Author Topic: Convert an alternator into a motor  (Read 18747 times)

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Telco

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Convert an alternator into a motor
« on: February 08, 2012, 01:52:35 PM »
Hi folks, new poster here.  I found this site while looking into a way to convert an old alternator I have into a motor.  I've found all sorts of information indicating it can be done, and all sorts of procedures that show how to convert an alternator into all sorts of other things.  But, I can't find anything that shows specifically how to convert an alternator into an electric motor.  Like say I want to use my alternator as the electric motor for a lawn mower.  Can anyone tell me what's required for this?  And, if it turns out to be something I can't handle, would an alternator rebuilding shop know what I was talking about or would they look at me funny? 

Also, would any control stuff be needed for this?  I saw mention about needing control circuitry to make sure it always spins the right direction.  And, should I decide to use this on a car to drive an accessory on my truck, would a control circuit be able to use the vehicle's VSS signal to control RPMs? 

Thanks for any information on this. 

JW

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Re: Convert an alternator into a motor
« Reply #1 on: February 08, 2012, 03:50:21 PM »
I have really great book on brushless commutation, its lost in a box someplace. This is the next best info I have at the time...

http://www.motion-designs.com/images/DTrends_May_2008.pdf

Notice the stator.

I know from reading some of the articles about Danb's workshops, that they will ocasionally make a dual/single rotor machine run as a motor with copper wire as a commutator. It's neat stuff.

Well your thread is in the right place "transportation"

JW


Telco

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Re: Convert an alternator into a motor
« Reply #2 on: February 08, 2012, 04:51:11 PM »
Thanks, but I'm afraid that was over my head.  I was hoping for more of a how to response.  I can pull an alternator down and put it back together OK, but wouldn't know the first thing about how to make modifications.

JW

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Re: Convert an alternator into a motor
« Reply #3 on: February 08, 2012, 05:02:28 PM »
The rotor on a standard alternator rotor has a fixed magnetic pole arrangement, sure you could keep the slip-rings working for the rotor or use permanent magnets. The problem is actually the stator because this is what you would have to drive the rotor with...

The way stators are wound in a auto alternator, its not optimized for motor, it works better as a 3ph alternator... The problem becomes to get optimized performance of the poles the motor would have to have to be usefull. In otherwords because of winding problems you would need a stator that magnetically was optimized for winding a brushless motor.

JW

dinges

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Re: Convert an alternator into a motor
« Reply #4 on: February 08, 2012, 05:48:10 PM »
Not sure what kind of alternator you have, but generally speaking, it's possible to use an alternator as motor or a motor as alternator.

For example, a 6V bicycle bottle dynamo (it's actually an alternator, not a dynamo, but everyone calls it a dynamo anyway....) will turn into a motor if you apply low-voltage AC to it (50/60 Hz). You may need to give it a push to get it to start though, because it's single phase.

Three phase alternators can be used as motors as well and will be self-starting. If you're talking about a car alternator, then it too could be driven as a motor. Not sure why you would want to do this, other than to show you can.

You would need a source of 3-phase alternating current power to drive the stator, and a source of DC to drive the rotor. If you don't have 3-phase power connection to your house and a suitable transformer to transform it down, you could try looking for the motor controllers that are used to power model aircraft using brushless DC motors. These motors often have very few windings of thick wire, requiring a power source of low voltage and high current. Plenty of those motor controllers can supply 50A or more and they may work to power such an alternator-motor.

So yes, you can use that alternator as a motor. But again, why you'd want to do that... I have no idea as to that. Makes more sense to me to find a plain DC motor that's optimized for the task if you want to build a low-voltage electric mower.

But theoretically, there's no reason you couldn't use an alternator as a motor. It would need some modification: remove the voltage regulator and make external connections to all three phases and to the DC for the rotor magnetization). In short, I don't think it's very practical, but it's certainly not impossible.
« Last Edit: February 08, 2012, 05:51:35 PM by dinges »
“Research is what I'm doing when I don't know what I'm doing.” (W. von Braun)

joestue

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Re: Convert an alternator into a motor
« Reply #5 on: February 09, 2012, 12:37:05 AM »
If you want an off the shelf solution they make outrunner motor drivers big enough to handle a car alternator running backwards as a BLDC 3 phase motor....$100-150 should be enough for one of them.
you'll have to upgrade the heatsinks because they are generally meant for mounting on the side of an airplane at 100mph windspeeds..

the only real problem is you will have to find the right rotor current (aka excition level, and this current also effects the power factor), also, you will need a motor controller rated for at least 50 volts, and to get any significant power out of the alternator you'll need at least 24 volts on the batteries.

also, you'll be burning up 30-60 watts in the rotor so if you want good efficiency you're going to have to pull at least 1 hp out at say, 3000 rpm to get say, 70% efficiency.

frackers

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Re: Convert an alternator into a motor
« Reply #6 on: February 09, 2012, 03:02:41 AM »
An alternator generates AC, a dynamo generates DC because it has a mechanical rectifier on it called a commutator . A dynamo can operate as a motor by applying DC to it as the switching of the commutator keeps the magnet field rotating and hence you get motion (shaft rotation) out. An alternator will operate as a motor if you put AC into it. Say on a car engine its driven from a 2:1 ratio fan belt and the engine can turn from 1000 up to 6000rpm - the alternator is doing 2000 to 12000 rpm. Convert that to revs per sec gives you the frequency of the AC you'll need to apply to it: from about 30Hz to 200Hz at something like 24volts with up to 50 amps. Thats one heck of a piece of kit!!

