3.13 AC Theory

Question 1

What is the most common type of current used in the mains supply of power in engineering?

Answer 1

AC

Question 2

What are the two main characteristics that distinguish AC to DC?

Answer 2

-the amount of both the voltage and current changes with time
-the direction of both voltage and current changes with time

Question 3

For common AC what is the average voltage and current?

Answer 3

They’re both zero due to the constant change in positive and negative

Question 4

What makes up one AC cycle?

Answer 4

A positive and a negative alternation

Question 5

The initial angle of a sinusoidal function at its origin is called?

Answer 5

Phase offset

Question 6

The phase of a periodic function is a measure of the fraction of the cycle that has been completed along what axis?

Answer 6

The x-axis

Question 7

What are the readings of a periodic function measured in?

Answer 7

Degrees˚ or radians π

Question 8

When is the first peak of the phase in a periodic function?

Answer 8

90˚ or π/2

Question 9

What is the top of a sinusoids wave called?

Answer 9

Amplitude or peak value

Question 10

What is the initial angle of a sinusoidal function called?

Answer 10

Phase offset

Question 11

If the phase offset starts at the origin what is the the phase offset?

Answer 11

Zero

Question 12

What is the phase difference?

Answer 12

The relative displacement between two points in a cycle

Question 13

What is the period of sinusoidal wave?

Answer 13

Time taken to perform one complete cycle

Question 14

The units for periods is given in?

Answer 14

Seconds

Question 15

What is the definition of frequency in a sinusoidal wave?

Answer 15

The number of complete cycle per second

Question 16

What are the units of frequency?

Answer 16

Hertz

Question 17

In a sinusoidal wave the higher frequency means a shorter?

Answer 17

Cycle time

Question 18

In a sinusoidal wave the lower the frequency means a longer?

Answer 18

Cycle time

Question 19

What is an instantaneous value of an alternating voltage or current?

Answer 19

Is the value at any particular moment in a wave

Question 20

How do you find the average value of a wave if it is always zero but work gets done?

Answer 20

You plot multiple instantaneous values on one half cycle

Question 21

When you’re find the average of half a cycle what are the equally divided portions called?

Answer 21

Mid-ordinates

Question 22

What is each quotient value of each portion of a mid-ordinate called?

Answer 22

Instantaneous value

Question 23

When finding the average of a half cycle, what happens if you use more mid-ordinates?

Answer 23

You get a more accurate value

Question 24

What is the definition of the root mean square value of an AC current/voltage?

Answer 24

The value in which when applied to a resistance will produce the same power output as an equivalent DC current/voltage

Question 25

Can you calculate a an entire sinusoidal wave by just knowing the peak value?

Answer 25

Yes

Question 26

What direction does a phaser rotate?

Answer 26

Anti-clockwise

Question 27

What are periodic voltage/currents that don’t follow a sinusoidal waveform called?

Answer 27

Non-sinusoidal AC voltages or currents

Question 28

What is a square waveform?

Answer 28

A waveform that rises quickly to a particular amplitude, re,aims constant for a time period and drops fast at the end

Question 29

In digital systems what is the normal waveform used?

Answer 29

A square waveform (binary)

Question 30

If the waveform is any other percentage than 50% what is it called?

Answer 30

A rectangular waveform

Question 31

What is a pulse waveform?

Answer 31

When it is on for a very small time before switching back off

Question 32

What are rectangular waveforms classed as?

Answer 32

‘Non-symmetrical’ waveforms

Question 33

On a square waveform what is the positive up line called?

Answer 33

Rising or leading edge

Question 34

On a square waveform what is the negative down line called?

Answer 34

Falling or trailing edge

Question 35

Triangular waveforms are said to be?

Answer 35

Bi-directional non sinusoidal waveforms

Question 36

Triangular waveforms oscillate between?

Answer 36

Positive and negative peak values

Question 37

What does a triangular waveform have to be converted into?

Answer 37

A square waveform

Question 38

What is a variation of a triangular waveform?

Answer 38

The sawtooth waveform

Question 39

In a triangular waveform what is the negative down line called?

Answer 39

Negative slope or ramp

Question 40

In a triangular waveform what is the positive up line called?

Answer 40

Positive slope or ramp

Question 41

What makes the sawtooth waveform different to the triangular waveform?

Answer 41

The rise time may be much higher than its fall time or vice versa

Question 42

Can sawtooth waveforms be a mirror image of themselves?

Answer 42

Yes

Question 43

Unless stated otherwise what are alternating quantities given in?

Answer 43

Root mean square

Question 44

What is a multi-phase system also known as?

Answer 44

Polyphase

Question 45

What is more efficient single-phase or multi-phase systems?

Answer 45

Multi-phase

Question 46

The number of poles in a single phase AC systems affects the?

Answer 46

Frequency of the wave

Question 47

What governs the amount of phases in a AC system?

Answer 47

The number of coils

Question 48

How many sine waves does a 2 phase AC generator produce?

Answer 48

2

Question 49

How many coils are used in a multi-phase generator?

Answer 49

Three

Question 50

What angle separates the stators in a two-phase generator?

Answer 50

90˚

Question 51

What angle separates the stators in a multi-phase generator?

Answer 51

120˚

Question 52

What takes high loads single-phase, two-phase, multi-phase?

Answer 52

Multi-phase

Question 53

What colours are the multi-phase system phases identified into?

Answer 53

-Red -Yellow -Blue

Question 54

What are the two ways you can connect a three-phase system?

Answer 54

Delta or star

Question 55

What are the two different voltages that can be measured in aircraft multi-phased systems?

Answer 55

-115V one phase to ground -200V phase to phase