Islanded Induction Alternator

[iceworm]

Started to hijack LMAO, Induction Motor regenerating back to inverters with questions about Induction Alternators So, new thread:

I've read about stand-alone Induction Alternators, but never seen one. Nor have I seen any peer reviewed papers.

... There might be s tiny amount of residual magnetism in the stator core, which is sometimes used in the home power market to make induction motors generate, but that requires capacitors to build up that voltage to the point of self excitation. ...

Q: Jraef – Do you have any leads on operational equipment or papers describing the physics?

Here's what I have in mind:
3ph induction motor, caps connected across windings

delta?
or grounded wye?
How much capacitance ? Maybe figure out the var loading from the pf and supply equivalent capacitance.​

Spin up the motor (what speed?)
Residual rotor magnetism bootstraps, caps supply excitation.
Voila! Generation happens.

However:
An induction alternator requires slip to generate.
The motor must overspeed the excitation frequency.
The output voltage is set by the excitation source voltage

So, when stand-alone:
What is the IA slipping against?
What frequency does the IA produce?
What is the output voltage?

If it does generate, (unknown frequency, unknown voltage), the best I could think of to harvest the energy is to rectify and feed an inverter. Of course, one would have to keep the voltage within inverter parameters.

Color me baffled.

All with knowledge are welcome. I’ll likely stay out – unless I have questions on physics.

 

[zbang]

Can't find the article I'd seen a couple of years ago, but Marathon has a piece about it- https://www.marathongenerators.com/generators/docs/manuals/SB317.pdf

 

[Jraef]

Started to hijack LMAO, Induction Motor regenerating back to inverters with questions about Induction Alternators So, new thread:

I've read about stand-alone Induction Alternators, but never seen one. Nor have I seen any peer reviewed papers.



Q: Jraef – Do you have any leads on operational equipment or papers describing the physics?

Here's what I have in mind:
3ph induction motor, caps connected across windings

delta?
or grounded wye?
How much capacitance ? Maybe figure out the var loading from the pf and supply equivalent capacitance.​

Spin up the motor (what speed?)
Residual rotor magnetism bootstraps, caps supply excitation.
Voila! Generation happens.

However:
An induction alternator requires slip to generate.
The motor must overspeed the excitation frequency.
The output voltage is set by the excitation source voltage

So, when stand-alone:
What is the IA slipping against?
What frequency does the IA produce?
What is the output voltage?

If it does generate, (unknown frequency, unknown voltage), the best I could think of to harvest the energy is to rectify and feed an inverter. Of course, one would have to keep the voltage within inverter parameters.

Color me baffled.

All with knowledge are welcome. I’ll likely stay out – unless I have questions on physics.

Don't get me wrong here, I don't recommend DOING this, I just meant it HAS been done because I have seen it demonstrated at a trade show for home power systems. The guy at the show was promoting the idea of using old ceiling paddle fans as wind generators and was selling a "black box" to make it work. So I searched on it afterward and saw what they were doing. As I interpreted it (it's been a while), they simply make sure that the prime mover drives the motor over its synchronous speed.

As to the amount of capacitance, that has a lot to do with the load. Voltage regulation is really difficult with this concept, and connecting a load makes it worse. The capacitance you pick will have to keep the motor excited enough after a step change in load, so the more you have the better.

This video shows the basics, but mentions the instability issue without addressing it. If you do a search on the term "self excited induction generator" you will get a lot of websites devoted to this idea, some with more details.

 

[GoldDigger]

Hopefully the self excited alternator is just producing AC to be rectified for battery charging, so the issue of synchronous speed does not come up.
Trying to make it grid interactive *safely* is far beyond the home installation scope.

 

[broadgage]

Induction motors HAVE been employed as rather crude alternators on small scale hydroelectric plant in remote places. So doing involves significant compromises. The motor run as an a alternator tends not to produce the voltage that it absorbs as a motor. I have heard of a USA type 60 cycle 480 volt motor being used to generate UK style 230/400 volt 3 phase.
The drawback is that 480 volt motors tend to be expensive specials in 230/400 volt lands.

The voltage tends to vary widely with the load, and such schemes only work reliably with a constant load.

I am aware of one scheme in the UK. It never worked reliably and has now been replaced by a water turbine that drives a couple of truck alternators to charge a 24 volt battery bank. Line voltage loads may be worked from an inverter.

An induction motor worked as an alternator can be used to feed power back into the utility system. At the very least, written permission of the utility will be required, and might not be granted for anything non standard.

There used to be a text book about use of water pumps as turbines, and induction motors as alternators.

 

[broadgage]

Link to a book on the subject.
https://www.nhbs.com/pumps-as-turbines-book