McCready00
Active Member
how do you actually know the power factor for a moving light?
power factor
- Thread starterMcCready00
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bishopthomas
Well-Known Member
What is the "power factor?" Based on another of your threads I am going to assume you mean the current draw. The most accurate way is read the manual. But if you want a close approximation then just divide the wattage of the lamp by the source power. So, a MAC250 on a 120 volt circuit will draw approximately 2.08 amps. Add a little to take into account the motors and other circuitry, and just call it an even 2.5 amps. So on a 20 amp circuit you can put up to 8 MAC250's. Of course, these are all approximations, and therefore I go with 6 on a circuit. For exact numbers one day I'll crack open the manual.
Actually, power factor is a technical term that does not mean current draw. It is related, though. Power factor has to do with reactance. Reactance and resistance together make up the impedance of a circuit. There is inductive reactance and capacitive reactance. In a traditional conventional unit (incandescent) the filament is the only real resistance, meaning there isn't any reactance and therefore the power factor is 1, allowing you to use W=VA to determine your power needs. The internal components of a moving light (motors, fans, etc.), however, combine to give the overall fixture a certain reactance. It is this reactance that determines your power factor. The only way of finding a fixtures power factor, short of finding the reactance of every internal component and adding them up, is finding it in the manufacturers literature. You seem to understand what/how to use a power factor, so I will not go into it unless you would like me to.
Hope this helps,
-Tim
Hope this helps,
-Tim
by Richard Cadena
JD
Well-Known Member
The easy way of looking at power factor is that the phase alignment between when peak current draw and peak voltage do not match.
The best example is cheap HID fixtures, such as the 70w HPS fixtures on sale at Home Depot and other places. They should be drawing 70 watts plus a few extra watts for what is wasted on the ballast. About 0.58 amps at 120 volts, right? Nope! Try over an amp! These fixtures have a power factor of 47 (or .47 depending on if real or %)
If you multiply the current draw and the supply voltage you come up with a figure MUCH higher than 70 watts. What is happening is that because of the reactor ballast, the current draw peaks -after- the line voltage has peaked. As the fixture is collecting it's energy at a point lagging behind peak voltage, it's current draw is much higher than you would expect. This can be corrected with a power factor correction capacitor. (Required on all indoor type fixtures, or more specifically, low power factor is not allowed.)
For the most part, movers, even the cheap ones, do include the PF cap.
Switch mode supplies also have a bit of a problem, but no where nears as dramatic. The supplies do draw peak current at peak voltage, but draw almost no current outside of the peak region! That is why in the computer world, battery backup systems are rated in VA. This gets confusing as one would think V time A = watts, right? Nope. In this case they are specing maximum current times maximum voltage, which is how a switch mode supply draws power. The maximum wattage output of the battery backups is actually derated from the VA rating. (Note the use of the word "maximum", not peak)
The best example is cheap HID fixtures, such as the 70w HPS fixtures on sale at Home Depot and other places. They should be drawing 70 watts plus a few extra watts for what is wasted on the ballast. About 0.58 amps at 120 volts, right? Nope! Try over an amp! These fixtures have a power factor of 47 (or .47 depending on if real or %)
If you multiply the current draw and the supply voltage you come up with a figure MUCH higher than 70 watts. What is happening is that because of the reactor ballast, the current draw peaks -after- the line voltage has peaked. As the fixture is collecting it's energy at a point lagging behind peak voltage, it's current draw is much higher than you would expect. This can be corrected with a power factor correction capacitor. (Required on all indoor type fixtures, or more specifically, low power factor is not allowed.)
For the most part, movers, even the cheap ones, do include the PF cap.
Switch mode supplies also have a bit of a problem, but no where nears as dramatic. The supplies do draw peak current at peak voltage, but draw almost no current outside of the peak region! That is why in the computer world, battery backup systems are rated in VA. This gets confusing as one would think V time A = watts, right? Nope. In this case they are specing maximum current times maximum voltage, which is how a switch mode supply draws power. The maximum wattage output of the battery backups is actually derated from the VA rating. (Note the use of the word "maximum", not peak)
Last edited:
McCready00
Active Member
Ok ill look at it tonight thanks..
Last question though, if by calculating on a fixture, it gives me a total of 0,60 for its power factor, usually, are all other fixtures of the same type will give me the same/or very close power factor number?
When i say "same type of fixture" i meant,the exact same type of fixture...
Last question though, if by calculating on a fixture, it gives me a total of 0,60 for its power factor, usually, are all other fixtures of the same type will give me the same/or very close power factor number?
When i say "same type of fixture" i meant,the exact same type of fixture...
