Cirrus SR20 Maintenance Manual page 2

C I R R U S
A I R P L A N E M A I N T E N A N C E M A N U A L
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.
D. DC Generation - Serials 2016 & subs w/ Perspective Avionics
During normal operation, the alternators feed their respective distribution bus independently (ALT 1
feeds Main Distribution Bus 1 and ALT 2 feeds Main Distribution Bus 2). The main distribution buses
are interconnected by an 80-amp (Serials 2016 thru 2240) or 60-amp (Serials 2241 & subs) fuse and
diode. The diode prevents ALT 2 from feeding Main Distribution Bus 1. Additionally, since ALT 2 voltage
is slightly higher than ALT 1 voltage, bus voltage is further assured.
The alternators are three-phase AC generator type; each alternator has an internal rectifier. Each
alternator uses two diodes for each phase to rectify the output. In addition, these diodes will block
reverse current. If a single output diode were to fail, the winding junction will be held to the output
potential, reducing the quantity of current the alternator can produce.
The alternators are regulated by the MCU and power the MCU via 6 AWG tin plated copper wires.
Alternator 1 is protected by an 100-amp fuse within the MCU while alternator 2 is protected by a 80-
amp fuse within the MCU. ALT 1 is connected to the Main Distribution Bus 1 and ALT 2 is connected to
the Main Distribution Bus 2. Each alternator system has its own 5-amp alternator circuit breaker
located in the circuit breaker panel. Each alternator is individually protected against overvoltage gener-
ation by the field control module, located within the MCU. ALT 1 is regulated to 28 volts and ALT 2 is
regulated to 28.75 volts.
Voltage output of each alternator is a function of engine RPM, alternator design, and load on the alter-
nator. During low RPM operation, the alternator will require higher engine RPM to provide the same
voltage to increased electrical loads.
The drop off RPM for each alternator will change whenever the electrical system loads are altered from
nominal. Any time an alternator drops off line, a Crew Alerting System (CAS) message will appear on
the MFD. If an alternator drops off line due to low engine RPM, the alternator can be restarted by sim-
ply increasing engine RPM. When alternator 2 drops off line, alternator 1 will power the Essential Bus
through the Main Distribution Bus 1 in the MCU. When alternator 1 drops off line, alternator 2 will
power the Essential Bus through the Main Distribution Bus 2 in the MCU. For additional information
concerning power distribution, see Electrical Load Distribution.
E. DC Generation - Serials 1337 & subs w/ Single Alternator SRV
Primary power for the airplane is supplied by a 28-VDC, negative-ground electrical system. The electri-
cal power generation system consists of an alternator controlled by a Master Control Unit (MCU)
mounted on the left side of the firewall and two batteries for starting and electrical power storage.
24-30
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15 Dec 2014
(See Figure 24-306)
(Refer to 24-50)
(See Figure 24-304)
EFFECTIVITY:
All
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