Fluorescent fittings and power factor.

[electroanorak]

Question: Does a four-tube fluorescent fitting consume less power if three of the tubes are removed?

Answer: No....

The place where I moved into was full of the office type fluorescent fittings (the type that go into suspended ceilings). They are made by Fitzgerald.There were both 2 foot and 4 foot types.

With so many units burning away, I decided to remove three tubes from each, to keep the electricity bill down. We are only doing building work, so that was fine.

After a few months, we got our first electricity bill of £969.00. I thought that it was a little high, so I questioned it. A check revealed that the meter was accurate, so I decided to find the problem.

I fitted a Gossen Merawat A2000 energy meter/logger to record our usage. It was whilst experimenting with the readings on the PC in the office when I turned on the lights (two of the 4 foot units with only one tube in each). I was shocked to see an increase in current on the blue phase of over 2 amps.
I thought that I would try adding more tubes and with each tube added, the current decreased until when they were all in, there was a reading of 1.2 amps.

I then tried it with the lights in the foyer area (12 of the 2 foot units). With just four tubes on (two in one unit and two in another), just over 2.1 Amps. With all tubes in (48 in all) and a reading of 2.6 Amps.

I can only put this down to the PF capacitors being calculated for all four tubes.

Any thoughts?

[Andy Green]

If you have a dual channel scope it would be interesting to put one channel across the mains and another across a small resistance in series with the neutral. Compare the waveforms phase relationship with 1 and 4 tubes in place. If PF is out one will lead the other, if not they will be in phase ( or poss 180 degrees out)

[Herald1360]

Were you measuring real power with the meter or VA (volts * amps)?

A fluorescent tube ISTR operates with about 80V across it, so for 40W, it takes half an amp. Mains is 240V so a choke is connected in series to drop the extra volts losslessly (well nearly). With no correction capacitor, your lamp is taking half an amp at 240V which is 120VA BUT it is only consuming 40W of actual power. The reason is that the current is no longer in phase with the voltage but lags it because of the choke. The power factor of this setup is 40/120 or only about 0.3- pretty awful. The electricity supplier doesn't like low power factors because they charge only for power consumed but would have to provide a network capable of delivering three times as much current in this simple example as for a unity power factor load of the same wattage. So for large installations, they insist on a decent power factor or else they will charge for current instead of power.

Without knowing exactly what the circuit of the fittings is, it's difficult to say anything precisely, but I would doubt that removing tubes increases the power consumed though it would probably worsen the power factor considerably. I would expect for minimum cost each fitting would have only one PF correction capacitor even if each tube has its own choke, so removing tubes will upset the power factor as there will then be too much capacitive (leading phase) current.

In a large installation, the fittings are probably equally distributed amongst the phases, so it might be possible to increase current on one phase at the expense of the others by selective tube removal.

[Herald1360]

Tried a simulation of four paralleled L-R series circuits in parallel with one PF correction capacitor. With one L-R removed, current fell slightly and PF dropped as phase shifted away from zero. With two L-R removed current was now higher than original and PF even worse. With three L-R removed current was now about twice original with PF even further down.

Actual power consumed of course dropped as expected to 3/4, 1/2 and 1/4 the original figure. (Well the simulated inductors were perfect )

[Nicklyons2]

Herald's experiment shows why the electricity suppliers desire the power factor to be as near '1' as possible; that increase in current causes resistive losses in cables etc to heat things up BUT the lousy PF does NOT show on a KWh meter the amount of energy actually being used. This means the suppliers get potential damage from heating in cables & transformers and don't even get paid for what (Watt) you're using. At one time the 'boards' penalised industrial consumers who did not keep the PF better than 0.8 - I'm a bit out of touch in this area these days - I'd be interested to know if this situation still exists.

[mike_newcomb]

It seems to me that nowadays many installations have far more fittings/tubes/bulbs than required to provide quality light.

For example, recently I was in a hospital waiting room, which was smaller than my own average sized living room. It was morning and perfect daylight was being supplied via large external windows.

However in the suspended ceiling were 12 light fittings, each with 2 bulbs, all switched on, appearing to be controlled from one light switch. Ok, the bulbs were low wattage but there is still a running cost. It went through my mind, how much would the NHS save if such installations and use of such were better thought out.

I used to look after a showroom, which had far more 4 tube flourescent light fittings than necessary. As the circuitry in each was duplicated, in each fitting we turned one pair of tubes 90 degrees such that they did not operate and also removed their starter. In addition we only switched on the lights when required, instead of being on throughout the (11 hour) day. This saved much.

In the same showroom were many spotlights. We found replacing 50 watt bulbs with 30 watt ones made no appreciable difference to light quality.

Choosing most suitable colour for tubes and bulbs is important for light quality.

Good Luck - Mike

[G6Tanuki]

Originally Posted by mike_newcomb

It seems to me that nowadays many installations have far more fittings/tubes/bulbs than required to provide quality light.

Lots of light-sources are the best way to minimise shadows in a working environment; it's hard to guarantee the recommended 500-750 Lux shadow-free illumination between 200 and 2000cm for a tyoical office-environment if you have only a few point-sources of light.

Linear tubes are better than old-style round incandescents. If you want to use spotlights you need lots of them to avoid the few-point-sources-cause-shadows thing.

My pet hate is how almost everywhere these days has "warm white" lighting (around 2700-3000K colour-temperature) which means white paper looks yellow and all the people look like they've got jaundice.
I recently switched most of the lighting at home from CFLs and GU10 Halogens to 6500K "Cool White" or "Ice-white" colour-spectrum LEDs and the improvement is dramatic: I really like 6-watt CREE LEDs! - except that the extra brightness and 'daylight' clarity really does show up the unevennesses in the plasterwork.

