Re: Generator RPM question


"Andy" <andysharpe@xxxxxxxx> wrote in message
news:1117740678.369443.108160@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> 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

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


.

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