🚀 go-pugleaf

nimkp@im.uib.no (Knut Pedersen) wrote:

>Is it possible to use a 3-phase motor as a generator?

Assuming this is an induction motor, not synchronous machine.

An induction generator is exactly this,  but it needs to be tied to a
grid (infinite source-sink at stable frequency)  By  powering the
motor a little above it's sycronous speed (1800 rpm for 60 Hz three
pole motor, overdriven to  say 2000 rpm) the thing actually generates
power.

In this way, you could buy less power from the utility, but I don't
think you could sell them any...they are not required to buy power
generated from non-renewable fuels. (My un-educated understanding
only...no expert on this point).  I know a former owner of a feed mill
that avoided high peak demand charges in exactly this way.  The
utility was thus always available to cover generator down time.

An induction generator is a pretty stable load.  You would want to
monitor output current to avoid burning it up, and also don't want to
be giving away power to utility, but other than that, you could just
open throttle on engine to an efficient operating level, and walk
away.

I have read (In alt.enrgy.renewable, I think) that just driving an
induction motor  with no utility intertie results in a "wild "
generator...that is to say with poor frequency and voltage regulation.
You may need to apply DC to two terminals briefly in order to provide
som initial magnitism for the thing to "build up" from.   Such "wild"
power is best rectified to DC, then converted to whatever you need.

If you will run without a utilitity intertie, I think you will end up
cheaper in the end to just use something intended for the job.
Conditioning the wild power from such a generator could easily cost
more than a "real" alternator. Modern brushless alternators are
elegant works of art...I would recommend them, as they are nearly
maintainace free.  Perhaps you could find one with a blown engine for
cheap.

Off topic aside:
I had the pleasure of watching a modern 500MVA alternator at a Swedish
nuke installation.  It was about the sixe of a Volkswagon bus, and the
exciter was about half that again.  Truely magnificent machine...

-KF-

I previously wrote wrote:

>(1800 rpm for 60 Hz three
>pole motor, overdriven to  say 2000 rpm)

Oops, I meant to say 6 pole motor.

Synchronous speed for 3 pole is 3600 rpm at 60 Hz.

-KF-

>Is it possible to use a 3-phase motor as a generator?
>I am thinking of using a disel engine as the power unit.


You can do this with a 3-phase induction motor, but it can't be used this
way as a standalone unit.  It has to be hooked to an existing power line to
work.  It's done by driving the motor in the normal direction of rotation
(relative to the direction it would rotate if it were used as a regular
motor), but faster than synchronous speed.  For example, a 6-pole motor on
60 Hz (you may have 50 Hz where you are) has a synchronous speed of 1800
rpm.  The rated speed will be about 1725, which means that the rotor lags
behind the field rotation by 75 rpm.  To make it run as a generator, you
have to run it about 1875 to 1900 rpm in the same direction.  The excitation
comes from the reactive power (vars) in the power line.  If it doesn't have
the power line to draw reactive power from, it can go wild and do just about
anything.  The 1900 rpm sounds like a good speed for a Diesel engine.

This method is popular for wind turbines since you don;t have to have
elaborate control systems to attach it to an existing line.  Just make sure
the phase rotation is right (a one-time job) and provide some overcurrent
protection.

-- pHil

nimkp@im.uib.no (Knut Pedersen) wrote:

>Is it possible to use a 3-phase motor as a generator?
>I am thinking of using a disel engine as the power unit.
>Any tips? thanks for help.
>Knut

Yes, but it has to be a synchronous motor.  Induction motors will
work, but they produce only reactive power.

J.L. Stillman, P.E.

103705.2167@compuserve.com (Joseph L. Stillman) wrote:

>nimkp@im.uib.no (Knut Pedersen) wrote:

>>Is it possible to use a 3-phase motor as a generator?
>>I am thinking of using a disel engine as the power unit.
>>Any tips? thanks for help.
>>Knut

>Yes, but it has to be a synchronous motor.  Induction motors will
>work, but they produce only reactive power.

>J.L. Stillman, P.E.

I disagree. The induction generator will absorb reactive power from
the power system so you must be interconnected, You can compensate for
this by installing static capacitors. If anyone would like to see a
working unit (hydroelectric), I can show it to them.

Some of the pump manufacturers a few years ago were pushing their
induction motor driven units as cheap but inefficient small
hydrogenerators. You just reverse the direction of rotation, the same
as large pump generators.


Regards,

John Phillips
Superintendent of Power
Niagara Power Project
New York Power Authority
work e-mail phillips@ip3gate.usa.com