8.4 Avoiding avoidable losses in small fluorescent lamps

Advertisements in favour of electronic ballasts occasionally claim that in magnetic ballasts »up to 30%« of the luminaire’s total power intake is absorbed as losses. First of all, it remains to be noted that a statement like »up to«, very popular though it may be, is also totally inappropriate to make any statement at all, unless simultaneously complemented by indicating the mean and the maximum values (see this, p. 289). The same here: The greatest relative losses occur with the smallest lamps. This can be traced back to a law of nature once called »Paradox of the Big Machine«. In a 58 W lamp, for instance, it is only 13% (see section 8.4). Moreover, the piece numbers of smaller lamps are also smaller, and so their overall contribution to the total losses is all the smaller. So the indication »up to 30%« tells nothing at all. While, on the other hand, it is even disexaggerating. For instance, when measuring the power shares on a TC-S lamp rated 5 W and operated with a conventional magnetic ballast, a lamp power magnitude of 5.6 W may be found, along with once again the same magnitude of ballast losses, so in this case you may very well speak of 50% losses. Generally, however, 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, for longer lamps a larger share of the voltage drops across the lamp and a smaller share across the ballast. At the same time the current rating is a bit lower with the longer lamps, while the ballast remains the same (Fig. 8.4).

Fig. 8.3: One and the same ballast is designed for 4 different single lamps and (not listed here for reasons of space) 3 possible tandem configurations

However, the ballast losses are approximately proportional to the square of the current. So if you replace the 5 W lamp in one and the same luminaire with a 7 W lamp, which is not a problem at all if only the greater lamp length can be accommodated, under the bottom line you receive more lamp power at lower power loss.

Fig. 8.4: One and the same ballast is designed for 4 different single lamps and 3 possible tandem configurations

However, the ballast losses are approximately proportional to the square of the current. So if you replace the 5 W lamp in one and the same luminaire with a 7 W lamp, which is not a problem at all if only the greater lamp length can be accommodated, under the bottom line you receive more lamp power at lower power loss.

But this is still not the full story, since 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 and thereby the resulting lamp power when operated in tandem lie slightly below the ratings. In order to minimize the deviation, the magnetic ballasts are designed in a way so that in single mode the current and power magnitudes are slightly above the ratings. In total, the effect is that the ballast is always less loaded, the more lamp power rating is connected to it. More lamp load leads to an absolute drop in losses and thus, in relative terms, saves duplicate (Fig. 8.5).

Fig. 8.5: TC-D lamp 18 W, energy efficient magnetic ballast and electronic ballast for this (top) and energy efficient magnetic ballast for a commonplace T8 lamp of equal power rating (bottom)

Simultaneously, the lamp efficiencies improve when the lamps are not operated at full power, and inversely efficiencies drop when lamps are operated at overload (more about this in section 8.4). This was revealed during a measurement carried out by a well respected and independent lighting institute , recording not only the electrical values but along with these the light output (Table 8.5). In this test the 9 W lamp turned out at the end of the scale, since the 5 W and 7 W lamps had already disqualified themselves to participate at all according to the results of a pre-test displayed in Fig. 8.5.

Table 8.5: Comparison of electrical data and light outputs with small fluorescent lamps

Albeit, the light output efficiency with a tandem connection of two 9 W lamps on one magnetic ballast – and even an old, less efficient one – turned out equal to that of a high-end CFL and 20% better than a cheap CFL from the DIY supermarket! It remains to be stated here that the operation of a CFL is always an operation with an (integrated) electronic ballast! So much about the better lamp efficiency with electronic ballasts. Compared to the single-mode operation of one 9 W TC-S lamp the 2*9 W tandem configuration turned out 25% more efficient – with the same ballast, after all! However, the light output is a bit less than double that of the single lamp. This remains to be considered when designing a lighting installation.

But the tandem circuit is also applicable to T8 lamps with a power rating of 18 W. Although in this case different ballasts are meant to be used for single and tandem configuration, the results are similarly profitable. Here, too, the finding is that the power loss in the class B1 ballast attributable to two lamps is even lower than that in the class B1 ballast for only one lamp (Fig. 8.6).

