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Generator Syncing

Discussion in 'Electrical Engineering' started by Jim Rojas, Oct 22, 2009.

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  1. Jim Rojas

    Jim Rojas Guest

    Is there an electronic solution to automatically sync AC generators,
    motors, or any other AC sources?

    At my home I have a micro hydro setup. I have (2) 3 phase AC water pump
    motors spinning backwards that each provide 240v 6amps, that I would
    like to sync together. At the present time, I run them separately
    running 2 different circuit breaker panels, and a set of capacitors as a
    low cost line conditioner to smooth out peak surges.

    I would like to parallel 2 more pumps as redundant backups in case one
    of them fail. They have been running this setup for 2 years now. My grid
    electric bill is down to just minimum service charges of about $17 a
    month. I average 700KW a month usage.

    My setup has a auto transfer switch from grid tie power to my motors and
    backup genset.

    Any advice would be greatly appreciated.

    Thank you

    Jim Rojas
  2. Jim Rojas

    Jim Rojas Guest

    Yes I believe they are synchronous motors.

    Jim Rojas
  3. Don Kelly

    Don Kelly Guest

    It would be nice to know for sure- water pump motors are likely to be
    induction machines- check the nameplate data- rated speed in particular.
    Do they generate without the backup genset? In that case do they always
    operate them with no connection to the genset (no synchronising with the
    genset)? The capacitors may provide some of the reactive needs but how much
    conditioning they do is unclear and such conditioning may not actually be
    needed other than reactive support. More information is needed- including
    the capacitor rating.

    By the way 700KW/month is meaningless. I assume that you mean 700KWH a
  4. Jim Rojas

    Jim Rojas Guest

    Ok. I will take a look on the nameplate tomorrow.

    700KWH is correct.

    Jim Rojas
  5. Jim Rojas

    Jim Rojas Guest

    I stand corrected. It is an induction motor.

    The info below comes from an overseas friend who help me set it up:

    The motor is 3 phase. In order to reduce it to single-phase, 3
    capacitors, one 70 μF and the other 140 μF (two 70 μF capacitors wired
    in parallel) were wired in a “C-2C†configuration, this process provides
    excitation. 70 μF is on one phase, 140 μF is on the other, and the third
    phase having none. We have plans to add a newer controller and a better
    ballast load, but this will have to wait until I return to Florida in
    the spring. Manually synchronizing these motors is pretty straight
    forward, but I would like to do it automatically when I am away. Either
    with an electronic circuit or even with a relays...either way it must be
    something any homeowner can handle without too much difficulty.

    Jim Rojas
  6. Jim Rojas

    Jim Rojas Guest

    My expertise is in electronic security. This is why I am asking for some

    Thank you

    Jim Rojas
  7. James Sweet

    James Sweet Guest

    Induction motors can be used as generators, sometimes there's enough
    residual magnetism to get them going. I recall reading that if you load
    them past a certain point, the output just falls to zero and you have to
    start over. I've never actually tried using one for that myself though.
    I don't think they're very efficient generators, but they are cheap.
  8. Jim Rojas

    Jim Rojas Guest

    The county I live in has a 5KW per single motor size limit before a
    laundry list of special permitting, engineers drawings, and inspections
    are required. They do allow paralleling of motors, up to 4, as long as
    they are all under 5KW each. I really need 10-15KW in the summer months.
    Everything in my home is electric. A 10+KW motor can easily weigh in at
    a couple of hundred pounds, to a ton. If a motor goes bad, I want to be
    able to replace it easily in a day, and not rent a bobcat, and start a
    huge permitting issue just on motor change out.

    It took me 3 years, and many letters to get the county to allow me to
    use water to power the motor. Once they saw that the diverted water
    quickly worked its way back into the water table, without any pollution,
    they signed off on it.

    Jim Rojas
  9. Jim Rojas

    Jim Rojas Guest

    This looks like what I need.

    Thank you all for your assistance.

    Jim Rojas
  10. Jim Rojas

    Jim Rojas Guest

    I found this one as well...far less complicated and far less to purchase.

    Jim Rojas
  11. James Sweet

    James Sweet Guest

    I sure wish there was something of that nature around me, there's no
    flowing water anywhere on my property though.

    Can you just wire them up in parallel? Certainly connecting a large
    generator out of sync to the grid is disastrous, but something of this
    nature I would have thought would sync up on its own just by switching
    it in, they are motors afterall.
  12. Jim Rojas

    Jim Rojas Guest

    I am no expert. But just paralleling AC motors would probably break the
    armature on one or both. I can also see the motor coils frying. You can
    easily parallel with DC motors. But DC motors require expensive
    controllers and inverters which makes it alot less appealing dollar wise.

