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Generator RPM question

A

Andy

Andy asks:

In the manual for my DeWalt power generator, it states
that the speed of the engine is electronically regulated
to 3700 ( yes, 3700) rpm.

In other manuals for other equipment, I see 3700 rpm referred
to as the speed for maximum current output at constant voltage
welding.....

However, the DeWalt generator is not a welder. It is used
only for home power backup.

I was under the impression that all of these generators
had to run at 3600 rpm (for 60 hz output). Yet the manual
plainly states 3700. The lady who answers at customer
service at DeWalt is of no help...

So, does anyone here have direct knowledge about this
number, or do I chalk it off to a missprint in the owners
manual ?

Thanks,
Andy
 
M

martin griffith

Andy asks:

In the manual for my DeWalt power generator, it states
that the speed of the engine is electronically regulated
to 3700 ( yes, 3700) rpm.

In other manuals for other equipment, I see 3700 rpm referred
to as the speed for maximum current output at constant voltage
welding.....

However, the DeWalt generator is not a welder. It is used
only for home power backup.

I was under the impression that all of these generators
had to run at 3600 rpm (for 60 hz output). Yet the manual
plainly states 3700. The lady who answers at customer
service at DeWalt is of no help...

So, does anyone here have direct knowledge about this
number, or do I chalk it off to a missprint in the owners
manual ?

Thanks,
Andy
Click to expand...
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
 
R

Roger Hamlett

Andy said:
Andy asks:

In the manual for my DeWalt power generator, it states
that the speed of the engine is electronically regulated
to 3700 ( yes, 3700) rpm.

In other manuals for other equipment, I see 3700 rpm referred
to as the speed for maximum current output at constant voltage
welding.....

However, the DeWalt generator is not a welder. It is used
only for home power backup.

I was under the impression that all of these generators
had to run at 3600 rpm (for 60 hz output). Yet the manual
plainly states 3700. The lady who answers at customer
service at DeWalt is of no help...

So, does anyone here have direct knowledge about this
number, or do I chalk it off to a missprint in the owners
manual ?
Click to expand...
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
 
J

John - KD5YI

Andy said:
Andy asks:

In the manual for my DeWalt power generator, it states
that the speed of the engine is electronically regulated
to 3700 ( yes, 3700) rpm.

In other manuals for other equipment, I see 3700 rpm referred
to as the speed for maximum current output at constant voltage
welding.....

However, the DeWalt generator is not a welder. It is used
only for home power backup.

I was under the impression that all of these generators
had to run at 3600 rpm (for 60 hz output). Yet the manual
plainly states 3700. The lady who answers at customer
service at DeWalt is of no help...

So, does anyone here have direct knowledge about this
number, or do I chalk it off to a missprint in the owners
manual ?

Thanks,
Andy
Click to expand...


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
 
B

Bob Eldred

Andy said:
Andy asks:

In the manual for my DeWalt power generator, it states
that the speed of the engine is electronically regulated
to 3700 ( yes, 3700) rpm.

In other manuals for other equipment, I see 3700 rpm referred
to as the speed for maximum current output at constant voltage
welding.....

However, the DeWalt generator is not a welder. It is used
only for home power backup.

I was under the impression that all of these generators
had to run at 3600 rpm (for 60 hz output). Yet the manual
plainly states 3700. The lady who answers at customer
service at DeWalt is of no help...

So, does anyone here have direct knowledge about this
number, or do I chalk it off to a missprint in the owners
manual ?

Thanks,
Andy
Click to expand...

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
 
A

Andy

Andy writes:

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
 
B

Bob Eldred

Andy said:
Andy writes:

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
Click to expand...

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
 
A

Andy

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
 
O

Ol' Duffer

In the manual for my DeWalt power generator, it states
that the speed of the engine is electronically regulated
to 3700 ( yes, 3700) rpm.
Click to expand...

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.
 
R

Rich Grise

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....
Click to expand...

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:
http://www.dewalt.com/us/service/

Thanks,
Rich
 
G

Gilbert

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
 
J

Joe

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.
 
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