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| Models: D18S and D18C, No. 9 |
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Issued: September 9, 1946
Reprinted: September 25, 1950
Revised: June 1, 1953 |
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REVERSE CURRENT RELAYS AND VOLTAGE REGULATORS
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In order to give better efficiency in the maintenance of the reverse current
relays and voltage regulators, located in the control panel box, this service
letter lists below several hints in the maintenance and care which may eliminate
your troubles on this subject.
1. NEVER ALLOW ANYONE TO ADJUST OR TAMPER WITH THE GENERATOR CONTROL PANELS
EXCEPT QUALIFIED ELECTRICIANS.
The generator control panels are tested and properly adjusted when the airplane
leaves the factory. The adjustments are very critical and must be made under
closely controlled conditions by qualified personnel.
2. DO NOT, AT ANY TIME, ATTEMPT TO RESTORE RESIDUAL MAGNETISM IN THE GENERATOR
FIELD BY MANUALLY CLOSING THE REVERSE CURRENT RELAY CONTACTOR.
This method, while it will restore the residual magnetism, will also severely
damage the contactor contacts and the voltage regulator. This could result in
unexpected failure in flight and early replacement of both the reverse current
relay and the voltage regulator.
If it becomes necessary to "flash the field" to restore residual magnetism, an
outline of the correct and recommended procedure follows:
Remove or disconnect the voltage regulator and attach a test lead to a point in
the generator control box at which battery voltage is available. With the
generator switches in "OFF" position and the engine operating at approximately
1600 RPM, touch the test lead to the "A" terminal on the voltage regulator base.
Remove the test lead, replace the voltage regulator and test voltage build up of
the generator. If after the flashing operation, the generator still fails to
build up voltage, connect a jumper wire between terminals "A" and "B" of the
voltage regulator. A voltage build up now will indicate a defective regulator
that must be replaced.
3. RELAY SETTINGS
To Check the relay setting, connect an accurate volt meter between the generator
post and ground. Slowly raise the generator speed and read the voltage at which
the relay operates the contactor. As shipped from teh factory, this setting is
26.0 and 27.0 volts. Now lower the generator speed and note the reverse current
when the relay opens. This should be 25 to 26 volts.
As an alternate method, instead of varying the generator speed, a variable
resistor of approximately 40 ohms and having fine steps may be temporarily
connected in series with the generator field and used to adjust the voltage for
test. Minor adjustment of the relay setting may be easily made. It should not be
necessary to change the spring adjustment unless the relay has been taken apart
or otherwise tampered with. However, see that the bent end of the spring rests
solidly in the hole in the end of the adjusting screw. If it does not, recheck
the voltage and reverse current setting carefully.
It is suggested that the generator switches be kept in "OFF" position during
idling, taxiing and landing operations because the vibrations present in the
airplane structure while the airplane is in operation are transmitted to the
contacts and induces them to chatter when the generator output drops below 26
volts.
4. NEVER USE THE AIRPLANE VOLTMETER TO SET THE VOLTAGE REGULATORS.
The voltage regulators must be adjusted by using a portable precision D-C
Voltmeter as follows:
a. Connect an accurate portable voltmeter between the "B" and "G" terminals of
the voltage regulator base. Open the generator switch. Run up the engine until
the generator is operating above its minimum rated speed. This would be
approximately 1800 rpm engine speed.
b. Allow the generators to run above minimum rated speed until their respective
regulators are hot. This procedure is necessary to insure that the control boxes
are adjusted for parallel operation at normal operating temperatures.
c. Adjust the voltage regulator to 28 volts as indicated on the portable
voltmeter by turning the rheostat adjusting knob clockwise (to increase voltage)
or counterclockwise (to decrease voltage). In order to simplify the paralleling
adjustment, it is very important that each voltage regulator be adjusted to
exactly the same no load voltage.
5. DO NOT ALTER THE POSITION OF THE PILE OR CORE ADJUSTING SCREWS.
Adjustments to these parts can be made only at a test bench by qualified
personnel and must not be attempted with the unit installed in the airplane.
6. PARALLEL ADJUSTMENT
After the above procedure has been accomplished both generator voltage
regulators must be adjusted for parallel operation.
a. Close both generator switches
b. Switch on a DC load equivalent to the full load rating of one generator
c. Check the generator ammeters. Each generator should take its share of the
load within 5 amperes. If such is not the case, readjust the regulators as
follows:
1. Turn the rheostat adjusting knob on the regulator of the generator which is
carrying the most load, ONE NOTCH ONLY in a counterclockwise direction.
2. Turn the rheostat adjusting knob on the regulator of the generator which is
carrying the least load, ONE NOTCH ONLY in a clockwise direction.
NOTE: A change in the rheostat setting on any one regulator will alter the
amount of load carried by each generator. Therefore, rheostat adjustments should
be made one notch at a time and both generator ammeters should be checked for
proper division of load after each rheostat adjustment.
3. Repeat the adjustments outlined above until each generator assumes its full
share of the load within 5 amperes.
4. Apply as much load as possible up to full-rated load per generator.
5. Check the reading of both generator ammeters. At full load, each generator
should take its share of the load within 5 amperes. If such is not the case,
readjust both regulators using the method described above and continue until the
load divides within the proper limits.
d. After the paralleling has been completed, the voltage should again be
checked. Connect the portable voltmeter between the positive buss bar and
ground.
e. Switch off all load except that which is permanently connected to the buss
bar. Take a reading on the portable voltmeter and if the voltage is not at its
proper value (28 volts), turn the rheostat adjusting knob of both regulators,
ONE NOTCH AT A TIME n the same direction clockwise to increase the voltage,
counterclockwise to decrease the voltage. Continue this procedure until the
proper voltage setting is reached.
f. Again switch on maximum load up to the full load per generator and check both
generator ammeters. If the generators do not divide the load within the proper
limits, repeat the entire procedure until with both regulators adjusted for
proper division of load, the voltage as measured with the portable voltmeter at
the buss bar is at the proper value. |
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