12 or 24 volt alternators

Ok to track your questions I'll number my reply.

1. Automotive alternators almost always produce three phase power prior to the rectifiers. This provides a reasonably smooth mechanical load on the pulley and maximizes drag to electric power efficiency.
2. In theory one could have an automotive alternator generate 120VAC. However, the winding insulation thickness and magnetic field density in all of the core materials will likely breakdown preventing this from being a practical method.
3. 1.5 HP ≈ 1120 Watts which will mean for a 12V alternator an output current of just under 100 Amperes. So if you can locate a 100 ampere 12V alternator and a suitable inverter then it is possible to operate a 1.5 HP table saw. But I won't say that this is very practical.

Its cheaper and better to buy a portable power unit with at least 1500watt.....

You have to get to understand that what you intended to do would probably need a specially made step up transformer which would be expensive, the power unit would be far cheaper......

A car alternator probably only puts out around 10 volts 3 phase, though I have never measured one....try it first.

Remember that the saw is probably an AC motor with brushes, so it will probably also work with DC too.....

If your car alternator can handle 120 amps or so, then just buy an inverter for the voltage required and let the car engine run while working.

Actually its not that hard to get a 120 volt DC output from a 24 volt alternator or from many 12 volt alternators as well. Just wire up the rotor power to the full rated voltage and spin the alternator fast enough. The good quality ones have diodes rated for around 200 -400 volts and those that don't can easily have them replaced with an external set that are capable of that voltage.

As far as the winding enamel and insulation alternators are wound with the same enameled wire as standard electric motors and the actual core slots have the same insulating material as the cores of electric motors as well. So over voltage will not be a problem.

The trick to getting the higher voltages is just removing the regulator system and them spinning them fast enough. Most have a safe RPM rating of at least 15 K RPM. Many can safely go far higher yet. The rotors are basically solid steel so they take tremendous forces to bend them even a little.

If you chose to you can also rewind one by hand to get different voltages as well. They are fairly simple. Just stop by a electric motor rewind and repair shop and pick up a some smaller gauge wire than whats in the alternator and some of the insulation paper and do a hand rewind. Its not all that hard once you see the pattern!

If your looking for a big and cheap one to experiment with just stop by the local scrap yard a pull bigger one off of a piece of old machinery. You will likely get it for less than $10 and if you do wreck it while playing with it well, it was scrap to start with!

Do a little mad scientist experimental work. You may just surprise yourself at what you can learn and do!

Thanks TCMTECH for some good info! Can you tell me the pitfalls if I (a) hook two alternators (AC output) in parallel? (b) Two (AC) alternators in series?

I read where the large mega Watt units have to keep them in phase when more than one is running. Would this apply to these small units? How do they lock them in step?

Thanks

afarmer; why do you want to use 2 alternators? what horse power will u use? perry

Perry---Good question! A lot of things I do involve using (old & new) existing off the shelf parts which are usually cheaper, Vs: having an entirely new one designed and mfg. It may be crazy but I was wondering if I could use multiple numbers of alternators to achieve a desired output. I have an extra stand alone engine of 60HP and it would turn several alternators if needed. i.e, how many used alternators could you buy for the price of one new 14KW Generator Head and belt drive them? This would also recycle alternators etc. AFarmer

You sort of don't. With the small ones your best off bringing the three phase out of them and then externally rectifying it so that it is no longer tied to the case and just putting the alternators in series.

Many of the commercial alternators however have both positive and negative lugs that have no internal references or connections to the case so they can be easily wired in series without any modification. Plus the built in regulators are mounted on the back of the case and are easily removed or bypassed.

The down side to alternators is that they produce higher frequency AC, up to 1200 - 1800 Hz, which is useless for most anything but incandescent lights. Its best off rectified and turned into DC where it can run any universal AC/DC power tool or device. Most CFL's will work just fine on 120 - 180 VDC as well.

<<< Just wire up the rotor power to the full rated voltage and spin the alternator fast enough >>>

Machines are manufactured to a max specified speed as all the moving parts, specially bearings and strructure will not be able to withstand 3 time the rated speed. Yes you can run it as a TEST-RUN but not continuous run & Load.

