Alternator Off Car Testing

In this exercise we had to dismantle an alternator and carry out different tests to see if the alternator was working properly and if not, what component would be the source of this failure.
Following the given list of instructions in the booklet, we took apart the alternator and began testing.
Starting on the Rotor windings, we tested it to ground. (Multimeter set to 2K on ohms scale.) To do this, we placed the black lead from the multimeter on on the centre of the rotor shaft and the red lead on the slip ring. The meter should end up with no reading. Also, the rotor winding internal resistance is tested. For this the leads are to be placed on the slip rings. The meter should read between 2-6 volts as specified. The reading achieved was 3.2 (minus the internal resistance of the multimeter) which gives a final result of 2.8 ohms. Results of these tests are within specs and pass.

Then further opening the alternator, the stator winding resistance is tested. This is achieved by placing the black lead of the meter on the common terminal (terminal with the most wires leading to it) and the red on each of the other terminals to gain readings of each. The spec for this was to be between 0 - 2 ohms. 0.2, 0.1 &0.1 were the readings of the terminals which makes them within specifications and therefore also pass. Then to make sure that the common terminal is grounded, the black lead is placed on the body of the alternator whilst the red is on the common terminal. To pass, there will be no reading on the meter. This was the case which means that there is no circuit between the stator winding and ground.

The meter then needs to be chnaged to diode test mode to test the rectifier positive diodes. Testing each of these, Black lead on a common terminal and the red on each of the rest one at a time. There were 3 terminals which gave relatively similar readings (0.580, 0.590, 0.579) and the other gave no rading at all. This is so as one of these terminals is the common ground and as such should have no circuit. Therefore all of these passed. This test is then reversed and all results should read infinity (no reading at all). All results were as required.
Testing the rectifier negative diodes is slightly different as the red lead is placed on a body terminal and the black is placed on each of the 4 other terminals one at a time. The results were the same however.

Testing the voltage regulator, we first had to find out the make of it, this was done (personally) by comparing the regulator to the diagrams in the booklet and finding a match. Carrying out a Transpo regulator test, we had to make sure the short circuit light was of (Pass), The warning light came on and stayed on (Pass), the field light should flash continuosly (Pass) and the reading (12.5v) was what it needed to be (Pass). This means that the regualtor is not short circuited and is doing its job correctly.
Then using a vernier caliper we checked the length of the brush protrusion. This is very important as if they are too short the brush springs can not apply enough pressure to maintain contact and in turn reduces output of the alternator. The length we got was 10.2mm and 10mm. This means it was well above the minimum length of 4mm.


REPORT:

Overall, the alternator passed, meaning that it operates as its supposed to and each component is working well.
Testing the rotor winding internal resistance gave a measurement of 2.8 ohms which is between recommended specifications of 2 and 6 ohms. Being that the rotor is an electromagnet if resistance is too high, this causes the magnetic field to become weaker which in turn decreases the field current of the circuit. If this happens the voltage regulator will have less field current to adapt to constant voltage at the stator output. This will also reduce the field current that will be induced into the stator windings and therefore have less output to the terminals of the coils with the same result of rectifier which will draw more energy from the battery to compensate for the fault, greatly decreasing the battery's state of charge over time.
It is very important that the rotor winding internal resistance is between specified values, if not they may need to be replaced.