King Fahd University of Petroleum and Minerals
1
Electrical Engineering Department
Major II Solution Key- 171
EE-306 Electromechanical Devices
Thursday, 14 Dec 2017, 6:00PM - 7:30PM
First Name
Last Name
Muhammad Khalid
Ibrahim Habiballah
ID
Sec 1
Sec 2
Sec 5
Sec 6
Sr. #
Muhammed Worku
Sec 4
Guidelines
Read the following instructions:
• Check that your full name and ID are written in the appropriate boxes above.
• This exam solution has
10 pages including all blank pages. All pages are numbered.
• Respond to all questions. Clear hand writing is required.
• Multiple responses to the same question will not be corrected.
• Cell phones should be turned off and exchange of calculators is not permitted.
• Do not leave the examination room without the permission of the Instructor.
• Ask the Instructor for any assistance.
Grading
Table 1: MCQ Answers
Q1
Q2
Q3
Q4
Q5
Q6
Q7
Q8
Q9
Q10
(d)
(b)
(a)
(c)
(b)
(a)
(c)
(a)
(a)
(b)
MCQs
Problem 1
Problem 2
Problem 3
Problem 4
Marks Obtained
/ 30 Marks
/ 15 Marks
/ 20 Marks
/ 15 Marks
/ 20 Marks
/ 100 Marks
King Fahd University of Petroleum and Minerals
Electrical Engineering Department
2
Multiple-Choice Questions. (10 x 3 = 30 Marks)
Note: For each independent question, CIRCLE one and only one answer among the proposed responses. Put
your answers in the Table 1 at the cover page.
Q1
A wave connected triplex wound 4-pole DC machine generates 1440 V running at 1800 rpm when
excited by the magnetic flux of 0.25 Wb.
The total number of conductors in the armature winding should be:
(a) 144
(b) 864
(c) 48
(d) 288
Q2
In a 12-pole simplex lap-wound DC machine, each current path can carry a current of 100 A.
The rated armature current of this machine is:
(a) 600 A
(b) 1200 A
(c) 2400 A
(d) 3600 A
Q3
A 220 V, 7 hp DC series motor has armature resistance (Ra ) and series field resistance (Rse ) of 0.6
Ω and 0.4 Ω respectively. The motor is running at 300 rpm and draws a current of 15 A when
supplying a certain load. Neglect rotational losses, and assume that there is no external resistance
connected to the armature circuit.
The torque developed by this machine is:
(a) 97.88 N m
(b) 310.4 N m
(c) 155.2 N m
(d) None of the above
EE-306 Major II
171
King Fahd University of Petroleum and Minerals
Electrical Engineering Department
3
Q4
In a DC series motor, if the field current is reduced by 50%, the torque of the motor will be:
(a) 50 % less
(b) the same
(c) 75 % less
(d) 25 % less
Q5
DC machines can operate under any power factor conditions.
(a) True
(b) False
Q6
Suppose a 3-phase synchronous motor is running in a clockwise direction. In case the direction of
its field current is reversed (i.e, field excitation is reversed), the result will be:
(a) The motor will continue to run in the same direction
(b) The motor will run in the reverse direction
(c) The winding of the motor will burn
(d) The motor will stop
Q7
A 3-phase, 230 V, 4 pole, ∆-connected synchronous machine has an induced emf of 225∠150 V.
The machine is operating as a:
(a) Synchronous motor
(b) Load information is required
(c) Synchronous generator
(d) Cannot be determined
EE-306 Major II
171
King Fahd University of Petroleum and Minerals
Electrical Engineering Department
4
Q8
Identify the incorrect statement from the following:
(a) The frequency of currents in the rotor of a synchronous machine is equal to the stator frequency
(b) Synchronous machine having a salient pole rotor type construction has a non-uniform airgap
(c) Cylindrical type of rotor construction is suitable for high speed operation
(d) Synchronous machines can operate at any power factor
Q9
A 3-phase, 200 hp, 2400 V, 60 Hz, 4 pole, Y connected synchronous motor with synchronous reactance of 12 Ω per phase produces a maximum torque of 2575 N-m.
The induced emf in the motor is:
(a) 1401 V
(b) 700 V
(c) 2802 V
(d) 350 V
Q10
A 3-phase, 1500 kVA, 13.2 kV Y-connected synchronous generator, is supplying a real power of
120 kW and a reactive power of 900 kVAR to a 3-phase load. Identify the correct statement about
this machine.
(a) The field winding of the generator is under-excited
(b) The field winding of the generator is over-excited
(c) The induced voltage (EMF) in the generator is smaller than the terminal voltage
(d) The generator is operating at leading power factor
EE-306 Major II
171
King Fahd University of Petroleum and Minerals
Electrical Engineering Department
5
Problem 1 [15].
A short-shunt DC generator has armature, shunt field and series field winding resistance of 0.2 Ω,
200 Ω, and 0.1 Ω, respectively. The terminal voltage (or load voltage) is 240 V when the generator’s
induced emf is 255 V.
Calculate the power delivered to the load.
Solution
EE-306 Major II
171
King Fahd University of Petroleum and Minerals
Electrical Engineering Department
EE-306 Major II
171
6
King Fahd University of Petroleum and Minerals
Electrical Engineering Department
7
Problem 2 [10+10 = 20].
A 250 V, 1600 rpm, DC shunt motor has an armature resistance of 0.1 Ω and a field resistance of 125
Ω. The motor draws a current of 24 A when delivering rated output at its rated speed. Assume
contact voltage drop per brush of 2 V.
(a) Calculate the developed power
(b) The efficiency of the motor (neglect rotational losses)
Solution 1
EE-306 Major II
171
King Fahd University of Petroleum and Minerals
8
Electrical Engineering Department
Solution 2
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EE-306 Major II
171
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King Fahd University of Petroleum and Minerals
Electrical Engineering Department
9
Problem 3 [12+3 = 15].
A 3-phase, 20 kVA, 400 V, 4 pole, 60 Hz, ∆-connected synchronous generator has an armature
resistance of 0.25 Ω per phase and a synchronous reactance of 4.0 Ω per phase respectively. The
field current and mechanical power are adjusted such that the generator delivers 8 kVAR at 0.85
power factor lagging.
(a) Determine the voltage regulation
(b) Find the power angle
Solution
EE-306 Major II
171
King Fahd University of Petroleum and Minerals
Electrical Engineering Department
EE-306 Major II
171
10
King Fahd University of Petroleum and Minerals
Electrical Engineering Department
11
Problem 4 [10+10 = 20].
A 3-phase, 50 hp, 400 V, 60 Hz, Y-connected synchronous motor has a full load efficiency of 88%.
The synchronous reactance of the motor is j1.6 Ω per phase where armature resistance is neglected.
The excitation of the motor is adjusted so as to give 0.9 leading power factor.
Calculate the following at full load conditions:
(a) The magnitude of induced emf (line) of the given 3-phase motor
(b) The total mechanical power developed
Solution
!End of Exam Solutions!
EE-306 Major II
171