Fig. 8.6: Split of total luminaire power intake for different TC-S lamp configurations with the same ballast

Now there are some more lamp types with a rating of 18 W available on the market, e. g. the TC-D lamp. But this one has a much higher operational voltage drop and can therefore not be operated in tandem mode. But since the voltage drop across the lamp under normal operating conditions is greater, the voltage drop across the ballast is smaller. So the required reactive power rating of the ballast is also selected accordingly smaller – and thereby the whole ballast is. But this is not yet all. When the lamp voltage is greater, the lamp current is also smaller and reduces the required reactive power level again (see section 5.2). Therefore a magnetic ballast for a TC-D lamp can be built extremely small, even when desigen according to efficiency class B1 – even smaller than a commensurate electronic ballast (Fig. 8.7)! So especially a luminaire with a TC-D lamp and a high-efficiency magnetic ballast saves space, production costs and energy in one go.

Fig. 8.7: 18 W fluorescent lamps in single and tandem mode comparison

The latter finds its confirmation when you add another light output measurement. For this reason the single and tandem operation modes of class B1 magnetic ballasts for 18 W and 2*18 W, respectively, were compared to a single and twin operation mode on an electronic class A2 ballast rated 18 W or 2*18 W, respectively. The result is compiled in 3 blocks of 7 measurements of the light flux F each, displayed in Table 8.6:

Table 8.6: Compilation of measuremets on 18 W fluorescent lamps with magnetic and electronic ballasts – a 240 V magnetic ballast having been measured in the middle erratically assuming it was a 230 V model; measurement therefore repeated in the row below

• One single T8 lamp,
• two T8 lamps in tandem or twin mode, respectively,one TC-D lamp,

with the following ballasts and data:

• Electronic ballast at the lower voltage tolerance limit 90% (207 V),
• electronic ballast at rated voltage (230 V),
• electronic ballast at the upper voltage tolerance limit 110% (253 V),
• magnetic ballast at the lower voltage tolerance limit 90% (207 V),
• magnetic ballast at rated voltage (230 V),
• magnetic ballast at the upper voltage tolerance limit 110% (253 V)
• magnetic ballast at the voltage magnitude where the light output equals that of the same lamp with electronic ballast at 230 V.

For measuring the T8 lamp in single-mode, a single-lamp electronic ballast was used instead of using the twin-mode one and connecting only one lamp, which would have been possible but would have yielded wrong results. The most crucial results can be found in Table 8.6, represented as the light efficiency ()tot in lumens per watt electrical power intake of the whole lamp and ballast system. The light efficiency cannot be given in per cent because regarding brightness the human eye is differently receptive to light of different colours. Therefore the sensitivity of a standardised average eye is already integrated into the unit for brightness. This unit is called lumen (simply the Latin word for light). So the efficiency of lamps and lumiaires has to be given in lumens per watt. So this and only this unit is adequate to assess which technical device provides the greatest brightness per power intake. Of course the share of ballast losses in the total power intake can be given as a percentage – as done in the last column of the table. However, with the electronic ballasts the required measurement of the lamp power, the ballast output power to the lamp so to say, was not possible due to the high output frequency. Therefore the efficiency η_Lamp of the lamp alone could not be calculated. Nevertheless, the following results can be read and conclusions drawn from the table:

1. The advantages of the tandem configuration and of the TC-D lamp already found in the pre-measurement with respect to reactive power find their confirmation.

2. The magnetic ballast power loss increases highly over-proportionally to the systems operating voltage. At 253 V the power loss is usually double as high as at 207 V. Together with the slight increase of lamp efficiency η_Lamp the voltage reduction practice results as an efficient means of loss reduction for all magnetic ballast configurations.

3. Inversely as with 58 W lamps (see section 8.4), the lamps are about 4% brighter with electronic than with magnetic ballasts. With the twin electronic ballast compared to the magnetic tandem configuration the difference is even 8%. The operating voltage on the tandem has to be turned nearly up to the upper tolerance limit of 244 V before the same brightness as with the electronic twin ballast is achieved.

