PI Manufacturing AC generators - Manual (Page 30)

Rectifier Diodes
The diodes on the main rectifier assembly can be checked
with a multimeter. The flexible leads connected to each diode
should be disconnected at the terminal end, and the forward
and reverse resistance checked. A healthy diode will indicate
a very high resistance (infinity) in the reverse direction, and a
low resistance in the forward direction. A faulty diode will give
a full deflection reading in both directions with the test meter
on the 10,000 ohms scale, or an infinity reading in both
directions.
On an electronic digital meter a healthy diode will give a low
reading in one direction, and a high reading in the other.
Replacement of Faulty Diodes
The rectifier assembly is split into two plates, the positive and
negative, and the main rotor is connected across these plates.
Each plate carries 3 diodes, the negative plate carrying
negative biased diodes and the positive plate carrying positive
biased diodes. Care must be taken to ensure that the correct
polarity diodes are fitted to each respective plate. When fitting
the diodes to the plates they must be tight enough to ensure a
good mechanical and electrical contact, but should not be
overtightened. The recommended torque tightening is 4.06 -
4.74Nm (36-42lb in).
Surge Suppressor
The surge suppressor is a metal-oxide varistor connected
across the two rectifier plates to prevent high transient
reverse voltages in the field winding from damaging the
diodes. This device is not polarised and will show a virtually
infinite reading in both directions with an ordinary resistance
meter. If defective this will be visible by inspection, since it will
normally fail to short circuit and show signs of disintegration.
Replace if faulty.
Main Excitation Windings
If after establishing and correcting any fault on the rectifier
assembly the output is still low when separately excited, then
the main rotor, exciter stator and exciter rotor winding
resistances should be checked (see Resistance Charts), as
the fault must be in one of these windings. The exciter stator
resistance is measured across leads X and XX. The exciter
rotor is connected to six studs which also carry the diode lead
terminals. The main rotor winding is connected across the two
rectifier plates. The respective leads must be disconnected
before taking the readings.
Resistance values should be within +/-10% of the values
given in the table below :-
Exciter Stator FRAME
SIZE
Main
Rotor Type 1 Type 2* Type 3**
Exciter
Rotor
UC22C 0.59 21 28 138 0.142
UC22D 0.64 21 28 138 0.142
UC22E 0.69 20 30 155 0.156
UC22F 0.83 20 30 155 0.156
UC22G 0.94 20 30 155 0.156
UC27C 1.12 20 - - 0.156
UC27D 1.26 20 - - 0.156
UC27E 1.34 20 - - 0.182
UC27F 1.52 20 - - 0.182
UC27G 0.69 20 - - 0.182
UC27H 0.82 20 - - 0.182
UCD27J 2.08 20 - - 0.182
UCD27K 2.08 20 - - 0.182
* Used with 1 phase transformer controlled 3 phase or 1
phase generators.
** Used with 3 phase transformer controlled 3 phase
generators.
7.5.1.2 UNBALANCED MAIN TERMINAL VOLTAGES
If voltages are unbalanced, this indicates a fault on the main
stator winding or main cables to the circuit breaker.
NOTE : Faults on the stator winding or cables may also cause
noticeable load increase on the engine when excitation is
applied. Disconnect the main cables and separate the winding
leads U1-U2, U5-U6, V1-V2, V5-V6, W1-W2, W5-W6 to
isolate each winding section. (U1-L1, U2-L4 on single phase
generators).
Measure each section resistance - values should be balanced
and within +/-10% of the value given below:-
AVR CONTROLLED GENERATORS
SECTION RESISTANCES
FRAME
SIZE
WINDING
311
WINDING
17
WINDING
05
WINDING
06
UC22C 0.09 0.14 0.045 0.03
UC22D 0.065
0.1 0.033 0.025
UC22E 0.05 0.075 0.028 0.02
UC22F 0.033 0.051 0.018 0.012
UC22G 0.028 0.043 0.014 0.01
UC27C 0.03 0.044 0.016 0.011
UC27D 0.019 0.026 0.01 0.007
UC27E 0.016 0.025 0.009 0.008
UC27F 0.012 0.019 0.007 0.005
UC27G 0.01 0.013 0.006 0.004
UC27H 0.008 0.014 0.004 0.004
UCD27J 0.006 0.009 - -
UCD27K 0.006 0.009 - -
TRANSFORMER CONTROLLED GENERATOR
SECTION RESISTANCES, 3 PHASE WINDINGS
380V 400V 415V 416V 460V
FRAME
SIZE
50Hz 50Hz 50Hz 60Hz 60Hz
UC22C 0.059 0.078 0.082 0.055 0.059
UC22D 0.054 0.056 0.057 0.049 0.054
UC22E 0.041 0.05 0.053 0.038 0.041
UC22F 0.031 0.032 0.033 0.025 0.031
UC22G 0.022 0.026 0.028 0.021 0.022
Measure insulation resistance between sections and each
section to earth.
Unbalanced or incorrect winding resistances and/or low
insulation resistances to earth indicate rewinding of the stator
will be necessary. Refer to removal and replacement of
component assemblies subsection 7.5.3.
7.5.2 EXCITATION CONTROL TEST
7.5.2.1 AVR FUNCTION TEST
All types of AVR's can be tested with this procedure :
1. Remove exciter field leads X & XX (F1 & F2) from the
AVR terminals X & XX (F1 & F2).
2. Connect a 60W 240V household lamp to AVR terminals
X & XX (F1 & F2).