De-geeking the gobbledegook of thyristor control

The company wants to de-geek, as it outs it, the entire gobbledegook of thyristor control.

“CD Automation has been dealing with thyristor applications for 27 years, and in this time we’ve learnt how to tame the fire in this magical electronic device,” explains Jez Watson, managing director of CD Automation UK. “If you have peak current in the mains supply, you generate flicker depending on how strong the supply is. This manifests itself in both domestic and industrial applications as flickering lights."

“If you start a large industrial motor, the lights will dim briefly because of the large peak inrush current. Preventing peaks on the mains supply can be achieved by using phase angle as a firing method for example.”

Thyristor dimmers switch on at an adjustable time (phase angle) after the start of each alternating current half-cycle, thereby altering the voltage waveform supplied to light and changing its RMS (Root Means Square) effective value.

The advantage of phase angle mode is that its continuous – it does not generate peak current, continually reducing true and RMS voltage to control an electric load from zero to 100 percent without interruption.

The resistance of the tungsten element in a lamp when cold can be as low as one twentieth of its nominal operating load, which translates to 20 times the nominal current when it is first switched on.

With industrial tungsten heating elements and other heating elements made of carbon and molybdenum, the resulting mains flicker is something electrical grid suppliers very much frown upon. In fact, European directives have banned equipment that produces such flicker. Phase angle is a soft starting method similar to that used on electric motors to avoid current peaks.

Every advantage has a disadvantage, however. In this case, phase angle generates harmonic disturbances and reduces power factor – something else grid suppliers dislike equally, especially in hospitals and chemical processing facilities where sensitive instruments can be affected.

To prevent such disturbances you need to use large, heavy and expensive electric filters in the 150-550Hz harmonic range. At such low frequency you need large wound component ferrite inductors.

For example, a filter for a 75kW phase angle load will weigh about 55kg. Otherwise you can only use phase angle for a short period to prevent inrush current, and then switch to burst modulated firing mode.

This is used for infrared lamps between 500W and 1kW, the tungsten elements of which are typically heated from 500-700 degrees centigrade and used for heating glue, drying paint or blowing a bottle using a small lump of plastic.

When a bank of eight infrared lamps for example need to be modulated from 50% of full load power, then a load manager may be needed to avoid all lamps coming on at the same time and causing electrical disturbances. CD Automation’s REVO PC provides effective power control of inrush current to minimise load current taken from the mains supply.

The advantage of burst firing is that it doesn’t generate harmonics. Starting at zero voltage, it ends at zero current crossing, so if current is not synchronised with voltage and goes to zero, burst firing stops.

So next time you listen to the song ‘Wotan’s Fire’ from Swedish metal band Thyrfing, you will know that the band’s name has nothing to do with thyristors, though it should be pointed out we have shifted to Norse mythology at this point as the band is named after a Tyrfing, a Norse royal sword.