The second thing all the modern Alternators have a "Slip-Clutch-Pully" which will not rise above the manufactured max speed.

You should keep in mind that the regulation is not only controlled by the control of Exciter current & other means of electronic controls but also by the "Slip-Clutch-Pully".

3rdly I have not encountered still an embedded in chassis of Auto-Alternator-rectifier section rated above 50V.

I will be obliged if you can link me to such rectifier Diode data sheet.

I am also after finding the Electronic components used in such regulator; Diodes & transistor .

and

Regards.

Haajee I purchased an Alternator (12V @ 200 Amps) and installed it as a second unit on a 5.7 liter V8. They sell units up to 350 amps Try the following--AFarmer

www.nationsautoelectric.com/highoutput.html

So where is this slip clutch thing on all modern alternators?

I currently have about 20 alternators for all makes models and ages and have recently replaced a few on newer model vehicles and farm machinery with brand new factory replacement units and not one had anything other than the standard direct drive metal pulley on the alternator shaft.

My pickup has a roughly 1:4 pulley ratio difference from the crankshaft to the alternator and a engine redline of about 6000 RPM. (1999 Ford V10) My math says thats around 24K RPM at the alternator and I know for a fact that has no clutching any place between the crankshaft and alternator or in the alternator as well!

Got any examples of these modern alternator slip clutches and where they are actualy found? I would like to see one!

I have never seen one either. Maybe he is mistaking the clutch for the AC for it?

While I no longer do my own automotive repair, I have heard of alternator deigns that incorporated a mechanical coupling angular speed control on an alternator. Maybe this was only implemented on the turbine based vehicles that Chrysler attempted back in the 60's. (Yes, I am old enough that this counts.) But with today's techniques of embedding everything even mechanical systems into tiny places, I would not preclude any approach just because I didn't knowingly see it before.

So your still considering 1960's experimental tech modern equipment?

Wow! The stuff they now use 50 years later is going to blow your mind then!

How are those polyester pants holding up?

In assembly with me I can see in the centre of pully side face in the cetre a rubber seal covering the clutch.

Automotive alternators are in the class of medium frequency alternators.

Obtaining 120 or 220 Volt output is possible. Either you rewound the stator or use a step up three phase transformer.

The point I would like to state here is the frequency. Their frequency in their nominal speed range is near to 400 Hz. There is no use unless you rectify it.

This means you cannot use this frequency at any household units, of which rated for 50 or 60 Hz.

Correct!

I think that he was concerned about frequency while automobile alternator is not a constant Frequency device otherwise.

Exactly. At their effective speed range, they produce high frequency, due the number of poles of the claw type armature at the rotor .

If the speed is decreased by using a suitable belt and pulley mechanism to match the 50, or 60 Hz, at the output, then the alternator will be unefficient, of which only producing a few decade of volts even if their rotor armature is full excitated.

Therefore it's better to use them in their normal speed and normal frequency (Approx 400 Hz.) and then rectify the output for battery charge purposes, and then convert the stored power to mains voltage and frequency by using a static invertor.

<...will either one of them power a 1 1/2 HP table saw...>

It would be better to have a mechanical power take-off for the saw and drive it directly, cutting out the alternator and the motor from the build.

The largest problem with making something work that was developed for something else, is the technology was developed to make it work most efficient for its original propose. Buy the time you have altered it. You discover it won't work and it is now broken.

I would install a separate alternator and tie it into an inverter (they are fairly inexpensive) through a disconnect. If you burn it up it won't effect the operation of the vehicle.

Also if you truly are AFarmer you know what a PTO is. You can connect up a generator with it. Check out the Northern Tool Catalog on line, there is also a company in Minnesota called Katolight that specialized in PTO Generators.

On last note, I have found that advertised HP on manufactured tools are highly suspect. Try to check the true rating on the nameplate of the motor.

It can be single or three phase,but it is rectified and inverted to produce AC output.

