Set No. 1
Code No: M0221/R07
IV B.Tech I Semester Supplementary Examinations, December
2013
POWER SEMICONDUCTOR DRIVES
(Electrical & Electronics Engineering)
Time: 3 hours
Max Marks: 80
Answer any FIVE Questions
All Questions carry equal marks
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1. Give the circuit layout for single phase DC drives .Enumerate the various single
phase DC drives used.
[16]
2. The speed of a 25 HP 380V 1800 rpm separately excited dc motor is controlled by a
three phase semi controlled converter and is fed from a 415V, 50 hz supply .motor
parameters are Ra=0.9Ω, La=10mh.Ka=50.Calculate the speed of the motor at a
torque of 50 Nm when the converter is fired at 450. Neglect losses.
[16]
3. A dc shunt motor has the armature resistance of 0.04Ω and the field winding resistance of 10Ω. Motor is coupled to an over hauling load with a torque of 400N-m.
Following magnetization curve was measured at 600 rpm:
Field Current, A
2.5 5 7.5 10 12.5 1.5 17.5 20 22.5 25
Back emf, V
25 50 73.5 90 102.5 110 116 121 125 129
Motor is braked by self excited dynamic breaking with a breaking resistance of 1
ohm. At what speed motor will hold the load?
[16]
4. A 230V, 960rpm and 200A separately excited dc motor has an armature resistance
of 0.02Ω. The motor is fed from a chopper, which is capable of providing both
motoring and braking operations. The source has a voltage of 230V. Assuming
continuous conduction: When motor is operated in Dynamic breaking, with Braking resistance of 2 ohms
(a)calculate Duty ratio of chopper for a motor speed of 600 rpm and breaking
torque of twice the rated value
(b)What will be the motor speed for a duty ratio of 0.6 and motor torque equal to
twice its rated value?
[16]
5. Discuss why the rotor of an Induction motor should have very large rotor resistance
when it is controlled from a three phase ac voltage controller.
[16]
6. Compare the performance of VSI & CSI fed.IM drives.Mention advantages & Disadvantages.
[16]
7. Discuss briefly about the traditional Scherbius system. Explain how the same
operation can be achieved using solid state Scherbious system.
[16]
8. (a) Why a self controlled mode is free from hunting oscillations?
(b) When operating in true synchronous mode, why the frequency must be changed
in small steps?
[8+8]
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Set No. 2
Code No: M0221/R07
IV B.Tech I Semester Supplementary Examinations, December
2013
POWER SEMICONDUCTOR DRIVES
(Electrical & Electronics Engineering)
Time: 3 hours
Max Marks: 80
Answer any FIVE Questions
All Questions carry equal marks
⋆⋆⋆⋆⋆
1. A dc series motor has Ra = 3Ω , Rf = 3 Ω and Maf = 0.15 H. The motor speed
is varied by a phase-controlled bridge. The firing angle is π 4 and the average speed
of the motor is 1450 rpm. The applied ac voltage to the bridge is 330 Sin wt.
Assuming continuous motor current find the steady state average motor current
and torque. Sketch the waveforms for output voltage, current and gating signals.
[16]
2. A 3Φ half wave bridge comprising three thyristors is fed from a 277rms , line to
neural , 60Hz supply and provides an adjustable dc voltage at the terminals of a
separately excited dc motor. The motor has Ra=0.02, La =.001H, Ka=1.2 and full
load Ia = 500A. Find the firing angle so that the motor operates at full load current
and at rated speed of 200rps. Assume continuous conduction and neglect thyristor
forward voltage drop.
[16]
3. With a neat diagram, explain the operation of a dc drive in all four quadrants when
fed by a single phase dual converter with necessary waveforms and characteristics.
[16]
4. Explain the Dynamic Breaking of Separately excited D.C motor by Chopper control.
[16]
5. (a) How the speed control of I.m is achieved by1Q A.C voltage controller
(b) Derive O/P voltage of a 1Q AC voltage controller and Draw O/P voltage &
Current waveforms.
[6+10]
6. Derive torque speed characteristic of an Induction motor operating on variable
frequency supply for
(a) V/f=Constant.
(b) E/f =Constant.
[8+8]
7. A 3 Phase,400V,50 Hz,10 KW 960 rpm, 6 pole star connected slip ring Induction
motor has the following constants referred to the stator.
Rs = 0.4 Ohm, R’r = 0. 6 Ohm, Xs = Xr ’ = 1.4 Ohm. The motor drives a fan
load at 960 rpm. The Stator to rotor turns ratio is 2.
What resistance must be connected in each phase of the rotor circuit to reduce the
speed to 800 rpm?
