electric generator question

[cameron sydenham]

any input would be great. we are using a 5000 watt generator, (7hp i think) to do some tests. is the input to turn this or any generator linear?? meaning, this generator says max output at 3500 rpm, what i am wondering is, at 3500 rpm, the motor needs to be at least 7 hp to turm it, is this right, and does this mean a 3.5 hp motor would only turn this generator to only 1750 rpm??? or is there a "spike" in input needed to turn it at higher rpms verses lower or vice versa?

what i am trying to get at is, is there any way to figure out how much hp i am using to turn the generator at a lower rpm than the max of 3500???

[khabe]

If I understood you correctly what you are asking ... then all dependes are upon about the generator,
loosely speaking:
When permanent magnet generator then all is in linear addiction to RPM (up to max power ( depends about copper wire, iron core, magnets ...)) voltage as well as power,
When wound rotor then linear dependency, when independently stabilized excitation (for example 12V direct to rotor coil) then output voltage changes linear to RPM - exact like PM Gen.
When to use step-down voltage regulator, then output power will depend about RPM up to max perf. (of course you have to reckon with minimum RPM too ...)
When wound rotor connected with output without any stabilizing circuits then output voltage will be very-very non-linear to RPM ...
When wound rotor and voltage stabilizer (like car alternator) - then output voltage is stabil - max power depends about RPM (op till declared max power).
means
When you driving your gen (voltage stabilisator through rotor coil)  5000W / 3500 RPM then with 1750 rpm you can get only half of output power.
Possible it was not what you asked,
my English is true DIY (selfmade ;-)
Cheers,
khabe

[cameron sydenham]

kind of answers me, i am not sure what type of generator this is, it was a standard gas generator we purchased, removed that gas motor and we are using the actual electric generator. what i am trying to figure out is can i use a simple graph to estimate the input power required to turn the generator from 0 rpm to 3500.
example, if i can turn the generator at rpm of 500, 750, 1000, 1500, can i deduce that it requires X Horsepower to do this??? based on the graph ?

[mscoffman]

I take it that this is a generator used to replace 50Hz utility power?

If it is, the answer would have to be generally "No". All generators that
are meant to service demand electrical load need to have a regulator
that dictates how much current is allowed to flow through to it's field coils.
So the generator will adapt to electrical load resistance demand by altering
it's field strength, trying to keep it's output voltage constant. Then it expects
the prime mover to change it's output torque via a throttle governor to keep
the RPM's constants.

An AC generator needs to have a precise RPM to have it's n* magnetic poles
hit per second to generate the correct Hz frequency for the AC output. This
controller could very well be resonant. So drifting off the correct RPM and not having
the prime mover torque be controlled by a governor sounds like a recipe for system
instability. Most likely the regulator won't even enable the electrical output until RPM's
are within 10% bounds of those required to prevent from damaging the load electrically
as it first comes up or engine shuts down.

There is also something called a "digital" generator that do things somewhat
differently, using an alternator and an electronic DC to AC inverter, though I
doubt that they are in that 5Kwatt power range at this time.

What you need is a dynamo generator meant for a "Brake Horsepower Dynamometer".
That would be expected to run at different RPM's and generate calibrated DC power output.
It may have the same mechanicals as any other generator but the field current regulator
will be very different. Alternatively one could see what is required and redesign the
utility power generator regulator to perform like one in a dynamometer and get rid of
any resonant aspect of the utility generator. Also, most AC stand-alone generators
have an "exciter" which is a small permanent magnet magneto for generating the
field-current initially while the generator section comes on-line.

:S:MarkSCoffman

[cameron sydenham]

the generator isnt being used for its power output, yet, we are only using it as a "load" on our motor and what we do know is if we can turn it , the generator at 3500 rpm, put a load on it, by plugging components in to it, and are able to continue turning it at the rpm needed, we can make our evaluations.