Specifications

6 of 10 P/N 11934-S10
Section 9 Cirrus Design
Supplements SR20
Section 7 - Systems Description
Electrical System
The airplane is equipped with 28-volt direct current (VDC) electrical
system. The system provides uninterrupted power for avionics, flight
instruments, lighting and other electrically operated and controlled
systems during normal operation.
Power Generation
Primary power for the SR20 is supplied by a 28-VDC negative-ground
electrical system. The electrical 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, contactors 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 aircraft electrical system during
normal system operation. The forward, belt-driven alternator is
designated 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 output 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
alternator off line, the associated ALT FAIL light illuminates, and the
other alternator will provide 100% of the electrical power requirements.
Original: 09-28-99