"Dimitrios Tzortzakakis" wrote in
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Tzortzakakis Dimitri?s
major in electrical engineering, freelance electrician
FH von Iraklion-Kreta, freiberuflicher Elektriker
dimtzort AT otenet DOT gr
? "klasspappa[remove]" ?????? ???
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Asynchronous motor CAN be a generator...
It must turn faster than the magnetic field and you may apply some power
to it, then it will transfer power back to source...

I just run through the book of electrical machines I, one of my old
subjects, and it doesn't mention asynchronous generators at all.It only
mentions alternators.If one generator needs applying power to it to
generate
electricity, it loses the purpose of being a generator.

Then you need to find another book. An asyncrhonous generator needs an
external source for reactive power / excitation. That is not the same as
'applying power to it to generate'. When properly excited, an asyncrhonous
generator can supply much more power to the electrical system than it draws
in excitation energy. Much like a large synchronous machine needs an
external supply to its field to operate, yet it results in a net input of
power to the electrical system.

The proof of this is
our friend's experience;in his post he mentions the asynchronous motor
when
operated as a generator has an output of only 0.1 V RMS, which is useless
for any purpose at all.

Because 'our friend' didn't supply an external excitation. A synchronous
machine with an electromagnet field (not permanent magnets) also outputs
very low voltage when spinning with no DC field current. Are you suggesting
that synchronous machines are 'useless for any purpose at all'? Of course
not. Yet you suggest that an *improperly operated* asyncrhonous machine is
proof that all such machines are 'useless'.

In no lab have we had a project of converting a motor
to a generator;always in motors and generators the apparatus was
separate.Talking about asynchronous generators.


Possibly because your lab just didn't bother to explore this area. An
async. generator operating in a stand-alone fashion has many drawbacks. The
voltage control is difficult unless you have either a wound-rotor machine
and control the rotor circuit resistance, or variable capacitance connected
to the line terminals.

Because of the torque/slip characteristics of induction machines, the output
frequency is also very difficult to control. If the prime mover speed is
held constant, the output frequency varies with the amount of real load and
the amount of slip (remember in an async. generator, the output frequence is
lower than rotor speed. To maintain the output frequency in a stand-alone
machine, you must increase the rotor speed as load is applied.

But, if the machine is connected to a 'stiff' grid that approaches the
theoretical 'infinite bus', then many of these issues fade. The bus
maintains a constant frequency and voltage and the real power output of the
machine is controlled by controlling the speed of the prime mover. As speed
is increased above the synchronous speed of the machine, more power is
supplied to the bus.

Work out how the torque and power vs speed curves of an induction motor
behave and simply extrapolate beyond the synch. speed.

daestrom