Ac Theory

Question 1

How is AC produced

Answer 1

Rotating field Generators

Question 2

What does the term period mean

Answer 2

Time taken to complete 1 full wavelength cycle

Question 3

How do you find average value

Answer 3

Peak value x 0.637

Question 4

How do you find rms value?

Answer 4

Peak value x 0.7071

Question 5

What are examples of non-sinusoidal waveforms

Answer 5

Square, triangular, sawtooth

Question 6

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

Answer 6

Oscilloscope using peak to peak values

Question 7

How do you calculate current in a purely resistive circuit

Answer 7

I=V/R
No reactance values

Question 8

How does an inductor cause phase shift between V and I

Answer 8

Delays the current in the circuit so voltage leads

Question 9

What is Impedance of an AC circuit defined as

Answer 9

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

Question 10

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

Answer 10

Current totals

Question 11

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

Answer 11

Current decreases
Therefore phase angle increases

Question 12

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

Answer 12

I (inductance) will decrease
Therefore phase angle will increase

Question 13

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

Answer 13

I (capacitance) will decrease
Therefore phase angle will decrease

Question 14

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

Answer 14

True power

Question 15

What is meant by reactive power

Answer 15

Power that is stored in reactive components

Question 16

What is apparent power

Answer 16

Power supplied to the circuit

Question 17

What is power factor

Answer 17

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

Question 18

What symbol is used to represent power factor

Answer 18

Lambda (upside down y)

Question 19

How do we correct for power factor caused by inductance

Answer 19

Capacitors in parallel configuration with load

Question 20

What are advantages of high power factor

Answer 20

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

Question 21

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

Answer 21

QC - QL

Question 22

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

Answer 22

Lagging because current is lagging voltage

Question 23

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

Answer 23

Leading because current is leading voltage

Question 24

What is the main issue with a low power factor

Answer 24

Large currents are required for the same amount of true power

Question 25

What is the definition of resonace frequency

Answer 25

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

Question 26

At high frequencies, is the reactance most capacitive or inductive

Answer 26

Inductive

Question 27

What is the impedence value at resonate frequency

Answer 27

Same value as R

Question 28

In what ways is series resonace dangerous

Answer 28

Large currents produced High voltages across reactive components

Question 29

In a resonant parallel circuit, what characteristics can we expect

Answer 29

XL = XC Current drop High impedence

Question 30

What are the dangers of resonace frequently in parallel circuits

Answer 30

There is no dangers

Question 31

What are advantages of 3 phase over single phase

Answer 31

Less conductors needed to transfer same power over distance More power produced Self starting motors Smoother rectification

Question 32

What is phase sequence

Answer 32

The order at which the windings reach their max positive values

Question 33

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

Answer 33

Red

Question 34

Why is correct connection of 3 phase important in rotating machinery

Answer 34

Can affect the direction of rotation

Question 35

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

Answer 35

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

Question 36

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

Answer 36

3 or 4 depending on neutral lead

Question 37

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

Answer 37

Loads have same impedence Same magnitude and phase angle

Question 38

What are the 6 types of angles in order

Answer 38

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

Question 39

What are the terminology angles and what are they

Answer 39

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

Question 40

What is a radian

Answer 40

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