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Need something else to worry about?    When is the last time you checked the frequency of the power at the outlets? Power companies are responsible for this and usually get it right, but not always. This occurred to me while I was servicing my Generac for the upcoming winter and found it was running about 68 Hz.

I'd suspect your measurement equipment before the power company frequency.  Any significant deviation of the frequency would cause the generating station to trip off the line.  If the generator outputs are more than about 0.02% from 60hz, the generator risks tripping off the line, being over 10% off frequency would be impossible!  All the generators on the grid have to remain lock-step in phase or all **** will break loose!  68hz, that would be impossible within the US power grid, that would force an immediate trip and kick that generator off the grid.  Even the tiny fractional variations of frequency are made up over a 24 hour period with TPC (Time Period Correction) of the grid frequency.

If your Generator is outputting 68hz then it has serious problems. We run quite a few diesel powered generators for center pivot irrigation and the most they ever get out sync is 61.5hz. At that point they get pulled and serviced.

The power at our mill and grain leg have never seen anything beyond 60.2hz and I'll bet that .1hz of that reading is instrumentation error.

Last edited by H1000

My power company delivers a perfect 120 at 60 Hz. I'm not worried about them, they're big enough to be sued so they can afford to do it right. Not everyone is on the national grid.

@H1000 posted:

If your Generator is outputting 68hz then it has serious problems. We run quite a few diesel powered generators for center pivot irrigation and the most they ever get out sync is 61.5hz. At that point they get pulled and serviced.

The power at our mill and grain leg have never seen anything beyond 60.2hz and I'll bet that .1hz of that reading is instrumentation error.

It's a 8kW portable, the engine speed controls the frequency. It is a simple throttle adjustment to set it back to 60.

My Generac does not connect to the grid. After the grid power is off for a minute, the generator kicks on running on natural gas, connected to the house only. When the power comes back on, the generator quits and the big 200 amp relay reconnects the house to the grid. So at 68 Hz, only the stuff in the house would suffer…but of course the frequency is one of those things the guy checks every fall, changes the oil, checks the battery, sends me a bill.

Short story version, the anti-slop spring on the throttle mechanism was bent enough to upset the spring balance and allow the throttle flop back and forth ever so slightly. So it would surge a little. I was lucky that straightening out that spring fixed the surge. I'm sure that's when the throttle setting got messed up.  I pulled the gas tank and completely cleaned the carb trying to solve the surge. I was running the generator with the tank off to the side and noticed the linkage open and then get pulled closed and realized the anti-slop spring was pulling too hard. Sure enough it was bent which effectively shortened it. To set the frequency, Generac service manual says to set the engine rpms at 3600. I don't have a tach so I just watched to no load frequency until it was back below 61 Hz.

I try to save money where I can and this generator may well run my layout some snowy night.



It's a 8kW portable, the engine speed controls the frequency. It is a simple throttle adjustment to set it back to 60.

How did this problem occur in the first place? If your genset isn't auto-regulating the throttle to compensate for the correct voltage and frequency properly then (to me) it has serious problems.  So many of these emergency gensets will only monitor & correct the output speed of the engine to operate at 1800 RPM (usually through a mechanical governor) and assume that the generator is working just fine which should give the operator a nice 60hz wave at around 110 to 130 volts. Rarely are they configured to also monitor the quality of the power output and make corrections to engine speed to ensure power output is within spec. So what did you did you do correct the problem, manually adjust the throttle lower to get 60hz and now you only have 105 volts? If so, then there is an issue with the generator component itself that needs to be addressed.

Last edited by H1000

Well, if the engine is operating at the correct RPM, that's about all they would be able to do to insure 60hz power, right?

You can deploy a VFD which can correct the frequency to the correct level to an extent. We use a logging & monitoring systems on our gensets, which don't actually do any corrections, but Monitor, alert and shutdown the engine if the voltage or frequency falls out of spec for a predetermined amount of time. I have also seen models that do this plus correct the frequency (VFD), change engine speed to compensate for load variables and generator offsets on the fly.

I'm not sure I see how you're correcting the frequency of the gas powered generator short of changing the engine RPM, it's a fixed ratio.  If you're saying your VFD is adjusting the engine speed, that's all I'd expect you could do.  Obviously you can monitor the frequency and kill the generator, but that's probably not desirable in many cases.

It's a Variable Frequency Drive (VFD). It can take the input frequency and modify to a higher or lower level of any input source (generator or grid). We use them a lot on center pivot irrigation with variable speed motors. Instead of adjusting the voltage of a fixed votage AC motor that is designed only for 480 volts, we lower and raise the frequency of the power to slow down or speed up the motor. VFD's are also commonly seen used with Boiler pump motors to change the flow rate again not by changing the voltage but but changing the frequency of the power going to the motor.

Last edited by H1000

Voltage regulation on this Generac is a big cap, so if the frequency is correct the voltage regulation is also correct.

It is just old enough that parts are no longer available. It keeps the lights on, runs the boiler and well pump. I have to roll it out the back door, manual transfer switches, even the battery died years ago. It still starts on the first pull and has more than paid for itself over time.

@H1000 posted:

It's a Variable Frequency Drive (VFD). It can take the input frequency and modify to a higher or lower level of any input source (generator or grid). We use them a lot on center pivot irrigation with variable speed motors. Instead of adjusting the voltage of a fixed votage AC motor that is designed only for 480 volts, we lower and raise the frequency of the power to slow down or speed up the motor. VFD's are also commonly seen used with Boiler pump motors to change the flow rate again not by changing the voltage but but changing the frequency of the power going to the motor.

That's variable speed control. My variable speed well pump is digitally controlled by varying the frequency. It was this system that clued me in that there was a problem when I flipped the trans switches to start the well controller. It would throw up a a big FAULT alarm but didn't really explain what the issue was. Obviously the generator was suspect.

I didn't mean to start off on a tangent, but this experience got me thinking how I mostly take frequency for granted.

Last edited by turkey_hollow_rr

For high power, that seems a very expensive way to do the job!  Converting to DC and back to AC ain't cheap for high power!

Still way more reliable than some type of variable speed drive gearbox. Generally a VFD control box on on one of our pivots will run about $350. I've only had to replace one in the last 10 years. Considering that I can loose up to $500 per day of down time on a single pivot due to yield loss, yeah they're cheap.

That's variable speed control. My variable speed well pump is digitally controlled by varying the frequency. It was this system that clued me in that there was a problem when I flipped the trans switches to start the well controller. It would throw up a a big FAULT alarm but didn't really explain what the issue was. Obviously the generator was suspect.

I didn't mean to start off on a tangent, but this experience got me thinking how I mostly take frequency for granted.

I agree, I don't want to steer the thread to far off into the woods. You do have something that will tattle on your generator, which is good!  There are VFD controls available that can correct the output of a genset and do so automatically with or without adjusting the speed of the engine, but they are pricey!

Even the latest Genrac generators still just vary the engine RPM to adjust the output frequency, no fancy electronics or VFD boxes here.

Do they adjust for the frequency, voltage output or generator shaft input speed (1800 RPM)? If the generator component starts going south, you might have 1800 RPM and only 55 Hz at 120 volts or 1950 RPM at 60 Hz and 140 volts. Higher end gensets will monitor all three and alarm/shutdown when stable output (120v @ 60 HZ) can't be maintained by simply controlling engine speed. Even higher end units will alarm/compensate and or shutdown to maintain a perfect power output, generally only found in hospital settings.

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