Therefore when assessing the light efficiency two different approaches have to be considered:

4. Either the luminaires are operated at rated voltage in either case. The comparison will then be closer to what will usually happen in practice, though it is not objective. We are then talking about a systems power of 19.13 W with electronic ballast versus a systems power of 24.47 W with magnetic ballast. A payback time for the well over 5 W saved cannot be given, as the impact of the price premium for an electronic ballast upon the price for a complete lighting installation is subject to substantial variances. However, with an energy price of 10 c/kWh it takes 1872 operating hours to save the first Euro. This cornerstone can be used for the according conversions: At 5 c/kWh it takes 3744 hours, at 20 c/kWh it takes 936 hours to save 1 Euro.

5. Or you calculate objectively. Nobody will increase the line voltage in order to achieve precicely the same brightness with the used / planned magnetic ballast as with the electronic ballast not used, but the lighting planner might include a few more lamps if the decision for magnetic ballasts has been taken. This would have practically the same effect as if the same number of lamps were connected to a line voltage of 241.7 V, which would be equivalent to the difference between 19.13 W and 26.18 W systems power, say 7 W. So the real, effective »savings cornerstone« is then 1418 operating hours per Euro saved at 10 c/kWh.

6. Moreover, it becomes obvious that the limits of the EU directive, which is 24 W systems power in class B1 and 19 W in class A2, are in principle not complied with, neither by the magnetic nor by the electronic ballast. Only by being rather lenient accounting to metering inaccuracy the EEI classes can still be seen as just about fulfilled.

But by all means this mode of operation does not represent the optimal combination. The power loss in a 36 W ballast is not double the loss in an 18 W ballast (»Paradox of the Big Ballast«), about the triple advantage of the tandem mode not even to speak. Rather, the respective conclusions to above items 4 to 6 for the twin or tandem modes of two 18 W lamps will be:

7. Comparing the operation at rated voltage in either case, the difference between magnetic and electronic ballast operation is now only more 2 W per system, whereas a system now comprises two lamps and one ballast. So with an electricity price of 10 c/kWh it takes 5000 operating hours to save one Euro. Or, selecting a different example: At uninterrupted permanent duty with 8760 h/a and an electricity price which is usually quite inexpensive for such use, e. g. 5.7 c/kWh, the electronic ballast saves precisely one Euro per year.

8. With equivalent brightness, that is, assuming corrected voltage for the magnetic ballast (although, as mentioned earlier, hardly anybody will ever do this in practice) the difference is 6.6 W per system. With an electricity price of 10 c/kWh one saves one Euro in about 1500 operating hours.

9. Although the directive provides a separate line with limits for two lamps being operated on one ballast, the values per lamp are identical to those for the single-mode operations as under item 6. Very much unlike with the configuration described under item 6, however, the limits are by far kept here: The electronic ballast remains well over 1.5 W below the class A2 limit, the magnetic ballast even falls 3.5 W below the B1 limit.

On the TC-D lamp the following can be observed:

10. The efficiency is about 5% to 10% poorer than that of the T8 lamp. This may be due to the compact design which leads to a part of the light generated hitting the lamp itself.

11. Here the use of the electronic ballast results in an uncommonly high saving of 28% on equal voltage or 34% at equal light output, respectively. It by far fulfills the requirements for class A2, while the magnetic one does not really match the limit for class B1. The magnetic one may have been designed a bit too small in favour of facilitating the design of very small luminaires (Fig. 8.7 top right), and in electrical engineering skimping on active material (magnetic steel and copper) always comes at the price of reduced efficiency. It has to be considered, however, that these two measurements possibly cannot really be compared because they could not be carried out on the same lamp. The TC-D lamp for magnetic ballast operation is equipped with an integrated starter and therefore has only two connections (Fig. 8.7). The starter is wired internally. The version for electronic ballast operation requires four pins.

12. Unlike the other electronic ballasts used in this test, the one for this lamp is not equipped with an electronic power stabilisation to offset variances of the input voltage.