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They switch the current on and off very rapidly for varying periods according to the average that is required. When the current required is high they will be on most of the time As the demand falls it changes 90% on 10% off, through 50% on 50%off, to 10% on and 90% off and so on.
The thyristor itself does not make any noise but the magnetic and electrical fields as the current passes through various components produces mechanical vibration at a frequency (or pitch) which will depend on the rate of on and off switching.
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