# midterm elmach 2020

Fatih Sultan Mehmet
Vakif University
Dept. of Elec. Electronics Eng.
EEM 19372E-Electrical Machines
Dr. Mehmet Onur GULBAHCE
2019-2020 Spring Sms-Midterm Exam
Name-Surname:
Student ID:
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Questions:
Points:
1
30
2
30
3
20
4
30
Total
110
BEFORE STARTING THE EXAM ,
The exam will be held on Zoom Video Communication System synchronously.
All notes are open and Matlab/Simulink is free to use during the exam. However, you should not communicate
with each other.
After the exam period is over, you must save the exam answer sheets with the name-surname.pdf and upload
them to the MOODLE system within the loading time given to you. Otherwise, all responsibility belongs to
you.
There is no relationship between the order of questions and their diculty. It is recommended that you do
not start answering unless you are sure you have read and understood all questions.
Exam duration is 120 minutes.
Exam papers with improbable errors are considered as cheating and the parts whose logical structure is not
understood are not included in the assessment.
QUESTIONS
30
1. a) Explain the production of the rotating magnetic eld in a three-phase alternating current winding.
b) Sketch the speed-torque curve of the induction machine. Show the characteristic points on the
curve; motor, generator and brake operating regions.
c) What does braking mean in an induction motor? Describe based on the denition of slip. Give
the names of braking methods of induction motor. Describe one of your choice.
d) Why should we use some starting methods when starting the induction motor? Explain softstarting method briey.
e) What is the operating principles of Doubly-Fed Induction Generators (DFIG). Why we use it
mostly in wind turbines.
20
2. A three-phase, 4 kW, 380V, 50 Hz, wye connected induction motor draws 7.8A from the source at
0.85 lagging power factor. The friction and vantilation loss is 55W and eciency of motor is 85%.
Calculate:
a) The shaft power (Pshaf t ),
b) Induced shaft power (Pim ),
c) Rotor copper losses (Pcu2 ),
c) Air-gap power (Pi ),
d) Input power (P1 ),
e) Stator copper losses and iron losses sum (Pcu1 + Pf e ) in rated operation.
30
3. A three-phase, Y-connected, 460-V (line-line), 25-kW, 60-Hz, four-pole induction motor has the
following equivalent-circuit parameters in ohms per phase referred to the stator:
Midterm Exam, Page 1/2
Dr. Mehmet Onur G&Uuml;LBAH&Ccedil;E
R1 = 0.103 Ω
EEM 19372E-Electrical Machines
0
R2 = 0.225 Ω
X1 = 1.10 Ω
0
X2 = 1.13 Ω
Xm = 59.4 Ω
The total friction and windage losses may be assumed constant at 265 W, and the core loss may
be assumed to be equal to 220 W. Rated slip of motor is 2%. With the motor directly to a 460-V
source,
a) Calculate the input current of motor
b) Calculate the eciency of motor
30
4. In order to determine the &quot;T&quot; equivalent circuit parameters of the induction motor, following
experiments are carried out after the machine reaches the permissible winding temperature:
Experiment 1: When 3V DC voltage is applied to two of the stator terminals of the star connected
motor, it draws 10A current.
Experiment 2: When 400V is applied between phases while the motor is no-load condition, it
draws 3.1 A current and 1540W power from the network.
Experiment 3: When 350V is applied between phases while the motor is no-load condition, it
draws 2.71 A current and 1200W power from the network.
Experiment 4: When the motor shaft is prevented from rotating and 110V is applied between
phases, it draws 10.5 A current and 1260W power from the mains.
Experiment 5: In blocked rotor test, when 110V is applied between phases, it draws 10.5 A
current and 1260W power from the network.
Based on the given results of the experiments, calculate the approximate &quot;T&quot; equivalent circuit
parameters of the induction motor. (Note that friction and ventilation losses do not change with
the voltage applied to the stator windings)
May the Lord give clarity of mind ,
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