(AUTONOMOUS)
B. Tech VII Semester Supplementary Examinations, May 2018
(Regulations: VCE-R11/R11A)
POWER SEMICONDUCTOR DRIVES
(Electrical and Electronics Engineering)
Date: 21 May, 2018 AN Time: 3 hours Max Marks: 75
Answer ONE question from each Unit
All Questions Carry Equal Marks
Unit I
1. With neat circuit diagram and relevant waveforms, explain the operation of single phase
semi controlled converter feeding armature of separately excited DC motor operating in
continuous conduction mode. Derive an expression for average value of an output
voltage.
7M
A 200V, 10.5A, 2000rpm, shunt motor has armature and field resistances of 0.5Ω and
400Ω respectively. It drives a load whose torque is constant at rated motor torque.
Calculate motor speed if the source voltage drops to 175V.
8M
2. With neat circuit diagram and relevant waveforms, explain the operation of three phase
fully controlled converter feeding armature of separately excited DC motor operating in
continuous conduction mode with firing angle 600. Derive an expression for average value
of an output voltage.
7M
The speed of a 20HP, 210V, 1000RPM series motor is controlled by a single phase semi
converter. The combined field and armature circuit resistance is 0.25Ω. Motor constants
are Kaf=0.03N-mA2 and Kres=0.075V-s/rad. The supply voltage is 230V. Assuming
continuous and ripple free motor current, determine the following for a firing angle of 300
and speed N=1000RPM:
i. Motor torque
ii. Motor current
iii. Input power factor
8M
Unit II
3. What is dynamic braking? With neat sketch and waveform, explain how dynamic braking
is achieved by DC chopper in separately excited DC motor.
7M
With a neat sketch and waveforms, explain the operation of separately excited DC motor
fed from step down chopper obtain the expression for average value of output voltage.
8M
4. Draw and explain the four quadrant operation of chopper fed DC drive. 7M
A 220V, 70A DC series motor has combined armature and field resistance of 0.12 Ω. This
motor is under the process of regenerative braking by a chopper with a source voltage of
220V. From magnetization characteristics, it is noticed that, Ia=70A and N=600RPM and
(back emf) E=202V
i. Calculate motor speed for a duty ratio of 0.5 at motor braking torque equal to the
rated motor torque
ii. Calculate maximum allowable motor speed for a maximum permissible current of 70A
and maximum permissible duty ratio of 0.95
8M
Unit III
5. Discuss about the closed loop operation of induction motor drives. 8M
A 2200V, 2600kW, 735rpm, 50Hz, 8-pole, 3-phase squirrel cage induction motor has the
following parameters referred to the stator: Rs=0.075Ω, Xs=0.45Ω, Xr'=0.55Ω.
Stator winding in delta connected and consists of two sections connected in parallel.
Calculate starting torque and maximum torque as a ratio of rated torque, if the motor is
started by star-delta switching. What is the maximum value of line current during
starting?
7M
Cont…2
2
6. Discuss the operation of cycloconverter controlled induction motor. 7M
With a neat circuit diagram and waveform, discuss about the pulse width modulated
voltage source inverter fed induction motor drive.
8M
Unit IV
7. Discuss about static kramer's drive for recovering slip power in induction motors. 7M
A three phase, 400V, 6-pole, 50Hz delta connected slip ring induction motor has rotor
resistance of 0.2Ω and leakage reactance of 1Ω per phase referred to stator. When driving
a fan load it runs at full load at slip. What resistance must be inserted in the rotor
circuit to obtain a speed of 850rpm? Neglect stator impedance and magnetizing branch.
Stator to rotor turns ratio is 2.2.
8M
8. Mention the advantages of rotor side control methods over stator side control methods
for an induction motor drive. And mention the applications of rotor side control method.
8M
What are the advantages of employing slip power recovery schemes? Compare static
scherbius drive with static kramers drive.
7M
Unit V
9. Discuss the operation of a cycloconverter controlled synchronous motor drive. Also
mention why a cycloconverter controlled synchronous motor drive is preferred over
inverter controlled synchronous motor drive for low speed applications.
8M
A 6MW, 3-phase, 11kV, Y-connected, 6-pole, 50Hz, 0.9pf (leading) synchronous motor
has Xs=9Ω and Rs=0. Rated field current is 50A. Machine is controlled by variable
frequency control at constant V/f ratio up to the base speed and at constant voltage
above base speed. Find the armature current and power factor for half the rated motor
torque, 1500rpm and rated field current.
7M
10. List some advantages and applications of synchronous machines. 5M
Draw and explain the torque speed characteristics of a synchronous motor drive for
frequency control.
10M