[Boom]

Replace the choke with a 100W light bulb and it will kill the PF stone dead to give true readings.

[julie_m]

But using a resistive ballast will mean you are paying for what it drops, instead of just storing up the energy and releasing it half a cycle later.

What you really need to do is to run half your lamps with series chokes, and the rest with series capacitors

[Boom]

Originally Posted by ajs_derby

But using a resistive ballast will mean you are paying for what it drops, instead of just storing up the energy and releasing it half a cycle later.

Not really true. Chokes get very hot in use and the heat has to be paid for.

[Herald1360]

But "very" hot for a choke is only about "ouch" or 60degrees. To raise the choke temperature that much might need about 10W. Much less than the 80W or so a resistive (bulb) ballast would waste. Also, you would get no useful inductive kick from a resistor to help the tube strike when the starter operates.

I think the basic answer to the original question is that removing tubes will reduce power consumption but will also increase current taken and provide a lousy power factor load. To keep things sensible PF wise, you would need to remove all the tubes from some of the fittings, which might not be particularly good for looks or even illumination, rather than some of the tubes from all of the fittings..

[broadgage]

There are numerous different fluorescent lamp circuits for multi lamp fittings.

Four lamps each 2 foot would normally be two series pairs each in series with a choke, four lamps, two chokes.
A single capacitor would be connected accros the mains to correct the power factor.
Use of only one pair of lamps would result in the power factor capacitor being too large, and a slightly leading power factor.
I would not expect the power factor in two lamp mode to be highly leading, remembering that in four lamp mode the capacitor is sized to only correct up to about 0.8, not to unity.

The watts used in two lamp mode MUST be about half that used in four lamp mode, since two lamps and one choke are using energy, rather than four lamps and two chokes.
A true watt meter is needed to determine current, not a current meter.

A less common four lamp circuit for two foot lamps again consists of two series pairs of lamps, one has a standard series choke and the other pair has a non standard series choke AND a series capacitor.
This circuit gives a good power factor but is slightly more expensive, and has the drawback that the actual lamp wattage varies if the capacitor value is different from the nominal.
The lamps with the series capacitor tend to have a shorter life.
If such a circuit is operated with only two lamps lit then the power factor will be strongly leading or lagging according to which pair is used.
The true watts used as determined by a proper wattmetter will about halved by use of two lamps.

I have seen four lamp fittings in which BOTH pairs of lamps had a series capacitor, this would give a strongly leading power factor and be useful for replacements or additions in an existing installation in which the power factor was excesively lagging.

[broadgage]

Originally Posted by Boom

Not really true. Chokes get very hot in use and the heat has to be paid for.

Chokes do indeed get very hot and the energy thus used has to be paid for, but chokes produce less heat and waste less energy than a simple dropper resistance, which is why they are used.
Fluorescent tubes can be used with a resistance dropper but this is not often done due to the waste of energy.

[Andy Green]

I remember seeing some really old fluorescent fittings in a derelict building about 30 years ago which had incandescent bulbs incorporated at the end of the fitting. Presumably they were ballast and may have also helped even out the colour temperature? That's what the radio engineer who had access to this building told me at the time anyway!

[Lucien Nunes]

The fluorescent lamp itself is non-ohmic, therefore with a resistive ballast the the distortion power factor deviates from unity even without phase displacement, making accurate calculations of real power impossible from current and voltage measurements. This is a separate consideration from the reduced displacement power factor incurred by wirewound ballasts that can be corrected with capacitors.

Lucien

[broadgage]

Originally Posted by Andy Green

I remember seeing some really old fluorescent fittings in a derelict building about 30 years ago which had incandescent bulbs incorporated at the end of the fitting. Presumably they were ballast and may have also helped even out the colour temperature? That's what the radio engineer who had access to this building told me at the time anyway!

Yes some fittings did use a series tungsten lamp as a ballast, the idea never found much favour because the lamps were expensive and non standard types, and even small changes in the supply voltage resulted in very significant changes in the brightness of the tungsten lamp.

Some fluorescent fittings incororporated filament lamps that were standard 240 volt lamps, not ballasts. This was done to improve colour rendering, provide directional light by use of reflector filament lamps, give a choice of lighting levels, provide emergency lighting, or for other reasons.

[turretslug]

Originally Posted by broadgage

Some fluorescent fittings incororporated filament lamps that were standard 240 volt lamps, not ballasts. This was done to improve colour rendering, provide directional light by use of reflector filament lamps, give a choice of lighting levels, provide emergency lighting, or for other reasons.

Some workshops with high ambient noise levels and exposed rotating machinery had combined lighting- the discharge light produced the efficient bulk of the illumination, the incandescent component prevented the possibility of dangerously deceptive strobing effects.

[broadgage]

Yes the avoidance of stroboscopic effects is most important if moving machinery is innvolved.
If half the lamps are run via series capacitors, then that not only improves the power factor but also nearly eliminates stroboscopic effects. Presuming of course that each machine etc is lit by roughly equal numbers of lamps with series capacitors or without.

The use of low power mains voltage filament lamps is nearly as bad as fluorescent lamps for stroboscopic effects.
Higher wattage lamps are fine as are reduced voltage lamps, since the degree of flicker is inversely related to lamp current.

In the early days of electric power for factories this was often at a much lower frequency than today and lamp flicker was obtrusive, and in the presence of machinery potentialy dangerous.
Solutions included retaining gas lighting or a seperate DC supply for electric lighting. Small lamps on machine tools sometimes used 6 volt lamps supplied from a transformer, the very thick filament of a 6 volt lamp not being liable to flicker even on 25 cycles.