Slow, small self-excited induction generator

[[email protected]]

Hello, sem.

I've been trying to put together a small, slow AC induction generator
for quite some time now, and may be ready to put this one to bed.

Yes, I know small self-excited induction generators (SEIGs) don't work
well, and neither do slow ones, but how small and how slow can you go?

I've got two of Burden's Surplus Center #10-1134 motors mounted back to
back with a pipe nipple in the end bells, and a threaded rod joins the
shafts. These are 400 rpm synch, and 225 rpm full load speed, 1/12 HP
motors running on 110 VAC, 1A full load.

One works as a motor to drive the other for generator tests.

These are split-phase permanent capacitor motors. The generator has its
main phase wired to a GC cap sub box in parallel with a 10/20/20
microfarad sub box I built. I find a capacitance of abou 25 ufd
provides best wave shape and output intensity, but there has been no
useable output.

What I am wondering is, when you drive an SEIG below synchronous speed,
should it output essentially nothing (just remanence induced voltage),
or should it, at such speeds, be well on its way to building up full
output?


I know a synchronous machine operates as a motor below synch speed and
a generator above. Does an SEIG show the same behavior, or does it
output zero at zero speed, with increasing output to some point above
synch speed at which it generates full output?

If the SEIG behavior emaulates the former, I have some hope for
success. If it is the latter, I'd better cut my losses now, and move
on.

Yours,

Doug Goncz
Replikon Research
Falls Church, VA 22044-2536

 

[webpa]

Long time since EE101...but I think this is correct: You are probably
never going to get any useful output from the driven motor. It is
carefully and specifically designed as a MOTOR, after all (for ceiling
fans, in your case). The magnetic circuit tricks used in such motors
to create a rotating field make it virtually impossible to use the
machine as a generator (alternator, actually).

Recommend: Discard the second motor. Connect instead an automotive
alternator with the diodes removed...which is very easy to do. This
gives you direct access to the armature and field coils. Most used
alternators have more than enough residual magnetism in the armature to
be self-exciting. If you've got a used alternator, be sure to check
and replace as necessary the brushes and bearings. Both are part of the
"alternator rebuild kit" appropriate for the specific device.

Paul Weber
Albuquerque, NM

 

[[email protected]]

Long time since EE101...but I think this is correct: You are probably
never going to get any useful output from the driven motor. It is
carefully and specifically designed as a MOTOR, after all (for ceiling
fans, in your case).

Well, many SEIGs are made from motors. Few are designed as generators.
The market is too slim.

My understanding is remanence in the rotor is spatially varying, and
when turned, induces a time-varying voltage in the stator. A capacitor
in parallel across the output leads allows this voltage to induce a
current time-varying current. This current induces a stronger field in
the rotor, and the system bootstraps.

The magnetic circuit tricks used in such motors
to create a rotating field make it virtually impossible to use the
machine as a generator (alternator, actually).

No, this is a common practice. My question has to do with how small a
motor will be a useful generator. Many small generators are around a
single HP.

Recommend: Discard the second motor. Connect instead an automotive
alternator with the diodes removed...which is very easy to do. This
gives you direct access to the armature and field coils. Most used
alternators have more than enough residual magnetism in the armature to
be self-exciting. If you've got a used alternator, be sure to check
and replace as necessary the brushes and bearings. Both are part of the
"alternator rebuild kit" appropriate for the specific device.

I have tried a fairly hefty DC servo motor as a generator and
understand how an alternator could be used, but I am trying to drive
small 110 VAC, 50-60 Hz power supplies so that a variety of information
technologies can be used in my mobile application: GPS units, Pocket
PCs, laptop computers... Yes, I know the output of a generator can be
inverted to AC.

Paul Weber
Albuquerque, NM

Thanks.

Doug