Ac Theory Flashcards
How is AC produced
Rotating field Generators
What does the term period mean
Time taken to complete 1 full wavelength cycle
How do you find average value
Peak value x 0.637
How do you find rms value?
Peak value x 0.7071
What are examples of non-sinusoidal waveforms
Square, triangular, sawtooth
What is the best way to measure the output from non-sinusoidal waveforms
Oscilloscope using peak to peak values
How do you calculate current in a purely resistive circuit
I=V/R
No reactance values
How does an inductor cause phase shift between V and I
Delays the current in the circuit so voltage leads
What is Impedance of an AC circuit defined as
The total opposition to AC current of a circuit containing resistance, inductance and/or capacitance
In parallel RLC circuits, what is used to calculate the values?
Current totals
In a para RLC circuit, if R is increased relative to XL and XC, what happens to phase angle and current
Current decreases
Therefore phase angle increases
In a para RLC circuit, if XL is increased relative to R and XC, what happens to phase angle and current
I (inductance) will decrease
Therefore phase angle will increase
In a para RLC circuit, if XC is increased relative to R and XC, what happens to phase angle and current
I (capacitance) will decrease
Therefore phase angle will decrease
In an AC circuit, what is the power dissipated in the resistance of the circuit called
True power
What is meant by reactive power
Power that is stored in reactive components
What is apparent power
Power supplied to the circuit
What is power factor
The factor or ratio at which apparent power is multiplied to get the true value
What symbol is used to represent power factor
Lambda (upside down y)
How do we correct for power factor caused by inductance
Capacitors in parallel configuration with load
What are advantages of high power factor
Less power wasted
Less apparent power needed
Lower currents needed
Reduced costs
Doesn’t disturb other loads on power grid
What is the net reactive power in a circuit containing both an inductor and capacitor
QC - QL
In a predominantly inductive circuit, will the power factor be leading or lagging
Lagging because current is lagging voltage
In a predominantly capacitive circuit, will the power factor be leading or lagging
Leading because current is leading voltage
What is the main issue with a low power factor
Large currents are required for the same amount of true power
What is the definition of resonace frequency
The frequency where XL = XC
voltage and current are in phase
At high frequencies, is the reactance most capacitive or inductive
Inductive
What is the impedence value at resonate frequency
Same value as R
In what ways is series resonace dangerous
Large currents produced
High voltages across reactive components
In a resonant parallel circuit, what characteristics can we expect
XL = XC
Current drop
High impedence
What are the dangers of resonace frequently in parallel circuits
There is no dangers
What are advantages of 3 phase over single phase
Less conductors needed to transfer same power over distance
More power produced
Self starting motors
Smoother rectification
What is phase sequence
The order at which the windings reach their max positive values
What colour phase is used as the reference phase in 3 phase systems
Red
Why is correct connection of 3 phase important in rotating machinery
Can affect the direction of rotation
What is the key controllable factor in operating an AC alternator that will determine the value of the EMF generated
Strength of magnetic field since most generators are at set rpm so can’t be changed
How many leads can you expect to find on the output of an AC generator
3 or 4 depending on neutral lead
What requirements would the loads have to meet in order to be part of a balanced system
Loads have same impedence
Same magnitude and phase angle
What are the 6 types of angles in order
Acute 0⁰-90⁰
Right 90⁰
Obtuse 90⁰-180⁰
Straight 180⁰
Reflex 180⁰-360⁰
Full turn 360⁰
What are the terminology angles and what are they
Vertically opposed aXb
Corresponding a/—-b/—–
Alternate a\/b
What is a radian
1 radian is the angle subtended at the centre of a circle by an arc that is equal to the length of the radius of the circle
2pi radians=360⁰
180⁰/pi = 1 radian = 57.2958⁰
