AIRCRAFT SYSTEMS ELECTRICAL SA227‐ SERIES/FCOM/VTM 6.24.1 ELECTRICAL SYSTEMS The aircraft is equipped with a DC and an AC power system. The DC power distribution system is a segmented three bus system consisting of two essential buses and one nonessential bus. Each bus may be selectively disabled and is over voltage and overload protected. Redundant circuitry is provided to ensure the operation of all essential and emergency electrical and electronic systems. The 115 VAC and 26 VAC buses provide power for AC equipment. Either of two AC inverters powers those buses. DC POWER DISTRIBUTION As shown in Figure 6.24‐1, the battery bus forms the central distribution point for power. Each battery is connected, through a battery relay, to the battery bus relay and then to the battery bus itself. A 150 amp circuit breaker supplies power to the nonessential bus. Power to each bus is controlled by a bus tie switch. Each generator supplies power to the battery bus through a 325 amp circuit limiter. Power supplied to each bus is distributed to the various circuit breakers by smaller bus bars. SIMPLIFIED DC DISTRIBUTION SCHEMATIC
Figure 6.24‐1
Revision: Original Aug 1,2013
GO/FCOM/SA227‐SERIES/VTM
AIRCRAFT SYSTEMS ELECTRICAL SA227‐ SERIES/FCOM/VTM 6.24.2 BUS TRANSFER Nine essential items (normally switched to the left essential bus) may be transferred from one essential bus to the other. The transferrable items are: 1. Turn and slip indicator (pilot’s) 2. Fuel crossflow. 3. Windshield heat (pilot’s) 4. Landing gear control 5. Landing gear position 6. Surface deice boots 7. Cabin pressure dump 8. L intake heat 9. R intake heat Figure 6.24‐2 In some avionics installations, a tenth transfer switch may be installed to allow transfer of power to selected pilot’s avionic equipment (usually the pilot’s DC attitude indicator). DC POWER SOURCES BATTERIES Two 24 volt, sealed lead‐acid batteries are installed in the aircraft. They are located, one in each wing, inboard of the nacelles. Each battery is rated at 26 ampere hours (Ah) capacity at (1) hour rate. The original nickel cadmium batteries have been replaced by the lead acid batteries; the temperature monitoring system has been deactivated. The BATT TEMP circuit breaker is pulled and collared. The Battery Temperature Indicator is removed and/or placarded.
Figure 6.24‐3
GENERATORS The primary power for the aircraft DC system is supplied by two engine‐driven starter‐generators. The generators are rated at 200 amperes (300 amps, later aircraft) continuous operation. The generators will deliver power to the DC system at all engine speeds at and above ground idle.
Revision: Original Aug 1,2013
GO/FCOM/SA227‐SERIES/VTM
AIRCRAFT SYSTEMS ELECTRICAL SA227‐ SERIES/FCOM/VTM 6.24.3 EXTERNAL POWER SUPPLY A ground power unit receptacle normally is located on the outboard side of the right hand engine nacelle. The receptacle location may differ, depending upon customer option. This receptacle may be used to supply auxiliary DC power during routine servicing and engine starts. Power from the ground power unit is connected directly to the aircraft bus system. Auxiliary power is not connected to the aircraft bus system until either battery switch is turned on.
Figure 6.24‐4
Revision: Original Aug 1,2013
GO/FCOM/SA227‐SERIES/VTM
AIRCRAFT SYSTEMS ELECTRICAL SA227‐ SERIES/FCOM/VTM 6.24.4 AC POWER DISTRIBUTION Either of two DC powered 350 volt‐amp static inverters provides AC power for the aircraft. Inverter operation is pilot selectable. In case of inverter failure, the pilot must select the other inverter. OPERATION The two inverters are controlled by a switch installed on the copilot’s switch panel. Power for No. 1 inverter comes from the left essential bus and power for No. 2 inverter comes from the right essential bus. When an inverter is selected, power from the one amp inverter control circuit breaker is used to close the inverter power relay. The inverter relay powers the respective inverter and the AC switching relay. The inverter output is both 115 volts and 26 volts and is fed to the AC buses. The AC distribution system is shown in Figure 6.24‐5. Bus tie circuit breakers between both the 26 volt and the 115 volt buses allow AC power to flow to all buses. NOTE The inverter switch is designed to permit operation of only one inverter at a time.
