On 2013-11-17, Jon Elson wrote:
Ignoramus26083 wrote:

On 2013-11-17, Jon Elson wrote:
Ignoramus26083 wrote:


I believe that if I excite one leg with DC, it will statr producing
AC. Right?
No, it will burn out. That's DC injection braking, what most VFDs
use for the final stage of stopping a motor. It may be necessary
to blip some DC on an induction alternator to get it excited,
but apparently with a cap back across the windings, they usually
will self-excite as long as there's no load on it.

Jon

So, what is a difference between a 3 phase generator and a 3 phase motor?
Absolutely no difference between an induction motor and an induction
alternator, except the cap bank for excitation.

Now, a real 3-phase alternator has a DC winding in the rotor
and a regulating device to control that DC excitation based on
output voltage and phase angle. Modern portable generators
use permanent magnets in the rotor and get rid of the regulator.

An induction AC motor uses the field of the stator to create an
induced field in the shorting bars of the rotor. This requires
the stator current to always have a lagging power factor.
The principle of the induction alternator is to get the same
induced field in the rotor by making the stator always have
leading current in it. (Everything has to be backwards as the
direction of power flow is backward.) Placing capacitors
across the line terminals assures a leading power factor, and
that will keep the alternator excited. Then, you have to
apply a load to the alternator to prevent the voltage from
soaring. You may also have to cut capacitors in and out
as load varies.

All electrical machine are supposed to be reversible.

Jon

Thanks. This actually makes sense. It would seem that it is
straightforward to build a 3 phase alternator to produce power, as you
described.

It may require some initial flashing to get started from black start,
but it will work. At the same time, such a system would be difficult
to regulate for constant voltage as load varies.