Cirrus SR20 Maintenance Manual page 14

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
G. Alternator 1 (See Figure 24-307)
Alternator 1 is mounted directly to the front, LH side of the engine and is used in all electrical configu-
rations.
Alternator 1 is a 75-amp belt-driven alternator. Alternator 1 has sufficient capacity to power the entire
aircraft electrical load and charge the battery during normal system operation. The alternator and bat-
tery are designed to function in parallel or independently and can be alternately connected to the pri-
mary bus. Alternator 1 will continue to generate electrical power in the case of battery failure or
malfunction. Alternator 1 is self-exciting, not self-starting. The battery must energize Alternator 1
before it can produce electrical power.
H. Alternator 2 (See Figure 24-308)
Serials 1005 thru 1267: The optional dual alternator system adds a 40-amp engine-driven alternator 2
to the single alternator system. An alternator switch, ammeter switch, circuit breaker, function module,
alternator fail lights, and alternator relay is also added in this configuration. Alternator 2 is mounted
directly to the aft engine accessory pad.
On the optional dual alternator system, alternator 1 produces approximately 60% of the system voltage
and alternator 2 produces approximately 40%. If alternator 1 fails during heavy loading, it may be nec-
essary to shed electrical load to prevent over-burdening alternator 2. The alternator fail lights are
located to the immediate right of the annunciator panel. The lights provide warning of a failed or inop-
erative alternator. If an alternator is not providing adequate proportional power, the appropriate alter-
nator fail LED will illuminate. In addition, both lights will illuminate during a battery discharge situation
(approximately a 3-amp draw).
Serials 1268 & subs w/o single alternator SRV: Alternator 2 is a 20-amp engine-driven alternator
mounted directly to the aft engine accessory pad. Alternator 2 supplies electrical power to the essen-
tial bus through a 25-amp circuit breaker located in the MCU.
I. Master Control Unit (MCU)
The MCU is an integrated component which ensures proper electrical output over the entire tempera-
ture and RPM range of the alternator system by regulating and distributing alternator output. The MCU
is mounted to the firewall, directly behind the engine.
(1) Master Control Unit - Serials 1005 thru 1267
The MCU used in the single alternator system contains a landing light relay, ammeter trans-
ducer, alternator control unit (ACU), three 25-amp distribution feeder fuses, buses, and contac-
tors for the alternator, start, master battery, and ground power circuits.
The MCU used in the optional dual alternator system contains a landing light relay, 15-amp land-
ing light circuit breaker, 15-amp auxiliary power circuit breaker, two ACUs, two function modules,
three 25-amp distribution feeder circuit breakers, buses, and contactors for the start, master bat-
tery, and ground power circuits.
Note:
The MCU will trip the alternator circuit breaker(s) whenever there is more than 6-10 amps
reverse current. The pilot has access to the alternator circuit breaker(s), which are located in the
circuit breaker panel, during flight. If the system is generating more than 31.75 ± 0.5 V, the corre-
sponding alternator circuit breaker will open, preventing further power generation by that alterna-
tor. The other alternator will regulate at a higher rate, 29.0 +0.25, -0.5 V during the overvoltage.
After the circuit breaker opens, the other alternator will provide 100% of the load at the nominal
voltage.
The MCU contains a current sensor in the battery bus, this current sensor sends a signal to the
ammeter (battery charge or discharge). The ammeter only indicates a discharge when the elec-
trical draw on the system exceeds the quantity of power generated by system. In the dual alter-
nator system, the ammeter will only indicate a discharge when the ammeter select switch is in
24-30
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15 Dec 2014
(See Figure 24-309)
The MCU on these airplanes is not field repairable and must be replaced if faulty.
EFFECTIVITY:
All
M O D E L S R 2 0