re-using an AC generator as a motor

Thread Starter

nota33

Joined Jul 9, 2023
25
why do it
I trash picked a honda clone 13 hp gas generator 6500watts so no cost to start with
use
to power a 25 ft sail boat I have with a very dead diesel 10 hp motor removed
I have seen on u-tube videos of converting a car alternator to a motor
and think the modern home generator is NOT actually a gen but really like the car alt

so can maybe be reused as an about a 10hp motor
I understand some limits to this idea one needs to retain the gas motor as an end bearing or make something there
and the tapered connection on the shaft

but lots of expensive copper in hopefully the right places with a few wires moved to make a motor

eventually I plan to use battery power LiFeP + inverter but trial use will be powered by a real honda generator that I have for storm use
 

Alec_t

Joined Sep 17, 2013
14,220
Welcome to AAC!
If the re-purposed generator is run as a motor from a 24V battery battery via an inverter and you want 10HP, that's equivalent to 7460W in theory. Allowing for conversion efficiency of ~80% you'd need around 9000W. The current draw would therefore be ~ 375A, so you'd need a pretty big battery to get more than a few minutes run-time.
 

BobTPH

Joined Jun 5, 2013
8,664
It is even worse than that. The motor would not put out 6000W at 24V in. You would need an inverter that raises the voltage.
 

Thread Starter

nota33

Joined Jul 9, 2023
25
plan is for 240v ac input first from a generator I own 13 hp real honda 7500 rated watts run

later after getting everything to work swap to battery if affordable with a big inverter at 240v ac

will use car battery packs from wrecked el cars NO older TESLA units as no LiFeP into an inverter if found

yes I understand BUDGET limits rule this project key words free cheap existing as in have the gen and the boat already
need to spin a prop 10 hp is a guess from down rate of 13 existing gas motor hp on both the real honda gen rated 7500 and the clones gen that will be the swaped into the motor I hope
and the big current needed but only for brief time as it is a sail boat with out constant batttery drain
like a few min to go thru an up wind bridge or channel or near a dock
so yes it needs real power but not for long

so my questions to this board is can I do it ?
will the standard china made 5/10 year old 6500w 120/240v generator sold everywhere in the USA [really an alternator ?]
work as a motor in a similar way as the u-tuber car units show it done [basic moving wires]

goal is to do this for a few very few hopefully hundreds not thousands with the existing boat and two generators
I donot want parts catalog expensive new stuff just working with existing junk ideas to convert the gen to a motor
as that bit is the first step before spending on the batterys and inverter [hope to find used cheap]

thanks for any help clues links other people who try such insane cheap BS ?
 

LowQCab

Joined Nov 6, 2012
3,937
"" so my questions to this board is can I do it ? ""

The short answer is NO.

Alternators / Generators are specifically designed to
produce Electrical-Power from Horsepower ( Torque ).

Motors are specifically designed to produce Torque from Electrical-Power.

When either design is used "in reverse", there is an extreme loss of EFFICIENCY.

The loss of efficiency can easily be as much as ~50%.

A truly efficient ~10-HP Electric-Motor is huge,
and might easily weigh as much as ~200-pounds or more.
.
.
.
 

BobTPH

Joined Jun 5, 2013
8,664
I don’t think you can run it as a motor with single phase power. If you have a 3- phase alternator maybe. But I am no expert on this.
 

Thread Starter

nota33

Joined Jul 9, 2023
25
NOTED THAT REAL 10HP EL MOTORS ARE A BIG HEAVY EXPENSIVE BEASTS
that I can't afford or power as most are 3 phase higher volts weight alot also
as they are made to run 24/7/365 on mini current draw

that is exactly why I have interest in this light free unit and no I do not expect efficiency or continued operation for long stretches
if I only get 50% of work done per unit input at free costs vs 100% at a higher then I can pay rate
I still see the free as the big ++++
if a few years from now after a 100 hours run time in small bits it goes boom so what

this is working with what we got
not how the best way is to do without costs considered

also we plan on solar and or prop spin when sailing to recharge
so marginal power efficiency is not a deal killer
 

schmitt trigger

Joined Jul 12, 2010
843
Let me ask you a question: when you look inside de rotor assembly, do you see some slip-rings and brushes?
If so, then the answer is no, unless you plan to use it as a synchronous motor. Which has some major considerations :
-It only operates at a single, fixed speed, synchronized to the incoming AC frequency.
-The synchronous motor is not self starting. You have to bring it to speed via an external torque, synchronize the mechanical phase with the electrical phase, and then throw the switch.
-Unless is a permanent magnet type, you must supply an external DC voltage for the exciter field.
 

Thread Starter

nota33

Joined Jul 9, 2023
25
I thought ac motors without brushes are fixed speed [fan]
and ac motors with brushes were variable speed ? skillsaw or drill type

so why is the gen/alt with brushes fixed speed only as a motor ?
and non self starting also ?
 

LowQCab

Joined Nov 6, 2012
3,937
Efficiency, Efficiency, Efficiency,

If a device of any kind has an efficiency of, let's say, ~50%,
then ~50% of the energy going into the device will be turned into HEAT,
and that HEAT will destroy the device.
.
.
.
 

LowQCab

Joined Nov 6, 2012
3,937
Don't believe simple Math equations ???
Go ahead and try it.

Just make sure You make a video to share with us on YouTube,
we could use a good laugh now and then.