[16]
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Set No. 2
Code No: M0221/R07
8. In variable frequency control of asynchronous motor why (V/f) ratio is maintained
constant up to base speed and V constant above base speed. Draw the relevant
characteristics.
[16]
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Set No. 3
Code No: M0221/R07
IV B.Tech I Semester Supplementary Examinations, December
2013
POWER SEMICONDUCTOR DRIVES
(Electrical & Electronics Engineering)
Time: 3 hours
Max Marks: 80
Answer any FIVE Questions
All Questions carry equal marks
⋆⋆⋆⋆⋆
1. Derive the Speed, Torque Equations of a fully controlled converter connected to
D.C series motor with continuous current operation with necessary waveforms.
[16]
2. The speed of a 25 HP 380V 1800 rpm separately excited dc motor is controlled by a
three phase semi controlled converter and is fed from a 415V, 50 hz supply .motor
parameters are Ra=0.9Ω, La=10mh.Ka=50.Calculate the speed of the motor at a
torque of 50 Nm when the converter is fired at 450. Neglect losses.
[16]
3. Explain the four quadrant operation of dc motors using dual converters.
[16]
4. (a) List the advantages offered by dc chopper drives over line-commutated converter controlled dc drives.
(b) A dc chopper controls the speed of dc series motor. The armature resistance
Ra = 0.04Ω, field circuit resistance Rf = 0.06Ω, and back emf constant Kv =
35 mV/rad/s. The dc input voltage of the chopper Vs=600V. If it is required
to maintain a constant developed torque of Td = 547N-m, plot the motor
speed against the duty cycle K of the chopper.
[8+8]
5. A 3 phase Squirrel cage Induction motor having 6 poles operating at 50 Hz has
rotor resistance and stand still reactance referred to stator are 0.2 ohm and 1 ohm
per phase when it is operated at rated voltage and rated frequency has a slip of
4 percent at full load . At what speed the motor will operate when the stator
impressed voltage is reduced to 1/1.414 times the rated voltage. Assume that the
load torque remains constant at the rated motor torque.
[16]
6. Why Current source inverter fed Induction motor is operated at constant rated
flux? Explain the principle of operation of Current source Inverter fed Induction
motor.
[16]
7. Draw the circuit diagram and explain the operation of rotor- resistance control
using chopper. Mention the advantages and disadvantages of the above method of
control.
[16]
8. Explain separate control & self control of synchronous motor.
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[16]
Set No. 4
Code No: M0221/R07
IV B.Tech I Semester Supplementary Examinations, December
2013
POWER SEMICONDUCTOR DRIVES
(Electrical & Electronics Engineering)
Time: 3 hours
Max Marks: 80
Answer any FIVE Questions
All Questions carry equal marks
⋆⋆⋆⋆⋆
1. (a) What are the assumptions made while doing the steady-state performance of
the converter fed dc drives. Justify your answers.
(b) Explain the use of freewheeling diode in the converter fed dc drives. Take an
example of 1-phase fully controlled converter for explanation. How it is going
to affect the machine performance?
[8+8]
2. The speed of a separately excited dc motor is controlled by means of a 3 phase semi
converter from a 3 phase 415V 50Hz supply. The motor constants are inductance
10mH, resistance 0.9 ohm and armature constant 1.5v/rad/s.calculate speed of the
motor at a torque of 50 Nm when the converter is fired at 450. Neglect losses in
the converter.
[16]
3. (a) What is 4-quadrant operation and explain with converters?
(b) Give a simple circuit for the speed control of a dc separately excited motor.
[8+8]
4. A dc series motor is powered by a simple (first quadrant) chopper from a 600V
dc source. Ra=0.03Ω, Rf =0.05Ω. the back emf constant kv=15.27Mv/A-rad/s.
Ia=450A. The armature current is continuous and negligible ripple. If duty cycle
is 75%, find
(a) input power
(b) equivalent input resistance
(c) speed and torque developed of the motor.
[16]
5. Discuss in detail with suitable diagrams how the speed of a 3 phase cage type
Induction motor can be varied using A.C. voltage controllers. Also draw the various
types of AC voltage controllers which are used for speed control of Induction motor.
[16]
6. Explain the following for variable frequency control of Induction motor.
(a) The motor has higher efficiency and better low speed performance when fed
from a pulse width modulated inverter instead of six step inverter.
(b) Cyclo-converter is suitable for low speed applications only.
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[16]
Set No. 4
Code No: M0221/R07
7. Explain static motor resistance control for speed control of I.M. Draw speed &
torque charecterstics.
[16]
8. With suitable circuit diagrams explain the principle of operation of Current source
Inverter fed Synchronous motor drive.
[16]
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