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ac theory > Ac Theory > Flashcards

Ac Theory Flashcards

1
Q

How is AC produced

A

Rotating field Generators

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2
Q

What does the term period mean

A

Time taken to complete 1 full wavelength cycle

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3
Q

How do you find average value

A

Peak value x 0.637

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4
Q

How do you find rms value?

A

Peak value x 0.7071

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5
Q

What are examples of non-sinusoidal waveforms

A

Square, triangular, sawtooth

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6
Q

What is the best way to measure the output from non-sinusoidal waveforms

A

Oscilloscope using peak to peak values

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7
Q

How do you calculate current in a purely resistive circuit

A

I=V/R
No reactance values

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8
Q

How does an inductor cause phase shift between V and I

A

Delays the current in the circuit so voltage leads

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9
Q

What is Impedance of an AC circuit defined as

A

The total opposition to AC current of a circuit containing resistance, inductance and/or capacitance

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10
Q

In parallel RLC circuits, what is used to calculate the values?

A

Current totals

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11
Q

In a para RLC circuit, if R is increased relative to XL and XC, what happens to phase angle and current

A

Current decreases
Therefore phase angle increases

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12
Q

In a para RLC circuit, if XL is increased relative to R and XC, what happens to phase angle and current

A

I (inductance) will decrease
Therefore phase angle will increase

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13
Q

In a para RLC circuit, if XC is increased relative to R and XC, what happens to phase angle and current

A

I (capacitance) will decrease
Therefore phase angle will decrease

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14
Q

In an AC circuit, what is the power dissipated in the resistance of the circuit called

A

True power

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15
Q

What is meant by reactive power

A

Power that is stored in reactive components

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16
Q

What is apparent power

A

Power supplied to the circuit

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17
Q

What is power factor

A

The factor or ratio at which apparent power is multiplied to get the true value

18
Q

What symbol is used to represent power factor

A

Lambda (upside down y)

19
Q

How do we correct for power factor caused by inductance

A

Capacitors in parallel configuration with load

20
Q

What are advantages of high power factor

A

Less power wasted
Less apparent power needed
Lower currents needed
Reduced costs
Doesn’t disturb other loads on power grid

21
Q

What is the net reactive power in a circuit containing both an inductor and capacitor

A

QC - QL

22
Q

In a predominantly inductive circuit, will the power factor be leading or lagging

A

Lagging because current is lagging voltage

23
Q

In a predominantly capacitive circuit, will the power factor be leading or lagging

A

Leading because current is leading voltage

24
Q

What is the main issue with a low power factor

A

Large currents are required for the same amount of true power

25
Q

What is the definition of resonace frequency

A

The frequency where XL = XC
voltage and current are in phase

26
Q

At high frequencies, is the reactance most capacitive or inductive

A

Inductive

27
Q

What is the impedence value at resonate frequency

A

Same value as R

28
Q

In what ways is series resonace dangerous

A

Large currents produced
High voltages across reactive components

29
Q

In a resonant parallel circuit, what characteristics can we expect

A

XL = XC
Current drop
High impedence

30
Q

What are the dangers of resonace frequently in parallel circuits

A

There is no dangers

31
Q

What are advantages of 3 phase over single phase

A

Less conductors needed to transfer same power over distance

More power produced
Self starting motors
Smoother rectification

32
Q

What is phase sequence

A

The order at which the windings reach their max positive values

33
Q

What colour phase is used as the reference phase in 3 phase systems

A

Red

34
Q

Why is correct connection of 3 phase important in rotating machinery

A

Can affect the direction of rotation

35
Q

What is the key controllable factor in operating an AC alternator that will determine the value of the EMF generated

A

Strength of magnetic field since most generators are at set rpm so can’t be changed

36
Q

How many leads can you expect to find on the output of an AC generator

A

3 or 4 depending on neutral lead

37
Q

What requirements would the loads have to meet in order to be part of a balanced system

A

Loads have same impedence
Same magnitude and phase angle

38
Q

What are the 6 types of angles in order

A

Acute 0⁰-90⁰
Right 90⁰
Obtuse 90⁰-180⁰
Straight 180⁰
Reflex 180⁰-360⁰
Full turn 360⁰

39
Q

What are the terminology angles and what are they

A

Vertically opposed aXb
Corresponding a/—-b/—–
Alternate a\/b

40
Q

What is a radian

A

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

ac theory (1 decks)

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