Introduction:
A full wave rectifier is a circuit that converts an alternating current (AC) input into a direct current (DC) output. In a 12 phase full wave rectifier, the AC input is divided into 12 equal phases, resulting in a smoother output compared to a single-phase rectifier. The ripple frequency refers to the frequency at which the output voltage fluctuates.

Explanation:
To understand why the ripple frequency in a 12 phase full wave rectifier is 720 Hz, let's break down the process step by step.

1. Rectification Process:
In a full wave rectifier, the input AC signal is converted into a pulsating DC signal. This is achieved by using diodes to conduct current in only one direction. Each diode conducts current during a specific phase of the AC input.

2. Ripple Frequency Calculation:
The ripple frequency is determined by the number of times the output voltage rises and falls within a given time period. In a 12 phase full wave rectifier, each diode conducts during one-twelfth of the input cycle.

The input frequency is given as 30 Hz. To calculate the ripple frequency, we need to determine how many times the output voltage rises and falls in one cycle.

- In a single-phase full wave rectifier, the output voltage rises and falls twice per input cycle. This results in a ripple frequency of 2 times the input frequency.
- In a 12 phase full wave rectifier, each diode conducts during one-twelfth of the input cycle. Therefore, the output voltage rises and falls 12 times within one cycle.

3. Calculation:
To find the ripple frequency in a 12 phase full wave rectifier, we multiply the number of diode conduction times within one cycle by the input frequency.
Ripple frequency = Number of diode conduction times * Input frequency

In this case, the number of diode conduction times is 12, and the input frequency is 30 Hz.
Ripple frequency = 12 * 30 Hz = 360 Hz

However, in this case, the correct answer is given as 720 Hz. It is possible that there is a mistake in the question or answer choices.