Systems 2 AC circuits
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What is the typical AC Voltage and Frequency on large aircraft?
Formula for calculating Frequency output of alternators with more than 2 poles
What is the value of RMS
RMS = peak value x sin 45o
RMS = peak value x 0.707
How does current wave compare to voltage wave
Current LAGS voltage by 90o
How is the voltage output of an AC generator controlled?
By varying the magnitude of the current flowing to the field windings
What is a CSDS and what does it do?
Constant Speed Drive System
Hydro mechanical differential transmission converting variable input (engine speed) to constant output (Gen) speed.
Variable delivery hydraulic pump
What is an IDG and what does it do
Integrated Drive Generator
- CSDS and Gen in 1.
- Saves on size and weight
What is the main difference between IDG and separate CSD and Gen systems
Instead of being Ram air cooled, the IDG is cooled by oil spray from CSD which is then cooled in a heat exchanger.
Reasons for, actions after, and consequences of unacceptable fluctuations in AC supply freq from CSD
Low Oil Pressure or High Oil Temp
Can be manually disconnected from flight deck (provided engine is running in normal operating range)
Cannot be re-instated in the air, only my engos on the ground
What is a VSCF
Variable Speed Constant Frequency.
- AC gen connected directly to engine
- Produces Freq Wild AC
- Then rectified to DC and then fed to a static inverter to give 3 phase AC at 400hz
How does a RAT work and at what RPM?
blades drive Hydraulic pump
pump drives hydraulic motor
motor drives a Gen at 12,000 RPM
produces 3 phase to power essential items and try to keep batteries charged
What are the advantages of solid state inverters over older rotary ones?
No moving parts so reliable
Very little heat generated so low cooling needed
No arcing at low air pressures like a rotary.
how is AC fed to essential consumers if the GEN fails
Battery DC is fed to a static converter to produce AC
When is a diode rectifier unsuitable? and then what is used?
At very high currents diodes break down
Rotary rectifier - (AC motor driving a DC gen) "an inverter in reverse"
Transformers convert what?
AC at constant frequency
Steps Voltage up or down, depending on number of coils of input and output winding's.
What are the 2 classes of Transformers
Voltage (power) transformers
What is a TRU and it's function?
Transformer Rectifier Unit
For converting AC and high voltage to DC at a lower voltage.
3 phase AC is stepped down and then rectified
Advantages of AC power
AC Gens are lighter and simpler
Lower transmission loss at high voltages and smaller diameter wire saves weight.
Voltage can be stepped up or down easily with a transformer at almost no loss.
DC voltage can be easily available with TRUs
With no Commutator, AC Gens don't arc at high voltages and low pressures.
disadvantages of AC
Relies on external excitation - Unless a complex internal excitation mechanism is used
Induced voltages cause by AC can lead to stray currents, radio interference and extra gen loads.
AC motors have similar advantages to AC Gens, although DC motors have higher starting torques.
What would you like to do?
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