PMG Exciter

[propel7]

What would you get if you used a small PMG to energize an Induction motor that is being driven to produce power as a generator? would you need to operate at a specific speed? what type of output would you get and what would you do with it? How would the frequency of the PMG affect the rotational speed of the Induction motor or would it matter?   Thanks,  Devon

[drdongle]

Firstly you don't need a PMA to excite an induction machine. You just have to put caps across the output ( value to determined by experimentation) and then overspeed it ( 10% or so) and possibly flash the windings momentarily with a DC current to start the excitation. Sear the archive here for "induction generator".

[Flux]

propel

I think you are confusing a line excited induction alternator with a brushless alternator.

You can excite an induction machine from a pma but not if they are coupled on one shaft.

If the pma is driven independently then it will act in the same way as a normal mains supply. The pma will determine frequency and the induction machine will need to operate within its slip speed above pma frequency.

Unless the pma is large in relation to the induction generator I don't think the system will be stable. The pma must be able to hold base frequency. The only case I can see to consider this would be if you have a large engine or hydro driven pma and you want to add a wind turbine as a power saver.

Flux

[Ungrounded Lightning Rod]

A very interesting raw power supply was used for the CDC 1604 (Cray's first "mainframe").  It had:

 - An 60 Hz induction motor.

 - A wound-field 400(ish) Hz generator, and

 - A tiny exciter generator

all on one shaft.

The exciter was about the size of the motor

on an oscilating fan.  The generator and motor

were each about 3 feet long and the same in diameter.

(I don't know the power or horsepower

rating but I'd guess maybe 30ish horses, 40 ish Kw.)

The exciter had a stator field and a rotor output.

But instead of slip rings or a commutator it had

some diodes to rectify the result, and feed it to

the generator.

The generator had a rotating field and a stator

output.

The result was that the rotating part had electrical

wiring within it, but no connection to the outside

world.  The regulator fed the field of the little

exciter, which amplified that signal to generate the

excitation current for the big genny.

Regulator had to be designed to take into account the

two-stage nature of the response to its drive.  Output

voltage went up with the square of input current.