Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Trying to understand capacitors and inductors

  1. Feb 22, 2013 #1
    Recently got hired at a local electric utility and unfortunately my college did not offer a lot of power classes so I am struggling with some things that really come down to how capacitors and inductors interact. I think I have some ideas but am really looking for some reassurance that I am right or why I am wrong.

    1) If inductors make the current lag and you add caps to make the voltage lag this is just bringing the p.f. closer to unity. But whether it is at unity or not the inductors still need vars to build up the magnetic field. Are the added caps now supplying the energy needed for the inductors magnetic field? (when I say supply I mean the capacitor is now releasing the stored energy in it's E field which the inductor now uses to build its magnetic field)
    2) If the caps are now supplying the energy to build up the magnetic field how is this happening? In the first quarter of the sine wave, when the energy is being stored in the inductor, woundn't the capacitor be charging and not be able to supply the energy needed? If someone could help me understand what happens in a cap and inductor in the first half of the sine wave I would appreciate it.
    3) If there is a capicitor 1 span (say 50 ft of wire) away from an inductive load, and I am correct in thinking the caps supply the vars to the inductor, then would the current in this 1 span of wire be higher than the span before it since there is additional current flowing between the cap and inductor?
    4) How does a cap bank raise the voltage? My thinking was since there is less current being supplied by the generator then there are less line losses and that raises the voltage.

    If anyone can help me understand any of these questions a little better I would greatly appreciate it. Thanks.
  2. jcsd
  3. Feb 22, 2013 #2
    Hello DJS - Welcome to PF :

    I also started in field service as an EE but had no 3 PH power education. You seem to be on the right track ( ahead of me) in your thinking.

    As for item 1: Probably best to think of caps causing a current lead (as in leading power factor), so that your phase reference is always the voltage. But your logic is correct. Since there is no net energy consumed ( converted ) in the Mag filed of the inductor ( or motor) or the Elec Field of the Capacitors - they each alternate charging and discharging - passing the energy back and forth. (props for recognizing that we are talking about energy - not just v and I...)

    2) Since a capacitor has a "90 deg leading PF" and the Inductor 90 Deg lagging PF - they are 180 our of phase with each other. At the peak of the Voltage sine - the Capacitor is reaching full charge ( 0 Current) and the Inductor ( is near full current) -- this is best for a Sketch of the sine waves or a simple LTSPICE simulation will kick this out asap.

    3) Well only if you are comparing to the Inductive load and caps on the same pole ( by span I am assuming you are talking about power poles. The Inductive load draws the same current - the only difference is inductive load vs inductive load + Caps. - Also see #4

    4) Nail on the head - keep in mind the MANY transformations and distance of wire the power travels from the the generation source to the load. If you need current that is not being used for power ( i.e. VARS) - the whole system has to support that current - supporting the current requires power ( via losses ) - but delivers no power to the user.

    Bonus Question -- particularly for starting motors, why is it a bad idea to have too much capacitance close to the load ( i.e. a net leading power factor before the motor starts.)

    Good Luck

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