Saab SONETT III Service Manual page 59

Terminal
(anade)
Terminal
(cathode)
Conducting
direction
1
Non-conducting
direction
Conducting
direction
1
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 , 1 1 1
—
HL
Casing
Casing
+ Diod
Di
od
S 2227
Testing alternator components
Testing the rectifiers
Use only a testing lamp of not more than 24 V or an
ohmmeter. To test the conducting and non-conducting
directions of the silicon rectifiers, the phase terminals
must first be disconnected, as it is otherwise impossible
to tel) which diode is faulty. The positive diodes between
terminal B+ and phase conduct from the terminal to the
casing but not from the casing to the terminal. The nega-
tive diodes connected between phase and B— (inverse
polarity) conduct from the casing to the terminal but
not from the terminal to the casing (see illustration).
To test this, connect the testing lamp in series with the
diode under test. The lamp should light up when B+ is
connected to the anode of a diode with normal polarity.
The lamp should not light up when B+ is applied to the
casing. If the diode has inverse polarity, the lamp will
light up when B+ is connected to the terminal (cathode)
but not when
the ri imrtion of currpnt flnw ic rpivorcpri .
A faulty diode that fails to pass current in the conduct-
ing direction has been overheated by excessive current
intensity. A diode that passes current in both directions
has probably been subjected to excessive voltage.
Testing rectifiers with an ohmmeter
The rectifiers can also be tested with an ohmmeter. In a
properly work ing diode, resistance is low in the conduct-
ing direction (e.g. a few ohms) but much greater in the
non-conducting direction (e.g. severa) kiloohms).
Testing the stator windings
Short to ground (rectifiers unsoldered)
The stator windings can be tested for shorting in the nor-
mal manner with a testing lamp of not more than 40 V.
The test can be performed with the rectifiers connected,
provided that the non-conducting direction of the diodes
is allowed for. The positive contact of the testing lamp
must be connected to the stator winding and the nega-
tive contact to the diode casing; if this method is adopted,
the test voltage must not exceed 24 V.
Short in the windings (rectifiers unsoldered)
The test for shorting in the stator windings can be made
with the probe of an winding tester. This test can only
be made when the alternator is disassembled.
A further possibil ity is to measure the joint resistance in
two phases. With three measurements, any deviation in
the resistance of the stator winding can be detected. If
the phases are marked U, V and W, measure U—V, U—W
and V—W. In each case the reading should be:
0.26 ohms + 10% (at 68°F/20°C).
Testing the exciter winding (rotor)
Short in the winding
Use an ohmmeter to measure the resistance of the winding
from slip ring to slip ring. The resistance should be
4.0 + 0.40 ohms.
Short to ground
Test the insulation of the exciter winding and slip rings
by checking for shorts to ground (test voltage 40 V).
321-9
SAAB Nov. 1972