Question

A) The order of first 3 harmonics on the output voltage of a controlled 3-phase rectifier in CCM operations are;

B) The order of first 5 harmonics on the input current of a controlled 3-phase rectifier in CCM operations are;

Answer 1

A) The output voltage waveform of a controlled 3 phase rectifier in CCM mode contains 6 ripples per time period of input voltage as it has 6 times the fundamental frequency of the applied input voltage. So if we consider the applied voltage frequency or input frequency to be fundamental frequency instead of the lowest frequency in the output voltage, all the frequencies in output voltage are multiples of 6 and so the order of 1st 3 harmonics will be 6, 12 and 18. Note that DC is the 0th harmonic and so not considered a harmonic.

B) The input current waveform of a controlled 3 phase rectifier in CCM mode shows half wave symmetry i.e when shifted by half time period, the resultant waveform when reversed via y axis gives original waveform. So by it contains only odd harmonics and all even harmonics vanish. Also as it is 3 phase rectifier, all triplen harmonics or multiples of 3 are absent. As the fundamental or 1st harmonic cannot be considered a harmonic, the 1st 5 harmonic orders will be 5, 7, 11, 13 and 17.