Difference between Natural commutation vs forced commutation for SCR thyristor

This page compares Natural commutation vs forced commutation and mentions difference between Natural commutation and forced commutation types for semiconductor thyristor devices such as SCR in power electronics.

Commutation is the process of turning off a conducting thyristor. There are two types of commutation viz. current and voltage based.

There are two methods for commutation viz. natural commutation and forced commutation. These are two different methods used in power electronics to control the switching of power semiconductor devices like thyristors and transistors in electronic circuits.

Natural Commutation | Line Commutation

In natural type of commutation, the switching of a semiconductor device from its conducting state to the non-conducting state occurs automatically without the use of any external circuitry or additional components. It relies on the inherent properties of the circuit. The switching process in natural commutation is automatic and is determined by the circuit conditions, such as the reversal of voltage or current.

natural commutation circuit

➨It occurs in AC circuits i.e. when supply voltage is AC. Due to this, SCR turns off when negative voltage appears across the SCR. As there are no special circuits needed to turn off the SCR (thyristor), this type of commutation is known as natural commutation.
➨Natural Commutation in Thyristors take place in Phase controlled rectifiers, AC voltage controllers and Cyclo converters.

natural commutation waveforms

Figure-1 depicts circuit in which natural commutation occurs. It is also known as "Line Commutation". Figure-2 depicts related waveforms of this commutation type. As shown here tc should be greater than tq, where
tc = time offered by circuit during which SCR should be OFF completely.
tq = turn off time of SCR

If the above condition is not taken care then, SCR will get forward biased before it has been off completely. Due to this, SCR will start conducting even without the application of gate signal.

Forced Commutation

In forced type of commutation, on the other hand, involves the use of external circuitry or components to actively turn off the semiconductor device such as thyristor (SCR, Diac, Triac etc.) transitioning it from the conducting state to the non-conducting state. In forced commutation, the switching process is controlled by an external circuit, which actively triggers the turn-off process.

➨It is Applied to dc circuits.
➨Forced Commutation is achieved by reverse biasing SCR device or by reducing SCR current below the holding current value.
➨Commutating elements such as inductance and capacitance are used here.
Forced commutation is applied to choppers and inverters.

Following are the methods used in forced commutation:
• Self commutation
• Impulse commutation
• Resonant pulse commutation
• Complementary commutation
• External pulse commutation
• Load Side Commutation
• Line Side Commutation

Difference between natural and forced commutation

Following table summarizes difference between natural and forced commutation types.

Feature Natural or Line commutation Forced commutation
Definition Switching occurs automatically without external control. Active control using external circuitry to switch off the device.
Control Determined by circuit conditions e.g. voltage or current reversal Actively controlled by external circuitry
Energy consumption Generally lower energy dissipation. May involve additional energy dissipation due to active control.
Complexity Generally simpler due to reliance on natural circuit behavior. Can be more complex due to the need for external circuitry and control mechanisms.
Examples Line-commutated thyristor converters in HVDC systems. Circuits using diodes, capacitors, or additional transistors for active control.
Applications Common in AC to DC converters, e.g., line-commutated thyristor converters. Used in applications requiring precise switching control, e.g., high-frequency inverters.

Conclusion : In summary, natural commutation is based on the inherent properties of the circuit and occurs automatically, while forced commutation involves active control using external circuitry to switch off the semiconductor device. The choice between these methods depends on the specific requirements and characteristics of the power electronic application.

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