5
3.8 Mains voltage interruptions and
3 8 Mains voltage interruptions and
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
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
3 9 Harmonic distortion
All
This is graphically represented as a square wave voltage, made up by Fourier analysis as the fundamental
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.
Vm
Vl
I
Fig. 122. Square wave form of lamp voltage.
128