Introduction:
An SCR (Silicon Controlled Rectifier) is a type of semiconductor device that is widely used in power electronics for controlling and converting electrical energy. It is a four-layered device that acts as a switch, allowing current to flow in only one direction. The natural commutation of an SCR refers to the process by which the device turns off automatically without the need for any external control signals.

Explanation:
The natural commutation of an SCR occurs when the anode current becomes zero. Let's understand why this happens.

1. SCR Operation:
Before discussing natural commutation, let's briefly understand the operation of an SCR. An SCR has three terminals - an anode, a cathode, and a gate. It acts as a diode in the forward direction, allowing current to flow from the anode to the cathode. However, it behaves as an open circuit in the reverse direction.

2. Forward Conduction:
When a positive voltage is applied across the anode and cathode of an SCR, it enters the forward conduction mode. At this stage, the SCR behaves like a closed switch, allowing current to flow from the anode to the cathode.

3. Gate Triggering:
To turn on the SCR, a positive gate current is applied. This triggers the device, causing it to enter the forward conduction mode. Once triggered, the SCR remains in the on-state until the anode current falls below a certain threshold called the holding current.

4. Natural Commutation:
The natural commutation of an SCR occurs when the anode current becomes zero. This happens when the external circuit connected to the SCR is designed in such a way that it provides a path for the anode current to naturally decay.

5. How Anode Current Becomes Zero:
When the voltage across the SCR is reversed (i.e., becomes negative), the anode current starts to decrease. This is because the negative voltage opposes the forward conduction of the SCR. As the anode current decreases, the voltage across the SCR also decreases. Eventually, the anode current becomes zero, and the SCR turns off automatically.

Conclusion:
In conclusion, the natural commutation of an SCR occurs when the anode current becomes zero. This happens when the voltage across the SCR becomes negative, causing the anode current to decrease until it reaches zero. The natural commutation process allows the SCR to turn off automatically without the need for any external control signals.