A single-phase, full-bridge diode rectifier fed from a 230 V, 50 Hz sinusoidal source supplies a series combination of finite resistance, R, and a very large inductance, L. The two most dominant frequency components in the source current are

The correct option is 50 Hz, 150 Hz.

Step 1: Analyze the source current waveform
A single-phase full-bridge diode rectifier supplying a load with a very large inductance (L) operates in continuous conduction mode. The highly inductive load acts as a constant current source, maintaining a constant DC output current, I_d.

As a result, the current drawn from the AC source (i_s) switches alternatingly between +I_d and -I_d every half period of the supply voltage. Therefore, the source current waveform is a symmetrical square wave of amplitude I_d and fundamental period equal to that of the AC supply.

Step 2: Fourier Series expansion of the square wave current
Since the source current i_s(t) is an odd symmetrical periodic function with supply frequency f₁ = 50 Hz, its Fourier series expansion consists only of odd harmonic components:

$i_s\left(t\right)=\frac{4I_d}{\pi }\left(sin(\omega t)+\frac{1}{3}sin(3\omega t)+\frac{1}{5}sin(5\omega t)+\cdots \right)$

Step 3: Identify the dominant frequency components
From the Fourier series expression, the present frequencies are the fundamental frequency and odd harmonic frequencies:

• Fundamental component (n = 1):

$f_1=50\text{Hz}$

• 3rd harmonic component (n = 3):

$f_3=3\times 50\text{Hz}=150\text{Hz}$

• 5th harmonic component (n = 5):

$f_5=5\times 50\text{Hz}=250\text{Hz}$

The amplitudes of the harmonic components decrease with the harmonic order n (proportional to 1/n). Therefore, the two most dominant (largest magnitude) frequency components present in the source current are the fundamental (50 Hz) and the 3rd harmonic (150 Hz).