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Thread: Backup electrical generator question about loads

  1. #1
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    Default Backup electrical generator question about loads

    I have a 15,000 watt, 240v, tractor PTO generator. I have learned to pay close attention to frequency as the main criteria for engine speed. I use a Fluke meter to make my adjustments to as close as possible to 60Hz. Unfortunately I do not have automatic throttle adjustment and I have a few loads that cycle on and off as needed. I try and keep the tractor somewhat under load as that seems to keep the fluctuation at a minimum. I have the ability to add heater loads or fan loads to accomplish this.

    So my question is one type of load better that the other for maintaining frequency? I need to run some of each but I can add more of either if it would help. I'm asking because we are on standby power now and it seems a little more fluctuating than normal.

    Thanks,
    Abner

  2. #2
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    I would think better to use a heater load since most small electric heaters would put a 1500 watt load on, which is a pretty big fan and the varying freq won't bother the heater.

    Steve

  3. #3
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    The real problem your fighting is the type of governor on a tractor vs on a generator. Variable speed governors used on tractors have a speed difference between high idle speed and high full load speed known as droop. Usually about 8%. So if your full load full speed is 2100 rpm you'll find if you rev up all the way with no load it will run about I think 2305RPM. So any change in load will cause a corresponding change in rpm and therefor frequency. Generator engines use a synchronous governor with effectively no droop. Steady loads would help maintain a steady frequency but that is not really practical for standby use. Set it a little fast, maybe 61hz so it doesn't drop off too much when a big load comes on.


    Good luck. Stay safe.

  4. #4
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    Dunno about no droop.... on an Onan JC type, the limits are 64 Hz no load, and 57Hz at max load, for instance.

    There is an adjustment for governor sensitivity, but there will always be some droop with any mechanical governor, as there is movement required to correct the error. All you can do is keep it to a few percent range.

    I would have said the heater load, but the fan load might be just as good if not better. The fan (may depend on type) generally has a cube law power requirement, so if the cycling load drops frequency, the fan will slow down, and drop load at the cube rate, faster than the heater, and should act to keep the voltage / frequency closer.
    1601

    Keep eye on ball.
    Hashim Khan

  5. #5
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    The generator has no voltage and frequency regulation of it's own. Solely dependent on the PTO speed?

  6. #6
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    Quote Originally Posted by reggie_obe View Post
    The generator has no voltage and frequency regulation of it's own. Solely dependent on the PTO speed?
    The voltage is covered by the regulator and the frequency is dependent on the rpm of the generator. Only the new inverter type generators regulate the voltage and frequency just like a regular vfd.
    Helder Ferreira
    Setúbal, Portugal

  7. #7
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    Quote Originally Posted by reggie_obe View Post
    The generator has no voltage and frequency regulation of it's own. Solely dependent on the PTO speed?
    That is correct. Faster tractor RPM =higher frequency. Old school, Onan brand so it's not junk.

    The tractor sits at the main panel about 150' away. 8" of snow, likely a couple of days with no power...

    So I turned on 3 - 500 watt bathroom heaters and then started adding lighting loads as balanced as possible across the 240. Got in down from 61.8 HZ to 60.5 and every electrical appliance appears to be happy including the battery back ups. I have several fan loads (5 at 1/5 to 1/3 Hp) also running.

    Thanks J Tiers that was my question, I was hoping you would respond. Knowing that a RPC can generate the 3rd leg I was wondering if the fan loads would work a little better.

  8. #8
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    Not entirely true.....

    The voltage is dependent on excitation (field voltage), and the frequency is linked to the voltage for any particular excitation level.

    It is a bit different when you have a load lower than the generator max output, vs an effectively infinite load, as in the case of a generator connected to the grid. With this independent setup, you can actually change frequency as you open the throttle. In the case of a grid connected generator, you essentially just change the shaft torque with the throttle, outputting more or less power.

    Field voltage works in both cases, but for independent gensets, it changes output voltage, and for a grid connected genset, it changes the VAR output, because the grid locks the genset as to voltage and frequency.
    1601

    Keep eye on ball.
    Hashim Khan

  9. #9
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    Quote Originally Posted by Noitoen View Post
    The voltage is covered by the regulator and the frequency is dependent on the rpm of the generator. Only the new inverter type generators regulate the voltage and frequency just like a regular vfd.
    I didn't think you statement is 100% true. See post #9: https://www.smokstak.com/forum/showthread.php?t=97240

  10. #10
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    Nice machining project. Ball governer for the tractor engine
    Helder Ferreira
    Setúbal, Portugal

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