AC fundamentals ch 4

Question 1

What is an AC and DC quantity?

Answer 1

changes its value & direction with respect to time.
which doesn’t change its value & direction with respect to time.

Question 2

Advantage soft AC over DC

Answer 2

Easy generation
easy distribution efficiency
easy design of Ac machines
use of tranformer is possible.

Question 3

Construction and Working of AC waveform

Answer 3

It has a permanent magnet and a rectangular coil made of copper. The coil can rotate on its own axis in either a clockwise or anticlockwise direction. Electric connections are attached to the two ends of the coil

When the coil rotates in an anticlockwise direction, it cuts through the magnetic field produced by the permanent magnet. This action, according to Faraday’s law of electromagnetic induction, leads to the generation of an electromotive force (EMF) in the conductor.

Question 4

Representation of sin wave

Answer 4

Where, v(t) = Instantaneous voltage
Vm = Peak Value
F = Frequency
It can also be represented as,

V(t) = Vm sinωt or Vm Sinθ
where θ = ωt and ω= 2πf

For current,
i(t) = Im Sin 2πft

Question 5

What is Instantaneous value, Cycle and time period

Answer 5

*value of a quantity at particular time.
*Cycle is a one positive and one negative proportion of waveform represented again and again.
*time taken by the waveform to complete one cycle.

Question 6

What is frequency, Amplitude and Angular velocity

Answer 6

*the number of cycles completed in 1 second unit hertz(Hz). F=1/T
*max value of an AC quantity denoted by Vm or Im.
*rate of change of angle with respect to time respect to time. 2π

Question 7

What is RMS value, average value, Form factor and crest factor

Answer 7

RMS value is the value of DC current required to produce the same amount of heat produced by AC current provided the resistance and time for which these current flow are same Vrms=0.70Vm Irms=0.70Im.
average of all Instantaneous values a period of half cycle V avg=0.637Vm I avg=0.637Im
*Form factor(Kf) is the ratio of RMS value to average value. Kf=V rms/V avg
*Crest factor(Kp) is the ratio of Peak value to RMS value. Kp=peak value/RMS value

Question 8

What is the difference between Phase angle and Phase difference?

Answer 8

*Phase angle:
The equation of the sine wave is given by
v (t) = Vm sin ωt
= Vm Sinθ
where, θ is the angle made by the conductor with reference
axis and is called as Phase angle.
* Phase Difference:
It is not necessary that the two voltage or current waves
originate at the same instant of time.
* Here VA and VB don’t have the same zero crossover point.
* They have same frequency and same peak
value.
VA = Vm sinθ
VB = Vm sin (θ + φ)
The angle φ is known as Phase difference between VA

and VB .

• If measured in radians phase difference takes value between 0 to 2π.
• If measured in degrees phase difference takes value between 0 to 360°.

Question 9

difference b/w Leading Phase Difference and Lagging Phase difference

Answer 9

If the phase angle “φ” is positive
If the phase angle “φ” is negative

Question 10

difference b/w In phase voltages and Out of phase voltages

Answer 10

The two AC voltages are said to be
“in phase” if the phase difference between them is
equal to zero.
The two AC voltages are said to
be “out of phase” with each other if the phase
difference between them is 180°.

Question 11

What is reactance and state its types

Answer 11

Opposition to Flow of current
i) Inductive Reactance(Xl):opposition to flow of AC offered by an inductance.
ii) Capacitive Reactance (Xc):opposition to flow of AC offered by a capacitor.

Question 12

Impedance(Z):

Answer 12

The combination of Resistance, Inductive Reactance , Capacitive Reactance

Question 13

Impedance Triangle

Answer 13

right angled triangle whose base, perpendicular and
hypotenuse represents Resistance, Reactance and Impedance
used to find the value of impedance.

Question 14

Apparent Power, Active Power, Reactive Power and Power Factor

Answer 14

Apparent Power: product of rms values of voltage(V) and current (I). It represents total power supplied by the source. It is denoted by S and measured in volt-amperes or
KVA.
Active Power: the average power consumed by the
ac circuit.
Reactive Power: It is the part of total power which is not consumed by the Load.
Power Factor: as ratio of True Power to Apparent Power.

Question 15

Polyphase Circuit

Answer 15

In a three-phase AC generator, three coils are rigidly attached together and spaced 120 degrees apart. These coils rotate in a uniform magnetic field, generating EMF (electromotive force) with a phase difference of 120 degrees. The coils are labeled as a1a2, b1b2, and c1c2.

The induced EMFs in the coils can be described as follows:
ea1a2 = E0 sin ωt
eb1b2 = E0 sin (ωt - 2π/3)
ec1c2 = E0 sin (ωt - 4π/3)

Here, E0 represents the maximum magnitude of the induced EMF, ω is the angular frequency, and t represents time.