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AC Generators

  1. Dec 9, 2007 #1
    I have a question in relation to electrical generators, 240VAC 50 Hz. A colleague wants to run a piece of gear that draws around 10A @ 240VAC with a pf of 0.8. He has indicated that running two 2kVA generators in parallel will provide greater ampage output, thus allowing the equipment with power draw greater than the output of a single 2kVA (I estimate 6.6A @ pf of 0.8) plant to be run.
    My first thought is OMG, what about the phases of each, are they in sync?
    Am I off track and there is no problem? If there is a problem what will be effected, the equipment being run (designed for 240VAC 50Hz) or the generators? My gut is telling me that there is a problem and that it will most likely damage the generators. Your help is greatly appreciated.
     
  2. jcsd
  3. Dec 10, 2007 #2

    stewartcs

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    Well if the generators are running in parallel, then they must be in sync. The problem of synchronizing generators comes in when attempting to parallel one to the bus. If they are out of phase (depending on how far out they are) when trying to parallel them, then the oncoming generator will suffer mechanical damage. I've personally seen a guy parallel a gas turbine generator set 180 degrees out of phase and it sheared all of the pins on the interface coupling.
     
  4. Dec 10, 2007 #3

    Astronuc

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    In large power plants, the generator is synched (frequency and phase) to the grid at very low power before the breakers are closed. stewartcs's example is one in the extreme of what can go wrong if the generator is not properly synched.
     
  5. Dec 10, 2007 #4
    Thanks for the input. The type of generators are the portable ones. Each generator has a power outlet (GPO) to connect the extension lead. He was thinking of making up a cable, via a junction box to power the plant (like a 'Y' extension lead) from two generators.

    This would mean that you would plug them in, start both generators (one at a time) and then turn the power circuits on at the running generators (one at a time) and finally start the load. In my mind, once the two machines were running and one is switched on (the other is running but not turned on [GPO is switched off]), you could potentially have two machines of different phases. I would have thought that when you switched the second one on, at that instance you could damage something?? Maybe the generator?
     
  6. Dec 10, 2007 #5

    stewartcs

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    I would strongly recommend NOT doing this.

    Correct.

    Correct again.
     
  7. Dec 10, 2007 #6
    Thanks for that. He owes me a beer.
     
  8. Dec 10, 2007 #7

    mgb_phys

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    I asked a heavy power engineer once, how you brought a new station online and connected it to the grid ?
    "Very very carefully" he replied.
     
  9. Dec 10, 2007 #8
    A cheap way to to phase 2 generators up is to connect the 2 neutrals together and then connect the 2 hots together through 2 light bulbs wired in series of the same wattage. As the phase changes the lights will grow from bright to dim. When they are completely out the generators can be directly connected together. I don't recommend doing this sort of thing on a small scale for any length of time. These small generators can have too many problems running on their own. Fuel consumption can easily get away from you and you have one generator running out of gas, etc.
     
  10. Dec 10, 2007 #9
    Small gen's such as the ones you've mentioned here are not intended to be operated in parallel.
    I wholeheartedly agree with stewartcs. "I would strongly recommend NOT trying this".
     
  11. Dec 10, 2007 #10

    Astronuc

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    The big nuclear stations get turbines rolling at about 15% of rated power. The generator is synched. AFAIK, they compare the frequency of the generator field with that of the grid, and they have to be right on within fractions of each other before they close the breaker.
     
  12. Dec 11, 2007 #11
    Forgive a long time luker, but relatively new poster from jumping here. I was here for something else and saw this topic which caught my eye and here I am.

    A caution. Many of these newer portable generators are not your simple "analog" AC sychronous machines. Many are actually an AC source driving an inverter which produces the output single phase wave form. That is, it is AC -- > DC -- > AC again.

    Why would you do that Rube Goldberg mess, you might wonder. Well, efficiency and frequency considerations. A 3-phase machine is more efficient and smaller in general than an equivalent single phase machine of the same rated output. And the faster you can turn it, the more voltage you can make with a given field. And second, you have to worry about frequency.

    So take a 3-phase alternator, run it fast and don't worry too much about speed control. If the load gets high, just speed up, and idle when she's low. Much like the basic principle behind a car alternator. Take that wild AC output and drive an inverter to give you your regulated output. :) The solid state electronics revolution has made these kinds of thing cheap and feasible. Some use full inverters, while some cheaper ones use cycloconverter designs, the details of which I forget.

    At any rate, you obviously don't parallel these things. :) However, some of the models have provisions to parallel by linking up the control electronics.


    -Richard
     
  13. Dec 12, 2007 #12
    Sorry if I am intruding this thread; am trying to educate myself:

    If I understand correctly; installing a second alternator on a car will not - charge the battery faster - or charge a bigger battery ?

    Would it be feasible using a kind of electronic regulator?

    Thanks :)
     
  14. Dec 12, 2007 #13

    How fast a battery will charge depends on much voltage you apply (and there's a limit there, IIRC, too much current and you're basically just electrolyzing the water -- that happens especially when the battery is near fully charged and is the bubbling you see). If your source can't pump the necessary current, then the voltage drops, it overheats or whatever. In that case, adding a second source would divide the load and give you more capacity.

    While car alternators are three-phase internally, the output is rectified DC and so paralleling that is simply paralleling two DC sources with diodes between them. Current can't flow backwards through the rectifying diodes. Putting the output DC stages in parallel just gives you more total current capacity.

    That has nothing to do with the problem I mentioned above about trying to parallel to inverter output stages. That will liberate the smoke unless the inverters are electronically synchronized.

    -Richard
     
  15. Dec 12, 2007 #14
    Thanks Richard :)
     
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