as the topic suggests, has anyone made an alternator test stand ? if so, what rpm electric motor was used, what pulley diameter was used, was a regulator built into the electrics, or did you just spin the alternator at a specific rpm and measure the output ? any other ideas or suggestions ? i have several i need to test, and i hate taking them all to the rebuild place just to see if they work. thanks in advance for any comments and advice !

just me I would swap em into your running vehicle and check output voltage at a fast idle, you would have an answer for ll three in less than an hour. If making a stand (going with the fast idle) alot of crank pulleys are 7.25" and there are 2 sizes of alt pulleys that I have seen, a 2&3/4" comes to mind for one of em. Been trying to find an electric mtr to power my air compressor & I see 1750 RPM on the side of several of em. You'd wire it up like it is in the car.

Without some sort of load your bench test is useless.

Would the battery provide the load?

My local guy has a 3-4 horse Briggs to run his. Cheaper than a suitable electric motor.

Unless you have a multiple horsepower $$$ electric motor, any load you put on the alternator to test it will just make everything come to a grinding halt.

Kevin

Depends if the battery was in need of a charge.

You can use a headlight as a load. More head lights = more load.

so it would need a good sized motor to turn the alt just from full fielding the alt? I know on my air compressor its looking like it will need a several horse elec mtr to turn it. I think I'm also gonna get my 3 HP Briggs/Stratton gas engine going & use it rather than lay down $100+ for an electric motor.

Full fielding the alternator will only show maximum voltage output (probably around 17 volts?). To really test the alternator you would need the voltage regulator wired in and an adjustable carbon pile load tester that measures volts & amps. I use an old Sun VAT40, you can purchase $50 carbon pile load tester at Harbor Freight which says it measures volts & amps but I don't see an inductive amp clamp in the picture. When testing an alternator you really need to measure voltage and amperage output.

The FSM has a pretty good testing procedure in it.

Some ideas: One HP = 745.7 Watts
Alternator output is usually around 14 volts, and Power is Current * Voltage, so the one HP should spin the alternator up to about 50 Amps (53.26 Amps), but that is just the power needed for the electrical load, you also need extra power because the alternator has a cooling fan, and I have no idea how much power it consumes?
Anyhow, the higher the current output, the more HP required to spin the alternator.

As for a test stand, it depends on if you just want to see the maximum the alternator will put out, or if you want to see the output power curve?

To map the power curve, you would need a way to vary the alternator speed. If you don't care about the curve, or lower RPM output, then you can just set up pulley sizes to spin the alternator fairly fast, maybe 5,000 RPM at the alternator shaft?

Then, as mentioned, you need a way to place a load on the alternator output. Normally the load would be adjustable so you can dial in the load until you see the output voltage start to drop. Then you measure the current. An inductive current meter would be ideal, but a current shunt could also be used.

For the load, Resistance = Voltage / Current, so 14 volts / 25 Amps = 0.56 Ohms, but he power load (or power rating od he resistance) is I^2 * R.
That is Current Squared times the resistance, 25 * 25 * 0.56 = 350 watts.

If you wanted to test 50 Amp output, that would be a 0.28 Ohm, 700 Watt load.

Got it covered. Got it from a shop that closed.

My NAPA store has one like John's, ask around your area, most auto stores will test for nothing.

Joe

I'm in the same boat - a stack of alternators...
Most testers use an electric motor. Two Hp should cover it as a test card I have for a 94mp 12SI shows maximum output of 1411 Watts (104 amps at 14.6V 6000 rpm (alternator). Turn on speed for that alt is 1100 rpm)

You'll have to do the math on the pulleys. Most alternators are being driven signifcantly faster than the crank. Just measure the crank pulley diameter and the alt pully and you'll have it.

Off the top my head...
Carbon pile will let you put load on. If it has an ammeter on it, it will be internal, in series. The ammeter itself will have an internal parallel shunt.
A battery and regulator, or power supply for field.
Motor with a stack of pulleys like on a drill press or band saw will let you do different rpms. Motor with variable rpm would be really good. I looked into this and now can't remember what's involved.

There was some useful info on alternator performance posted on Speedtalk a year or two ago - mostly by "BC Johnny". How Alternators are Upgraded p. 2

well, it seems i forgot about the load induced aspect of the testing procedure. rats ! i have a pile of these things from different applications, and nothing running at this time that would be applicable to test each without pulley swaps and bracketry fabrication. my local shop will test for free, but i don't want to take advantage of him by bringing a basket load of 'em to test. for some reason, i was just thinking i may be able to spin up to "x" rpm and see what voltage was being produced. at this point, i guess i'll just call him and see what he will charge me to check them. thank you guys for pointing out my feeble old mind forgot about things that were important for this endeavor !

drop em off & ask him to do em in his leisure/when he can, he'll appreciate the no pressure.

In olden times, that is what I used for a camping generator. The donor was a Toro reel type lawn mower. Put as big as a pulley that would clear on the drive shaft on B&S and a V belt to the alternator. Used a 72 Mopar regulator and alternator wired to a deep cycle battery. Worked like a champ. There were no fancy Hondas and AC generators were extremely expensive. Ran my Coleman crank up tent trailer with it.
Craig

that's what i was thinking. they have been laying around this long, another few weeks won't hurt !

I have a friend that uses a chry.100a alt.driven by an elect.motor for welding.

The master technician MTSC books on mymopar.com and E-bodies.Com have bench testing procedures as well.