Electrical Flashcards
System voltage & battery
28-VDC, dc power provided by one 34 amp-hr, air cooled, 20 cell, nickel cadmium battery & two 250 amp starter/generators connected in parallel
What bus is the battery always connected to?
Hot battery bus
How does the battery provide power to the whole system?
Battery switch closes a bus tie and a battery relay
The airplane utilizes a ________ system
Multi-Bus
How many buses are there? What are they?
5
L/R generator Bus, Center Bus, Triple-Fed Bus, and battery emergency bus
What is the triple fed bus powered from?
Battery and both generator buses
What is the center bus fed from?
Two generator buses and the battery, connecting both components when the bus ties are closed
What is provided to isolate a fault in the system?
High current (Hall Effect) sensors, bus tie relays, and current limiters
The DC power distribution is commonly called a _________ system
Triple-fed
What are the 3 DC power sources available?
One 24 volt, 34 amp-hr, nickel cadmium battery
Two 28 volt, 250 amp starter/generators
What happens when the battery switch is turned on?
Bus tie relays close
How is the battery power routed?
Battery relay to the triple fed bus, and through the battery bus tie relay to the center bus and to both starter relays
What 3 buses are tied together by the center bus?
Generator bus, hot battery bus, and the battery
How is the electrical system protected from excessively high current flow?
The bus tie system
With no power applied to the electrical system, how many bus tie relays are left open?
All 3
Why is the coil circuit of the bus tie relay latched open?
To prevent the bus tie relay from closing
When the bus tie relay has been opened by excessively high current flow through the Hall Effect sensor, how can it be reset?
Momentarily activating the BUS SENSE switch to RESET
Momentarily activating the BUS SENSE switch to TEST connects bus voltage to ________ current sensor test circuits.
All 3
Manually closing the generator bus tie relays will connect the generator buses to what bus?
The center bus, providing power to the whole system
Can a generator bus tie relay be manually closed if a fault opened the tie?
No, the BUS SENSE switch must be activated to RESET to reset the tie.
What is bypassed during a cross-generator engine start?
The high current sensors and current limiters
What happens if an over current situation causes a current limiter to open?
It causes bus isolation
What is the protection for the triple-fed bus?
60-amp current limiters
When will triple-fed bus isolation occur?
If all 3 of the limiters are opened
Why is load shedding beneficial?
Electrical system automatically will remove excess loads
What happens, during load shedding, when both generators are offline?
The generator bus ties open and the bus loads are shed
What is the best choice to make if load shedding occurs during flight?
To land as soon as practicable, unless it can be remedied, and one generator be brought back online.
Is the lead acid battery air cooled?
No
The starters are dual purpose, ________ units.
Engine driven
A series starter winding is used during _________ operation and a shunt field winding is used during ________ operation.
Starter, generator
What is the regulated output of the generator?
28.25 +- 0.25 volts with a maximum continuous load of 250 amps.
During an engine start, the battery is connected to the _________ by the starter relay.
Starter/generator
Normally one engine is started on ______ power, and the other is ______ started.
Battery, cross
During a cross generator start, the generator control panel closes what relay?
Cross-start, which allows for a bypass of the generator bus, current limiter, and bus tie relay.
When a starter is selected, what happens to the bus tie sensors?
They are disabled to prevent them from opening their respective bus tie relays.
Is the generator system self exciting or require external power for operation?
Self-exciting
How many generator control units is the generator operation controlled through?
2
What are some functions of the Generator Control Unit (GCU)?
Voltage regulation, over voltage protection, paralleling/load sharing, reverse-current protection, cross-start relay activation
What happens if the generator output voltage is within 0.5 volts of the aircraft bus voltage?
GCU sends a signal to the line contractor, which will close and connect the generator to the aircraft bus and will close both generator ties to connect the center bus and the generator buses.
During single-generator operation, what does the GCU do to the line contractor?
Opens it and isolates the inoperative generator from its bus.
What provides over voltage protection to prevent excessive generator voltage from being applied to aircraft equipment?
The GCU
What is the max voltage of the generator output before being detected by the over-excitation circuits?
32 volts
What happens during an over-excitation from a generator?
The GCU over-excitation circuit will disconnect generator from electrical system.
What is paralleling/load sharing?
Averages the output of the both generators to equalize the loads
When does paralleling of the GCU become operative?
When both generators are online
What percentage are the GCUs designed to balance loads to?
2.5%
What is reverse current protection provided by?
Ex: When shutting an engine down, enough power isn’t provided so it begins to pull from elsewhere.
The GCU.
During cross-starts, what sensors are disabled to prevent bus isolation?
The Hall Effect sensors
How is AC power provided to avionics and engine instruments?
By 2 inverters
What are the 2 levels of power that inverters can provide?
115 volts, 400Hz for avionics and 26 volts, 400Hz for the engine instruments, and some avionics
What is the standard rated output of an inverter?
250 volt-amps (optional: 300 amps)
Where are the inverters located?
The wing, outboard of each nacelle
What is the purpose of an inverter?
To change AC power to DC power
What are indications of an inverted failure?
Getting INVERTER annunciator, erratic behavior, or loss of AC powered instruments (torque gage), or AC avionics
Switches that have a white ring around them receive power from which buses?
The center bus and the triple-fed bus
