5.2 Special aspects when compensating small lamps

The operating voltage drop across smaller, i. e. shorter fluorescent lamps of the same type family is lower than with the longer types of the same series. Thereby a larger part of the voltage drops across the ballast, and this voltage drop is greatly – in the ideal case would be wholly – inductive. So on the one hand the smaller lamp has a lower active power intake, but on the other hand it has a higher reactive power dissipation. Commonly, these two effects lead to a substantially lower power factor for the lower lamp power rating. So the compensation investment increases inappropriately. This can be observed very clearly on TC-S lamps with 5 W, 7 W, 9 W and 11 W power rating, since these 4 models are all operated on the same ballast (Fig. 5.8). However, the operating voltage drop across the TC-S lamps rated 5 W, 7 W and 9 W is so low that the common mains voltage of 230 V allows two of these lamps to be operated in series on one ballast. In effect, this doubles the operating voltage drop again, of course. Since the same ballast is used for this so-called tandem connection as for the single operation, the actual current when operated in tandem lies slightly below the lamp current rating – though not very much, since the inductive voltage drop still prevails. One of the advantages of this operating mode is that two lamps together use less reactive power than one of them already does in single mode (Fig. 5.9). But the tandem configuration may very well claim even more advantages than this (see section 8.3).

Fig. 5.8:One and the same ballast is designed for 4 different lamp types as well as for 3 tandem connections (only one of them listed here for reasons of space); the power factor increases substantially with the lamp power rating connected

Fig. 5.9: Power factor as the ratio of active power (grey benches) plotted against reactive power (blue benches)