1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Electrical Voltage, a appliance problem
  1. Voltage problem (Replies: 1)

  2. Voltage Problem (Replies: 1)

  3. Problems with voltage (Replies: 15)

  4. Voltage of electricity (Replies: 13)

Loading...