Ac motor as generator and starter

[veggie]

Just wondering.....

I have seen some discussions about using an AC motor as a generator.
Maybe a dumb question, but could that same motor not be used for starting???

As an example, a 5HP motor can be used to make 3.5 kw. Just right for a 6/1 listeroid.
Why not wire some circuitry allowing a push button to start, and on release of the button, the generator circuit is enabled?

(Unless there is some permanent modification done to the motor which makes it useless and a starter motor)

Possible ? or not ?

Veggie

[mobile_bob]

i suppose it could be made to work, but
you likely would need a decompressor because most induction motors
probably don't have enough starting torque to get over the hump of compression

or maybe use a clutching mechanism?

then once running it would need to be geared so that it was running the motor
over syncronous speed by about the same amount of rpm as the rated slip
(1800/1740 as motor, 1800/1860 as generator) so getting the gearing right is important

then you have to decide whether you want to remain connected to the grid after startup
and have another layer of legal issues, or whether to run in "island" mode, where you will need
some run caps to provide for excitation...

a few other issues remain depending on whether you are thinking single phase or 3 phase motor

Bill Rogers book is very good at explaining pretty much what you wanna do

utterpower.com sells his book and in my opinion should be in ever diy'ers library

bob g

[BruceM]

I wonder if the and ST-3 or ST-5 would have enough starting torque if driven as a motor?  A shaft position sensor might be needed.  The rotor would have to be fed near it's rated voltage from a DC-DC converter (Assuming 12v battery for starting.)  Then inverter generated AC (slowly ramping frequency?) fed to the stator.  The real time switchover from starter to power has me confused at present.

[veggie]

So, if I understand you guys so far, a system like this "might' work.
(In simple terms)
- Larger battery bank for off grid power (so no legal issues).
- An inverter capable of providing for the dwelling and also for motor starting. (Eg: 7.5 kw normal, 15kw for motor starting).
- Capacitors in place as per Bob's comment.
- ST head (or motor) turns the engine with decompression open.
- Close decompression lever.
- Once the engine fires, the starting power supply is terminated.
- Motor becomes a genny and the battery bank gets recharged

Anybody done this ?

Veggie

[BruceM]

Re: synchronous generator/starter
As a generator, the capacitance needed varies with load.  

As a starter, the motor size is relatively huge, not very practical to start on an inverter.

That's why I was thinking about driving an ST head as a motor.

[mobile_bob]

i am likeing Bruce's idea of using the st head as a starter motor

the st head already has two pole sets on the stator that are 90 degree's displaced
so with some switching electronics a rotating field could be established, that might be one
method.

another as mentioned would be some sort of position sensor to trigger the power to the
rotor or stator, seems plausible

either way it out to be much more powerful on startup torque and take straight battery power
which would negate the need to another conversion (battery to inverter) and the end result
as a generator would be in my opinion superior to that of an induction generator for all but
grid tied units.

again my opinion only, but as with everything there are those a heck of a lot smarter than i am
that seem to be able to do some really interesting things that on the surface look to be impossible,
or fly in the face of conventional wisdom.

i would really like to see a workable system based on the ST head made to work as a stater/generator
if for no other reason than it ought to be quieter than a conventional starter and more reliable i would think.

Bruce,, tag your it buddy!
i can't think of anyone more qualified or likely to put together such a system.

btw, i think i have some large SCR's or Triacs on heat sinks that i would donate to the project if they would
be useful.

bob g

[veggie]

I Found a small paragraph on page 107 of Bill Rogers' book relating to the use of an AC motor as both a Generator and a Starter.
He states that it is possible by adding some switchgear to temporarily reverse the motor connections thereby turning it back into a motor.
In that same chapter is does state that the motor capacitors must be sized to the load in order to achieve any decent efficiency.
The more on deviated from the optimal load, the less efficient the system.
Voltage regulation is not so simple, and for that reason, he only covers "grid feed" systems where the grid is used as the ultimate regulator.

Veggie

[mobile_bob]

ah, you have master Bill's book!

very good resource that breaks things down for us meer mortals, i bought it for the discussion of power factor
and power factor correction,, and as far as i am concerned that alone was worth the purchase price.

now we gotta figure out how to get him to join our little forum, would be nice to have an EE on tap around here.
(not that we don't already have one,, i just don't know everyone that well)

bob g

[veggie]

Bob,

Yep, it's a great little book but IMHO he stopped short.
The title "Home Power Producer's Guide..." implies coverage of much more than the generator head itself.
The home producer needs info about chargers, battery banks, voltage regulators, and Inverters.
That's what I was hoping for when I bought the book.
If anyone knows a good book that covers off grid power generation in a comprehensive manner please post it here.

Cheers,
Veggie

[mobile_bob]

one of the best books on the subject i have found although a bit dated

how to make home electricity form wind, water, and sunshine
by john a kuecken
tab books 1979

most of his info is solid and he outlines the use of about every possible way
of making power for home use.

can be found used on ebay or amazon from time to time

certainly worth the 10-15 bucks it generally can be bought for.

bob g

[mobile_bob]

if anyone has a definetive work on the topic of offgrid power generation
that covers all aspects, from engine driven genset, windpower, batteries, inverters
hydro, solar etc etc i would really appreciate the heads up as well.

not having found that one source, my approach has been like a logic problem
wherein i had the goal, then formulated a series of questions i needed answers to,
then started the research to get those answers, and figured that one source for an answer
was not sufficient

so i would continue until i found 3 solid sources that agree'd that were based on solid science
and not anecdotal evidence.

some of this has taken the better part of 10 years of digging to get to the bottom of it, and the
reason why i get really excited when i find a book that has an answer to a specific question.

sometimes i will pay 20 bucks for a book because of one sentence or a paragraph that answers a question that fits into the big puzzle i have setup in front of me.

i wouldn't be surprised if i had over 100 books on everything known to man that relates to power generation, storage, transmission, etc.

some of them i have about wore out going back to them over and over again finding more info
that is important to something i am trying to understand now.

Bill Rogers book is an example of what i found very interesting
the power factor question is interesting in that you can't find two electrical engineers
that can explain why it is important to an offgrid power producer, or how it will effect
generator efficiency (some will tell you that it has no effect) etc etc.
i spent years trying to get my head around that topic, and Bill presented it in such a
clear and understandable manner that i could apply.

my problem is "if i can't apply it, i have a problem getting my head around it"
if i can apply it, i will do what i have to do to understand it.

bottom line is this
i don't think there is a single definitive book that covers all aspects of offgrid power production
i would also expect that if someone were to take up the task of writing such a book it would sell
very well.

i know i would buy a copy!

bob g

[mobile_bob]

two more books that are more current and very useful

living on twelve volts


wiring for 12volts 

both from amplepower

http://www.amplepower.com/products/living/index.html

frequently they turn up on ebay
and are excellent sources of info about all sorts of things
relating to power production, batteries, inverters, refrigeration etc
and doing it efficiently which is very important in my opinion

bob g