3.13 AC Theory Flashcards
What does AC stand for in AC waveform?
Alternating Current
True or False: The AC waveform is characterized by a constant voltage level.
False
What is the shape of a typical AC waveform?
Sine wave
Fill in the blank: The maximum value of an AC waveform is known as the _______.
Peak value
What is the term for the time it takes for one complete cycle of an AC waveform?
Period
What is the frequency of an AC waveform?
The number of cycles per second, measured in Hertz (Hz)
True or False: The RMS value of an AC waveform is equal to the peak value divided by √2.
True
What does RMS stand for?
Root Mean Square
Which value indicates the average power delivered by an AC waveform?
The RMS value
Fill in the blank: The _______ value is the average of the absolute values of the waveform over one complete cycle.
Average
What are the two main types of AC waveforms?
Sine wave and square wave
What is the phase angle in an AC waveform?
The angle that represents the position of the waveform with respect to time
True or False: In a purely resistive AC circuit, the voltage and current are in phase.
True
Fill in the blank: The _______ is a measure of how much the AC waveform deviates from a pure sine wave.
Total Harmonic Distortion (THD)
What does the term ‘peak-to-peak value’ refer to?
The difference between the maximum positive and maximum negative amplitudes of the waveform.
What is the significance of the zero-crossing point in an AC waveform?
It indicates the point where the voltage changes from positive to negative or vice versa.
Which characteristic of the AC waveform affects the design of electrical systems?
Frequency
True or False: An AC waveform can have multiple frequencies.
False
What is the relationship between frequency and period?
Frequency is the reciprocal of the period (f = 1/T).
Fill in the blank: The _______ value of an AC waveform can be calculated as the peak value divided by √2.
RMS
What is the effective value of an AC voltage or current?
The value that produces the same power as a DC voltage or current.
True or False: The frequency of AC power in most countries is 60 Hz.
True
What happens to the waveform during the charging and discharging cycles in a capacitor in an AC circuit?
The waveform is shifted in phase.
What is a square wave?
A waveform that alternates between a maximum and minimum value with a constant amplitude and a sudden transition.
Fill in the blank: The _______ of an AC waveform is the maximum amplitude reached by the waveform.
Peak value
What does the term ‘phase shift’ refer to in an AC circuit?
The difference in phase angle between voltage and current waveforms.
What is the peak value of the voltage or current in an AC waveform called?
Amplitude
Amplitude is usually shown as Vp or Ip.
How is amplitude measured?
From the zero reference line to the positive or negative peak value of voltage or current
Amplitude is expressed in volts or amperes.
What does the instantaneous value of an alternating voltage or current represent?
Its value at a particular instant
This value may be zero or equal to the peak value depending on the timing of measurement.
How can the instantaneous value of current be calculated?
Using the formula I = Ip * sin(θ)
Where Ip is the peak value and θ is the phase angle.
What is the average value of current or voltage for a sinusoidal waveform over a full cycle?
Equal to zero
This is due to the symmetry of the sinusoidal waveform about the X-axis.
What happens to work done in each half cycle of an alternating waveform?
It does not cancel out over a full cycle
Work is done regardless of the direction of current flow.
How is the average voltage calculated for one half cycle?
By adding mid-ordinate values and dividing by the number of mid-ordinates
This process approximates the average voltage.
What is the average voltage value approximation if the maximum voltage is 340 V?
214.6 volts
The true value approaches 240.4 volts with more ordinates.
What does the Root Mean Square (RMS) value represent?
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.
How is the RMS value of a sinusoidal AC voltage calculated?
Divide the peak value by √2 or multiply by 0.707
RMS value is critical for power calculations.
What is the positive or negative maximum value of the AC current or voltage called?
Peak value
Denoted as Vp or Ip.
How can the instantaneous value at a specific angle be calculated?
Using the formula V = Vp * sin(θ)
This shows how the sine wave can be represented at any angle.
What does a phasor represent?
An alternating quantity in peak value and phase
It is assumed to rotate in an anti-clockwise direction.
What is the difference between a vector and a phasor?
A vector represents magnitude and direction; a phasor represents peak value and phase
This distinction is crucial in AC circuit analysis.
How can a sine wave be mathematically represented?
By a mathematical equation showing amplitude and phase
This is necessary for circuit calculations involving alternating current.
What does the peak-to-peak value mean?
Twice the peak value
This is the measurement between the positive and negative maximum values of a sine curve.
What is the formula to calculate the average voltage from mid-ordinate values?
Average Voltage = (Sum of mid-ordinates) / (Number of mid-ordinates)
This method is used for approximation in AC analysis.
