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Posted by Andrew Mawson on August 9, 2006, 12:50 pm
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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
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Posted by Mark Rand on August 9, 2006, 1:35 pm
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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
>
If it's an induction motor then you have to supply the magnetizing VARs
externally. This can be in the form of the mains and its "infinite bus*". If
you try to run the motor faster than synchronous speed the it'll generate.
When you use two phases and a capacitor you are supplying the magnetizing VARs
to the third phase from the capacitor. Voltage regulation isn't as good, but
it'll work.
If you run a single phase motor as a generator, the mains is still providing
the magnetising VARs, but there are larger circulating currents in the rotor
squirrel cage windings... it'll still generate.
If you throw away the squirrel cage rotor and put your own wound rotor in with
a simple voltage regulator feeding the windings, then you have a large noisy
lump in the garden!
People in the business talk about electrical machines rather than motors or
generators because the only difference is whether you take mechanical power
out or put it in.
* An infinite bus is an electrical circuit in which the voltage and frequency
are unaffected by the amount of current you take out or put in to it.
Don't know if that has muddied things up more or not.
regards
Mark Rand
RTFM
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Posted by Greg on August 9, 2006, 6:05 pm
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> * An infinite bus is an electrical circuit in which the voltage and
frequency
> are unaffected by the amount of current you take out or put in to it.
Although the mains is often referred to as an infinite bus, and it's true
that you won't change it's frequency unless they are supplying your area
from a portable generator while maintaining the network, you can all too
easily change it's voltage as can be seen by the lights dipping when you
start a lathe etc. I suppose the best way to describe the mains is an
infinite bus with an impedance in series with it.
Greg
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Posted by John Stevenson on August 9, 2006, 4:54 pm
Please log in for more thread options 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
>
Andrew, The best motor to convert to a generator is the slip ring type
AC motor, found in overhead cranes etc.
These can be easily excited and controlled by stuffing DC into two of
the three slip rings.
Can't remember off hand what's needed, 90 volts seems to ring a bell.
Try some of the rewind people as they aren't a common motor but they
are the best.
--
Regards,
John Stevenson
Nottingham, England.
Visit the new Model Engineering adverts page at:-
http://www.homeworkshop.org.uk/
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Posted by Greg on August 9, 2006, 6:17 pm
Please log in for more thread options > 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).
That's essentially what a rotary converter is, once the start cap is
disconnected you are feeding one phase in and getting three phase out,
although they usually have running capacitors to balance the voltages and
improve the power factor. Note however that if you want 400V out you have to
feed 400V in so have to use a step up transformer, this is because the input
is applied across two phases of the motor not a phase and the star point, if
indeed there is one.
Greg
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>
>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
>