TM 55-1520-240-23-7
9-30
ALTERNATING CURRENT (AC) POWER DISTRIBUTION
(Continued)
9-30
THEORY OF OPERATION
The four sources of ac power for the helicopter have the
following order of operation:
NO BREAK - WORK HARDER
When one or both engines are running, the No. 1 and
No. 2 generators supply power to their respective buses.
If either generator fails or is shut down, that source will
be isolated from its bus and the remaining generator
will then supply both buses through crosstie circuitry.
With either or both of the main generators operating, the
APU generator and the external power receptacle are
disconnected from the buses and can supply no power.
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If the APU is operating while both engines are shut down
or both generators are manually switched off. The APU
generator is automatically connected to both ac buses.
The external power receptacle remains disconnected
from the buses.
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Only if both main generators and the APU generator are
not operating is it possible to apply external power to the
buses. If, while an external power source is connected,
any of the three generators is turned on, the external
power source will be disconnected at the buses.
NO BREAK - WORK HARDER
In all cases, the appropriate power source is connected
to the buses by power contactors, operated by 28 vdc.
MAIN AC GENERATOR AND CONTROL UNIT
Each of the generators consists of three units: a
permanent magnet generator (PMG), a main exciter and
rotor assembly, and the main generator stator windings.
NO BREAK - WORK HARDER
The output of the PMG is fed to a generator control unit
(GCU) in one of the electrical equipment compartments.
Sensing circuits in the GCU average the three phases
of generator output voltage and apply this signal to
an integral voltage regulator. The voltage regulator
uses this signal to regulate the current applied to the
generator exciter and rotor section, keeping output
voltage between 113 and 117 volts.
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If generator output voltage drops below 102 volts
for more than five to seven seconds, or exceeds
122 to 125 volts for a period of time depending on
the magnitude of the overvoltage, relays within the
GCU will shut down the generator and de-energize
generator contactor 241 K1 in the PDP, disconnecting
the generator from the bus.
NO BREAK - WORK HARDER
Similarly, if generator frequency drops below 330 Hz for
three to seven seconds, a relay within the GCU will
disconnect the generator from the system. However, the
generator will continue to operate, and if frequency rises
above 335 Hz, it will automatically be reconnected to
the system.
NO BREAK - WORK HARDER
If generator frequency drops below 320 Hz, the
generator is disconnected from the bus and is also shut
down. To get the generator back on line after it has been
shut down, the generator control switch on the overhead
ELECT panel in the cockpit must be cycled from ON to
OFF/RESET to ON.
APU GENERATOR AND CONTROL UNIT
The APU generator system is similar to that of each
of the two main generators. The generator supplies
power to the No. 1 and No. 2 ac buses when the main
generators are not running. Like the main generators, it
consists of three units: a permanent magnet generator
(PMG), an exciter generator, and a main generator
section.
NO BREAK - WORK HARDER
The PMG is used to monitor generator frequency for
input to the generator control unit. It supplies ac and dc
power as soon as the generator begins turning. The
ac terminals (P1-PN) provide a minimum of 20 volts
at no load. The dc terminals (+ -) provide between 29
and 40 volts.
NO BREAK - WORK HARDER
The exciter generator provides main generator section
excitement and regulation. Terminals (F1 -A-) of the
exciter section supply 3 to 14 volts during operation.
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The main generator section supplies 115/200 volt, 400
Hz ac output to the APU contactor in the No. 2 power
distribution panel. Cross-tie to the No. 1 PDP is through
external power control relay 241K5 in the No. 1 PDP.
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