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Homework Help: Power factor correction

  1. Dec 22, 2011 #1
    1. The problem statement, all variables and given/known data

    2. Relevant equations
    If I want to figure out how much money we are losing because of poor power factor and the information I have is Power Factor 60% and total KWH I can take 60% of my total and then subtract that from my total then multiple the subtracted number by my current rate and it will tell me how much money we are throwing away because of poor power factor?

    3. The attempt at a solution
  2. jcsd
  3. Dec 22, 2011 #2
    there is not enough info here to calculate anything

    Power factor is (real power/apparent power) it is a measure of how much power is real and is calculated as a cosine of the phase angle.

    power companies charge for volt amps so the worse your power factor is then the more you will pay. i guess an example would help !!

    lets say you own a great big motor 20 kw your power factor was 0.7 lagging and the price per unit is 1 euro per kilowatt hour. you want to run your motor for one hour.

    because your power factor is bad you are paying for allmost 29 Kw when a good power factor would be considered as 22 Kw
  4. Dec 22, 2011 #3
    It doesn't make sense to say they charge your more for a worse power factor because you are simply getting less power. Why would they charge you more? It would make more sense for them to charge you more for a better power factor because you are getting more power.

    If they charge you per amount of real power, you'd want to get your power factor as close to unity as possible. (For maximum power that is)

    You can always tweak your power factor by adding either a capacitor or an inductor bank. (It will either consume or supply VAr's)
    Last edited: Dec 22, 2011
  5. Dec 22, 2011 #4
    If you want to figure out how the power factor is affecting your intake of real power you need the complex power, S. (This is in volt-amps)

    Then your real power is going to be,

    [tex]P = Scos(\theta)[/tex]

    Where the cosine portion is your powerfactor,

    [tex]PF = cos(\theta)[/tex]
  6. Dec 22, 2011 #5
    ok this the all the information that I have:
    KW 98
    KVA 130
    So PF is 75%

    That is all three phases added together.
    The voltage is 283 per phase and amps is 472 for all three phases

    Lets say I only pay 8 cents a Killowatt.. how do I figure out how much money I am losing? I guess that is the confusing part for me. Do I need for info?
  7. Dec 22, 2011 #6
    it has nothing got to do with the consumer getting more or less power man !!

    the whole idea behind powerfactor is the simple fact that it costs more for the supply company to generate to customers with a bad power factor i.e the need more volt amps on the transmission line.
  8. Dec 22, 2011 #7
    your soloution !! [i suspect this is homework so you can figure this out yourself]

    your power factor is correct

    now do an arccos on this value, this is the angle between your real power and apparent power. when you draw this your apparent power phasor will be the hypotenuse of a right angled triangle. straight away you can see that this line is longer that your real power ?
    (by the way its normal to put real power on your x axis)

    and thats it.... is reduce the angle reduce the cost good luck !!!!
  9. Dec 22, 2011 #8
    This is not my homework. Im just curious as to how much money I can tell my boss we could save by installing capacitors at our poor power factor locations. I know the phase angle, its 40 degrees. I have an energy system that lets me see all this real time. I'm sorry if my question comes off confusing because you think this is my homework. I'm just looking for some better examples of how to quickly calculate how much money we are throwing away using the Data I stated in my previous post.
  10. Dec 23, 2011 #9
    not homework then ea !!

    Power factor cannot be solved on this forum properly, because it needs a detailed study of plant and equipment and how it might be connected. I don’t recommend telling your boss anything unless you completely correct, as it is normal that supply companies dump huge fines on to customers with bad power factor, so he should know if he does not. That said I’ll use your example to demonstrate but you will need to fill in some blanks yourself ok?

    Your power factor angle loos like being 41.1 degrees (you dont mention lagging or leading so im going with the persumption of lagging)

    p = 98
    s = 130

    and most importantly Q = 85.46 kvars lagging (take out your trigo book to see where this comes from)

    you need capacitors which are rated in kvars to reduce Q (ie capacitors/inductors dont use real power but the create reactance or kvars) this is just the opposite of your problem.

    ive allready told you how to calculate the cost of your bad power factor, if you are indeed curious then you should not have a problem turning the handle on this from here.

    good luck
  11. Dec 23, 2011 #10


    User Avatar

    Staff: Mentor

    That's true. Not enough information.

    Wrong. Power companies charge domestic consumers for the actual power used.

    It would make a lot of sense (i.e., make more money for the power company) if they were to charge according to kVA but they don't, and I think we should not encourage them to consider doing otherwise. :smile:

    Power authorities prefer that you have a power factor close to 1, and will probably pressure big companies to take steps to do so, but for small domestic consumers, there is generally no incentive to consider powerfactor correction.

    EDIT: Just noted where raiderUM indicates his employer is a medium commercial user of power. In that case, it is just possible that the power company may be imposing a charge for kVA. It would make sense to have that written into the supply contract, as an incentive for a good powerfactor. But we can't know unless OP is able to find out.

    So to put it starkly, the boss may possibly be paying for 130 units of supply, when he need only be paying for 98 units. This is 32% higher than if the pf were unity. You can only know by looking at the account, or reading over the contract, whether the power supplier is imposing a penalty for a poor pf.

    If you really are paying per kilowatt, then you are losing no money. You are paying only for the actual power you use. On the other hand, if you were paying per kilovolt-amp then there is a potential saving of 32% to be made if you fixed your poor powerfactor to make kW ≈ kVA. You need to scrutinise the quarterly account to see whether powerfactor is somehow being factored into the bill.
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