Fluorescent myth

1 min read

Andrew Porter’s letter ’LED is the answer’ (Talking Point, 21 May) is rather misleading, and it would be unfortunate if it were to create an urban myth about fluorescent lamps since they are undoubtedly very effective in reducing the power demand from lighting.

Traditional fluorescent lamps with magnetic ballast, sold for domestic use, can as Mr Porter says have PF as low as 0.3. This means the current is rather more than three times that needed for the power. The result is an increase in losses in the supply network and the generator, which requires an increased excitation to produce the VAr. There is no added load on the prime mover other than the very small additional network losses.

Even if we take a very pessimistic supply resistance of 0.4W (taken from IEC 60725) then the VAr from a 40W lamp with 0.3 power factor causes an increased loss of only 0.11W. However the 100W incandescent bulb it replaces causes nearly as much loss (0.07W) — let alone the additional 60W of heat produced.

But in reality things are not even this bad. Traditional fluorescent lamps for commercial premises have internal power factor correction, and the most popular domestic compact fluorescent lamps have factors of more like 0.5, so the supply current and network losses are still less than for the equivalent incandescent bulb.

Finally, the low PF of CFLs is caused not by a phase-lag in the current but by the harmonic currents generated by the rectifier — in the same way that rectifiers of most electronic equipment generate harmonics, including the power supplies for LEDs. The harmonic current does have a potential EMC impact, but this can be managed quite readily by using electronic power factor correction where necessary.

Colin Hargis

Oswestry, Shropshire