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The voltage regulator opens and closes the circuit to the rotor windings, which act as the field in order to maintain system voltage in the 13.5-14.5 volt range. The raw output is three-phase AC, which is then converted by the rectifier bridge and diode trio to a DC voltage, and a typical voltmeter will measure the average.
If you look at output on a fast scope it will have a sawtooth appearance.
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While the test can be performed on a bench, one will have to be careful with the interpretation of the results.
Due to the (no load) shape of the output voltage, the measured value may be much less, if a RMS voltmeter is used. This is because the RMS (Root Mean Square) meter will disect the wave and tries to put it back together in order to get an average value. With a triangular wave it is fairly easy to estimate the average, but one needs to have some electical / electronics background to visualize it.
Are you sure that your Current Clamp can read DC?
Most clamps in the lesser price range are AC clamp meter only. There are DC clamps, but get more expensive, since the way the current is picked up is more difficult. They measure the magnetic field around the wire and run it through a certain algorithm to finally display it. The result may not be very accurate, because of movements, angle and possibly other absorbing obstructions can influence the reading, during measurement.
The best and easiest way to guestimate / test it, would be to connect a battery and spin the alternators axle shaft at (I believe double the engine RPM, depends on ratio) and measure the battery voltage.
If the Battery is fully charged, there will be very little current flowing into the battery and the voltage is close to the alternator (correction "Regulator") output e.g. 13.8 VDC. If it is higher (with battery) careful here, your battery may get a shortened lifespan.
The alternator has to be able to apply a higher than nominal voltage across the battery, otherwise there will be no current flow into the battery.
In general a Starter Motor can pull about 280-350 Amps during cranking. This number depends on the mechanical load e.g. engine compression and anything else that is mechanically attached to the crankshaft.
Right after the engine started, the alternator is now giving whatever it has to give. For example: if the alternator is a 72 Amp alternator, you should see a number not far below. Again, this number largely depends on condition of the battery and how long the starter motor was cranking. The current will very quickly be reduced , because the battery's potential increased e.g. the voltage rose.
A good battery has a larger capacity e.g. can handle a large load for a longer time. Therefore the discharge is not as muich as it would be with a smaller capacity battery. The alternator has to re-charge the battery as fast as possible, therefore will deliver as much current as possible.
As soon as the battery reaches its fully charged state, the voltage regulator will open and the current will stop. Since there are consumer attached at all times, the battery will discharge again, but at a lower rate. The regulator sees the lower voltage and puts the alternator back to the battery to charge it again. This goes on as long as the car is driven / engine is running.
I better stop here!!!!
A word of caution!
With fairly large currents flowing, always make sure that the wiring can handle the current.
If the wire gauge is too small, it can cause a fire or at least the wire will vaporize.