Frequency Spectrum Generated by Thyristor Control

This paper describes the measured harmonics in the load currents of thyristor circuits and shows that with firing angle control the harmonic amplitudes decrease sharply with increasing harmonic frequency, but that they extend to very high harmonic orders of around 6000. The amplitudes of the harmonics are a maximum for a firing delay angle of around 90°. Integral cycle control produces only low order harmonics and sub-harmonics. It is also shown that with firing angle control apparently random inter-harmonic noise is present and that the harmonics fall below this noise level at frequencies of approximately 250 KHz for a switched 50 Hz Waveform and for the resistive load used. The noise amplitude decreases with increasing frequency and is a maximum with 90° firing delay.