Cirrus SR20 Maintenance Manual

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
1. DESCRIPTION
This section covers the systems to generate, regulate, control, and indicate DC electrical power. The DC
generation portion of the system includes the batteries, alternators (with internal rectifiers), and regulator.
A. DC Generation - Serials 1005 thru 1267 w/ Single Alternator
Primary power for the airplane is supplied by a 28-VDC negative-ground electrical system. The electri-
cal power generation system consists of a 24-volt, 10-amp-hour battery, a 75-ampere alternator, a volt-
age regulator and an over-voltage protection system. The battery is an aviation grade, 12-cell lead-
acid type with non-spill vent caps. The battery is used for engine starting and as an emergency power
source in the event of alternator failure. The 75-ampere alternator provides constant charging current
for the battery and primary power to the aircraft electrical system during normal system operation. The
voltage regulator provides transient suppression and constant voltage regulation of the unfiltered alter-
nator power. To protect sensitive instruments, the over-voltage protection system monitors the primary
power bus and automatically limits the peak voltage to 28.5 volts.
B. DC Generation - Serials 1005 thru 1267 w/ Dual Alternators
Primary power for the airplane is supplied by a 28-VDC negative-ground electrical system. The electri-
cal power generation system consists of a 24-volt, 10-amp-hour battery, two alternators, and a master
Control Unit (MCU). The MCU contains an Alternator Control Unit (ACU) for each alternator, contac-
tors for starter, battery, and ground power, a landing light relay, circuit protection for the circuit breaker
panel buses, and modules for other protection and annunciation functions. The battery is an aviation
grade, 12-cell lead-acid type with non-spill vent caps. The battery is used for engine starting and as an
emergency power source in the event of alternator failure.
Two rectified alternators provide constant charging current for the battery and primary power to the air-
craft electrical system during normal system operation. The forward, belt-driven alternator is desig-
nated ALT 1. The aft, engine-driven alternator is designated ALT 2. Although each alternator produces
the same amount of power at a given rotational speed, ALT 1 rotates faster and is rated at 75 amperes
while ALT 2 is rated at 40 amperes. Paralleling circuits in the function modules balance alternator out-
put so that, under normal operating conditions, ALT 1 provides 60% of the electrical power and ALT 2
provides the remaining 40%.
Each alternator's ACU provides transient suppression and constant voltage regulation of the alternator
output. To protect sensitive instruments, over-voltage circuits monitor each alternator's output and
automatically limit peak voltage to 28.5 volts. In the event an over-voltage or an overload condition
occurs, the associated ACU automatically opens the affected alternator's circuit breaker. With the alter-
nator off line, the associated ALT FAIL light illuminates, and the other alternator will provide 100% of
the electrical power requirements.
Should both alternators fail, the battery will supply system current and a discharge rate will be indi-
cated on the ammeter. Under these conditions, depending on electrical system load, the LOW VOLTS
warning light will illuminate when system voltage drops below approximately 24.5 volts.
C. DC Generation - Serials 1268 thru 2015, 2016 & subs w/o Perspective Avionics
303)
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-
EFFECTIVITY:
All
DC GENERATION
M O D E L S R 2 0
(See Figure 24-301)
(See Figure 24-302)
(See Figure 24-
24-30
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15 Jun 2010