Another Generator Question

Married2Allis

Well-known Member
Have a 13kw backup generator and ran it today. I checked the house current and it was showing 121 volts @ 59.4 hz. I ran the generator and at 59.4 hz it was putting out 115 volts. To try and match the house current at 121 volts, the generator was up to 61.7 hz. Generator is just 2 years old and was checked out by a good electric motor shop when I bought it.

It puts out clean power +/- 5% THD, but I am worried about damaging my HVAC unit. What should I set it to run at, is 61.5 hz too high? - thanks
 
The line voltage coming into my place varies quite a lot. I bought one of those cheap plug in meters talked about on here, so I could set my alternator by cycles, rather than voltage. It only registers every 10 cycles- 40-50-60 ! So it does me no good. If I had a precise way to set my speed/cycles, I'd set it at 60 and take whatever the voltage is.
 
I think you are worried about nothing. Especially true with the Hertz cycles. The 115-121 volts you are reading is just an
average reading anyway. Grid power is often 170 volts, back and forth with the highs and lows of the cycles. I know I have
run my PTO generator to power our house and accidentally had it near 130 volts on the built-in panel volt-meter. Nothing in
the house was hurt. A common volt-meter cannot accurately measure AC voltage unless the power has a perfect sine-wave. If non-
perfect, you need a RMS voltmeter to get a good reading. Just one person's opinion, but from the figures you give - I don't
think you have anything to worry about.
 
JDEM has it about right. Your AC or heat pump may not like it more than 2 or 3 hz off, but should run on anywhere from 110 to 130V without problems.
 



As one of the engineers said; "The only thing that cares about being a few cycles off is.... a cheap motor driven clock."

Computers use a crystal/pll for timing,, as well as most electronics with active cpu/displays. The non active electronics does not care.. the active, has its own timing base. The government had a plan to drop all power to 50 cps for 30 seconds to warn the US of a nuclear attack.. And that would not hurt anything.
 
I'm listening too. The old military generator I just resurrected runs at 61.5 htz when I set the engine governor run the genny at 240 no load volts. When I slow it down so it puts out 60 htz the volt meter shows 230 no load volts. The volt meter on the generator coincides with my hand held volt meter so I fairly well trust it. I ended up setting the engine speed to get the generator to put out a little over 240 no load. Next week I plan to put it to work on the farm's grid to see what happens.
 
Wow this is going up and down a little. Voltage from your gen set and
out on the line can be quite a few volts up and down. That is
definitely true. With modern electronis especially control systems you
want as close as possible to 60Hz as you can get. If the frequency
gets too far out of wack you can fry some pretty expensive circut
boards to say the least. Go to Lowes or Depot and pick up a good
digital meter that has a range for Hz. In the $30.oo range and you
will have a good meter for lots of other projects. When you plug it
into a socket it should read a rock solid 60.0 hz. ON THE NOSE! Your
generator will drift a tiny bit just running maybe a couple of tenths.
Under a load the govenor MUST come back to 60!!!! If it doesn't you
either have a crummy generator engine, or you need a much bigger
generator. I would tell you a 6000 is just enough for a home. Go for
an 8000 if you can afford it. HVAC really can pull a load that a
small generator will really struggle with. I have a 8000 unit that
really kicks it. Please don't go cheep on your emergency power. Also
you must run Hi-Test gas. Those units work sooooo much better on that
stuff. Any more questions give us a yell.
 
The main thing is to have enough voltage UNDER LOAD AT THE LOAD. Crank up your AC and and measure the voltage as close as you can get to the AC compressor. Anything less than 110 volts is asking for trouble. (If it's a 240 volt unit, you need at least 220V.) +/- 5 percent on the frequency shouldn't be a big deal.
 
Bob Bancroft,

The algebra for frequency is easy to figure out if you have an accurate tachometer. The frequency is equal to the speed (in rpm) times the number of generator poles, all divided by 120.

f=(np)/120

D.
 
