# ME2205 Electrical Drives and Control Nov Dec 2011 Question Paper

Anna University Chennai Question Paper
B.E./B.Tech. DEGREE EXAMINATIONS, NOV / DEC 2011
Regulations 2008
Third Semester
Common to Mechanical Engineering, Production Engineering and Chemical Engineering
ME2205 Electrical Drives and Controls
Time: Three Hours
Maximum: 100 marks

Note: New Subject Code in R-2013 is EE6351 Electrical Drives and Controls

Part A – (10 x 2 = 20 marks)
1. Mention any four factors influencing the choice of Electric drives.
2. Define the thermal overload factor for a motor working on short time duty.
3. It is said that the speed of a DC motor depends on the back e.m.f. and on the flux produced. State what kind of proportionality exists between these quantities.
4. Why i s DC series motor used for traction purpose?
5. What i s the basic difference between a three-point starter and a four-point starter?
6. What is the advantage of three phase slip ring induction motor?
7. State whether the speed and torque will increase or decrease when a resistance is inserted in the field dircuit of DC shunt motor.
8. Define Duty cycle of a DC chopper.
9. How to keep the magnetic field in the stator of induction motor approximately constant in the operating range of speeds?
10. Name two methods adopted for varying the slip frequency of induction motor.

Part B – (5 x 16 = 80 marks)
11. (a) (i) Briefly explain, with suitable sketches, the heating and cooling curves of electric motors. (6)
(ii) In a certain electric drive, the temperature rise is 25?C after one hour and 37.5?C after two hours, starting from cold conditions. The ambient temperature i s 30?C.
(1) Find the final steady temperature and the heating time constant.
(2) When disconnected, the temperature falls from the final steady value to 40?C in 1.5 hours; Calculate the cooling time constant. (10)
OR
11. (b) (i) Sketch the relevant load diagrams and explain how the power rating for drive motors is selected for the following classes of load duty:
(2) short time duty (8)
(ii) A motor i s required to deliver a load which follows the following cycle:
50 kW for 10 minutes; no load for 4 minutes; 25 kW for 10 minutes; no load for 6 minutes.
The cycle is repeated indefinitely. Find the suitable capacity of a continuously rated motor for the purpose. (4)
(iii) A motor whose continuous rating i s 20 kW, has a heating time constant of 60 minutes. What load it can deliver for 10 minutes, if this is followed by a shut down period long enough for it to cool? (4)

12. (a) (i) Sketch the following characteristics of DC series motor and derive their corresponding equations:
(1) Torque vs Armature current (3)
(2) Speed vs Armature current (3)
(3) Speed vs Torque (3)
(ii) A DC series motor runs with a speed of 800 r.p.m. and draws a current of 20 A from a supply at 250 V. The armature and series field winding resistances are 0.2 ? and 0.3 ? respectively. A load change causes the current to increase to 50 A. Assuming the flux produced is proportional to current, calculate the speed of the motor at the new load. (7)
OR
12. (b) (i) Draw the approximate equivalent circuit of a three-phase induction motor and derive the torque equation. Sketch also a typical torque-speed characteristic of such a motor and indicate the different operating regions. (10)
(ii) The rotor resistance and reactance at stand-still condition of a three phase four-pole 440 V induction motor are 0.5 ohm and 0.8 ohm respectively per phase. The supply frequency i s 50 cycles/s. Neglect the stator impedance. Calculate the starting rotor current and also the rotor current when the speed is 1440 r.p.m. (6)

13. (a) (i) With a neat sketch, explain the working of a four point starter for a DC shunt motor. (8)
(ii) A 230 V shunt motor has an armature resistance of 0.2 ohm. The starting armature current must not exceed 50 A. If the number of sections of resistances in the starter is 5, find the value of resistance in each section. (8)
OR
13. (b) (i) Sketch a schematic circuit diagram of a stator-resistance starter for a three phase induction motor. Derive an expression for the ratio of starting torque to full-load torque when such a starter is used. (8)
(ii) A three phase, 400 V, 22 kW, 500 r.p.m. squirrel cage induction motor has a full load efficiency of 0.85 and a power factor of 0.88. The motor takes a short circuit current of 150 A at 0.25 lag p.f. The full load slip is 5%. A starting torque of at least half the full load torque i s required. If a stator-resistance starter i s used, determine the magnitude of the required resistance in the stator circuit in each phase. (8)

14. (a) (i) Sketch the necessary schematic circuits for the following methods of controlling the speed of a DC series motor.
(1) Armature diverter control
(2) Tapped field control
Mention clearly whether the speed will increase or decrease, in each case, with reasons. (4 + 4 = 8)
(ii) A 250 V DC series motor drives a fan, the load torque being proportional to (speed)1:5. At a certain speed, the motor takes 40 A. The armature resistance is 0.6 ohm. Find the extra resistance needed to reduce the speed to one half of the original speed. Saturation may be ignored. (8)
OR
14. (b) (i) Sketch a basic step-down chopper circuit and explain its working. Distinguish between ‘Time ratio control’ and ‘Current limit control’. (10)
(ii) A DC shunt motor takes a current of 100 A on a 500 V supply and runs at 1000 r.p.m. Its armature resistance is 0.25 ohm and the field resistance is 125 ohm. A chopper is used to control the speed of the motor in the range 400-800 r.p.m having constant torque. The ‘ON’ time of the chopper is 2.5 ms. Determine the chopper frequencies at 400 r.p.m and at 800 r.p.m. The field is supplied directly at 500 V supply. (6)

15. (a) Using speed-torque characteristics, explain how stator voltage change can be used for speed control of three phase induction motors. What are the drawbacks of this method of speed control? Briefly explain two conventional methods of implementing stator voltage control. (6 + 4 + 6 =16)
OR
15. (b) (i) What i s the effect of injecting voltage in the rotor circuit (wound rotor) of an induction motor? Sketch typical changes in torque-speed characteristics due to injection of rotor voltage. How this concept be used to control the speed? (8)
(ii) Sketch a block diagram of a conventional Kramer system of slip power recovery scheme for a three-phase wound-rotor induction motor and explain its working. (8)