CORK INSTITUTE OF TECHNOLOGY
INSTITIÚID TEICNEOLAÍOCHTA CHORCAÍ
Semester 2 Examinations 2014/15
Module Title: Alternating Current Theory
Module Code:
ELEC6028
School:
Mechanical, Electrical and Process Engineering
Programme Title(s):
Bachelor of Engineering (Honours) in Electrical Power Systems
Programmes Code(s):
EEPSY_8_Y1
External Examiner(s):
Mr Traoloch O’Brien, Ms Valerie Kenneally
Internal Examiner(s):
Mr Sreto Boljevic
Instructions:
Answer FOUR Questions.
Each question carries equal marks
Duration:
2 Hours
Sitting:
Summer 2015
Requirements for this examination:
1
Question 1
An alternating voltage v is given by: v=120sin314t – 0.25) V
Find:
a)
b)
c)
d)
e)
f)
g)
h)
i)
j)
k)
The maximum value
Angular frequency
Frequency
Time period
Instantaneous value when t is 2 sec and 5sec
Average value
R.M.S value
Peak factor
Form factor
Peak to peak value
Sketch the wave form of the alternating voltage
(2)
(2)
(2)
(2)
(4)
(2)
(2)
(2)
(2)
(2)
(3)
Total (25)
Question 2
a) A series R-L circuit supplied by 230V, 50Hz takes a current of 12A, lagging 20◦.
Draw the circuit diagram and
Find:
i)
ii)
iii)
iv)
v)
vi)
The resistance
The reactance of the circuit
Impedance of the circuit
Inductance of the circuit
Power factor
Draw the phasor diagram of the circuit showing the voltages
across the resistor, inductor and supplied voltage.
(12)
b) A series R-C circuit supplied by 230V, 50Hz takes a current of 12A, leading 20◦. Draw
the circuit diagram and
Find:
i)
The resistance
ii)
The reactance of the circuit
iii) Impedance of the circuit
iv) Capacitance of the circuit
v)
Power factor
vi) Draw the phasor diagram of the circuit showing the voltages
across the resistor, inductor and supplied voltage.
(13)
Total
2
(25)
Question 3
A coil of resistance 6Ω and inductance 100mH in series with a 100µF is connected to a
230 V, 50Hz supply. Draw the circuit diagram and calculate:
i) The circuit current
(5)
ii) The phase angle between the supply voltage and current
(4)
iii) The voltage across the coil
(3)
iv) The voltage across the capacitor
(2)
v) Power factor of the circuit
(3)
vi) Total amount of watts,VAr and VA consumed by the circuit
(4)
vii) Draw the circuit phasor diagram
(5)
Total
(25)
Question 4
A coil of negligible resistance and 100mH inductance is connected in series with a
capacitance of 4µF and resistance of 12Ω across a 100V, variable frequency supply.
Draw the circuit and calculate:
i)
ii)
iii)
iv)
v)
vi)
The resonant frequency
The current at resonance
The voltage across the coil and capacitor at resonance
The Q – factor of the circuit
Total amount of watts consumed at resonant frequency
Draw the phasor diagram of the circuit at resonant frequency
(3)
(4)
(6)
(3)
(3)
(6)
Total
(25)
Question 5
A coil of 150mH inductance and 35 Ω resistance is connected in parallel with 25µF
capacitor across a 230V, 50Hz supply. Draw the circuit diagram and calculate:
i) The current in the coil and its phase angle
(3)
ii) The current in the capacitor and its phase angle
(3)
iii) The circuit impedance
(2)
iv) The supply current and its phase angle
(4)
v) Power factor of the circuit
(3)
vi) The total amount of watts, VAr and VA consumed by the circuit (5)
vii) Draw the circuit phasor diagram
(5)
Total
3
(25)
Question 6
For a single-phase motor that takes 50A at a power factor of 0.7 lagging from 230V, 50
Hz supply, determine:
i) The current taken by a capacitor connected in parallel with the motor
to correct the power factor to unity.
ii) The value of the supply current after power factor correction
iii) The capacitance of the capacitor
iv) kVAr rating of the capacitor
v) Using the results obtain from above discussed the benefits
of unity power factor
Total
4
(6)
(6)
(4)
(4)
(5)
(25)