    Jim Rojas
  13. James Sweet

    James Sweet Guest

    It could be a real problem if the generators were driven by something
    with a lot of torque and rotational mass, like a reciprocating engine,
    but I would think a generator in that power range driven by a small
    hydro turbine would not have any problems. Remember that in normal use
    they are abruptly connected to the AC line when used as a motor, and
    they spin up to speed on their own driving a load. The only difference
    here is that the motor is already spinning when you connect it to the
    grid. You certainly won't fry the coils.
  14. daestrom

    daestrom Guest

    What you're saying about AC synchronous generators could be true,
    depending on the sizes and such.

    But if I understand you so far, these are really induction generators
    (AC induction motors driven at greater than synchronous speed).
    Induction generators don't 'lock' in to synch with the running supply.

    The nature of induction generators can make operating them in parallel a
    bit tricky as well. When unloaded, the AC output is very close in
    frequency to the synchronous output (f= N*P/120 where N is RPM and P is
    number of poles). But as they are loaded, the speed must be raised if
    you want to maintain a constant frequency output. This is because the %
    slip between output frequency and rotor speed rises with load.

    Because induction generators don't have their own internal excitation,
    you might find it easiest to connect the incoming machine by just
    running it at synchronous speed without any capacitors and connect it to
    the line. This is equivalent to connecting an induction motor across
    the line when it's already spinning at rated speed. No large surge

    After it's on the line, then connect the capacitors to provide the
    excitation needed.

  15. Jim Rojas

    Jim Rojas Guest

    Thank you for the advice.

    Jim Rojas
  16. Don Kelly

    Don Kelly Guest

    As Daestrom indicates, induction motors are easy and synchronization is not
    needed. However, for best operation, they should be connected to the system
    as the system must provide reactive (and this will be appreciable).
    With the system connection, the system dictates the frequency independently
    of the speed of the motor but, if it can't supply the needed reactive, the
    voltage will be low. Capacitors in parallel will help provide this
    reactive. Parallel operation shouldn't be a problem if there is sufficient
    capacity, system + capacitors to supply the reactive.
    In more remote parts of Scotland, induction motors have been used in that
    Daestrom's statements appear to be based on stand-alone setups and there
    could be problems there as you then do get into control of the sources
    driving the two motors to try and maintain the same frequency (if not
    quite the same, there will be some messy circulating currents as one motor
    may try to accelerate the other until they are at the same speed This may
    not fit the conditions imposed by the prime movers and water flow).
    In your case it appears that you do have a standby generator (substitute for
    the grid system and governed to control frequency which can provide the
    necessary frequency control and take care of excitation and load reactive
    with the assistance of parallel capacitors. How much? As a rough measure-
    the no load current and KVA (volt-amps) which would be near 50% of the
    motor rating.

    In any case the more information available - the better an answer can be.
    For example, you don't specify how much of your load is single phase nor the
    rating of your backup generator. What is the maximum power load that you
    want to deal with? Do you run the two induction generators independently of
    this backup generator? If so, how well do they hold frequency? It all
    boils down to what you actually have and how you are using this at present.
  17. Don Kelly

    Don Kelly Guest

    It is intended for synchronous machines- both phase (speed of prime mover)
    and voltage control. On the basis of the description given- don't bother
    wasting your money.

    You don't have either when dealing with induction motors. If you are
    connected to the grid, and they allow it, then you can simply open the gates
    to let the water run and then connect the induction machine to the grid. The
    amount of power produced is at grid frequency and is dependent on the "slip
    speed" of the machine. This also means that excess is fed back to the grid.
    For example, a motor rated at 10KW (input) at 5% slip (say 1710 rpm for a 60
    Hz 4 pole machine) will produce something near (but under) 10KW at 1890
    rpm. What you actually get is dependent on the prime mover as well as the
    motor/generator used.
    Don Kelly
    cross out to reply

  18. Which is why it ran at 375, instead of 360.
  19. daestrom

    daestrom Guest

    You may find it interesting, but this is *exactly* how a 1000 MW nuclear
    plant (BWR) syncs onto the grid as well.

    We use the governor in speed-control mode to get the 'scope going very
    slowly in the 'fast' direction (clockwise). And we wait a little closer
    to 'midnight', but the basic steps are the same.

    Then we run the governor 'up out of the way' and let the pressure
    controls for the plant control the loading. (in BWR's the turbine is
    slaved to follow reactor power instead of like PWR's where it's the
    other way 'round)

    We do have 'synch-check' relays, that interlock the closing circuit, so
    that if an operator screws up, he *can't* close the breaker more than a
    few degrees out of phase.

    P.S. But these are synchronous generators, not the induction generators
    the OP mentioned.
  20. daestrom

    daestrom Guest

    Nah, that don't sound right. If synchronous speed were 360, that would
    be a 20 pole machine. At 375, that would be 62.5 Hz instead of 60. You
    can't parallel with 2.5 Hz difference. Most synchroscopes won't even
    rotate continuously with that much frequency difference, they just
    'stutter'/wiggle back and forth.

    To get the 'scope to rotate 'slowly in the 'fast' direction' takes only
    about 0.2 Hz or less difference.

    More likely it ran at 375 because it was a 16 pole *50* Hz machine
    ('English Electric' might be British).

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