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M3 > 3.13 AC Theory > Flashcards

3.13 AC Theory Flashcards

1
Q
A
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2
Q

What does AC stand for in AC waveform?

A

Alternating Current

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3
Q

True or False: The AC waveform is characterized by a constant voltage level.

A

False

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4
Q

What is the shape of a typical AC waveform?

A

Sine wave

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5
Q

Fill in the blank: The maximum value of an AC waveform is known as the _______.

A

Peak value

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6
Q

What is the term for the time it takes for one complete cycle of an AC waveform?

A

Period

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7
Q

What is the frequency of an AC waveform?

A

The number of cycles per second, measured in Hertz (Hz)

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8
Q

True or False: The RMS value of an AC waveform is equal to the peak value divided by √2.

A

True

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9
Q

What does RMS stand for?

A

Root Mean Square

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10
Q

Which value indicates the average power delivered by an AC waveform?

A

The RMS value

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11
Q

Fill in the blank: The _______ value is the average of the absolute values of the waveform over one complete cycle.

A

Average

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12
Q

What are the two main types of AC waveforms?

A

Sine wave and square wave

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13
Q

What is the phase angle in an AC waveform?

A

The angle that represents the position of the waveform with respect to time

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14
Q

True or False: In a purely resistive AC circuit, the voltage and current are in phase.

A

True

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15
Q

Fill in the blank: The _______ is a measure of how much the AC waveform deviates from a pure sine wave.

A

Total Harmonic Distortion (THD)

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16
Q

What does the term ‘peak-to-peak value’ refer to?

A

The difference between the maximum positive and maximum negative amplitudes of the waveform.

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17
Q

What is the significance of the zero-crossing point in an AC waveform?

A

It indicates the point where the voltage changes from positive to negative or vice versa.

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18
Q

Which characteristic of the AC waveform affects the design of electrical systems?

A

Frequency

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19
Q

True or False: An AC waveform can have multiple frequencies.

A

False

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20
Q

What is the relationship between frequency and period?

A

Frequency is the reciprocal of the period (f = 1/T).

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21
Q

Fill in the blank: The _______ value of an AC waveform can be calculated as the peak value divided by √2.

A

RMS

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22
Q

What is the effective value of an AC voltage or current?

A

The value that produces the same power as a DC voltage or current.

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23
Q

True or False: The frequency of AC power in most countries is 60 Hz.

A

True

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24
Q

What happens to the waveform during the charging and discharging cycles in a capacitor in an AC circuit?

A

The waveform is shifted in phase.

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25
Q

What is a square wave?

A

A waveform that alternates between a maximum and minimum value with a constant amplitude and a sudden transition.

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26
Q

Fill in the blank: The _______ of an AC waveform is the maximum amplitude reached by the waveform.

A

Peak value

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27
Q

What does the term ‘phase shift’ refer to in an AC circuit?

A

The difference in phase angle between voltage and current waveforms.

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28
Q

What is the peak value of the voltage or current in an AC waveform called?

A

Amplitude

Amplitude is usually shown as Vp or Ip.

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29
Q

How is amplitude measured?

A

From the zero reference line to the positive or negative peak value of voltage or current

Amplitude is expressed in volts or amperes.

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30
Q

What does the instantaneous value of an alternating voltage or current represent?

A

Its value at a particular instant

This value may be zero or equal to the peak value depending on the timing of measurement.

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31
Q

How can the instantaneous value of current be calculated?

A

Using the formula I = Ip * sin(θ)

Where Ip is the peak value and θ is the phase angle.

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32
Q

What is the average value of current or voltage for a sinusoidal waveform over a full cycle?

A

Equal to zero

This is due to the symmetry of the sinusoidal waveform about the X-axis.

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33
Q

What happens to work done in each half cycle of an alternating waveform?

A

It does not cancel out over a full cycle

Work is done regardless of the direction of current flow.

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34
Q

How is the average voltage calculated for one half cycle?

A

By adding mid-ordinate values and dividing by the number of mid-ordinates

This process approximates the average voltage.

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35
Q

What is the average voltage value approximation if the maximum voltage is 340 V?

A

214.6 volts

The true value approaches 240.4 volts with more ordinates.

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36
Q

What does the Root Mean Square (RMS) value represent?

A

The value which produces the same power output as an equivalent DC current or voltage

RMS values are typically quoted for AC unless specified otherwise.

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37
Q

How is the RMS value of a sinusoidal AC voltage calculated?

A

Divide the peak value by √2 or multiply by 0.707

RMS value is critical for power calculations.

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38
Q

What is the positive or negative maximum value of the AC current or voltage called?

A

Peak value

Denoted as Vp or Ip.

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39
Q

How can the instantaneous value at a specific angle be calculated?

A

Using the formula V = Vp * sin(θ)

This shows how the sine wave can be represented at any angle.

40
Q

What does a phasor represent?

A

An alternating quantity in peak value and phase

It is assumed to rotate in an anti-clockwise direction.

41
Q

What is the difference between a vector and a phasor?

A

A vector represents magnitude and direction; a phasor represents peak value and phase

This distinction is crucial in AC circuit analysis.

42
Q

How can a sine wave be mathematically represented?

A

By a mathematical equation showing amplitude and phase

This is necessary for circuit calculations involving alternating current.

43
Q

What does the peak-to-peak value mean?

A

Twice the peak value

This is the measurement between the positive and negative maximum values of a sine curve.

44
Q

What is the formula to calculate the average voltage from mid-ordinate values?

