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Mains voltage interruptions and short-circuiting
For various reasons, the supply voltage can be subject to deviations;
therefore a certain degree of deviation from the rated value has been
taken into account everywhere.With gas-discharge lamps deviations
of up to +/- 10 per cent of the rated supply voltage normally have no
detrimental effects.
Apart from such ‘normal’ variations, in practice three possible
uncontrolled effects can be distinguished:
1) Short-circuit of the mains voltage.
2) A dip in the power supply voltage.
3) Interruption in the power supply current.
These phenomena can occur during a thunderstorm, when switching
from one power supply source to another or when connecting heavy
loads to the mains, and are usually of very short duration.This is a
good thing too, since a single dip of 10 milliseconds (half a cycle) or
even less, can have a significant influence: the lamp will extinguish.
As the fluorescent lamp re-ignites in only a few seconds or even less,
these phenomena hardly give problems in practice (see also section
4.1.14: Effects of mains voltage fluctuations).
Harmonic distortion
All gas-discharge lamps stabilised by copper/iron ballasts have harmonics
in the lamp current.The first reason for this is that the lamp voltage
(= the voltage across the discharge tube) is more or less a square wave
of changing polarity every half cycle (see Fig. 122).
This is graphically represented as a square wave voltage, made up by
Fourier analysis as the fundamental sine-wave of the mains supply and
a large number of odd harmonics (see Fig. 123).
The voltage across the ballast is the vectorial difference between the
supply voltage and the lamp voltage, so the harmonics of the lamp
appear in the ballast voltage.As the ballast determines the current,
there will be only odd harmonics in the lamp current. Even harmonics
are not present.
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Fig. 122. Square wave form of lamp
voltage.
3.8 Mains voltage interruptions and short-circuiting
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