Voltage Regulator
Models: D18S and D18C, No. 9 Issued: September 9, 1946
Reprinted: September 25, 1950
Revised: June 1, 1953
REVERSE CURRENT RELAYS AND VOLTAGE REGULATORS
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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