SIMPLIFIED AC DISTRIBUTION SCHEMATIC
Figure 6.24‐5 WARNING AND MONITORING SYSTEMS
Revision: Original Aug 1,2013
GO/FCOM/SA227‐SERIES/VTM
AIRCRAFT SYSTEMS ELECTRICAL SA227‐ SERIES/FCOM/VTM 6.24.5 The pilot is informed of the condition and operational status of the various sources of electrical power.
CABIN DOOR
L BETA
L BETA
LOW SUCTION
BATTERY FAULT
L CHIP DET
R CHIP DET
CABIN ALTITUDE
SAS FAULT
L XFER PUMP
R XFER PUMP
GPU PLUG IN
CARGO DOOR
L BAT DISC
R BAT DISC
L SRL OFF
--------------
L AC BUS
R AC BUS
R SRL OFF
GEAR DOOR POSITION
L GEN FAIL
R GEN FAIL
NOSE STEER FAIL
BATTERY DISCONNECT LIGHT An amber battery disconnect light, when illuminated, indicates the respective battery relay has opened and disconnected the battery from the bus system. The circuit consists of contacts inside the battery relay which are shorted to ground when the relay is open. The disconnect light function is turned off during the engine start cycle. BATTERY FAULT LIGHT The red battery fault light illuminates when the fault protection panel senses a battery fault and disconnects the batteries. The circuit consists of a relay inside the fault control panel which provides power to illuminate the fault light. GENERATOR FAIL LIGHT An amber generator fail light, when illuminated, indicates the respective generator relay has opened, disconnecting the generator from the bus system. Contacts inside the generator relay are shorted to ground when the relay is open. Circuitry inside the annunciator panel to power the light also serves as a press‐to‐test function. AC VOLTMETER AND BUS FAIL LIGHTS A bus selectable voltmeter and two amber bus failure caution lights comprise the AC warning and monitoring systems. The AC voltmeter is powered from the 115 volt bus system. A selector switch allows either bus to be monitored. Each 115 volt bus also powers a bus failure relay. When voltage is present at the bus, the relay is energized and breaks the path for power to the AC bus caution light. If power is lost to the bus, the relay relaxes and the light illuminates, indicating a loss of bus power. Illumination of one AC caution light is normally an indication of an AC bus tie circuit breaker failure. Illumination of both caution lights is normally an indication of an AC power source failure. GENERATOR AMMETER Two ammeters are installed in the left side console to indicate the respective generator’s output. Each meter is powered by a shunt installed in the negative side of the respective generator.
Revision: Original Aug 1,2013
GO/FCOM/SA227‐SERIES/VTM
AIRCRAFT SYSTEMS ELECTRICAL SA227‐ SERIES/FCOM/VTM 6.24.6 VOLTMETER AND SELECTOR SWITCH A voltmeter and selector switch are used to monitor any one of six sources listed below. Each source providing a signal to the voltmeter contains a circuit breaker for protection. The functions of each switch position are as follows: 1. L or R battery position: Monitors the battery voltage of the battery side of the battery relay. In order to obtain an accurate reading, the battery switch should be moved to OFF to isolate the battery from the bus. If a reading is taken with the battery switch ON, the indicated battery voltage will be approximately equal to bus voltage. 2. L or R GEN position: Monitors the generator voltage of the generator side of the generator relay. In order to obtain an accurate reading, the generator switch should be moved to the OFF position, isolating the generator from the bus. If a reading is taken with the generator switch ON, the indicated generator voltage will be approximately equal to bus voltage. 3. BUS position: Monitors the voltage at the battery bus in the electrical panel behind the pilot’s seat. This voltage will be the average of the voltages applied to the bus. For example, with one battery switch on and no generators operating, the bus and battery voltage will be approximately equal. 4. GPU position: Monitors the voltage output of a GPU connected to the aircraft. Voltage upstream of the battery bus relay is monitored. After the battery switch is moved to ON, this voltage will be approximately equal to bus voltage. NOTE To avoid battery drain after shutdown, the voltmeter selector should be placed in the DC bus position.
Figure 6.24‐6
Revision: Original Aug 1,2013
GO/FCOM/SA227‐SERIES/VTM
Figure 6.24‐7