3.13 AC Theory Flashcards
What is the most common type of current used in the mains supply of power in engineering?
AC
What are the two main characteristics that distinguish AC to DC?
-the amount of both the voltage and current changes with time
-the direction of both voltage and current changes with time
For common AC what is the average voltage and current?
They’re both zero due to the constant change in positive and negative
What makes up one AC cycle?
A positive and a negative alternation
The initial angle of a sinusoidal function at its origin is called?
Phase offset
The phase of a periodic function is a measure of the fraction of the cycle that has been completed along what axis?
The x-axis
What are the readings of a periodic function measured in?
Degrees˚ or radians π
When is the first peak of the phase in a periodic function?
90˚ or π/2
What is the top of a sinusoids wave called?
Amplitude or peak value
What is the initial angle of a sinusoidal function called?
Phase offset
If the phase offset starts at the origin what is the the phase offset?
Zero
What is the phase difference?
The relative displacement between two points in a cycle
What is the period of sinusoidal wave?
Time taken to perform one complete cycle
The units for periods is given in?
Seconds
What is the definition of frequency in a sinusoidal wave?
The number of complete cycle per second
What are the units of frequency?
Hertz
In a sinusoidal wave the higher frequency means a shorter?
Cycle time
In a sinusoidal wave the lower the frequency means a longer?
Cycle time
What is an instantaneous value of an alternating voltage or current?
Is the value at any particular moment in a wave
How do you find the average value of a wave if it is always zero but work gets done?
You plot multiple instantaneous values on one half cycle
When you’re find the average of half a cycle what are the equally divided portions called?
Mid-ordinates
What is each quotient value of each portion of a mid-ordinate called?
Instantaneous value
When finding the average of a half cycle, what happens if you use more mid-ordinates?
You get a more accurate value
What is the definition of the root mean square value of an AC current/voltage?
The value in which when applied to a resistance will produce the same power output as an equivalent DC current/voltage
Can you calculate a an entire sinusoidal wave by just knowing the peak value?
Yes
What direction does a phaser rotate?
Anti-clockwise
What are periodic voltage/currents that don’t follow a sinusoidal waveform called?
Non-sinusoidal AC voltages or currents
What is a square waveform?
A waveform that rises quickly to a particular amplitude, re,aims constant for a time period and drops fast at the end
In digital systems what is the normal waveform used?
A square waveform (binary)
If the waveform is any other percentage than 50% what is it called?
A rectangular waveform
What is a pulse waveform?
When it is on for a very small time before switching back off
What are rectangular waveforms classed as?
‘Non-symmetrical’ waveforms
On a square waveform what is the positive up line called?
Rising or leading edge
On a square waveform what is the negative down line called?
Falling or trailing edge
Triangular waveforms are said to be?
Bi-directional non sinusoidal waveforms
Triangular waveforms oscillate between?
Positive and negative peak values
What does a triangular waveform have to be converted into?
A square waveform
What is a variation of a triangular waveform?
The sawtooth waveform
In a triangular waveform what is the negative down line called?
Negative slope or ramp
In a triangular waveform what is the positive up line called?
Positive slope or ramp
What makes the sawtooth waveform different to the triangular waveform?
The rise time may be much higher than its fall time or vice versa
Can sawtooth waveforms be a mirror image of themselves?
Yes
Unless stated otherwise what are alternating quantities given in?
Root mean square
What is a multi-phase system also known as?
Polyphase
What is more efficient single-phase or multi-phase systems?
Multi-phase
The number of poles in a single phase AC systems affects the?
Frequency of the wave
What governs the amount of phases in a AC system?
The number of coils
How many sine waves does a 2 phase AC generator produce?
2
How many coils are used in a multi-phase generator?
Three
What angle separates the stators in a two-phase generator?
90˚
What angle separates the stators in a multi-phase generator?
120˚
What takes high loads single-phase, two-phase, multi-phase?
Multi-phase
What colours are the multi-phase system phases identified into?
-Red
-Yellow
-Blue
What are the two ways you can connect a three-phase system?
Delta or star
What are the two different voltages that can be measured in aircraft multi-phased systems?
-115V one phase to ground
-200V phase to phase
