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Generator Residual Magnetism

  1. Aug 15, 2009 #1
    I plan to use a automatic genset controller to protect/run a synchronous generator. This controller determines when to disconnect the starter by determining the generator frequency. I plan to connect the exitor regulator to the main breaker so when the main breaker trips the regulator is shut down. Any fault condition will trip the breaker and also shut down the engine. It is my understanding that even with the regulator power supply disconnected there is still some residual magnetism in the rotor which causes the generator to still generate a small voltage.

    My concern is about what would happen if the engine shuts down and the breaker fails to trip, causing the load to still be connected to the generator as it slows down. Will this cause the generator to lose it's residual magnetism thus causing it to not generate at the next startup which would cause the starter to never disengage thus damaging it?

    I am aware that if an induction motor is stopped while a load is applied it will lose it,s magnetism causing it to need manual remagnetisation upon next startup. I am not sure if it applies the same to my application with a syncronous generator.


    Any confirmation of any answers will be nice!!
     
  2. jcsd
  3. Aug 16, 2009 #2
    There are Demagnetizing effects in electrical machine as a magnetizing system which involved to induction phenomenon if secondary current load exist, similar what where happen in transformers.
     
  4. Aug 16, 2009 #3
    Hi Josh,

    Sure enough, a frozen contactor can make things messy when you need to go offline. Personally, I'd put fuses in a set of series for such an emergency. Fuses are a pain, but they're reliable - not something you can say about many breakers.

    As for the magnetization, that happens with dyno-type generators, but the world doesn't use those much, except for welding machines. When the rotor gets its polarity reversed, or the user wants to reverse it, you have have to expose the commutator and apply a stout current (a car battery is good) to the brushes in order to reverse the polarity (while it's still!).

    As for an induction motor, they don't have a permanent field. Same's true with your synchronous generator.

    Remember that your synchronous generator will become a synchronous motor if you attempt to turn off the engine with the generator on line!

    Mike
     
  5. Aug 16, 2009 #4
    I do not plan to run generator online with grid.

    As far as the contactor/breaker goes, I plan to use a shunt trip breaker because I am monitoring for faults other than just overcurrent. A simple fuse will not just work in my application. I am monitoring voltage and frequency as well as other conditions. A fault will trip the shunt trip breaker.

    I thought that a synchronous generator had residual magnetism otherwise it would be necessary to introduce a voltage at each startup.

    The site below says that an induction generator can be self excited if capacitors are present and that the residual magnetism is what starts the induction

    process.www.qsl.net/ns8o/Induction_Generator.html

    My concern is about what would happen if the engine shuts down and the breaker fails to trip, causing the load to still be connected to the generator as it slows down. Will this cause the generator to lose it's residual magnetism thus causing it to not generate at the next startup which would cause the starter to never disengage thus damaging it?


    Sorry I was not clear earlier.
     
  6. Aug 17, 2009 #5
    Wow,

    That's an odd one. Back in the 70's, people were using induction motors as online generators, but my understanding was that you had to have the online voltage to ensure rotor magnetization - otherwise, it's output would die down.

    As for the capacitors, back then, they were there for power factor correction.

    Admittedly, I'm having difficulty wrapping my head around around this problem... For my own edification, I'd like to see how it turns out.

    Best Wishes,

    Mike
     
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