If u dont get 120v AC output,then get a transformer to step down to 120v AC.

To power yr table saw depends on yr alternator power rating and frequency,and also yr table saw frequency and voltage rating,which u did not supply us.But if the parameters of yr alternator agree with yr table saw parameters,then it can power yr 5.5hp table saw.

Patrick Whowha

If an alternator produces DC at 12 or 24 volts, how did you get the idea that it would produce anything like 120 volts on the AC side? or more as you spoke about a "step down" transformer.......

With the best will in the world I cannot follow your logic.....sorry......

I believe a "step up" would be more likely needed.....

Thanks Brave Sir: The raventech is VERY interesting! These guys seem to be thinking outside the box!

Interesting. It looks, from the rudimentary drawings and sketchy information, as though maybe they are using the properties of a ferroresonant transformer to give a steady state output. Kind of like a CVT with an external control loop to give an artificially stable output sine wave at the desired frequency. Clever if that works.

It sounds a very clever frequency multiplier.

The automotive type alternators are equipped with solid iron rotors, used as to be the inductor feed with DC, . I do not guess they could fed with the sine wave

Auto-Alternators are f12V, 24V, from 30A to 100A.

Which alternator you have.

Anyhow whichever you mean, most of the modern Vehicles have 3Φ inverteors but not 120VAC. If you afford to get it rewind it for 120V & every tech working on Re-winding can do it.

See the data on yours & calculate the power that is Voltage X Current = Power

Your requirement is approx 1 1/2 KW.

Now rest is your home-work.

Have a nice day !

Car alternators are rated for the voltages you have specified but that is a function of the regulator. An alternator without an internal regulator can produce high voltages An alternator with full field and enough rpm will produce a/c three phase 120 volts before the rectifying diodes.

....then someone will have to make their own regulator to get to such voltages as the standard one obviously cannot do that!!!

Someone here mentioned as a "ball-park" figure of 2 amps for the rotor field for 60 amps (at 12 volts) output......I wonder what voltage/current will be needed to get 120VAC out?

Using simple Ohm's Law, (which is probably over simplifying it a little!), just staying with a max of 60 amps (anything else will need a proper rewind of the stator coils), then we need 10x more energy out, which means 20 amps input to the rotor coil....are the brushes up to this is my first question? I somehow doubt that the manufacturer planned for a 10x increase in rotor current through the slip rings.

The only proper way to do it to my mind is to rewind the stator coils with thinner wire, but with more turns, effectively a wire that is 1/10 of the thickness of the original wire, which will give an approximate 10x increase in voltage, but 10x less current available BUT will allow this to be developed with the 2 amp rotor current that it was designed for.

A slight increase in rotor voltage will probably help as well to get things going better, but insulation qualities (as with the main field) for 120 volts are quite different to the requirements of 12 volts.....l.For instance no more rainwater can be allowed to pass through the alternator as with a standard car alternator....

I see a great deal of work if the alternator is to produce 120 volts at a reasonable amps directly. My personal take is that a step-up transformer would make things easier, but still a lot of design and test work as most of the alternators are 3 Phase, not single phase!!! So it really needs to be a single phase alternator to allow single phase 120 to be easily and cheaply developed using a transformer.....

Thats my 2 cents worth, have a great 2010!......

I'm not sure when it was; probably over 30 years ago; that there was an item in Popular Mechanics, I think, about this and I'm only going by what I remember but they did modify an alternator to produce about 120v. Of course output was 3 phase, the frequency was high and the regulator was only a resistor in the field, you had to speed up the prime mover to get the voltage. Ohms' law about having to have 60 amps field current only applies if you keep the same speed. Here we increase the speed to get the voltage as well. The voltage was not constantly adjustable to the load which meant you had to keep the same load on the alternator once you were up to voltage, but it was supposed to work. Not a practicable application and there are lots of better ways of getting useable 120V.

