CIRRUS AIRPLANE MAINTENANCE MANUAL MODEL SR20
24-30
Page 3
Serials 1337 & subs w/ Dual Alternator SRV
EFFECTIVITY:
Alternator 1 (ALT 1) is a belt-driven, internally rectified, 75-amp alternator regulated to 28 volts, and
mounted on the RH front of the engine. Alternator power is routed into the MCU, where it is distributed
to the Main Distribution Bus and the Essential Distribution Bus. The alternator is regulated by a voltage
regulator within the MCU, and can be disconnected from the distribution bus system by the ALT 1
switch located on the pilot's bolster switch panel. The alternator is self-exciting (not self-starting) and
requires battery voltage for field excitation in order to start up - for this reason, the batteries should not
be turned off in flight.
Battery 1 (BAT 1) is an aviation grade 12-cell, lead-acid, 24-volt, 10-amp-hour battery mounted on the
right firewall. BAT 1 is charged from the main distribution bus in the MCU. Battery 2 (BAT 2) is com-
posed of two 12-volt, 7-amp-hour, sealed, lead-acid batteries connected in series to provide 24 volts.
Both BAT 2 units are located in a vented, acid-resistant container mounted behind the aft cabin bulk-
head (FS 222) below the parachute canister. BAT 2 is charged from the circuit breaker panel Essential
bus.
The Master Control Unit (MCU) is located on the left firewall. The MCU controls ALT 1, starter, landing
light, external power, and power generation functions. In addition to ALT 1 voltage regulation, the MCU
also provides external power reverse polarity protection, alternator overvoltage protection, as well as
alternator fail and over-current annunciations. Alternator power is distributed to the airplane circuit
panel buses through main and essential buses in the MCU. The distribution buses are interconnected
by two 50-amp fuses and a diode. The diode prevents BAT 2 from feeding the main distribution bus.
F. DC Generation - Serials 1337 & subs w/ Dual Alternator SRV (See Figure 24-305)
Primary power for the airplane is supplied by a 28-VDC, negative-ground electrical system. The electri-
cal power generation system consists of two alternators controlled by a Master Control Unit (MCU)
mounted on the left side of the firewall and two batteries for starting and electrical power storage.
Alternator 1 (ALT 1) is a belt-driven, internally rectified, 75-amp alternator mounted on the RH front of
the engine. Alternator 2 (ALT 2) is a gear-driven, internally rectified, 20-amp alternator mounted on the
accessory drive at the rear of the engine. ALT 1 is regulated to 28 volts and ALT 2 is regulated to 28.75
volts. The output from ALT 1 is connected to the main distribution bus in the MCU through an 80-amp
fuse. The output from ALT 2 is connected to the essential distribution bus in the MCU through a 40-
amp fuse. Both alternators are self-exciting (not self-starting) and require battery voltage for field exci-
tation in order to start up - for this reason, the batteries should not be turned off in flight.
Battery 1 (BAT 1) is an aviation grade 12-cell, lead-acid, 24-volt, 10-amp-hour battery mounted on the
right firewall. BAT 1 is charged from the main distribution bus in the MCU. Battery 2 (BAT 2) is com-
posed of two 12-volt, 7-amp-hour, sealed, lead-acid batteries connected in series to provide 24 volts.
Both BAT 2 units are located in a vented, acid-resistant container mounted behind the aft cabin bulk-
head (FS 222) below the parachute canister. BAT 2 is charged from the circuit breaker panel Essential
bus.
The Master Control Unit (MCU) is located on the left firewall. The MCU controls ALT 1, ALT 2, starter,
landing light, external power, and power generation functions. In addition to ALT 1 and ALT 2 voltage
regulation, the MCU also provides external power reverse polarity protection, alternator overvoltage
protection, as well as alternator fail and over-current annunciations. Power is distributed to the airplane
circuit panel buses through main and essential buses in the MCU. During normal operation, the alter-
nators feed their respective distribution bus independently (ALT 1 feeds main distribution bus and ALT
2 feeds the essential distribution bus). The distribution buses are interconnected by two 50-amp fuses
and a diode. The diode prevents ALT 2 from feeding the main distribution bus. Additionally, since ALT2
/ essential distribution bus voltage is slightly higher than ALT 1 / main distribution bus voltage, ALT 1
will not feed the essential distribution bus unless ALT 2 fails.
15 Jun 2010
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