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PIPER AIRCRAFT
PA-42-720 (Advanced Trainer)
MAINTENANCE MANUAL
A. DC Generation (continued)
11. Description and Operation of Generator Control Unit
The generator control unit is designed to incorporate all the normal features of a direct current 28-
volt control system in one compact unit having three printed circuit boards. By using a comparator
amplifier with a regulated reference voltage, any difference between the reference voltage and the
generator output, as seen by the sensing line, is amplified and supplied to the comparator circuit,
which controls the shunt field excitation of the generator.
The paralleling portion of the generator control unit (one on each engine) incorporates the
advantages provided by integrated circuitry. The difference between the interpole voltage of the
local generator and interpole of the other generator is amplified, inverted, and filtered. Any
resultant difference is then coupled to the summing junction of the voltage regulating circuit. The
equalizing circuit continuously sums the difference between the interpole voltages of both
generators to zero.
The line contactor and reverse current control circuitry consists of two integrated circuits and
differential voltage sensing operations. When the generator has attained an output voltage equal
to the tie bus voltage, these circuits allow the line contactor to close. They also monitor the
difference in voltage across the interpole terminals of each generator. When this potential reaches
a positive voltage of 0.2 volts or more, the integrated circuits, after a time delay inversely
proportional to the magnitude of the voltage, cause the line contactor to open. When this happens,
the circuit automatically shifts back to differential voltage sensing.
12. Removal of Generator Control Unit
Location of generator control unit is on the aft engine nacelle electrical accessory panel, aft of
engine firewall at station 108.50 on respective engine.
(a) Remove the aft upper engine panel for access to generator control unit.
(b) Disconnect electrical plug from control unit.
(c) Remove mounting screws and lift unit from airplane.
13. Installation of Generator Control Units
(a) Position generator control unit on aft engine nacelle panel and secure with appropriate screws.
(b) Reconnect electrical plug.
(c) Replace aft engine access panel and secure with appropriate screws.
NOTE: The replacement of generator control units require a functional check for proper
operation (refer to Checking Generator Control Unit paragraph).
14. Checking Generator Control Unit
(a) Start engines and set to a gas generator rpm of 60%. Turn on all electrical equipment available
in aircraft; system voltage must remain at 28 + 1 volts.
NOTE: Voltage adjustment screw is located above connector of generator control unit,
between the two test jacks.
(b) Turn the left starter generator switch to the OFF position. This will transfer the full electrical
load to the right generator and generator control unit. System voltage must remain at 28 ± 1
volts.
EFFECTIVITY:
040-999
Jan
10/9024-30-00
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