It is not so sure whether the undoubted, measurable rest of the efficiency improvement beyond the 4% difference of light output with electronic ballasts really bases on the high frequency – or perhaps rather on the current waveshape fed into the lamp? It was tried to find this out by means of another measurement at a special independent lighting institute. The idea behind this was another found statement that the efficiency of a fluorescent lamp is not optimal at rated current but better at lower current, as is the case with a lot of electrical equipment, incandescent lamps exempted. If this is valid for the TRMS or arithmetic mean value of same current, then it also goes for each and every instantaneous value along the curve. So, with sine current, efficiency drops within the range around the peak, since most of the light is generated during this time span. If the output current of an electronic ballast were rectangular, then there would be no efficiency drop at any point of the curve – and energy efficiency would be better, because this constant value would be considerably lower than the peak value of a sine wave. Indeed the output current looks more like a rectangle than like a sinus (Fig. 8.8).
Fig. 8.8: Output current curve of an electronic ballast (H.-G. Hergesell, Paderborn Airport), recorded with 3 different power analyzers
Fig. 8.9: Test samples for the measurements documented in Fig. 8.10 and Table 8.7
If this is so, then it should be possible to achieve the same efficiency improvement by lowering the overall current. The generalizing conclusion may be justified that higher power intensity is bad for efficiency.
Fig. 8.10: Efficiency of various ballasts with the same lamp at varying voltages according to Table 8.7
The values in the Directive refer only to rated power, but what happens at reduced power, e. g. when a lamp with magnetic ballast is fed only with the power rated for operation with an electronic ballast (Table 8.1) or even substantially less? To find out, 5 different ballasts for a 58 W lamp were taken under test (Fig. 8.9):
Now on each of these 5 samples all required parameters were measured, always using the same lamp: Active and reactive power across the whole system, active power (loss) across the ballast, and of course the light output of the lamp. All of the results have been compiled in Table 8.7) but, the graphic evaluation of this verbose table included in the download version of this text. Among others the 4% difference of luminous density in favour of magnetic ballasts (»5000 lm at rated voltage versus »4700 lm with electronic ballast) finds its confirmation here, but beyond this, the graphic evaluation of this verbose table provides much more ease of interpretation (Fig. 8.10). Unfortunately, on account of the high output frequency at the terminals of the electronic ballast, it was not possible to measure its output power. This is not a tragedy, though, since the most important data, system input power and light output, could be measured. The following can be concluded from the results:
Table 8.7: Measurements on 5 different ballasts at different line voltages with the same lamp
The high variance of efficiency even with moderate voltage reduction on a lamp circuit with whatever type of magnetic ballasts has three main reasons:
To offset the lower absolute light output, about 150 magnetic ballast luminaires operated at 190 V would have to be used to replace 100 electronic ballast luminaires. Now since the 150 magnetic ballast luminaires are simultaneously the more energy efficient solution, a cost premium would be acceptable in replacing the electronic with magnetic ballasts in order to save energy, inverting the usual approach. Still, this need not necessarily be any more expensive. Cases have been reported where the solution with 100 electronic ballasts has been bid higher. So the payback time may assume a negative value! Adding the cost for voltage reduction, it is still very short. In two example cases from Switzerland 50 open longitudinal 58 W luminaires were bid alternatively with electronic ballasts at 2575 SFr and 50 commensurate luminaires with magnetic ballasts, regardless of efficiency class, at 1700 CHF. So no premium was charged at all for a better efficiency class of the magnetic ballast, but it was very well possible to get 150 lamps equipped with these at a lower price than 100 luminaires with electronic ballasts. Whether the price premium in such cases really improves the electrical contractors’ businesses or whether the electronic ballast merely adds to the turnover but cuts revenues is yet another question to be critically scrutinized in each individual case.
In May 2000, being informed about this, the EU made an amendment to their document that any other measure judged appropriate to improve the inherent energy efficiency of ballasts and to encourage the use of energy-saving lighting control systems should be considered.
Indeed, in Germany there are at least three manufacturers of dedicated voltage reduction plant that is meant to operate fluorescent lighting at reduced voltages. Refurbishment in existing installations is easy as long as dedicated power lines for the lighting have been installed. Occasionally voltage reducers are also offered for the general supply but these have to be treated with care. Many power consuming devices have the inverse behaviour as fluorescent lamps with magnetic ballasts. Incandescent lamps, whenever living a lot longer, yield a dramatic loss of energy efficiency. Induction motors as well as practically all electronic devices, including decent electronic ballasts with constant light output regulation, have an increased instead of decreased current intake with reduced line voltage. Ohmic losses in the mains and especially inside the motor increase instead of decreasing. Also electronic ballasts of the type tested here, with constant regulated light output, react in this way and therefore cannot be influenced by varying the line voltage. With fluorescent lighting, however, the loss of luminous density can be offset by installing additional lamps – or simply taken for granted, which often is acceptable.
On the other hand, the undervoltage extends the lamp life by about 33% ... 50%, the voltage reduction plant manufacturers claim. The trade association of German lamp manufacturers points out that also the opposite can happen because the optimum filament temperature is not reached. So far it can only be concluded from the conflicting statements that this issue has not yet been experimentally investigated. Life time tests of longlife devices take a long time by definition.