Class C commutation method uses an external pulse for commutation.

Explanation:
- Commutation in electrical engineering refers to the process of transferring current from one circuit element to another. It is an essential part of the operation of various electrical devices, such as motors and generators.
- Commutation methods can be classified into various classes based on the techniques used for commutation.
- Class A and Class B commutation methods are the most basic methods that do not involve the use of an external pulse for commutation. These methods rely on the inherent properties of the circuit elements for commutation.
- Class C commutation method, on the other hand, involves the use of an external pulse for commutation. This method is commonly used in electronic devices and switching circuits.
- In Class C commutation, an external pulse is applied to the circuit to trigger the commutation process. This pulse is typically generated by a separate circuit or device, such as a pulse generator or a timing circuit.
- The external pulse is used to control the switching of the circuit elements, allowing the transfer of current from one element to another. This pulse is synchronized with the operation of the circuit to ensure proper commutation.
- The use of an external pulse in Class C commutation provides greater control and flexibility compared to Class A and Class B methods. It allows for precise timing and synchronization of the commutation process, resulting in improved efficiency and performance of the circuit.
- Class C commutation is commonly used in applications where high-speed switching and precise control are required, such as in power electronics, communication systems, and digital circuits.
- Overall, Class C commutation method stands out among the given options as the one that uses an external pulse for commutation.

Class C commutation method uses an external pulse for commutation.

Explanation:
Commutation is the process of changing the direction of current flow in a motor or generator. It is an essential part of the operation of electrical machines, especially in DC machines. Commutation ensures that the current flows through the windings in the correct sequence, producing the desired torque or electromotive force.

There are several methods of commutation, including Class A, Class B, Class C, Class D, and Class F. Each method has its own characteristics and applications.

- Class A commutation: In Class A commutation, the current is commutated by a mechanical switch, such as a brush or a commutator. This method is commonly used in small DC motors.

- Class B commutation: In Class B commutation, the current is commutated by a combination of mechanical and electronic switches. This method is used in medium-sized DC motors.

- Class C commutation: In Class C commutation, an external pulse is used to commutate the current. This pulse is typically generated by a separate commutation circuit or controller. The external pulse ensures that the current is switched at the optimal time, improving the efficiency and performance of the motor. Class C commutation is commonly used in large DC motors and generators.

- Class D commutation: In Class D commutation, the current is commutated using electronic switches, such as transistors or thyristors. This method is used in high-power applications, such as traction motors.

- Class F commutation: Class F commutation is a variation of Class D commutation, where the current is commutated using a combination of electronic switches and mechanical switches. This method is used in specialized applications where precise control is required.

In summary, Class C commutation method uses an external pulse for commutation. This method is commonly used in large DC motors and generators to improve efficiency and performance.