What are you reading voltage with? Many engine-driven generators do not make perfect sinewaves. As a result - you can take
readings with half-a-dozen cheap voltmeters and they all will be wrong. The only way to be sure is - take a reading with a
RMS voltmeter - or - put a scope on it and verify you have a perfect wave-pattern. Conventional voltmeters are only accurate
when reading AC if the wave pattern is perfect like comes from the grid.

I just went through this with a generator that supposedly only ran at 108 volts AC. Put an RMS voltmeter on it and it read
122 volts. Note - the owner had no complaints with using anything off the generator. It was just the 108 volt reading with
his cheap digital voltmeter that made him nervous.
 
Thanks. Interesting. I have a digital photo/tach, which uses reflective tape. But I have no access to the inner workings of my PTO driven alternator. I was thinking the step up drive resulted in an armature speed of 1800 RPM, and it has three poles. But that doesn't figure out correctly. I'll have to do some investigating to really find out.
 
What you are showing is just fine. Most most items to day will kick off if they get out of range. When I first started 57 fully loaded to 63 no load
was normal.Has improved some since then. But I think you will be just fine.
 
> Many engine-driven generators do not make perfect sinewaves.

That's a good point. And most cheap digital multimeters are not true rms. In fact, some pretty expensive multimeters like my Fluke 77 are peak-averaging.
 
(quoted from post at 08:48:02 11/26/17) Thanks. Interesting. I have a digital photo/tach, which uses reflective tape. But I have no access to the inner workings of my PTO driven alternator. I was thinking the step up drive resulted in an armature speed of 1800 RPM, and it has three poles. But that doesn't figure out correctly. I'll have to do some investigating to really find out.
poles? Nope. Even numbers only, every south pole has a north Every genset I ever saw had 2 or 4, 2 poles @ 3600 rpm for 60 hz or 4 poles @ 1800 for 60 hz.
 
Bob Bancroft,

I re- read my reply, and to me, I sounded all too wrong and strong. I meant to give you a formula to figure things out. I did not intend to be a smart-aleck.

I hope you figure it out, just was hoping to get you some decent info.

I think you'll get this frequency item settled out in short order.

D.
 
No problem. If it was a little warmer, and I had not much going on, I might pull the back cover off my alternator, and also study the manual. But that probably won't happen right now, and then I'll forget! I can at least get the formula into the manual, or maybe even put it on the machine. Maybe it's got 4 poles. Then it works out.
 
Thanks. It must be 4. That makes it work out correctly. For some reason I've got a little picture of three in my mind. Maybe something about an automotive alternator?
 
(quoted from post at 13:15:06 11/26/17) Thanks. It must be 4. That makes it work out correctly. For some reason I've got a little picture of three in my mind. Maybe something about an automotive alternator?
oesn't matter what kind of alternator, has to have an even # of poles. With automotive, you don't need to worry about freqs because the first thing you do is rectify it. But they still have a north pole for every south one, all magnets do.
 
I have a Fluke that is also not RMS. I had too many generators and inverters around that I could not get accurate AC volt readings from. I finally bought a "cheap" Extech True RMS" for $75. Huge difference in readings. My 15 KW PTO generator reads aound 108 volts with a cheap digital (and pricey Fluke) and 122 volts with the RMS.

I had a brand new so-called "true sinewave" inverter that was only reading 96 volts. Everything seemed to work okay with it, but I thought it was defective. I called the company's tech line and they said that 96 volt reading was normal with a conventional voltmeter. Sure enough, I checked it with the RMS and it reads 118 volts instead of 96 volts.









r
 
Two 120v bulbs and a perfect match to power company frequency..........better than any frequency meter that you can afford.
g309ViS.jpg
 
(quoted from post at 13:25:01 11/27/17) JMOR I really like that drawing. Has a funny cartoon quality to it. The light bulbs sort of look like 'eyes'. :)
hey will even wink at you when frequency doesn't match. :)
 
You do have the very old way of doing it. Is kinda neat as they wink
back and forth till you get the Rpms. set correctly. You can also go
to you local electrical supply house and see if they have a 240 bulb
in stock. Another thing is use a couple of NEON indicator lites. They
will glow a little till the syn wave is right at zero. Either way it
will work . Nothing beets having a good meter though.
 
