Natural commutation vs forced commutation-Difference between Natural commutation and forced commutation based on Thyristor

This page compares Natural commutation vs forced commutation and mentions difference between Natural commutation and forced commutation types based on Thyristor device.

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.

Natural Commutation

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

➨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

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