Briggs and Stratton make nice small engines...
Robin Down Under (Or Are You Up Over)

Telco

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Re: Convert an alternator into a motor
« Reply #7 on: February 09, 2012, 02:54:05 PM »
JW, so I would need to get the alt rewound to do this, and replace the rotor?

Dinges, the reasoning is simple.  I have two alternators that I have no use for.  Both came off an S10 2.8L pickup, one I had rewound to produce 130 amps when I still had the truck.  Second, the alternators are very small in size, yet are supposed to be able to produce as much as 4HP continuous.  A 4HP motor capable of continuous operation that is purpose built is a lot larger than an alternator, so getting that kind of power out of a device that size is very attractive.


dinges

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Re: Convert an alternator into a motor
« Reply #8 on: February 09, 2012, 03:41:50 PM »
Quote
A 4HP motor capable of continuous operation that is purpose built is a lot larger than an alternator, so getting that kind of power out of a device that size is very attractive.

It sure sounds attractive! But I suspect there's a reason that real 3 kW DC motors are a tad larger than car alternators.... as in, "I have trouble lifting one from the floor without ruining my back". As opposed to a car alternator that fits in the palm of my hand.

If something sounds too good to be true, it probably is?

Anyway, my point: 4hp (3000 W) sounds like a lot to me, especially 3 kW continuously, from something so small as a car alternator.

My 2nd point: at what RPM would the alternator reach its rated 3kW ?....  And would that RPM be practical for a lawnmower?

3rd point: what voltage do you intend to use this lawnmower at?

4th point: how thick would the power cables have to be to deliver the required current from the batteries to your motor at your desired system voltage at the rated power of 3kW?

5th point: how do you intend to power this lawnmower? Batteries, or from the grid, or? If batteries, they would surely have to be big (and probably you'd need to drive the wheels as well, as it'd be near-impossible to push with human strength through a lawn).

I could probably think of a few other things that you might want to think about when continuing with this project, but I figure for the moment the main issues have been mentioned. Am interested in your response and how you would go about tackling those issues.

Personally, I think the easy part of your project will be the modification of the alternator. The pricy part will be the motorcontroller, the batteries (if used) and the cables. I expect that for the money you could buy a brandnew 3.5 hp B&S lawnmower. That would be the practical solution *I* would choose, anyway.

But your project surely makes for an interesting challenge! And I know I would not be up to that challenge....
« Last Edit: February 09, 2012, 03:43:29 PM by dinges »
“Research is what I'm doing when I don't know what I'm doing.” (W. von Braun)

joestue

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Re: Convert an alternator into a motor
« Reply #9 on: February 09, 2012, 04:07:43 PM »
it would appear i've underestimated how much power you can push through an alternator.

a little googling returns
http://www.rcgroups.com/forums/showthread.php?t=905411&page=10
(start at page 1 if you really want to understand this topic)

http://endless-sphere.com/forums/viewtopic.php?f=10&t=15057

if you want to get 4kw you're still going to have to put permanent magnets on the rotor, and you'll get 4 kw at something like uh, 12-18K rpm.

richhagen

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Re: Convert an alternator into a motor
« Reply #10 on: February 09, 2012, 07:08:41 PM »
Keep in mind that electric motors are much more efficient than gas engines in converting their fuel into work.  I know that a horsepower is a unit of power, and as such should be convertible to Watts, but in practice it does not always work out that way.  A rule of thumb for electric cars was that a gasoline engine roughly 2.5 times the horsepower of an electric motor delivered about the same power to the wheels.  The reason was that electric motors were rated for continuous duty, whereas most engines are rated for their peak horsepower and as such were not directly comparable figures.  To replace that 4hp gas mower engine with an electric motor, I would be looking at a motor in the range of 1.5 to 2 horsepower on the name plate.  Rich
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JW

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« Last Edit: February 10, 2012, 08:43:52 AM by JW »

Bruce S

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Re: Convert an alternator into a motor
« Reply #12 on: February 10, 2012, 08:26:18 AM »
This should be a useful link. Come to think about it, have never seen Kurt produce a shred of technical info.

Kurt who?
Let's NOT get into the incendiary remarks about people...again
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author "Bruce S"

TimS

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Re: Convert an alternator into a motor
« Reply #13 on: February 10, 2012, 01:26:58 PM »
Good remark.  I agree.  That's over............

equiluxe

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Re: Convert an alternator into a motor
« Reply #14 on: June 01, 2012, 11:57:30 AM »
To run an alternator as a synchronous motor you will need an inductance or a capacitor and a diode across the rotor field slip rings, this will allow self exciting of the field also you will need a solid state 3 phase inverter to give you your 3phase power for the stator wingdings. Although a car alternator may produce up to 4hp worth of electricity this will be on an intermittent basis you certainly will not be able to draw 4 hp from it for any length of time as it just does not have a continuous rating like most electric motors will have.