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General Metalworking - All aspects of working with metal.
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Posted by jk on July 18, 2008, 12:52 am
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>
>Im one of those unusual people that hasnt a supply off the grid.
> Weve made our own electric for some 35 yrs and have serviced,
>overhauled and struggled with ac generators up to 20kva size. So your
>welcome to my experience.
Your experience in of the grid generation is tripping you up here.
> 1stly
> the statement that all motors as is, are generators isnt true.
>Most ac generators, wether single or 3 phase has the field windings
>excited by a dc field. Then the rotational energy imparted by the
>engine, turns the armature windings through this field to create
>avoltage output. this can be dc as well as ac.
> So to get your 3 phase 125hp ac motor to give you an a output from the
>slip rings you will need to excite the fields with a dc current.
> How you do this will depend on how the motor is wound.
>The armature speed in rpm, will depend on how many pole pieces there are
>in the field. a 2 pole motor will need in the order of 3200 rpm , a 4
>pole 1600 rpm and a 6 pole around 1100 rpm. The precise figure will
>depend on what frequency your local ac grid happens to be.
He is using an INDUCTION machine, not a synchronous one.
> 2ndly, Your local power co will want you to maintain this frequency
>between rigid limits.
>Can your wood gas engine do this?
No they wont (OK yes they will, but you cant change it, so they don't
care.) Power output will be at EXACTLY what the grid frequency is.
> 3rdly, as its a 3 phase motor, and assuming youve met the above
>requirements, you certainly can supply one of the phases to the local
>grid,solong as the other 2 phases have the same kw load.
> To do this youll need to load up the non grid phases for example,
>with resistance loads such as immersion heaters in a bulk hot water
>storage system.
Even with synchronous generators you don't need to have the load
balanced. Granted, it IS the only way you will get maximum power out
of it, but it is not necessary.
>4thly, your engine govenor will need to be locked onto the grid
>frequency , this is going to be the most difficult part of this project.
>Ask your local supply co what their regulations are.
>Also what switch gear they require,
> Talk to the maker of the motor asking for a wiring diagram.
>Start looking for electronic engine govenors.
Again, since this is an induction machine, none of that is needed OR
useable. Power is transmitted across the air gap, by the rotor
attempting to be turned faster than synchronous speed. The difference
between the generator speed, and the synchronous speed is the slip
speed. This slip, (just like when used as a motor) is what determines
how much power is transmitted. The frequency however is unaffected.
All of this however is useless to you Ted, since you are off grid.
>this will give you some idea of the problems you have to face.
> good luck and keep us all posted as to the answers you get.
> Ted Frater
> Dorset
>UK.
>
>
>
>
jk
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Posted by Jim Wilkins on July 18, 2008, 6:06 am
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> Power is transmitted across the air gap, by the rotor
> attempting to be turned faster than synchronous speed. =A0The difference
> between the generator speed, and the synchronous speed is the slip
> speed. This slip, (just like when used as a motor) is what determines
> how much power is transmitted. The frequency however is unaffected.
>
> jk
I thought a motor sent power into the line when it ran at the
synchronous speed with its phase ahead of the line, and drew power
when the phase was behind the line.
I'm not an EE but I do their work.
Jim Wilkins
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Posted by Jon Elson on July 20, 2008, 1:25 am
Please log in for more thread options Jim Wilkins wrote:
>
>>Power is transmitted across the air gap, by the rotor
>>attempting to be turned faster than synchronous speed. The difference
>>between the generator speed, and the synchronous speed is the slip
>>speed. This slip, (just like when used as a motor) is what determines
>>how much power is transmitted. The frequency however is unaffected.
>>
>>jk
>
>
> I thought a motor sent power into the line when it ran at the
> synchronous speed with its phase ahead of the line, and drew power
> when the phase was behind the line.
It will send power to the mains when the speed is ABOVE
synchronous. At exactly synchronous speed, the mains will
excite the stator iron, but there will be (approximately) no
flux in the rotor, as there is no slip. Line current will be
minimal at sync speed.
To correct what someone said previously, when running above
sync. speed, the induction alternator will appear as a
CAPACITOR, not an inductor. The phase angle shifta as you cross
sync. speed, and induction motors appear as inductors, so
induction alternators must appear as capacitors. The phase
angle varies with load, just like on an induction motor. So, at
light load, the apparent capacitance is dominant, and the line
current will be nearly 90 degrees leading. At rated load, the
line current will be nearly in phase with the voltage. This
could be one reason to not use an insanely huge motor as an
induction alternator (compared to the size of the
engine/waterwheel, etc.), as the power factor will always be
low, and you will be feeding huge capacitive VARs to the mains,
possibly cooking the pole transformer.
Jon
Jon
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Posted by Jim Wilkins on July 20, 2008, 8:15 am
Please log in for more thread options > Jim Wilkins wrote:
>
> > I thought a motor sent power into the line when it ran at the
> > synchronous speed with its phase ahead of the line, and drew power
> > when the phase was behind the line.
>
> It will send power to the mains when the speed is ABOVE
> synchronous. =A0At exactly synchronous speed, the mains will
> excite the stator iron, but there will be (approximately) no
> flux in the rotor, as there is no slip. =A0Line current will be
> minimal at sync speed.
>
> Jon
I put 'synchronous' in the wrong place so the sentence appeared to
describe an induction motor.
> > I thought a synchronous motor sent power into the line when it was driv=
en
> > with its phase leading the line, and drew power
> > when the phase lagged the line.
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>Im one of those unusual people that hasnt a supply off the grid.
> Weve made our own electric for some 35 yrs and have serviced,
>overhauled and struggled with ac generators up to 20kva size. So your
>welcome to my experience.