automotive engineering schools: Alternators Off Car Testing

Wednesday 14 March 2012

Alternators Off Testing:

The purpose of this exercise is not only to learn how to dismantle an alternator and learn how to test the Rotor Shaft, Slip Rings, Stator Windings, Rectifier Positive Diodes, Rectifier Negative Diodes, Brush protrusion and Testing the voltage Regulator - Which the regulator can only be tested if it has voltage going through it.
But to know how the Alternator generates its voltage to charge the Battery & also to determine faults internally in the Alternator. If the Alternator has no internal faults then we know that the fault lies with the Regulator.

Here we have an Alternator that has been supplied to use by Unitec Northern Campus Albany.

What your seeing is a birds eye view of the Rectifier, Rotor shaft & Regulator.

To the right the Regulator & the Brush Holder have been removed. The Brushes connect to the Slip Rings on the Rotor Shaft and carry the DC electricity from the Slip Rings to the Regulator which then sends to the battery.

Birds eye view of Brushes & brushes holder

With testing the Brushes they have to be a minimum length of 4.0mm. If these are to short then the spring's that are connected to the Brushes are unable to apply enough pressure to hold the Brushes to the Slip Rings. If this is the case it will create sparks between the Brushes & the Slip Rings & which will damage the Slip Rings.
Our Brushes measured ( 5.6mm) measuring between the Slip Ring contact face & the face of the Brush holder.

Taking off the Rectifier

To do this we first unscrewed the Stator Winding Terminals,which connect to the Rectifier.

Testing the Rectifier positive diodes (common lead B)

Testing the rectifier once off is an easy process.
First set the multimeter to the diode test mode.
Put the black prob on the B terminal.
With the red prob, then touch each of P terminals.

The resistance should be in between 0.5 to 0.7, The resistance should be low.

Testing the Rectifier positive diodes (positive lead B)

Same as above - set the multimeter to diode test mode
Put the red prob on terminal B
Put the black prob lead on each of the P terminals

The resistance should be infinity ( 0 ). which tells us that there is no resistance/ the electricity flow is one way. If there resistance shows a reading then we know the diode is broken.

Below is a pic of testing the rectifier - positive diodes (which was taken from a Unitec workbook.

Testing the Rectifier negative diodes (using ground points).

Set the meter on diode test
Put the common lead on the E terminal/ Ground point.
Touch the Positive lead on each of the P terminals

The resistance should be infinity ( 0 ). If there is resistance then the diode is broken.

We then repeat the process, but changing the Positive lead on to the ground point
Putting the common lead on each of the P terminals

This should show a resistance reading 0.5 to 0.7, If there is no reading or reading is lower or higher then the diode is broken & needs replacing. Or replace the Rectifier.

Below is a pic of testing the rectifier - negative diodes (which was taken from the Unitec workbook).

Testing of the slip rings:

Rotor Ground Test:

The slip Rings is what the Brushes are attached too.
The small amount of is carried through the slip ring and past through to the brushes
To test these is an easy step
First put the black lead on the center of the rotor shaft
Put the red lead onto the slip ring
There should be no circuit between the two
If there is then the rotor winding has shortened and needs to be replaced.

Rotor Winding Internal Resistance Test:

First set the meter to 200 on the Ohms meter
Then place one end of each lead on the slip ring

The resistance should be 2 - 6 ohms.
All test's on our alternator passed we had nothing fail. The testing was a success.

Stator Winding Resistance Test:

With black lead attached to the common terminal which is the on with the most copper wires - 3
The red lead touching each of the other terminals
Resistance should be between 0.0 - .2 ohms

Stator Ground Test:

Put the red lead on the stator winding common terminal - the one with most wires
The black lead on the ground point/ Alternator housing. There should be no current between the two. If there is the stator has grounded and will need replacing.

The following pic's below is are of dismantling the alternator at Unitecs workshop Albany:

Rectifier, Regulator Cover