Yes, every fixture of the same make and model will have the same power factor. What fixture are you using with a power factor of .60? That is really low...
By definition, power factor is the ratio of the real power (in watts) to the apparent power (in VA). In a single phase system, it is mathematically the cosine of the phase difference between the voltage and the current.
You should be able to find a wattmeter which will measure real power and if you also measure voltage and current, you can find apparent power and then power factor...
A power factor of 0.6 is ugly. Most utilities will specify a minimum PF in the order of 0.9 if you want to be supplied...
However 0.6 is about the sort of power factor you get from a switchmode that does not have power factor correction installed...
You should be able to find a wattmeter which will measure real power and if you also measure voltage and current, you can find apparent power and then power factor...
A power factor of 0.6 is ugly. Most utilities will specify a minimum PF in the order of 0.9 if you want to be supplied...
However 0.6 is about the sort of power factor you get from a switchmode that does not have power factor correction installed...
Just to add to the confusion. Since Power Factor is a result of a reactive loads and manifests as a phase shift between the current and the voltage, it can either lead or lag depending on which is ahead of the other.
It is possible to compensate for a load with low power factor by introducing reactive components that provide an equal but opposite power factor into the circuit. There are a number of devices for doing this, like banks of capacitors or inductors, synchronous motors, and the like.
A qualified electrician should be able to point you at solutions to keep your electricity supplier happy.
It is possible to compensate for a load with low power factor by introducing reactive components that provide an equal but opposite power factor into the circuit. There are a number of devices for doing this, like banks of capacitors or inductors, synchronous motors, and the like.
A qualified electrician should be able to point you at solutions to keep your electricity supplier happy.
JD
Well-Known Member
The reason power factor is of concern to utilities is that the extra current used is very real. The result is that there will be extra heating on all of the supply equipment, and a greater deal of voltage drop. Because the equipment is "feeding" itself at a lower voltage (off phase), greater current demand is made to obtain a given wattage. The greatest offenders are induction motors running under little load, and HID fixtures that are not PF corrected.
Good example- We recently had 10 MH fixtures installed on the rooftop of our church, each at 100 watts. (Total 1000 watt load.) These replaced fixtures that had been incandescent 150 watt spotlights. So, even though we shed 500 watts, the breaker feeding them (20 amps) began tripping. This usually occurred after they had been on for a few hours. I took one of the fixtures apart and noticed the PF cap was not included (it was an "option".) I tried to explained to our Vestry that if we added this part the fixtures would draw less power. It was as if I was trying to get them to believe in Tinkerbell or the Tooth Fairy!! ... Finally got them to ok adding the caps, end of problem! The thing about PF is on the surface, it just seems counter-intuitive. Yet, there it is!
Good example- We recently had 10 MH fixtures installed on the rooftop of our church, each at 100 watts. (Total 1000 watt load.) These replaced fixtures that had been incandescent 150 watt spotlights. So, even though we shed 500 watts, the breaker feeding them (20 amps) began tripping. This usually occurred after they had been on for a few hours. I took one of the fixtures apart and noticed the PF cap was not included (it was an "option".) I tried to explained to our Vestry that if we added this part the fixtures would draw less power. It was as if I was trying to get them to believe in Tinkerbell or the Tooth Fairy!! ... Finally got them to ok adding the caps, end of problem! The thing about PF is on the surface, it just seems counter-intuitive. Yet, there it is!
John's example is a good illustration of the possible issues that can arise from power factor. Noteworthy with the increasing push to have evryone move to use CFLs instead of incandescant lamps. while it is unlikely that a homeowner will install enough CFLs on any circuit to trip the breaker it does add to the power factor problem for the hydro generation people - typical CFLs have a power factor of 0.5.
The power companies force commercial hydro customers to address the power factor issue by installing two meter types: the usual meter you see at home that measures "resistive power" where the voltage and current are in phase and the VAR meter that measures the power where the current and voltage are out of phase. the VAR meter determines the billing rate and the farther apart the readings are for the two meters the more you are penalised. This motivates the user to correct their power factor to as near 1 as possible.
The issue is for the hydro companies is more than just the extra power they need to generate it is also about matching the load to the generation capability to avoid line regulation issues.
The power companies force commercial hydro customers to address the power factor issue by installing two meter types: the usual meter you see at home that measures "resistive power" where the voltage and current are in phase and the VAR meter that measures the power where the current and voltage are out of phase. the VAR meter determines the billing rate and the farther apart the readings are for the two meters the more you are penalised. This motivates the user to correct their power factor to as near 1 as possible.
The issue is for the hydro companies is more than just the extra power they need to generate it is also about matching the load to the generation capability to avoid line regulation issues.
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