Attempt to build DIY high current power supply from car alternator?

Question

Recently I had posted about a thread related to a bench DIY high current transformer 12VDC supply that I'm trying to built. But due to some transformer and diode rectifier limitations that project may take a little more time to be completed until I get my hands on a much beefier transformer or two transformers combined enough to do the job. Meanwhile I have a used spare 100 or 120A alternator (I don't remember the exact current rating) in my possession. I was thinking that if I mounted the alternator with an AC induction or a brushed DC motor like from a treadmill I can use it to generate the max current capability of the alternator to run high current 12VDC motors that pulls close to 80-90Amps for a brief testing like under 10 seconds. I've a treadmill motor in my possession, which I think its rated for 90 or 100VDC that is rated for 3150rpm I guess. I can use a pulley system to run it to obtain higher rpms at the alternator. The voltage output of the alternator doesn't really matter too much for me as long as its more than 12VDC, since my goal is not to charge a battery.

What do you guys think about this project? Is it a good idea? Also what is the RPM range required to obtain that max. current capability of an alternator? If you need a specific alternator model I can look it up and let you guys know later.

Edit: I'm aware that I'll need a 12V supply to excite the field windings. Planning to use a 12V transformer with rectifier for that job.

Answer 1

Alternators are designed to generate enough voltage to prevent the battery from discharging at the engine idle speed. Speed and field current determine the voltage. At idle, the regulator lets the maximum field current flow to the alternator. As engine speed increases, the regulator cuts back the current to prevent excess voltage. For your purposes, you would only need to drive the alternator at the speed it would run at the normal idle speed of the engine. You can operate the alternator without a battery, but you do need the proper voltage regulator and a 12-volt DC power supply for the field. If you operate the alternator at a voltage that is too high, the initial starter current will be higher and more difficult to provide.

Once the starter is at full speed, the current should drop somewhat, but probably not as much as might be expected with no load, because the field is connected in series with the armature. I would think that a proper bench would include measuring the idle current. There must be some guidelines about that someplace.

One thing that might help with the testing power supply would be to put a small value of resistance, perhaps 1/20 ohm, in series with the starter before connecting it to the power supply and then short the resistor when the motor has picked up some speed. You might be able to make the resistor from a piece of stainless steel.