A

Average Voltage = (Sum of mid-ordinates) / (Number of mid-ordinates)

This method is used for approximation in AC analysis.

45
Q

Fill in the blank: The instantaneous value of voltage is calculated using the formula V = _____ * sin(θ).

A

Vp

Where Vp is the peak value of voltage.

46
Q

What are periodic voltages or currents that do not follow a sinusoidal waveform called?

A

Non-sinusoidal AC voltages or currents

47
Q

What is the condition for a supply to be considered a pure AC supply?

A

The area between the sine curve and the horizontal time axis is the same above and below the axis

48
Q

How are AC waveforms classified?

A

According to the shape of the waveform

49
Q

What indicates that current changes quickly between extremes in a waveform?

A

Square wave

50
Q

What does a triangular wave indicate?

A

A steady rate of change of voltage over each half cycle

51
Q

What is a square waveform characterized by?

A

Rises quickly to an amplitude, remains constant, and drops fast

52
Q

In digital systems, what type of waveform is the norm?

A

Square waves

53
Q

What is the frequency of a square wave calculated by?

A

The formula involving pulse width

54
Q

What is the duty cycle of a symmetrical square wave?

A

50%

55
Q

What happens if the duty cycle of a waveform is not 50%?

A

The waveform is called a Rectangular Waveform or a Pulse

56
Q

What defines a Rectangular waveform?

A

Non-symmetrical waveforms with unequal pulse widths

57
Q

What are Triangular Waveforms characterized by?

A

Bi-directional non-sinusoidal waveforms oscillating between positive and negative peak values

58
Q

What is required to convert a sine wave into a triangular waveform?

A

High gain amplification and a constant current source for charging and discharging a capacitor

59
Q

What is the key difference between Triangular and Sawtooth waveforms?

A

The rise time of the triangular wave is equal to its fall time, while the sawtooth may not be

60
Q

What practical use do Sawtoothed waveforms have?

A

They provide quality of sound or tonal colour in music synthesizers

61
Q

What is an alternating quantity?

A

Any quantity which reverses its direction at regular intervals

62
Q

What terms describe the waveform of an alternating quantity?

A
  • Amplitude
  • Cycle
  • Periodic time
  • Frequency
63
Q

What is the practical importance of instantaneous and average values of an alternating quantity?

A

Limited practical importance

64
Q

What is the Root Mean Square (RMS) value of an alternating quantity?

A

The equivalent value of a direct quantity which would produce the same power in a load

65
Q

What is the relationship between Peak value and RMS value?

A

Peak value equals 1.414 x RMS value; RMS value equals 0.707 x Peak value

66
Q

What should all alternating quantities be considered as unless otherwise stated?

A

RMS values

67
Q

What is a Phasor?

A

A line representing an alternating quantity, where length indicates amplitude and angle indicates phase

68
Q

How can a sine wave be mathematically represented?

A

Using a specific mathematical formula

69
Q

What is the main difference between single-phase and multi-phase systems?

A

Multi-phase systems are more efficient than single-phase systems

70
Q

How is a single-phase AC generated?

A

By spinning rotors that place coils in stators under a changing magnetic field, inducing EMF

71
Q

What is Faraday’s Law?

A

A principle that explains how an EMF is induced in coils when exposed to a changing magnetic field

72
Q

What happens during one complete revolution of a two-pole rotor in a single-phase system?

A

It produces a full sine wave

73
Q

In a two-phase system, how are the windings arranged?

A

Two windings A and B are 90° apart

74
Q

What is the phase difference between outputs in a two-phase generation?

A

90°

75
Q

What is the phase angle difference when combining two outputs in a two-phase system?

A

A new phase angle difference is created

76
Q

How are multi-phase systems constructed?

A

By placing three coils around the stator, each displaced by 120°

77
Q

What is the phase difference between each pair of coils in a three-phase system?

A

120°

78
Q

What type of power circuit is a single-phase power system?

A

A two-wire alternating current (AC) power circuit

79
Q

What is typically served by a single-phase power supply?

A

Residential homes

80
Q

What is a key difference between single-phase and three-phase power supplies?

A

Three-phase accommodates higher loads better

81
Q

What does a single-phase AC power system consist of?

A

A generator connected through two wires to a load

82
Q

What does a single-phase three-wire system typically include?

A

Two identical sources and a neutral wire

83
Q

How are three coils arranged in a three-phase system?

A

Mounted with a mutual displacement of 120°

84
Q

What are the typical color codes for phases in a three-phase system?

A

Red, Yellow, and Blue

85
Q

What are the two connection types for three-phase systems?

A

Delta and Star

86
Q

In a Star connection, what is connected to a common point?

A

One end of each winding

87
Q

What voltages are available in a Star connection?

A

Phase voltage and line voltage

88
Q

What is the relationship between Line and Phase Voltages in a Star connection?

A

The voltage between any two lines is the phasor difference between the phase voltages

89
Q

In a Delta connection, how are the windings connected?

A

From the finish of one to the start of the next

90
Q

What happens to phase currents in a Delta connection?

A

Each phase current is the difference between two line currents

91
Q

What voltage levels are typically measured in aircraft three-phase systems?

A

115 V phase to ground and 200 V phase-to-phase

92
Q

What condition allows for no current in the common return line of a three-phase system?

A

All three phases have the same loading

93
Q

Fill in the blank: A single-phase AC power system typically resembles a simple _______.

A

DC circuit

94
Q

True or False: A three-phase system can only accommodate a single voltage level.

A

False

95
Q

What type of loads are single-phase systems typically used for?

A

Lighting or heating loads

M3 (19 decks)

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