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

[Synchronous Generator] How exactly is power transferred?

  1. May 28, 2014 #1
    I appreciate your attention.

    This question concerns the generators connected to an infinite bus.

    I would like to know the details of the chain of event that leads from the rotating turbine to the electrical power "put onto" the grid. Just a simple mechanical to electrical energy conservation view won't help me here.

    first of all, I don't even know what electrical power is in this context. Don't just say p= VI with an angle, I m trying to understand this, not getting a number for an assignment. My understanding of electrical power is very simple: you create a tendency for electrons to flow, if the electron actually flows in your created tendency, power is generated by you. well my understanding is indeed p=vi here but in this case, there is ONLY you, you are the only one creating the tendency, and the electron flow is the result of only your tendency.

    Now if we are on the grid:
    Let's ignore transformers everywhere in the grid, and we will have the stator voltage being the grid voltage minus the pitful losses, the stator current is the grid current, the speed of rotor is the grid frequency (you know what I meant). How can you transfer electrical power? Don't you need higher frequency to drag up the waveforms, or higher voltage to create higher incentives for electrons to flow or adding more current to the system to "transfer" power?

    And then moving on to the question of "torque to electrical power".
    and please, try to steer the conversation away from anything about equivalent circuit (-jXI related stuff), it is a BS mathematical convenience, does not help a bit.

    When you push more water to the turbine (wicket gates open more), and with the speed of the turbine locked to the grid and staying constant, you are supposed to generate more power. How?

    Is it the armature reaction? I can see the total air gap flux shape being distorted, but how is that related to power? no idea.

    or is it the magnets attraction? higher load means higher armature current, higher armature current will create a bigger magnet for your rotor magnet to overcome? so the rotor as a magnet is always trying to spin faster (or a Phase lead) than the rotating magnetic field in the stator due to the grid? the current will be lagging due to the normally inductive load. With excitation current and VARs coming into the mix, I m feeling completely stupid.

    does energy transfer thru the M field? like the transformer?

    Thank you for reading this far. Thank you more if you can help me untangle some of the stuff in here.
    Last edited: May 28, 2014
  2. jcsd
  3. May 28, 2014 #2


    User Avatar
    Science Advisor

    The rate of rotation must be synchronous with the grid. The voltage induced in the phase stators is determined by the magnitude of the rotating field excitation.

    The direction of energy flow is determined by the sign of the VI product. The direction of current flow is determined by the difference between generator voltage and infinite bus voltage.

    As the voltage being generated by a synchronous machine begins to slightly exceed the voltage on the infinite bus, the product of current by voltage reverses and so the system changes from being called a motor to being called a generator.

    When water flow is increased, the synchronous generator field must be changed to alter the generator voltage and so transfer the additional energy available to the infinite bus. That must be done so as to maintain generation without a change of synchronous speed. The further analysis of synchronous generators is impossible without the consideration of phasors that you have precluded. Suffice it to say, that if the field excitation is not adjusted to transfer all the energy available, the generator phase will lead or lag the infinite bus.

    The momentum of water in the penstock does not permit sudden changes of flow rate. Regulation by field excitation can be difficult when the electrical load of the infinite bus is suddenly removed from a synchronous generator.

    If you want to understand the detail of the magnetic interaction of a rotating field with a fixed stator phase winding, then you should abandon the added complexity of water and synchronous alternators by studying a simple DC machine.
  4. May 28, 2014 #3

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    This question comes up periodically.

    Inside the alternator it is plain old F = QV cross B, right hand rule.

    The physical force is exerted on the charge in the armature conductors.

    See if this thread is any help, and the one linked to in fifth post.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook