Generator RPM question

Many modern generators, don't generate AC at all!. They use a DC alternator, and then electronically synthesise the AC, to give crystal controlled stability on the output. Even 3600RPM, only works for 60Hz AC, if you have a generator system that gives one cycle/rev. This is a relatively inefficient design. Most have more poles than this. Synthesising the output, has the big advantage, that the same unit can be used for different countries. I have a Honda unit, that offers 400Hz,

60Hz, and 50Hz all at different voltages, with a simple switch.

Best Wishes

I doubt if it would make any difference to your laptop. I think that the switch mode brick power supplies are pretty tolerant of frequency and voltage. You could of course measure the output frequency......

martin

I do not have direct knowledge of why DeWalt made that statement, but I can guess. Note that the fraction of 3700/3600 is about 1.02777... Less than 3% difference.

The speed of the generator determines output frequency. So, 1.02777.. times

60 is 61.666... Hz. Not enough difference to disturb any attached equipment that I can think of at the moment.

As the load increases, the frequency goes down (at least temporarily, until the governers kick in) so it is probably wise to set the engine a little high to compensate. Three percent is not much IMO.

Cheers.

John

Is there an inverter on the generator? If so, the inverter is an electronic switch that develops 60 Hz power from the generator input power. The frequency is determined by crystal controlled electronics in the inverter and not by the rotation speed of the generator. Such systems usually use induction motors as generators. If an induction motor is turned by an external force faster than the electrical rotation speed, it will deliver power to the line instead of absorbing it as is the usual case with a motor. The electrical rotation speed is 60 Hz or 3600 RPM, the over drive speed is

3700 RPM thus the motor (generator) delivers power to the inverter. If it were acting as a motor, the rotation speed would be slower than the electrical speed, something like 3450 RPM. This difference is called slip. Modern power inverters make this scheme practical giving several advantages over older synchronous designs.The chief advantage is that the engine speed is not critical over wide margins. This makes engine speed control simpler. That advantage is useful for wind turbines where the wind speed and resulting blade rotation are variables that are hard to control. Another advantage is simplicity in design. There are no rotor windings, no slip rings, no commutators, no field rectifiers, no exciter, no permanent magnets, or any of the other accouterments usually found on generators. I'm not sure that this is what you have, but it sounds like it based on what you have said. Bob

Thanks to all who replied. I am aware of generator/inverter combinations. It is NOT one of these.

I am also well aware that the 3% rotational speed difference makes no appreciable difference in operation of anything except synchronous clocks......

But my original question is:

Assuming a 2 pole alternator , like almost every 7kw generator manufactured by Coleman, Kohl, Genrac, etc,,,,,,,

Is the 3700 rpm a missprint, or, if it is not, where does it come from ????

Thanks again for the info, and a lot of good info was presented here, but the question was specific....

Andy

If it is truly a synchronous generator, then 3700 RPM is not correct as you suspect. Is the load specified for this speed? Maybe 3700 RPM is the regulated engine speed for little or no load, but under high loading, the speed droops down to 3600 RPM or even lower. In otherwords, the speed regulation is not tight enough to hold 3600 RPM at all loads so they boost the speed a bit as a factory setting knowing that it will come near 3600 RPM under average loads. Just a guess. Bob

Andy replies to Bob:

That was my guess also. But, nowhere in the manual does it say that 3600 is the run speed, only that "generator rpm is maintained at 3700".

Since nobody has suggested a good reason for 3700 that seems to apply, I suspect that DeWalt messed up and it wasn't worth reprinting the manual, since nobody usually gives a damn.....

I tried to find other DeWalt generators on the internet to look in the manuals for them, to see if it was universal. I can't find the generator owner manuals. Darn near every other product they make, tho.

I did notice that all the Honda engines that run generators have their horsepower spec'd at 3600 rpm, which I expected.....

Thanks, Bob. I always learn something when I post to this newsgroup, since there are so many people here with first hand experience....

Andy

The new Onans (Cummins) are specified at 2880. Go figure that! Lots of electronics in the new stuff. It obviously works, but I wonder about reliability.

I can't understand why you haven't called somebody at DeWalt and asked yet. We can speculate and invent all day, but that isn't really going to answer your question, is it? So, why not ask somebody who might actually _know_ the answer? Like, the guy who works at DeWalt. Maybe they have an email or something:

Thanks, Rich

He did. From Andy's original post:

RoyalHeart

--

A thoughtful pause, then resumes the prince, "We must Learn from 
Yesterday, Live for Today, and look Forward to Tomorrow, for The Past be 
The Present, and No Less The Future."

I have a coleman 1850 watts generator and I did a test with it: I connected a KillAWatt and a 1500 (900/600) heater to it. At no load I had around 62 hz and around 127 volts at medium load it had 60 hz and 117 volts at full load 1350 watts it had 58 hz and around 107 volts So the governor is not 100 percent exact.

Get one of those KillAWatt it's the best thing since the light bulb. You can find them in ebay or do a google search for it

00, "Andy" wrote:

Generator rpm is typically: (Frequency x 60 x 2) / (number of poles)

on a Catapillar 400 kW generator, 3 phase, 60 Hz system that had 4 poles on each phase we ran 1800 RPM.

On small home generators, they are typically 2 pole, so 60 Hz would be 3600 rpm and 3000 rpm for 50 Hz.

--
Joe
http://www.fcctests.com