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Posted by on July 18, 2008, 10:37 am
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On Tue, 15 Jul 2008 09:42:17 -0500, spaco
>Slightly off topic, but not totally:
> We are preparing to build 2 woodgas cogeneration units that will feed
>power back into the grid through a net metering contract. We have
>found that "any induction motor IS a generator". We will use that
>knowledge to send one phase's output of a 125 HP 3 phase induction motor
>back into the power line. We have to do it this way because we don't
>have 3 phase lines anywhere near our home. To bad, too, since, if we
>did have 3 phase, we could develop 45KWHR whereas, on single phase, we
>can only "sell" 25KWHR.
>
> My question: Can we successfully use just the one phase's output in
>this way without destroyiing the electric motor? Our plan is to run
>this system for one day a week to develop all the electric power we need
>for the following week or so. This means that the motor will be active
>for 8 to 10 hours at a time.
>
>Just to complicate things a little, we may also use one or two of the
>other phases to develop local power while we are "net metering".
>
>Pete Stanaitis
>----------------------
For an induction motor to self excite and act as a
generator it has to feed a capacitative load. It is a pretty
inconvenient arrangement. The output frequency is controlled by
shaft speed and there is no easy way of controlling output
voltage which is both load and speed dependent.
If the generating induction motor is directly
connected to a primary power line, this phase synchronises both
operating frequency and excitation and it becomes a useful and
manageable device.
Connected to the supply and running light it can
operate as a motor or, in mirror image fashion, as a generator.
If a load torque is applied which slightly slows the
rotor it acts as a motor and supplies mechanical power to the
load.
If negative torque (i.e.it is driven) is applied which
slightly speeds up the rotor it acts as a generator and feeds
electrical power back into the line.
The frequency of the generated power is independent
of shaft speed and always exactly equal to supply frequency.
Shaft speed must be slightly above synchronous speed
and is effectively a mirror image of the motor shaft speed
performance. If as a motor it delivers it rated output power at
100 RPM BELOW synchronous speed it will deliver a similar amount
of generated power at 100 RPM ABOVE synchronous speed.
This is true for both single and three phase machines
connected to single and three ase lines.
Provided the shaft speed is somewhere near its rated
speed and it is phase synchronised to the single phase power line
the generating motor can be loaded with any convenient
distribution of line loads provided these do not exceed the rated
line currents as a motor.
Since there is no active voltage regulation the outputs
from the two spare phases may be a bit or below nominal but
should be close enough for most purposes.
Hope this helps
Jim
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> We are preparing to build 2 woodgas cogeneration units that will feed
>power back into the grid through a net metering contract. We have
>found that "any induction motor IS a generator". We will use that
>knowledge to send one phase's output of a 125 HP 3 phase induction motor
>back into the power line. We have to do it this way because we don't
>have 3 phase lines anywhere near our home. To bad, too, since, if we
>did have 3 phase, we could develop 45KWHR whereas, on single phase, we
>can only "sell" 25KWHR.
>
> My question: Can we successfully use just the one phase's output in
>this way without destroyiing the electric motor? Our plan is to run
>this system for one day a week to develop all the electric power we need
>for the following week or so. This means that the motor will be active
>for 8 to 10 hours at a time.
>
>Just to complicate things a little, we may also use one or two of the
>other phases to develop local power while we are "net metering".
>
>Pete Stanaitis
>----------------------