( keep a big Fire-Extinguisher near-by and wear Safety-Glasses ).

You would be much further ahead if You would just
purchase a common 12-Volt "Trolling-Motor" and clamp it to the Stern of your Boat.
.
.
.
 
I thought ac motors without brushes are fixed speed [fan]
and ac motors with brushes were variable speed ? skillsaw or drill type

so why is the gen/alt with brushes fixed speed only as a motor ?
and non self starting also ?
Way too complex to explain in a forum post. Suffice to say is it depends on how is the field created, how are the field and armature connected, and the type of excitation.
Suffice to say that motors and/or generators with a shunt DC field and an AC armature, always run synchronously.
The brushed motors found in power tools are universal series field. A different animal.
 
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Thread Starter

nota33

Joined Jul 9, 2023
25
have several trollers
btw 12v ones are useless low amp / watts no real power
hardly move a little row boat
24v big ones run on 36 or 48v still lack power maybe 1 hp if overvolted 1/2 hp at 24v
I have a claimed 2 hp unit that costs $ 3300 new but not much power when needed rated 48v
good for a little boat not a 25 ft at 5000lbs

I do not need bullet proof for hours
would like 8 10 hp for 5 to 10 min
to shove the boat up wind/current
with the diesels prop
 

strantor

Joined Oct 3, 2010
6,778
"" so my questions to this board is can I do it ? ""

The short answer is NO.

Alternators / Generators are specifically designed to
produce Electrical-Power from Horsepower ( Torque ).

Motors are specifically designed to produce Torque from Electrical-Power.

When either design is used "in reverse", there is an extreme loss of EFFICIENCY.

The loss of efficiency can easily be as much as ~50%.
I disagree. Care to explain why an electric machine would be any more or less efficient when used as a motor than as a generator? And where this 50% figure comes from?
A truly efficient ~10-HP Electric-Motor is huge,
and might easily weigh as much as ~200-pounds or more.
That simply isn't true. Industrial motors with NEMA designations are 200lb beasts, yes. But there are plenty of motors that manage to put out that much power while not weighing as much. For example, the motor in a Tesla weighs 99lbs, outputs 450HP, and has 93-97% efficiency. Oh and it doesn't burn up from being used as a generator when regenerating either.
 

strantor

Joined Oct 3, 2010
6,778
@nota33 some people around here are too polite to say that they think an idea is stupid, and prefer instead to skirt (or trample on) the truth to make it sound impossible. It's done with good intentions (so you don't waste time on what they see as a lost cause) but IMO the truth is always the right answer.

The truth: yes, what you're describing should be possible. But there are much better ways.

A "generator head" (alternator) is a synchronous AC electric machine and while built for the purpose of generating power, should be capable of motoring as well. It would need to be controlled like any other synchronous AC motor (ex: AC servo motor, PMAC, PMSM), with shaft position feedback and an electronic drive. But additionally, it would need a purpose-built regulator to control current in the rotor windings in response to load. This would need to work together with (as part of) the electronic drive. I know of no such drive which has a regulator for the purpose of running a generator head backwards as a motor. You would need to design and build this drive yourself. This is not a weekend warrior project; it is something a team of power electronics engineers could spend months or years on.

But... the whole reason why a drive is required for synchronous motors is because they're almost exclusively started from 50/60Hz mains, and a synchronous motor just can't do that; it would be like trying to jump from a bridge onto a train going 50/60mph. But since you will be powering this from a generator, there is a small chance that it might just work, because both the motor and the generator will be starting at the same time from the same (0) RPM. More like boarding a train at a station and then accelerating with it up to 50/60mph. This assumes that your generator rotor is charged and the generator is outputting voltage below operating speed, which probably isn't the case (I think most don't start outputting voltage until they reach operating speed) so the generator may require modification for this to work. It also assumes you have solved the regulator issue, which you haven't, and that will still require some specialized circuits that you'll probably have to design yourself because you'll be the only person trying to make this work.

You'll be the only person trying to make this work, I'm sorry to say, because it's not a great idea. It's a whole lot of work to accomplish something that's already accomplished with better and cheaper technology.

You need look no further than this:
Far better to use the Honda engine to shaft drive the boat than muck around with electric drive.
More efficient, simpler, more robust, less opportunity for failure, just better in every way.

If this backwards generator idea is something you just must see come to life then go right ahead and make your dreams a reality, but don't expect much help along the way because people volunteering help need to believe in what they're doing and good luck getting anyone to believe in this.

I hope this candid answer is helpful.
 

Thread Starter

nota33

Joined Jul 9, 2023
25
Far better to use the Honda engine to shaft drive the boat than muck around with electric drive. Ive used aircooled engines in boats before.
if I had a new spare honda but I need the gen for hurricanes that is a real honda
and the free unit is a clone of a honda of unknown date from china called a atouan
that I never heard of [some are ok other pure junk] or trust
gas motor devalues a boat while el rig is the new trend almost diesel value added
 

Thread Starter

nota33

Joined Jul 9, 2023
25
so the gen or alt definition is not the question
as this would need a variable frequency control box if run from battery/inverter ?

can the unit be rewound to avoid fixed rpm runing
as a 60's teen we rewound el race car motors even used silver wires for minor advantage
inc balance and epoxy to hold the wires on + true com's ect
so is something I have done
 
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