CORK INSTITUTE OF TECHNOLOGY
INSTITIÚID TEICNEOLAÍOCHTA CHORCAÍ
Autumn 2015
Module Title:
Electrical Science – AC Theory
Module Code:
ELEC 6019
School:
Mechanical, Electrical & Process Engineering
Programme Title:
Bachelor of Engineering in Electrical Engineering – Year 1
Programme Code:
EELEC_7_Y1
External Examiner(s): Mr. Michael Radley, Ms Siobhan Wall
Internal Examiner(s):
Mr Sreto Boljevic
Instructions:
Answer any four questions.
Duration:
2 Hours
Sitting:
Autumn 2015
Requirements for this examination:
Note to Candidates: Please check the Programme Title and the Module Title to ensure that you are attempting
the correct examination.
If in doubt please contact an Invigilator.
Question 1
An alternating voltage is given by v=100sin(50πt – 0.3) Volts.
Find the following:
i)
The amplitude (or peak value),
ii) Peak – to peak value,
iii) The r.m.s value of the voltage
iv) The periodic time,
v) The frequency,
vi) Form factor,
vii) The phase angle (in degrees) relative to 100sin(50πt) sine wave.
Total (25 marks)
Question 2
A coil of resistance 5 Ohms and inductance 120 mH in series with a 100 micro Farad capacitor is
connected to a 300V, 50 Hz supply. Draw the circuit diagram and determine :
i.
ii.
iii.
iv.
v.
vi.
The current through the circuit
The phase difference between the supply voltage and current
The voltage across the coil
The voltage across the capacitor
Total amount of Watts, VAr, and VA consumed
Draw the phasor diagram of the circuit showing the circuit current and voltages across the coil and
capacitor
Total (25 marks)
Question 3
a) A coil of resistance 100 ohms and inductance 100 micro Henry is connected in series with
100 pico Farad capacitor. The circuit is connected to a 10V variable frequency supply. Draw the
circuit diagram and determine:
i)
ii)
iii)
iv)
b)
The resonant frequency
Current at resonance
Voltage across L and C at Resonance
Q factor of the circuit.
Considering the R-L-C series circuit, explain power triangle.
(18 marks)
(7 marks)
Total: (25 marks)
Question 4
Two impedances Z1= (6 + j6)Ω and Z2= (3 – j3)Ω are connected in parallel across 230V, 50 Hz supply.
Draw the circuit diagram and determine:
i) Current through each branch
ii) Total circuit supply current
iii) Circuit power factor
iv) Total amount of Active, Reactive and Apparent power taken by the circuit
v) Draw the phasor diagram showing the following phasors: circuit currents and supply voltage.
Total (25 marks)
Question 5
A 10Ω resistor, a 20mH inductor and 159µF capacitor are connected in parallel across the 200V, 50Hz
source. Draw the circuit diagram and determine:
i) Current through each circuit component
ii) Total supply current
iii) Power factor of the circuit
iv) Total amount of Watts, VAr, VA consumed by the circuit
v) Draw the phasor diagram òf the circuit showing the following phasors: circuits through each
circuit component, total supply current and supply voltage.
Total (25 marks)
Question 6
A 230V, 50Hz single phase motor takes 60A and operates at a power factor of 0.8 lagging. If a capacitor
of 159µF is connected in parallel with the motor. Sketch the circuit diagram and determine:
i) The new line current
ii) The new power factor
iii) Draw the phasor diagram of the circuit
iv) Using the results discussed benefits gained from power factor improvement.
Total (25 marks)