I assume you got the 60 amps from my post. That was the max rating for an average modern (small) car alternator working at only a nominal 12 volts with the original thick wire on the stator fields......it demonstrates basically the "starting point" for working with a car alternator at higher voltages........and the aproximate limits for the original fields....forgetting the need for upgrading the insulation etc..

BUT!! Getting 120 volts from the same thick windings will mean a dramatic increase in speed (as you said correctly), resulting in a very high frequency (as you also said!) and as I pointed out correctly, you will also need a dramatically higher current flowing through the rotor/slip rings to get to that higher output voltage......nothing else will achieve it (120 volts) using the original (number of) stator windings....

If the OP has absolutely no money and a pretty good workshop and a lot of spare time, it can possibly be achieved, but I personally (in spite of my Scottish blood) see no reason to do this. My private time is more valuable than that.....

For camping on sites with no mains, I use a big battery, and a car engine for re- charging it and a commercially produced inverter to produce mains voltage at the correct frequency of a quality that even a PC can use!!! Its only for driving a small TV and sat receiver.....and occasionally re-charging my Laptop battery....

More batteries would extend the time I could produce electricity of course, or I could buy a (small?) quality generator with a diesel or petrol driven alternator!! I did have one, but hated the weight and volume it took, gave it away to my son-in-law. It cost less than €100 new for 900 watts at 240 volts........but it was only 2 stroke and not as quiet as I would have liked.

Though I did make some easy, simple changes that resulted in it being far, far quieter, (and a little heavier!) but it only took an hour to make!! A few feet of silicon runbber tubing and a small cheap motorbike/Scooter silencer and some mounting clips.....it wasn't pretty but it worked.

Until someone takes the time to make a unit, we are all surmizing!!

I'm not sure of the name now, but a UK firm supplied
alternators for direct fixing to any vehicle engine.
(worked off the cooling fan pulley wheel.)

It supplied 230volt single phase and it had enough power
to drive almost anything. (also had 110V for commercial use)
I almost bought one at the time, but money was short.

I'm sure if you google the firm will show up.
Hope this helps.

jt.

This is supposed to be an Engineering forum folks. How about a true engineering approach to this.

An automotive alternator that is supposed to charge a 12V battery with the engine idling speed at 900 RPM must apply 12.7 volts at the battery. This means that the peak to peak voltage out of the alternator's stator windings must be at least 13.3 volts to forward bias the diodes. This means that the RMS voltage out of the stator windings can be found by this series of equations.

V_rms=V_p/(√2)=V_pp/(2*√2) → V_rms= 13.3V/(2*√2)≈4.70V_rms

This means that with the same rotor magnetic field spinning, the angular velocity must be increased to achieve a higher rms voltage. Since by Lenz's law the voltage produced is proportional to the change in magnetic flux. Since I stated for this initial analysis that the magnetic field was fixed for this calculation then all of this has to be from an increase in angular velocity. So

120 V_rms / 4.70 V_rms = ω/ → ω= ω_o* 120 V_rms / 4.70 V_rms =900_rpm* 120 V_rms / 4.70 V_rms ≈23000_rpm

Now this calculation tells you how fast the engine has to turn with the same pulley configuration and magnetic field to get the desired 120 VAC. Changing pulley ratios to speed up the rotation rate for the alternator will improve this, but this will likely not be practical for all of this change, for the alternator typically already spins faster than the engine. So then one must increase the magnetic field of the rotor. But in normal operations, the current applied to the rotor windings at idle is the maximum current condition. Increasing this current might saturate the core materials and produce no net effect in the B field magnitude.

This is why I said that taking a low voltage alternator to produce line voltages was theoretically possible but likely impractical. Can one luck into an alternator configuration that can do this, yes one can. But I will not vouch for how reliable, or safe this will be. You will notice that I've not considered at all what frequency the AC signal will be. To calculate that one must know the number of poles found in both the stator and rotor configuration. This will be difficult to find out without experimentation or disassembly since this information is pointless to document with normal use of an alternator.

The module that Brave Sir Robin found that was designed to do this is still the preferred method. (I never did give him a GA. I'll rectify that now. _{Pun intended.} )