4.1 Emission of disturbances

As mentioned before, magnetic ballasts provide a mature and long-proven technique unlikely to cause any trouble or damages to other power consumers or the supplying voltage. One possible source of disturbance, which is more likely today to cause damage or malfunction to modern sensitive equipment than was the case in the past, is the voltage peak generated by self-induction in its high inductance at the instance of turn-off. Normally this will not be a problem, since lamps are hardly ever operated in parallel with such equipment on the same circuit with a common switch, but in one case it did happen. This rather uncommon damage could only occur on account of this exotic constellation (Fig. 4.1) but it should be mentioned that it may be a bit less exotic to parallel magnetic ballast fluorescent lamps with electronic halogen lamp transformers. Cases have been reported where the latter have repeatedly been destroyed by the turn-off self-induction surges of the former, and a special surge protector has been developed. Yet the problem could as well have been avoided by paralleling the two lamp-ballast-units with a capacitor. An appropriately dimensioned compensation capacitor will form a resonance frequency equal to the mains frequency, and the AC will therefore softly sway out after the supply voltage is turned off. With a smaller capacitor the resonance frequency is higher, and the turn-off voltage peak is »only« substantially attenuated, not entirely avoided, but the height of peak is not as crucial for the likelihood of disturbances as the rise time edge, which is attenuated very much through even a small capacitance.

Fig. 4.1: Unhealthy parallel connection of an electronic load and a highly inductive load on one common power switch

In another case an old Commodore computer locked up every second time the 18 W fluorescent lamp in the bathroom of an old residential building was turned on. The home was TN-C wired, without a dedicated earth / protective conductor but only two cores in all single-phase supply lines and an interconnection between the neutral and protective earth connectors inside each single socket. This alone may have been the cause for the trouble or at least may have contributed to it [2], but anyway a capacitor connected in parallel with the ballast and lamp solved the problem.

It remains to be amended that there is no voltage surge when turning on the current in an inductance. The mentioned lock-ups in fact did not occur when pressing the light switch but when the starter tried to fire the lamp, which is basically a turn-off process of a reactor current, intentionally generating a voltage surge to get the lamp started (section 3.1).

Fig. 4.2: Symmetric ballast

In some cases sensitive equipment may be disturbed by the high frequency emissions that the lamp as a gas discharge device emits, even if operated at mains frequency. In these cases it sometimes helps to just swap the polarity. Care should be taken that the ballast is always connected to the phase and the lamp to the neutral as an earthed conductor, not vice versa. This reduces the likelihood of described trouble. If it still occurs, a so-called symmetric ballast may help, the inductance of which is split in two halves, each of them to be connected to one end of the tube (Fig. 4.2). Beyond, only the usual commonplace filters will help, whenever these, if used excessively, may cause a leakage current problem. Inrush currents, however, are generally not a problem with magnetic ballasts. Their inrush currents are not that high. Further attenuation can be achieved when serial compensation is applied (bottom of Fig. 3.1), while parallel compensation (chapter 5) adds the inrush current of the capacitor, which has very steep rise time edges and may therefore very well become a problem.

Fig. 4.3: Operating 3 fluorescent 58 W lamps with magnetic ballasts on 3 phases, sum of the phase currents forming the neutral current

When talking about the harmonic disturbances of electronic ballasts it is frequently alleged that magnetic ballasts also cause current harmonics, while this is not really so. The ballast itself is a linear element if designed properly, so as not to let the core material enter the range of magnetic saturation under normal operating conditions, which would be highly disadvantageous from a power quality as well as an energy efficiency viewpoint. Rather, the non-linear behaviour of the lamp itself causes an extreme magnitude of voltage distortion (Fig. 3.7, Fig. 3.8) but which on account of the high inductance of the ballast causes only little current distortion. So no nameworthy disturbance appears across the terminals of the luminaire. The harmonic load on the neutral conductor with lamps spread equally across the three phases is correspondingly low, in the case of 58 W lamps the simulation reveals about 35% of the phase current (Fig. 4.3).

Sometimes noise is mentioned as a type of disturbance from magnetic ballasts but this, if it occurs, is a case of faulty lamp design or fabrication. A faultless ballast alone does not produce any noise, but if it is fixed to a metal sheet surface in the luminaire this has to be done adequately: Tightly but including washers made of rubber or plastics. Otherwise mains frequency humming or buzzing may occur.

As another disturbance the inevitable permanent flicker at double the mains frequency is often mentioned. In some locations, where rotary machines are worked with, this can become dangerous on account of a stroboscopic effect that may make the rotating machinery appear to stand still or at least cause heavy optical misconception of its rotary speed or even the direction of its rotation. This, however, can easily be avoided by spreading lights equally across the three phases of the supply or by simply applying lead-lag connection (section 5).

Apart from this, it remains to be noted that with TV sets the 100 Hz technique is regarded as the latest flicker free development.