Different kinda question

[Fla._Deadheader]

Why does an automotive type Alternator HAVE to reach a high RPM to give near maximum output ???  I thought that was the reason Alternators became standard equipment, to charge at lower RPM ??? 

  What RPM, approximately, would be required for near max output on a 24V heavy equipment type Alternator ???

[Gary_C]

The old generators produced Direct Current and the Voltage they produced was dependent on the RPM's. At slow engine speeds they produced less than 12 Volts DC and thus were ineffective at slow speeds. The DC generators however have an advantage in they could produce 24 volts+ and are still used in those dedicated car and truck starters you see on service trucks.

Alternators produce an alternating current that is rectified to produce direct current. That is why they are capable of producing 12+ Volts even at slow speeds. They just will not produce very much charging current at slow speeds, however they will contribute to the load at slow speed.

You do not necessarily have to run an alternator at max speed to get maximum current output. The voltage regulator will turn the alternator on and off to try to regulate the battery voltage, and it will produce all the current it is able to produce during those on times. That max current is more than the rated output and it will burn up if it had to produce continously at that max output.

Most rebuilt alternators now come with a warning that an alternator is not a battery charger. If your battery is discharged, you are required to use a battery charger before installing the alternator or the warranty is void. They can tell if you return the alternator because it has overheated.

Hope this helps.  ;D

[Fla._Deadheader]

Thanks for the input, Gary_C. Everything you wrote is comparable to what I had learned over the years.

  Is it possible to change the Diode package in a 12V Alternator at 100 amps, to get 24V at 100 amps. The load would be the same on charge rate ???

[rebocardo]

> What RPM, approximately, would be required for near max output on a 24V heavy
> equipment type Alternator

It really depends on the alternator. Western Electric has a table for its bigger alternators and at what RPM they make max amps. Usually around 900 rpms Alt speed. With pulleys, that could be an engine speed 2x-4x of the alt. depending on application.

Most heavy equipment does not make over 1600 rpms or so, so they have to have big pulleys to get the alts to operate at engine speed. The only real way to change it is to get bigger pulleys.

The lower amps on a alternator on most cars at idle is okay because you do not need the higher amp output for the ignition system until you hit higher RPMs, you are unlikely to use very single power switch (seats, windows, a/c, radio, wipers, heater, rear defroster) all at once, you do not need faster wiper speeds at idle (as an example) so the alt with the battery as a slight reserve is more then able to keep up.

The problem is not only amp draw, but, cooling the alt (with rect.) at low RPMs. Some alt. have internal fans (gm type) to cool both the rect. and reg. at idle, but, if you increased the output over stock, you would need much more cooling, bigger (more) diodes, and a bigger heat sink to compenstate.

[Fla._Deadheader]

  Suppose that I were to install the Diodes and heat sink AWAY from the Alt frame, say, external mount ???  Would that work OK ???

[Gary_C]

There are 24 Volt alternators available. I have two right now. One is an older Case 1470 tractor that was made with a 24 Volt system. The other is a Valmet 840 Forwarder that has a 24 Volt system. So I am sure that an alternator can be found that will give you 24 Volts. I believe that many new Forestry machines come standard with 24 Volt systems because of the computers that control the operation and there are probably other farm tractors that were made with 24 Volt systems. Just find a rebuilt alternator that is made for 24 Volts. Should be cheaper than making your own.

There are two reasons that I have seen that cause alternator failures. The first is arcing caused by reverse hookup of jumper cables and the second is overheating of components. Many times the item that overheats is the wire connector bolt that gets so hot that it actually melts the insulation. The overheating is caused by too much resistance, not by lack of cooling. So just adding more cooling will not stop the heating and associated failures. Also the rectifiers are rated by what is called "Peak Inverse Voltage" that they can handle. If you exceed that rating, no amount of cooling will prevent failure.   ;D

[DanG]

FDH, you familiar with Ohm's Law?  12 volts x 100 amps =1200 watts.  24 volts x 100 amps =2400 watts.  Ya might could pull that for a few seconds on the way to the parts house. :D :D :D

The amount of current(amps) you use is controlled by the user, not the source.  The limiting factor is the size of the conductors within the alternator.  The diodes have nothing to do with it, other than being one of those limiting factors due to their size.  You cannot reconfigure them and expect the thing to work.  A diode merely blocks the flow of current in one direction.  The bank of diodes in an alternator serves as a rectifier to change the AC current to DC.  To my knowledge, the only way to get 24v out of a 12v alternator is to rewind it.  To double the power(watts) you'd have to also double the size of the conductors.

[Gary_C]

I can hear the wheels turning down there in CR but you haven't said anything about whatfor. So whatisit you are trying to do or is it illegal and you can't tell us?    ;D ;D

There are a lot of 24V high output alternators already available. They come standard on some John Deere and Case tractors as well as military vehicles (28 Volt) and I think some cars like police cars. I know you like to build everything yourself, but there seems no need here. The parts will probably cost you more than an off the shelf rebuilt one.   ???

To my knowledge, the only way to get 24v out of a 12v alternator is to rewind it.

You can easily get 24 Volts out of an alternator with out rewinding anything. You just need 24 volts for the fields and a 24 volt regulator. As you correctly pointed out, the problem is handling twice the power.   ;D ;D