King Saud University
College of Engineering
Electrical Engineering Department
EE435-Electric Drives
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‫اﻻﻣﺘﺤﺎن ﻣﻜﻮن ﻣﻦ ورﻗﺘﻴﻦ‬
2nd Semester 1426-1427H
Final Exam
Time: 3.0 hrs.
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:‫اﺳﻢ اﻟﻄﺎﻟﺐ‬
Answer All Questions:
Question1
(a) Derive and sketch the relation between angular velocity and induced torque for series
DC motor.
(b) Draw and explain the speed torque curves of series and separately excited DC motors
in speeds lower and higher than rated speed.
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(c) A 220 V, 50 Hz single-phase supply with 4mH source inductance is connected to
buck converter via single-phase full-wave uncontrolled rectifier. The buck converter has
the following values Vo=20 V, Io=10 A, C=500 μ F, L = 10μH and f s =40 kHz.
(i) Find the duty ratio. (ii) Calculate the ripples in output voltage, ΔVo.
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(d) The speed of a 125 hp, 680 V, 1200 rpm, separately excited dc motor is controlled
by a single-phase full bridge controlled converter.The converter is operated from a 600
V, 60 Hz supply. The rated armature current of the motor is 170 A. The motor
parameters are Ra = 0.1Ω , and Kφ = 0.4 V/rpm. The converter and ac supply are
considered to be ideal.
(i) Draw the converter output voltage and supply current at rated load and α = 30°.
(ii) Find the input power factor and THD of supply current at rated load and α =30°.
(iii) Find no-load speeds at firing angle α = 30°. Assume that, at no load, the armature
current is 12% of the rated current and is continuous.
(iv) Find the firing angle to obtain the rated speed of 1200 rpm at rated
Question 2
(a) Draw the three-phase PWM converter circuit and explain the operation of this
converter in controlling three-phase induction motor. Also, explain and draw the circuit
of the unipolar and bipolar PWM.
(b) In a constant flux control of 3-phase induction motor, prove that the maximum torque
is constant for any operating frequency in a < 1 range.
(c) A 480V, 60 Hz 4 pole 1710 rpm, Y-connected induction motor has the following
parameters per phase: Rs = 0.3Ω , Rr′ = 0.15Ω , X s = X r′ = 1.1Ω
X m = 40 Ω
The motor is controlled by variable frequency control at a constant flux of rated value.
(i) Calculate the speed and stator current at half the rated torque and 20 Hz supply.
(ii) Solve part (i) assuming the speed-torque curves to be parrallel stright lines for s < sm .
(iii) Calculate the frequency, the stator current, and voltage at rated breaking torque and
1400 rpm.
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:‫اﺳﻢ اﻟﻄﺎﻟﺐ‬
(d) Three phase 4-pole 380V 50Hz, Y-connected induction motor is connected to (600V
60 Hz single-phase supply) via single-phase full wave uncontrolled rectifier, boost
converter and 3-phase PWM inverter as shown in the following figure. This system is
used to control the speed of the induction motor by E/f control. The switching frequency
of the boost converter is 20kHz and its inductance is 1mH. The DC link voltage is
maintained constant at Vo =650V, the load torque is maintained constant at 70N.m. The
induction motor has the following parameters per phase: Rs = 0.28Ω , Rr′ = 0.17Ω ,
X s = X r′ = 1.2 Ω
X m = 42 Ω ,
Assuming the speed torque curves is parallel straight lines for s < sm .
1- If it is required to drive the motor at 800 rpm, determine;
(i) The slip of induction motor.
(ii) The modulation index m a of the PWM inverter.
(iii) The duty ratio of the boost converter.
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2- If it is required to drive the motor at 70Hz control signal frequency of the PWM,
determine:
(i) The speed of rotation.
(ii) The modulation index m a of the PWM inverter.
(iii) The duty ratio of the boost converter.
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