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Homework Help: Electrical Voltage, a appliance problem

  1. May 17, 2010 #1
    1. The problem statement, all variables and given/known data

    Anyway can you please help me out with the following?

    240V , 50 Hz electrical appliance is rated at 2 kW. It has a lagging power factor of 0.7
    (a) What is appliances power factor when it is used on a 60Hz supply.
    (b) What is supply voltage required to maintain appliance at its rated power when operated off a 60 Hz supply?

    2. The attempt at a solution
    No idea..!

    Can you suggest some relevant equations and guidance?
    Thanks in advance
  2. jcsd
  3. May 17, 2010 #2
    First find Z of the load using the relation P=V^2/Z. Then using the given power factor you can find the reactance X=Zsin(theta). Let this be X1. Now find new X (X2) for 60 Hz. Remember X is directly proportional to frequency for a lagging circuit since it is inductive. From X2 you can calculate the parameters you want to know.
  4. May 17, 2010 #3
    Hello, can you be bit more explanatory?
    i am not getting anything !
  5. May 17, 2010 #4
    You are asked to find pf on 60 Hz. You may remember that the impedance of the load Z=R+jX, where X is positive for inductive loads i.e., lagging pf and only X is dependent on frequency. Also pf=tan(X/R). Now if you change frequency of supply X changes proportionately. You know X=2*pi*f*L for inductive load. So for change in frequency from f1=50Hz to f2=60Hz , we have X2/X1=f2/f1. The first thing you have to do is find X1 i.e., X at 50 Hz using relations I mentioned earlier (Note pf=cos(theta) and so you can find theta). Then find X2 and from this since R is unchanged, you can find new pf.
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