 whizkid3Premium,MVM
join:2002-02-21
Queens, NY
kudos:8
4 edits | reply to SmokChsr
Re: AC /KW? Question for the electricians. said by SmokChsr:said by eightball009:SmokeChaser, how does the power factor fit in? That is the efficiency of the system. Wouldn't the RMS voltage be the right voltage to use?
No, Power factor and efficiency are totally different things. Exactly. Power factor has almost absolutely nothing to do with the efficiency.
Power factor results in current that must flow in the circuit, primarily to charge the magnetic field of an inductive load each half-cycle. This same current flows back to the utility each half-cycle. Its not used to do any Work (as in the physics sense of the word.) Energy related to power factor is known as reactive power; and is not lost or used. Generally, residential clients are not billed for reactive power usage due to low power factor. Commercial clients are also not generally billed directly for this reactive power. However, generally if commercial customer's power factor is below a certain threshold, they are charged a steep premium for their electricity. (These are generalizations. There are places that bill residential clients for low power factor; there are places where commercial client don't pay; and there are places where commercial clients are billed directly for reactive power used - even though its not used.) The problem with reactive power is the utility still must produce it, and transmit it. Their generators must be larger and so must their transmission system to carry it. This costs money.
Getting back to things, power factor and efficiency are two different animals. Efficiency is energy you use to perform Work. Unfortunately, its not for the work you want to do. If a motor is inefficient, it will waste energy by turning it into heat, rather than sending it to its shaft to drive its mechanical load. While you may not need the heat at all - this energy is produces does perform Work, by heating the space. And you may waste more energy, running an air-conditioner more to remove the heat generated from efficiency losses from the building - a double hit. Efficiency is simply the percentage of energy used to produce the required function (i.e. cool the air in an A/C), divided by the total energy consumed through the power cord.
As for RMS voltages - this is all we talk about. Nominal RMS voltages in North America are 120V, 208V, 240V, 277V, 480V etc. (There are a bunch more, but they are not as prevalent.) No one says '170V' (the peak voltage), when they are talking about the 120V (RMS) receptacle on their wall.
If one wants to determine (roughly) the energy usage, this is the formula:
(BTU/h) / (SEER) = Average power consumed in watts.
If one is trying to design the electrical system, these are the formulas showing the relationship between amps, volts & power:
Amps x Volts (RMS) * power factor / 1000 = kW
kVA * power factor = kW
said by SmokChsr:To go a bit further, it's most likely a 240V 30A circuit. If it's on a 30A circuit, that would say that the unit should draw less than 30 Amps. (WhizKid3 will know the % of overrate the circuit needs to be respective to the load) Absolutely true. If its on a 30A circuit - it does not draw 30 amps when running at full load.
There are (at least) two amp ratings that one finds on air-conditioner nameplates:
FLA = full load amps (the actual amps drawn when the unit is running at full load, which includes reactive power requirements)
MCA = minimum circuit amps (the minimum size in amps of the branch circuit wire and over-current protection that will meet NEC electric code requirements.)
If it simply has a cord & plug - well then there's no telling what amps may be on the nameplate. The manufacturer has already done all the design calculations for you, and provide a plug that is sized properly taking into account required up-sizing requirements etc. Interestingly enough, if the unit has a 30A plug - it may draw as little as 17A at full load. Since 17A x 125% exceeds the capacity of a 20A plug, the next size up is 30A. The only amp rating that can help even slightly towards determining power consumption, is the FLA rating. Even then, its almost worthless for determining power consumption when you factor in power factor; efficiency; and your local climate and usage. |
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