3 phase motor as a generator ?

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3 phase motor as a generator ? Andrew Mawson 08-09-2006
Posted by Greg on August 9, 2006, 6:21 pm
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> So if a 3 phase motor has one phase mains energised and is then spun
> up to synchronous speed will it generate the two other phases?

Reading it again, it would be best desribed as a motor having two phases
applied from the mains and generating the third phase, or putting it another
way it's working like a three phase motor on a three phase supply that has
lost one phase, yet keeps running.
Greg



Posted by on August 10, 2006, 4:59 am
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 On Wed, 9 Aug 2006 17:50:44 +0100, "Andrew Mawson"

>Can one use a 3 phase motor as a 3 phase generator?
>
>A rotary phase converter gets a normal 3 phase motor spinning using
>essentially 2 phases by using an artificially generated second phase
>via a capacitor, and the motor generates the third phase.
>
>So if a 3 phase motor has one phase mains energised and is then spun
>up to synchronous speed will it generate the two other phases? (I
>appreciate that a normal 3 phase generator would have an armature that
>is a DC magnet spinning in the three stator coils).
>
>AWEM
>

Not quite - it has to be beyond synchronous speed before it
it behaves as an induction generator.

The normal squirrel cage 3 phase induction motors work
because the rotor speed is slightly lower than the synchronous
speed of the rotating magnetic field. This generates large
currents in the rotor bars at this difference frequency and this
provides the motor output torque.

If it is mechanically driven to reach synchronous speed the
difference frequency falls to zero so that no current is induced
in the rotor bars. The machine is now passive - it neither
absorbs or generates power.

If the machine is mechanically driven ABOVE synchronous
speed large currents are again generated in the rotor bars but
this time the torque generated opposes the input drive. The
magnetic field from the rotor rotates at rotor rotation frequency
minus the difference frequency = synchronous frequency and this
generates power which is fed back into the supply. i.e although
the rotor speed is slightly above synchronous speed it feeds back
power at supply frequency.

If we plot shaft torque against shaft speed we get an "S"
curve lying on its back. Torque passes through zero at
synchronous speed and rises to a maximum at rotor speeds just
below and just above synchronous speed - absorbing power below -
generating power above.

Strictly speaking the above applies to a 3 phase machine
connected to a high power 3 phase supply. However, if only one
phase is connected, the machine will also act as a rotary 3 phase
converter. Three phase power will be available and power will be
fed back to the single energised phase.

Note - this only works because it is connected to a
relatively high power supply. This defines the synchronous speed
and permits a stable system.

Jim





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Posted by Colin Ager on August 13, 2006, 12:09 pm
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Andrew Mawson wrote:
> Can one use a 3 phase motor as a 3 phase generator?
>
> A rotary phase converter gets a normal 3 phase motor spinning using
> essentially 2 phases by using an artificially generated second phase
> via a capacitor, and the motor generates the third phase.
>
> So if a 3 phase motor has one phase mains energised and is then spun
> up to synchronous speed will it generate the two other phases? (I
> appreciate that a normal 3 phase generator would have an armature that
> is a DC magnet spinning in the three stator coils).
>
> AWEM
>
>
Hi Andrew. There is an article on using a three phase squirrel cage
motor as a generator in "Stationary Engine" magazine No 217 (March 1992)
A version is shown which produces about 6 kW

This is Based on information supplied by John Bonner.If needed I could
scan the piece.


Posted by Andrew Mawson on August 13, 2006, 1:43 pm
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> Andrew Mawson wrote:
> > Can one use a 3 phase motor as a 3 phase generator?
> >
> > A rotary phase converter gets a normal 3 phase motor spinning
using
> > essentially 2 phases by using an artificially generated second
phase
> > via a capacitor, and the motor generates the third phase.
> >
> > So if a 3 phase motor has one phase mains energised and is then
spun
> > up to synchronous speed will it generate the two other phases? (I
> > appreciate that a normal 3 phase generator would have an armature
that
> > is a DC magnet spinning in the three stator coils).
> >
> > AWEM
> >
> >
> Hi Andrew. There is an article on using a three phase squirrel cage
> motor as a generator in "Stationary Engine" magazine No 217 (March
1992)
> A version is shown which produces about 6 kW
>
> This is Based on information supplied by John Bonner.If needed I
could
> scan the piece.
>

Colin,

If you have the issue to hand that would be very kind.

Regards,

Andrew

ps not seen you at Marshall House for a long time !!!!!!!



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