classes A to E

Class	Method	Key Feature	Applications
A	Load Commutation	Uses the natural oscillation of the load	Inverters
B	Resonant Commutation	Capacitor discharge through SCR	Choppers, inverters
C	Complementary Commutation	Two SCRs are commutating with each other	Inverters
D	Auxiliary Commutation	Auxiliary SCR forces commutation	Choppers
E	External Pulse Commutation	Reverse pulse from outside circuit	High-frequency converters

Conclusion

Commutation is a vital aspect of SCR operation, especially in DC circuits where current does not naturally go to zero. Natural commutation is simple but limited to AC circuits, while forced commutation provides flexibility for DC applications using different circuit arrangements.

• Class A and B rely on resonant circuits.
• Class C and D use additional SCRs and capacitors for controlled commutation.
• Class E depends on an external pulse source.

By understanding these commutation techniques, engineers can design efficient power electronic systems such as inverters, choppers, and converters.

 ---------Industrial Electronics – I (Single-phase & polyphase controlled rectifier)-5.1 Next Page ----------