Can an alternator be tested for output off a car?

[deansplit]

Hi, I'm building a kit car. I have an alternator with no idea what Amp output it is. Is there a way of testing it with it not installed on a car?

 

[brianmoooore]

Apart from feeding any writing you find on it into Google, then no.
Alternators only push out the power required from them, and I assume that the requirements for a kit car are low, so almost any rating will do, with the only penalty being weight and size.
Little Mitsubishi alternators used to be a favourite, but there may be another favourite by now.

 

[deansplit]

Thanks Brian, but ignore the kit car par as it has no bearing... it's an alternator for a Porsche 911 cooling fan for a VW Type 1 engine. The one I have which came with the used cooling kit (no writing on it whatsoever) is fine for what I need, but I'm adding an electric heater so have been advised to up the alternator to 90amp. I just don't know what I have though.

 

[R-fur]

An Alternator can be tested off of a car but you need a test rig and a motor to turn it. Might be worth while taking to a local auto electrician to check it out for you.

 

[brianmoooore]

Test rig to see if it works is easy enough, but would this normally check its rating?
It occurs to me that I don't actually know what happens to an alternator at its maximum rated output. Does its output voltage start to drop off, limiting the current, or, do its output keep increasing until it self destructs?

 

[Rockingit]

Test rig to see if it works is easy enough, but would this normally check its rating?
It occurs to me that I don't actually know what happens to an alternator at its maximum rated output. Does its output voltage start to drop off, limiting the current, or, do its output keep increasing until it self destructs?

A crude one will just burn out the windings - no different to an alternator in a generator. When it hits the limits of it's inductance it just gets hot and melts! The voltage will always drop as current increases in the normal way but unlike a generator that has an AVR to keep the rotor voltage even, a mechanically driven alternator has no means to do that as it can only turn at the fixed speed of whatever drive/belt etc is making it move. So eventually all that heat gets the better of it.

 

[R-fur]

An alternator has a sense wire (sometimes an internal connection) that senses the voltage and turns the excitation on and off to control the output.
I agree with Brianmoooor that a simple test rig cannot check an alternators ratting.