Voltage Balance between Phases
The voltages between the three phases, and each phase to neutral, should be balanced, and if an unbalance is shown
on any phase of more than 1%, this indicates that a fault exists in the main stator windings. This test should be carried
out with all of the customer's external connections removed to eliminate the possibility of external shorts between the
machine and the main isolator. Further tests can be made on the resistance values of the stator windings
(see Winding Resistance Chart, at the end of this section).
Voltage Balanced but Reading Low when Separately Excited
This indicates that a fault exists in either the main rotating rectifier assembly, or one of the excitation windings i.e. the
main rotor and/or the exciter stator and rotor. First check that the d.c. separate excitation supply is not lower than
12 volts, and that the speed is correct
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.
Replacement of Faulty Diodes (See Fig. 11).
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 based diodes, and the positive plate carries
positive based 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 2.03 - 4.74 Nm.
Surge Suppressor
This is a protection device which prevents high transient voltages from damaging the main rectifier diodes. All
machines are fitted with a single surge suppressor connected across the +ve & -ve rectifier plates. The resistance of
this device varies considerably, depending upon the meter used to check it. As a general guide, the forward resistance
should be in excess of 500 ohms. The reverse resistance should be considerably higher i.e. above 10 k ohms. A faulty
surge suppressor will be either open circuit or short circuit in both directions.
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 and main rotor resistances, can be obtained from the connections to the main rectifier assembly 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.
A.V.R. Supply Leads from Main Stator
The final test which can be made in the separately excited condition is to ensure that the supply from the main stator to
the A.V.R. is correct. The voltages at the A.V.R. leads are derived from the main stator winding and the levels can be
determined by checking the connections and referring to the wiring diagram.
SECTION B THE ELECTRONIC CONTROL SYSTEM
A.V.R.
Should the previous tests prove successful, any faults can now be assumed to be within the voltage control system and
its respective wiring. This system can be affected by bad connections, therefore, the wiring between the auxiliary
terminals and the A.V.R. push-on terminals should be carefully examined for broken, loose or corroded connections.
The sensing circuit leads have been covered in a previous section.
Accessories
Forming an integral part of the electronic circuitry there may be fitted one or more electrical accessory. These items
must also be checked for loose, broken or corroded connections, and their resistance values checked. See Section 6 -
Accessories.
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