AC Electrics

Question 1

RMS voltage is … times the peak AC voltage

Answer 1

0.707

Question 2

aircraft ac generators generally have a … field coil producing a magnetic field which induces electricity in the … coils

Answer 2

rotating

stationary

Question 3

a three phase ac generator has how many stator coils?

Answer 3

3

Question 4

formula for output frequency of a multi phase ac generator?

Answer 4

(rpm x no. of pole pairs) / 60

Question 5

describe a three phase star connection arrangement

Answer 5

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

Question 6

what is phase voltage in a three phase ac star connection?

Answer 6

The voltage between a single output, a live line, and the return

Question 7

what is line voltage in a three phase ac star connection?

Answer 7

Voltage between two live lines

Question 8

RMS line voltage is … times phase voltage

Answer 8

1.73

Question 9

is line current equal to phase current?

Answer 9

yes

Question 10

capacitance is measured in …

Answer 10

Farads

Question 11

Capacitance is … proportional to the area of the plates and … proportional to the distance

Answer 11

directly

inversely

Question 12

value of dielectric constant (K) for fuel is about … compared to … for air

Answer 12

2

1

Question 13

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

Answer 13

current

voltage

Question 14

does current lead or lag voltage in a capacitive ac circuit?

Answer 14

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

Question 15

capacitive reactance (resistance) is affected by …

it is directly/inversely proportional to this

Answer 15

frequency

inversely

capacitors act as low resistors at high frequency and high resistors at low frequency

Question 16

what is Lenz’s law?

Answer 16

Induced current in a coil will create its own magnetic field that opposes the movement

Question 17

The effect of the back EMF (inductance) is to

Answer 17

resist current flow

Question 18

The unit of inductance is the

Answer 18

Henry (L)

Question 19

capacitive and inductive reactance are both measured in …

Answer 19

ohms

Question 20

Effect of inductance is similar to capacitance except that here current (leads/lags) behind voltage

Answer 20

lags

Question 21

inductors are frequency sensitive and reactance is (inversely/directly) proportional to frequency

Answer 21

directly

high resistance at high frequencies and low resistance at low frequencies (opposite of capacitors)

Question 22

The total resistance to current flow in an AC circuit with capacitive and inductive reactance is called

Answer 22

impedance

and is the vector sum of reactance and resistance

Question 23

What is this symbol?

Answer 23

inductor

Question 24

for total inductance, inductors are added in the same way as … and capacitors added the opposite way round

(how to remember?)

Answer 24

resistors

(similar circuit symbols)

Question 25

power in a capacitive or inductive circuit is (higher/lower) than a purely resistive circuit because …

Answer 25

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

Question 26

is work done by the generator useful or wasted in overcoming reactive loads?

Answer 26

wasted

Question 27

what is power factor of an ac circuit?

Answer 27

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)

Question 28

AC generators rated to take account of reactive loads and output power rating is given as …

Answer 28

KVA kilvolt hours

Question 29

KVAR =

Answer 29

reactive power

Question 30

Ac generator rating KW =

Answer 30

effective/useful power

Question 31

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 …

Answer 31

there is no control of rpm

resistance

frequency

Question 32

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

Answer 32

output torque

Question 33

how does a variable speed constant frequency ac generator drive work?

Answer 33

Takes the variable output from a frequency wild generator, rectifies it to DC and then converts it again to AC at constant frequency

Question 34

real load on an ac generator is adjusted with…

reactive load is adjusted with …

Answer 34

csd torque

field energising current

Question 35

synchronous ac motor speed is directly related to …

Answer 35

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

Question 36

synchronous ac motors are used for …

Answer 36

low torque, constant speed applications such as to drive a tachometer

Question 37

synchronous motor RPM = (formula)

Answer 37

(frequency x 60) / no. of pole pairs

(rearrangement of generator frequency formula)

Question 38

synchronous ac motors use … as the rotor

Answer 38

a permanent magnet

Question 39

how does an ac induction motor work?

Answer 39

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

Question 40

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?

Answer 40

ac motors

the same

greater

remembered by VDV (V delta V) voltages equal in delta connection

Question 41

Vital services are always … powered from the …

Answer 41

DC

Hot battery bus

Question 42

In a split bus ac system, the bus tie breakers are normally …

Answer 42

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

Question 43

In a paralleled bus ac system, the bus tie breakers are normally …

Answer 43

closed, because the generators are paralleled