2005 Electrical and Electronics 2005 Anna University B.E EEE SOLID STATE DRIVES Question paper

Course: B.E Electrical and Electronics
University: Anna University

2005 Anna University B.E EEE SOLID STATE DRIVES EE-432 Question paper
B.E./B.Tech. DEGREE EXAMINATION, NOVEMBER/DECEMBER 2005
Seventh Semester
Electrial and Electronics Engineering
EE 432 – SOLID STATE DRIVES
Time : Three hours Maximum : 100 marks
Answer ALL Questions
PART – A [MARKS 10 x 2 = 20]
1. Classify teh drive motors based on their speed torque characteristics.
2. List the factors that govern the choice of rating of a motor to be used as a drive element.
3. What is meant by critical speed in phase controlled dc drives?
4. What causes poor input power factor in phase controlled dc drives?
5. What is the need for constant v/f ratio in variable voltage induction motor drives?
6. Sketch the mechanical characteristics of a three phase induction motor with v/f control.
7. Give the expression for torque of a static rotor resistance controlled three phase induction motor.
8. Where are closed loop control schemes of induction motors preferred?
9. Name the different modes of operation of a synchronous motor drive.
10. What is meant by brushless excitation?

PART – B (5 x 16 = 80)
11. (i) Build up the speed torque curves of a DC series motor with shunted armature connection. [MARKS 5]
(ii) Distinguish between constant torque and constant H.P. operations of a DC motor. [MARKS 3]
(iii) Explain how you would determine the power rating of an electric motor when it is subjected to
continuous duty and variable load. [MARKS 8]
12. (a) (i) Describe a speed control scheme for a spearately excited dc motor using a single phase half
controlled thyristor bridge converter. [MARKS 8]
(ii) A separately excited dc motor is supplied from a 230 Volts 50 Hz source through a single
phase half wave controlled rectifiers. Its field is fed through a single phase semiconverter with
zero degree firing angle delay. The motor armature resistance is 0.7 ohms and motor constant
is 0.5V-sec/radians. For a rated load torque of 15 N-m at 100 rpm and for continuous ripple free
current, determine (1) the firing angle delay of the armature converter (2) input power factor of
the armature converter. [MARKS 8]
(OR)
(b) (i) Suggest a SCR based power circuit, from a dc supply to control the speed of a separately
excited dc motor in both directions and explain its working. [MARKS 8]
(ii) A 230 volts 960 rpm 200 Amps separately excited dc motor has an armature resistance of
0.02 ohms. The motor is fed from a dc source of 230 volts through a chopper. Assuming
continuous conduction (1) Calculate the duty ratio of chopper for motoring operation at rated
torque and 350 rpm. (2) If maximum duty ratio of chopper is limited to 0.95 and maximum
permissible motor speed obtainable without field weakening. [MARKS 4+4]
13. (a) (i) Using a diagram and torque speed curve, explain the stator voltage control scheme for the
speed control of a three phase induction motor. [MARKS 6]
(ii) Bring out the limitations of the above speed control scheme. [MARKS 3]
(iii) Explain how voltage and frequency are varied in voltage source inverter fed induction motor
drives. [MARKS 7]
(OR)
(b) (i) Induction motor speed control with constant supply voltage and reduced supply frequency is
rarely used in pratice. -Justify [MARKS 5]
(ii) Describe the stator frequency control scheme for the speed control of three phase induction
motors. [MARKS 6]
(iii) Why in this method of speed control an induction motor is said to be working in the field
weakening mode. [MARKS 5]
14. (a) (i) Explain using a power circuit how static rotor resistance control serves to vary the speed of a
three phase slip ring induction motor. [MARKS 7]
(ii) Bring out its advantages and limitations. [MARKS 3]
(iii) Derive the expression for torque of the induction motor, when it is working in above scheme.
[MARKS 6]
(OR)
(b) (i) Draw and explain the slip power recovery scheme applicable for three phase slipring induction
motors. [MARKS 8]
(ii) Show how the above shceme can offer both subsynchronous and super synchronous ranges
of speed. [MARKS 4]
(iii) Compare slip power recovery shceme with rotor resistance control. (4)
15. (a) (i) Describe using a circuit the self contrlled mode of operation of a synchronous motor. [MARKS 10]
(ii) Bring out the need and advantages of closed loop control in a synchronous motor drive. [MARKS 6]
(OR)
(b) (i) Suggest a scheme for speed control of synchronous motors using current source inverters and
explain its working. [MARKS 10]
(ii) Discuss how a synchronous motor operating in an overexcited state does not require a
commutation circuit and serves to improve the input power factor. [MARKS 6]