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!

B Electrical Generator -- why there is a mechanical load change with elecrical load changes?

  1. May 17, 2016 #1
    this might have been answer before. Question is, what in physics or chemistry creates a load on a generator when the demand increases. For example, If no lights are turned on connected to a generator, it will spin freely. but the more lights are turned on the harder is to spin the generator.
    What causes this increase in torque requirement. for example the windings in the generator can be mounted in the stator. it doesn't even touch the rotor. so why the increase in torque required to spin the generator as the load demand increases. what causes it?
    is it an increase in magnetic flux caused by the windings of the generator that oppose the magnetic field of the rotor?
    for someone that doesn't understand physics, it would be rational that the spinning torque required shouldn't change regardless of the electrical demand on the generator.
    thank you,
  2. jcsd
  3. May 17, 2016 #2
    Read a bit of this: https://en.wikipedia.org/wiki/Eddy_current

    The "demand" on the generator has to do with the electrical resistance in the circuit. Limiting the current flowing and thus the "drag" created by eddy currents.

    This is my lamens explanation as I understand it.
  4. May 17, 2016 #3


    User Avatar

    Staff: Mentor

    A generator and a motor are the same device. Why does the motor output increase when you increase the input power (torque, rpm, whatever)?

    The simple answer is that motors and generators push and pull electrons through wires, which is both a physical and electrical force.
    Last edited: May 18, 2016
  5. May 18, 2016 #4


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    One answer it that it has to be so in order to satisfy conservation of energy. You can't get more electrical energy out of a generator than the mechanical energy you put in. So if you draw more power out by increasing the load it's only reasonable you will have to supply more input power.
  6. May 18, 2016 #5


    User Avatar
    Science Advisor

  7. May 20, 2016 #6
    Remember that in order to have a magnetic force you have to have a wire moving through a magnetic field, breaking the magnetic field lines (or the opposite, thus magnetic field lines moving and breaking through the wire). When there's no resistance in the circuit then the generator pushes the electrons and they just start flowing at whatever speed the generator is asking them to flow, the motion of the magnetic field lines and the wires in the generator get synced up, there's no forces developed and the generator just kind of spins with the current or the current flows with the spin of the generator...whichever way you prefer to look at it.

    Apply a load and it will push back on the current, slowing it down. Now there's a mismatch in the motion of the magnetic field lines and the generator wires, the magnetic field lines get broken by the generator, and you get a force that tends to slow the generator down.

    I'm kind of a layman and it's been a while since I looked at electric circuits, so someone please correct me if I'm wrong.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted