Friday, 22 February 2008


They will need capacitors to work. The newer metals used in some motors are more efficient as far as the hysteresis losses go. That means a few of them don't have the
'fault' of having residual magnetism. What's good for a motor is, in
this case, bad for the generator start-up.

I think where the confusion is starting may be from the planned usage.
In a grid tied IMAG (Induction motor generator) the thing will spin like
a motor until you apply motive power to it. It has to spin a bit faster
than the synchronous speed in order to generate. This setup is pretty
much fool proof. You start it as a normal motor and should the grid go
down, it stops generating. Note that you HAVE to start it as an electric
motor FIRST, from the line.

Now, IF we don't want this IMAG running constantly, there has to be a
mechanism to ensure that it will be running in phase with the power
lines when we connect. It can get expensive, but a cheap method involves
the use of a light bulb placed in parallel with a breaker which is in
the line going to the grid.

The operation starts by bringing the motor up to speed with your prime
mover. When the generated power is in sync with the line, the light bulb
will go out and the breaker can be closed. The rotor is now locked to
the line frequency.

That last setup requires the capacitors you speak of in order to start
generation from the residual magnetism. It is an isolated system at
start up and needs its own reactive power.

For line connected, grid start motors:

The simplest inter-tie allowed is with an induction generator which is
excited by the grid, so *no* problems with synchronization. The
generator is turned somewhat faster than synchronous speed so there is
positive slip and generates power into the grid as long as the grid is
powered. This power grid is an "infinite buss" and gives all the kVAr
required to magnetize the generator and accepts all the kVA the
generator can produce.

So far so good! When the power goes off, then the induction generator
unloads and must be protected from overspeed etc, and even though there
may be fuel available and the engine could be run, it cannot generate
backup power.

Grid Connection Required

On the page about the permanent magnet synchronous generator we showed
that it could run as a generator without connection to the public grid.
An asynchronous generator is different, because it requires the stator
to be magnetized from the grid before it works.
*You can run an asynchronous generator in a stand alone system, however,
if it is provided with capacitors which supply the necessary
magnetization current.* It also requires that there be some remanence in
the rotor iron, i.e. some leftover magnetism when you start the turbine.
Otherwise you will need a battery and power electronics, or a small
diesel generator to start the system).

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