SNS COLLEGE OF ENGINEERING
Kurumbapalayam(Po), Coimbatore – 641 107
Accredited by NAAC-UGC with ‘A’ Grade
Approved by AICTE & Affiliated to Anna University, Chennai
SAMPLE QUESTION PAPER WITH ANSWER KEY
COURSE: B.E – MECHANICAL
EE6351 ELECTRICAL DRIVES AND CONTROLS
Class: III Sem Mech A, B & C
Answer ALL questions
PART – A
1. Define group drive system
Group drive consists of a single motor, which drives by one or more shafts. This line
shaft may be fitted by pulley or belts by means of group of machines to be operated called group
drive.
2. What is meant by dynamic braking?
When an electric motor rotates, kinetic energy is stored in its rotating mass. During
dynamic braking the kinetic energy of the motor is converted in to electric energy and this
energy is dissipated in resistive elements.
3. Examine the need for starters in electric motors?
When induction motor is switched on to the supply, it takes about 5 to 8 times full load
current at starting. This starting current may be of such a magnitude as to cause voltage drop in
the lines. So Starters are necessary to start, limit starting current and also for protection.
4. Why rotor resistance stator only suitable for slip ring induction motor?
To include resistance in the rotor circuit there by reducing the induced rotor current at
starting. This can be implemented only on a slip ring induction motor.
5. What is the use of chopper circuit in DC motor?
It converts fixed dc voltage in to variable dc voltage. Also it has high efficiency,
flexibility in control, light weight, quick response etc. Moreover it is highly used for speed
control of motors because it has low harmonics than any other devices.
6. Define term Slip.
7. What is mean by duty cycle?
It is defined as the ratio between on time chopper and total time of chopper.
8. How the speed of induction motor is more than its synchronous speed in static Kramer
system?
The Kramer system is only applicable for sub-synchronous speed because the slip power
fed back to the supply,
9. State the advantages of solid state speed control compared to conventional speed control.
* More Accurate.
* Flexible to control.
* Dynamic Performance Improved.
* Low cost & Less Maintenance.
* High Efficiency.
10. State any two merits of slip power recovery scheme.
1. Overall Efficiency is improved.
2. Slip power can be recovered and fed back to the supply.
PART – B
11. (a) (i) Explain the heating and cooling curves of an electrical machine with necessary
derivation.
11. (b) (i) With neat sketch, explain the types of classes of duty of an electric motor
12. (a) (i) Enumerate various braking characteristics of dc motor.
12. (b) (i) Apply the multi-quadrant operation for driving hoist load.
13. (a) (i) Illustrate the working of four point starter with neat diagram and demerits of
four point stator over three point starter.
13. (b) (i) Compare slip ring and squirrel cage rotor of an induction motor.
13. (b) (ii) A three phase, 4 pole, 440 V induction motor rotor resistance and reactance at
stand-still condition are 0.5 ohm and 0.8 ohm respectively. The supply frequency is 50 Hz.
Calculate the starting rotor current and also rotor current when speed is 1440 rpm. Neglect
stator impedance.
14. (a) (i) Draw the Schematic Diagram of Ward Leonard control system and explain its
operation.
14. (b) (i) Explain type-E chopper circuit and their function with neat diagram.
15. (a) (i) Explain the types of slip power recovery system with neat diagram.
15. (b) (i) Describe how the speed control of a current source inverter fed synchronous
motor is controlled in its self controlled mode.