(quoted from post at 19:18:28 11/27/17) You do have the very old way of doing it. Is kinda neat as they wink
back and forth till you get the Rpms. set correctly. You can also go
to you local electrical supply house and see if they have a 240 bulb
in stock. Another thing is use a couple of NEON indicator lites. They
will glow a little till the syn wave is right at zero. Either way it
will work . Nothing beets having a good meter though.
don't know about "nothing", as I believe the power generation syncs to the NBS atomic clock!
 
(quoted from post at 22:00:33 11/27/17)
(quoted from post at 17:02:47 11/27/17) ]I don't know about "nothing", as I believe the power generation syncs to the NBS atomic clock!
Huh? Where did that tidbit come from?
our of power generation plant. Do your own research.
 
> Two 120v bulbs and a perfect match to power company frequency..........better than any frequency meter that you can afford.

Hmm. It will tell you if your generator's no-load frequency matches the line frequency. It won't tell you what its frequency will be under load, unless you can figure a way to tap into the line while the whole house is disconnected from the line and powered by the generator.

The purpose of a synchronization light isn't just to get the generator running at the same frequency as the grid, it's to get it IN PHASE with the grid. Even if a generator is running at the same frequency as the grid, if the two aren't in phase the generator will be ripped from its mounts when it's brought on line.
 
(quoted from post at 22:34:12 11/27/17) > Two 120v bulbs and a perfect match to power company frequency..........better than any frequency meter that you can afford.

Hmm. It will tell you if your generator's no-load frequency matches the line frequency. It won't tell you what its frequency will be under load, unless you can figure a way to tap into the line while the whole house is disconnected from the line and powered by the generator.

The purpose of a synchronization light isn't just to get the generator running at the same frequency as the grid, it's to get it IN PHASE with the grid. Even if a generator is running at the same frequency as the grid, if the two aren't in phase the generator will be ripped from its mounts when it's brought on line.
ome can only give criticism, never a solution. Some call then a 'troll'. So, do you have a frequency meter better than the power company?
 
> Some can only give criticism, never a solution. Some call then a 'troll'.

And some are a bit thin-skinned.

> So, do you have a frequency meter better than the power company?

The issue is not whether the line frequency is maintained accurately, but whether or not your method is a good way to check a generator's frequency UNDER LOAD. Sure, you can do it if you don't mind tapping into the line side of your transfer switch. Otherwise, the generator won't be under full load and the frequency will droop when you do load it.

As for accuracy, using the light will get you close to line frequency, maybe within one percent (which is plenty close). You'll never be dead on because once you get to the point the light is slowly flashing, you have no way to tell if the generator is faster or slower than the line. So it will get you within one or two percent. Sorry, that's nowhere close to the accuracy of the crystal in my 30 buck Kill-a-Watt meter.
 
(quoted from post at 07:00:34 11/28/17) > Some can only give criticism, never a solution. Some call then a 'troll'.

And some are a bit thin-skinned.

> So, do you have a frequency meter better than the power company?

The issue is not whether the line frequency is maintained accurately, but whether or not your method is a good way to check a generator's frequency UNDER LOAD. Sure, you can do it if you don't mind tapping into the line side of your transfer switch. Otherwise, the generator won't be under full load and the frequency will droop when you do load it.

As for accuracy, using the light will get you close to line frequency, maybe within one percent (which is plenty close). You'll never be dead on because once you get to the point the light is slowly flashing, you have no way to tell if the generator is faster or slower than the line. So it will get you within one or two percent. Sorry, that's nowhere close to the accuracy of the crystal in my 30 buck Kill-a-Watt meter.
elieve what you want on that $30. Sorry! :)
 
One thing not mentioned but has to be remembered is that diesel engines have a fine tuned governor or use to compared to a gas
powered engine which will help keep the Hz more stable. This was years ago though. Engines are mostly electronically governed now
days. A worn engine to generator coupling will cause the hertz to fluctuate and never hold a frequency.
 

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