1. Limited time only! Sign up for a free 30min personal 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!

Importance of the frequency of AC mains power lines

  1. Apr 13, 2015 #1
    From textbook: "In much of the world (Europe, Australia, Asia) the rms voltage is 240 V, so the
    peak voltage is 340 V. The line voltage can vary, depending on the total load; the
    frequency of 60 Hz or 50 Hz, however, remains extremely steady"

    Why is it so important to keep it steady? what happens if fluctuations occur?
  2. jcsd
  3. Apr 13, 2015 #2
    You wouldn't want different frequency sinusoidal outputs overlapping from different power stations.
  4. Apr 13, 2015 #3


    User Avatar
    Gold Member

    I guess it is important to keep the frequency steady because otherwise all the devices that contain capacitors and/or inductors would have their power rating altered because the reactances of the inductors and/or capacitors inside the device depends on the frequency, and the power which depends on current, which depends on reactance. It is not only that the power rating will be altered but the operation of the device will be disturbed because the distribution of currents within the device elements will change (again due to the reactance changes due to frequency change).
  5. Apr 13, 2015 #4
    I guess TV's and motors are also design for a specific Htz
  6. Apr 13, 2015 #5


    User Avatar
    Gold Member

    Now that i read it more carefully, i think what the book is trying to say is that while load affects the rms/amplitude of the voltage of the line, load does not have any effect on the frequency of the line. What happens if fluctuations occur in the frequency (due to problems in the generators and the power plants i guess), me and Puma tried to answer in the previous posts.
  7. Apr 13, 2015 #6

    Doug Huffman

    User Avatar
    Gold Member

    To be operated in parallel, AC generators must be in synchronous frequency and providing proportional load, else the lagging machine will be driven as a synchronous motor with resultant increase in current. Load should not affect regulated voltage, imbalanced voltage regulators cause circulating reactive currents that will overload a machine just as real current.

    look to the definition of reactance for the effect of varying frequency from nominal.
  8. Apr 13, 2015 #7


    User Avatar
    Gold Member

    The generating stations are connected together in a grid system. If one station senses that the frequency of the grid is falling, it will automatically work harder to inject more energy. In addition, if the frequency changes, all the generators in the grid, which have a lot of inertia, will have to adjust their speed whilst remaining in synchronism.
    The frequency is sensitive to load, because the turbines slow down if they have to work harder. With an electricity grid, all the alternators run in synchronism, but each one has a "cruise control" which senses frequency changes in the grid and controls the amount of steam being delivered to the turbine. The frequency is kept as constant as possible because that indicates that the generating capacity in the grid is equal to the load. If the frequency is allowed to change too quickly, generators will not be able to stay in synchronism, because of their large rotating mass.
  9. Apr 13, 2015 #8


    Staff: Mentor

    Tech99 gave the best answer, "as constant as possible".

    In huge grids, such as the USA eastern interconnection, frequency variations are tiny, because changes in load or generation are tiny compared to the grid.

    In small grids, such as on an isolated island, the frequent variations are much larger, because variations in load can be large compared to the grid. I recall vacationing on Grand Canary Island and listening to the refrigerator motor rev up,and down. Another case in Queensland Austrailia, they had a drag bucket machine for coal mining. Every time that bucket hit the ground it added 25% to the total grid load.

    Big or small, "as constant as possible" is the best answer.
  10. Apr 15, 2015 #9


    User Avatar
    Science Advisor
    Homework Helper

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

Similar Discussions: Importance of the frequency of AC mains power lines