Fill in the blank: The instantaneous value of voltage is calculated using the formula V = _____ * sin(θ).
Vp
Where Vp is the peak value of voltage.
What are periodic voltages or currents that do not follow a sinusoidal waveform called?
Non-sinusoidal AC voltages or currents
What is the condition for a supply to be considered a pure AC supply?
The area between the sine curve and the horizontal time axis is the same above and below the axis
How are AC waveforms classified?
According to the shape of the waveform
What indicates that current changes quickly between extremes in a waveform?
Square wave
What does a triangular wave indicate?
A steady rate of change of voltage over each half cycle
What is a square waveform characterized by?
Rises quickly to an amplitude, remains constant, and drops fast
In digital systems, what type of waveform is the norm?
Square waves
What is the frequency of a square wave calculated by?
The formula involving pulse width
What is the duty cycle of a symmetrical square wave?
50%
What happens if the duty cycle of a waveform is not 50%?
The waveform is called a Rectangular Waveform or a Pulse
What defines a Rectangular waveform?
Non-symmetrical waveforms with unequal pulse widths
What are Triangular Waveforms characterized by?
Bi-directional non-sinusoidal waveforms oscillating between positive and negative peak values
What is required to convert a sine wave into a triangular waveform?
High gain amplification and a constant current source for charging and discharging a capacitor
What is the key difference between Triangular and Sawtooth waveforms?
The rise time of the triangular wave is equal to its fall time, while the sawtooth may not be
What practical use do Sawtoothed waveforms have?
They provide quality of sound or tonal colour in music synthesizers
What is an alternating quantity?
Any quantity which reverses its direction at regular intervals
What terms describe the waveform of an alternating quantity?
- Amplitude
- Cycle
- Periodic time
- Frequency
What is the practical importance of instantaneous and average values of an alternating quantity?
Limited practical importance
What is the Root Mean Square (RMS) value of an alternating quantity?
The equivalent value of a direct quantity which would produce the same power in a load
What is the relationship between Peak value and RMS value?
Peak value equals 1.414 x RMS value; RMS value equals 0.707 x Peak value
What should all alternating quantities be considered as unless otherwise stated?
RMS values
What is a Phasor?
A line representing an alternating quantity, where length indicates amplitude and angle indicates phase
How can a sine wave be mathematically represented?
Using a specific mathematical formula
What is the main difference between single-phase and multi-phase systems?
Multi-phase systems are more efficient than single-phase systems
How is a single-phase AC generated?
By spinning rotors that place coils in stators under a changing magnetic field, inducing EMF
What is Faraday’s Law?
A principle that explains how an EMF is induced in coils when exposed to a changing magnetic field
What happens during one complete revolution of a two-pole rotor in a single-phase system?
It produces a full sine wave
In a two-phase system, how are the windings arranged?
Two windings A and B are 90° apart
What is the phase difference between outputs in a two-phase generation?
90°
What is the phase angle difference when combining two outputs in a two-phase system?
A new phase angle difference is created
How are multi-phase systems constructed?
By placing three coils around the stator, each displaced by 120°
What is the phase difference between each pair of coils in a three-phase system?
120°
What type of power circuit is a single-phase power system?
A two-wire alternating current (AC) power circuit
What is typically served by a single-phase power supply?
Residential homes
What is a key difference between single-phase and three-phase power supplies?
Three-phase accommodates higher loads better
What does a single-phase AC power system consist of?
A generator connected through two wires to a load
What does a single-phase three-wire system typically include?
Two identical sources and a neutral wire
How are three coils arranged in a three-phase system?
Mounted with a mutual displacement of 120°
What are the typical color codes for phases in a three-phase system?
Red, Yellow, and Blue
What are the two connection types for three-phase systems?
Delta and Star
In a Star connection, what is connected to a common point?
One end of each winding
What voltages are available in a Star connection?
Phase voltage and line voltage
What is the relationship between Line and Phase Voltages in a Star connection?
The voltage between any two lines is the phasor difference between the phase voltages
In a Delta connection, how are the windings connected?
From the finish of one to the start of the next
What happens to phase currents in a Delta connection?
Each phase current is the difference between two line currents
What voltage levels are typically measured in aircraft three-phase systems?
115 V phase to ground and 200 V phase-to-phase
What condition allows for no current in the common return line of a three-phase system?
All three phases have the same loading
Fill in the blank: A single-phase AC power system typically resembles a simple _______.
DC circuit
True or False: A three-phase system can only accommodate a single voltage level.
False
What type of loads are single-phase systems typically used for?
Lighting or heating loads
