johnc
Model 4390: Have one of those tiny tachs on gen. , reads 3800 rpm so brought super good tach from work to double check, also reads 3800. All electric output readings are good, gas pressure is good. Do I change the engine speed by adjusting the governor? Then go back and readjust electrical settings? Does the fuel reagulator change the engine speed? If I remember right, when I installed the tiny tach a couple yrs ago, the rpm's where close to 3600.
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78buckshot
Hi John, yes you will adjust two governor speeds, idle ( no load) and high speed (loaded). Your manual should describe how to do it. It would be good if you also can read the frequency as you adjust the speeds, after you set the speed then you can adjust the voltage regulator but it might be OK. The incoming gas pressure should have little to no effect on the speed even if the pressure has changed, the mixture screws on the regulator will have quite an effect on engine speed but I wouldn't mess with them if your machine starts well and stays steady when heavily loaded.
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johnc
Buckshot: Engine runs smooth , no problem there. Always good to know proper sequence when changing things, as one adjustment can affect another. I'll go thru the lo and hi speed governor adjustments tomorrow, thanks for the info. Checking HZ no problem..
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78buckshot
The high speed adjustment screw is hard to see, it's under the air box towards the front of the engine. In the battery compartment you'll find a plastic plug above the battery on the bulkhead, pull the plug out and use a long thin phillips driver to adjust the speed, I have a hex extension for my drill that works pretty well, I don't use the drill to adjust it, only the extension.
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johnc
Buckshot: Went thru the governor setup. No load and hi load adjustment. All the while trying to keep the gen. around 3600 rpm. Finally decided the rpms aren't that important, as long as the hz. are right at load and no load conditions. Voltage is right on. I figure if rpms were a big deal, they would specify that in the manual.
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78buckshot
I use only frequency to make the adjustments, like you said-RPM isn't what we really care about.
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Canada_Guy
johnc;11185 wrote:
Finally decided the rpms aren't that important, as long as the hz. are right at load and no load conditions.


Engine RPM's and the frequency of the generated voltage are directly related. The frequency that the generator produces is based on the number of revolutions the motor turns the rotor per second.

60 revolutions per second times 60 seconds = revolutions per minute (RPM)

60 times 60 = 3600.

If the motor is truly running at 3800 RPM, then it will be producing voltage at 63.33 Hz (3800 RPM divided by 60 seconds).
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johnc
Exactly what the directions dictate. Set the load hz. at 58, I set mine slightly higher 58.6. Set the unload hz. at 62.5, mine is set at 63. Volts at noload 246. Manual states that under loaded conditions hz. should not drop below 58 and volts not less than 230 + - 2%. Never could get rpms and volts and hz. to all agree with specs. The manual never once mentions rpms. Just specifies volts and hz. agreeing at certain load conditions.
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Skip Douglas SkipD
johnc;11185 wrote:
I figure if rpms were a big deal, they would specify that in the manual.


Canada_Guy;11191 wrote:
Engine RPM's and the frequency of the generated voltage are directly related. The frequency that the generator produces is based on the number of revolutions the motor turns the rotor per second.

60 revolutions per second times 60 seconds = revolutions per minute (RPM)

60 times 60 = 3600.


johnc;11215 wrote:
The manual never once mentions rpms. Just specifies volts and hz.
I suspect that the reason that Generac does not specify RPM is because measuring the AC frequency is also measuring RPM and most folks who work on their generators have a multimeter that will show the frequency but may not have a tachometer. In fact, either measurement provides the same information (if one does the math in the post by Canada_Guy above).

Actually, a tachometer is a far superior instrument to use because it does not depend on the generator's ability to function to measure engine RPM. I have and use both instruments, by the way.
Skip Douglas
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johnc
I think I was under the misconception that the gen. should not run higher than 3600 rpm. That is were all the confusion began.
But as C.G. stated hz changes as rpms change. All is relative, isn't it?
Buckshot says he adjusts by hz and volts, pays no attention to rpms.
My problem was trying to keep the gen. at 3600 rpms while changing the load conditions, that was impossible. That made me realize that as one thing changes others have to change accordingly. Hope that is right.
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78buckshot
I didn't mean to imply that Rpm is irrelevent, if I were concerned only about the engine operation then speed would play a part in my adjustment. My objective is to check and adjust for proper alternator output. If I find the frequency way out of spec then my diagnostic shifts to speed and then the alternator. Sorry for any confusion or poor info.
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johnc
So what is everyones asssessment? Does the gen run at a constant 3600 or as load increases the speed can increase? The output hz and volts are perfect at load and no load conditions. If rpms dropped below 3500, volts and hz. went to crap. But under hi load conditions, 3800 was about max. Overspeed shutdown occurs at 4320 rpm 72 hz. I assume that nothing is wrong with seeing 3800 rpms under certain conditions.
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Skip Douglas SkipD
The 4390 series is not, according to what information I have seen, designed to run at a constant 3600 RPM (60 Hz) but is designed to operate from about 58 to 62 Hz. That translates to between 3480 and 3720 Hz. I could be wrong, so let's hear from the bonafide Generac techs who've been around for a while....
Skip Douglas
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douglas123
If your genset does not have electronic stepper motor ie mechanical governor the engine will run 62 hz no load and 58 hz full load. If you have newer models with stepper motor the engine will be at 60 hz with or without load. Some of the fairly new models actually run at 58 hz until a load is applied. No wonder techs stay confused. Hope this helps with some of the confusion.
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johnc
Thanks Doug. That answers the question perfectly
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