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Car alternator max frequency?

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Austin the Ozone

Mon Jul 11 2011, 09:07PM

Registered Member #3989 Joined: Thu Jul 07 2011, 05:10PM
Location: In a van down by the river.
Posts: 52

Probably been posted before but I could not find a satisfactory answer so here goes. Anybody ever bypassed the diodes in a car alternator and then spun it really really fast to see what frequency it can get up to? I have a old ford and an old chevy alternator here to play with and was wondering this. Like how many rpms can be safely turned before it flies apart and what hertz and if the voltage would keep climbing with speed. The size of the alternator pulley seems about 1/3 or 1/4 the size of the crankshaft pulley that drives it so if car motor can turn 5k rpm than that would be 15-20,000 rpm for the alternator which seems kinda high but who knows. Could this not be then used to direct drive a step up transformer etc. Thank you

Dr. Slack

Tue Jul 12 2011, 05:57AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

All your thoughts +1

The point about an alternator is that the rotor construction is very simple and strong, to give decent output at the low end while being able to survive the high end of the speed range. If it spins to 20k in you car, I don't see why it shouldn't spin to 20k on your bench. But, car components are specified not to fail, it's really bad business if even a few cars in a million fail badly, so it's likely that this 20k estimate is conservative. But how conservative?

There are only a few companies who make the bulk of components for cars. It might be an idea to see if you can dig out some specifications for a particular alternator type, they ought to specify a max survival rpm. I know it's heresay to suggest that a 4HV user ought to find specifications rather than just turn up the wick on something to see whether it survives. However, if you do want to sacrifice one of your alternators in the interests of science, make sure that flying pieces aren't likely to go through people or property when it goes.

I would expect that the windings will be OK to whatever voltage you care to generate. The nominal output is 10s of volts. The maximum output is limited by the iron saturation and the speed, so you can't generate more than an order of magnitude above the nominal output. Standard copper windings are typically specified to 50v on the insulation, but will take a lot more if they are good condition.

Ash Small

Tue Jul 12 2011, 06:32AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Most are rated for 20k. I looked into this 20 odd years ago.

I'ts the regulation that keeps the volts down. By de-regulating you can achieve much higher voltages. There are several sites on the internet that describe how to do this so that you can use them for welding, etc. (or there used to be, I've not looked recently).

You can't drive a transformer directly from an alternator (at least you can't drive a 3.5-5 kVA welding buzzbox from a 7.5 kVA generator) because of 'inductive loading' or something (someone else will have to explain this in detail, I only know it doesn't work, something about phase shift, or something). (The person who designed my generator explained this to me on the phone
ten years ago, he told me to 'crank up' the RPM to 3,600 to give 300 volts @ 60 Hertz (instead of 240 @ 50), to strip and overhaul the alternator (re-machine the commutator, etc) and to connect a resistive load as well, but it still only works at very low amperages)
If you have resistive loads on the alternator as well you can drive a transformer, but it has to be smaller than the output of the alternator. Power companies seem to get round this, I don't know the full story, all I know is that I need to build (or buy) an inverter welder if I want to power a welder from my 7.5 kVA generator.

Austin the Ozone

Tue Jul 12 2011, 07:55AM

Registered Member #3989 Joined: Thu Jul 07 2011, 05:10PM
Location: In a van down by the river.
Posts: 52

Thanks for the replies, I found a site http://autonopedia.org/renewable_energy/Generators/Alternator_Secrets.html that indicates car alternators are three phase, which I did not know. I don't fully understand how three phase supply works yet, but it is three sepparate sine waves at the same time right? Can they be combined into a single output tho for more hertz or more volts?

I like the part at the end where he indicates using a old style motorized clock to count the frequency. LOL time flys when your having fun! Especially at 20,000 HERTZ!!! The second hand turning at ~300 rpms like a fan blade!

Ash Small

Tue Jul 12 2011, 08:07AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Austin the Ozone wrote ...

.car alternators are three phase, which I did not know. I don't fully understand how three phase supply works yet, but it is three sepparate sine waves at the same time right? Can they be combined into a single output tho for more hertz or more volts?

Some can be re-configured to single phase. On some it's just a case of how the windings are connected together. (star or delta, I think)

I once re-configured a single phase six volt motorcycle alternator by connecting two windings in series so that it gave out six volts from one winding, to run the ignition system, and 12 volts from the other two to run a 12 volt lighting system, etc.

EDIT: The link above doesn't appear to work.

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