AC Electrics Flashcards
RMS voltage is … times the peak AC voltage
0.707
aircraft ac generators generally have a … field coil producing a magnetic field which induces electricity in the … coils
rotating
stationary
a three phase ac generator has how many stator coils?
3
formula for output frequency of a multi phase ac generator?
(rpm x no. of pole pairs) / 60
describe a three phase star connection arrangement
Each pair of stator coils is connected at one end to a common star, or neutral point and at the other end to the output, through a load and then back to the star connection through a return line
what is phase voltage in a three phase ac star connection?
The voltage between a single output, a live line, and the return
what is line voltage in a three phase ac star connection?
Voltage between two live lines
RMS line voltage is … times phase voltage
1.73
is line current equal to phase current?
yes
capacitance is measured in …
Farads
Capacitance is … proportional to the area of the plates and … proportional to the distance
directly
inversely
value of dielectric constant (K) for fuel is about … compared to … for air
2
1
Max charge across the plates of a capacitor occurs when supply … is zero, just before it reverses. The maximum discharge current occurs when the supply … is zero
current
voltage
does current lead or lag voltage in a capacitive ac circuit?
leads
in a purely capacitive AC circuit, the capacitor has the effect of shifting the current and voltage 90 degrees out of phase; current leads voltage by 90 degrees. However, there will always be some resistance in a circuit so it will never be purely capacitive so all we can say is that current leads voltage in a capacitive circuit
capacitive reactance (resistance) is affected by …
it is directly/inversely proportional to this
frequency
inversely
capacitors act as low resistors at high frequency and high resistors at low frequency
what is Lenz’s law?
Induced current in a coil will create its own magnetic field that opposes the movement
The effect of the back EMF (inductance) is to
resist current flow
The unit of inductance is the
Henry (L)
capacitive and inductive reactance are both measured in …
ohms
Effect of inductance is similar to capacitance except that here current (leads/lags) behind voltage
lags
inductors are frequency sensitive and reactance is (inversely/directly) proportional to frequency
directly
high resistance at high frequencies and low resistance at low frequencies (opposite of capacitors)
The total resistance to current flow in an AC circuit with capacitive and inductive reactance is called
impedance
and is the vector sum of reactance and resistance
What is this symbol?
inductor
for total inductance, inductors are added in the same way as … and capacitors added the opposite way round
(how to remember?)
resistors
(similar circuit symbols)
power in a capacitive or inductive circuit is (higher/lower) than a purely resistive circuit because …
lower
the voltage will be out of phase with the current and therefore power is no longer a product of voltage and current at their peak values, but the product of lesser values
The answer is to balance a capacitor with an inductor to bring the current and voltage back into phase – a resonant circuit where the impedance is totally resistive
is work done by the generator useful or wasted in overcoming reactive loads?
wasted
what is power factor of an ac circuit?
Ratio of useful power to reactive power and is a measure of the efficiency of a circuit
useful work / total work (KW/KVA)
(Since capacitive and inductive reactance are affected by frequency, any change in the frequency produced by an aircraft generator will affect power in the circuits)
AC generators rated to take account of reactive loads and output power rating is given as …
KVA kilvolt hours
KVAR =
reactive power
Ac generator rating KW =
effective/useful power
ac generators will be frequency wild if … and can only be used for systems that rely upon … only and are not affected by changes in …
there is no control of rpm
resistance
frequency
a CSD unit is hydromechanical using its own oil supply
as engine speed or load on an ac generator changes, the csd alters its … to keep the generator running and the same rpm and frequency
output torque
how does a variable speed constant frequency ac generator drive work?
Takes the variable output from a frequency wild generator, rectifies it to DC and then converts it again to AC at constant frequency
real load on an ac generator is adjusted with…
reactive load is adjusted with …
csd torque
field energising current
synchronous ac motor speed is directly related to …
stator input voltage frequency
(if not overloaded)
Under higher loads, the rotor falls behind the rotating magnetic field reducing efficiency, torque then reduces in turn and the motor slows and quickly stalls
synchronous ac motors are used for …
low torque, constant speed applications such as to drive a tachometer
synchronous motor RPM = (formula)
(frequency x 60) / no. of pole pairs
(rearrangement of generator frequency formula)
synchronous ac motors use … as the rotor
a permanent magnet
how does an ac induction motor work?
First off, there is no electrical input to the rotor coils
The changing electromagnetic field produced by the stator coils induces a current in the rotor coils, which in turn produces its own magnetic field
The magnetic field of the rotor is attracted to the rotating magnetic field of the stators, so the rotor ends up chasing the stator field
As rotor speed builds up toward the speed of the stator field, the relative movements of the fields reduce and thus the induced EMF and ultimately torque that makes the rotor turn reduces, meaning that the rotor can never quite catch up with the stator
The difference in speed between the stator field and the rotor is called the slip speed and this will increase as torque load on the rotor increases
Delta connections used with?
differ from star connections in that the line voltage is … phase voltage, and line current is … phase current
How to remember this?
ac motors
the same
greater
remembered by VDV (V delta V) voltages equal in delta connection
Vital services are always … powered from the …
DC
Hot battery bus
In a split bus ac system, the bus tie breakers are normally …
open, because there is no control to keep the generators frequency, phase and output exactly the same (they cannot be paralleled) and therefore cannot be connected to the same bus
The BTB will close if one generator fails to allow the failed busbar to be powered
In a paralleled bus ac system, the bus tie breakers are normally …
closed, because the generators are paralleled
