# Question Papers

B.E. (Electrical) INDUSTRIAL DRIVES & CONTROL (2008 Pattern) (Sem. – II)

Time :3 Hours]                                                                           [Max. Marks :100

Instructions to the candidates:-

1)            Answer 3 questions from section I and 3 questions from section II.

2)            Answers to the two sections should be written in separate books.

3)             Neat diagrams must be drawn wherever necessary.

4)             Use of logarithmic tables, slide rule, Mollier charts, electronic pocket calculator and steam tables is allowed.

SECTION – I

QI) a) Explain the different types of load in industry with suitable load torque characteristics and suitable choice of motor with example.                                                          [8]

b) A horizontal conveyor belt is moving at a velocity of 1.5 m/s and moves load at the rate of 60,000 kg/hr. The belt is 90m long and is driven by a motor with the speed of 960 rpm. Determine the equivalent rotational inertia at the shaft of the motor.       [8]

OR

Q2) a) What is steady state stability of a drive explain with suitable example explain how we determine stability of operating point.                                                            [8]

b)            A motor is used to drive a hoist motor. Characteristics are given by Quadrants I, II 8 IV : T = 200-0.2N, N-m Quadrants II, III 8 IV: T = -200-0.2N, N-m Where N is speed in rpm.

When hoist is loaded, the net load torque TL = 100 N-m and when it is unloaded, net load torque TL = -80 N-m. Obtain the equilibrium speeds for operation in all the four quadrants.                                                                                                                               [8]

Q3) a) Draw the speed torque characteristics of DC series motor under dynamic braking. Draw circuit diagram and explain the operation with equation. [8]

b) State and explain how armature current and speed of a DC separately excited motor will be affected by each of the following changes in its operating conditions.    [8]

i)             Halving armature voltage with field current and load torque remaining constant.

ii)           Halving armature voltage with field current and power developed remaining constant.

iii)        Doubling the flux with armature voltage and load torque remaining constant.

iv)         Halving both armature voltage and field flux and developed power constant.

OR

Q4) a) Compare regenerative braking and plugging of 3 phase induction motor with suitable characteristics.                                                                                                    [8]

b) A 3 phase, 440 V, 50Hz, 6 pole, star connected induction motor has following parameters referred to stator.

R: 0.5Q, R’r = 0.6Q; X = X’ = IQ

Stator to rotor turns ratio is 2. If the motor is used for the regenerative braking, Determine                                                                                                                               [8]

i)             Maximum overhauling torque it can hold and the range of the speed in which it can safely operate.

ii)           The speed at which it will hold a load with a load torque of 160 N-m.

Q5) a) Draw circuit diagram of single phase fully controlled rectifier control of DC separately excited motor. Draw voltage 8 current waveforms. Also draw speed-torque characteristics 8 explain the operation with mathematical equation.     [10]

b) Explain the operation of a closed loop speed control scheme with inner current control loop. What are the various functions of inner current control loop?                   [8]

OR

Q6) a) A 220V, 1500 rpm, 10A, separately excited DC motor is fed from a single phase fully controlled rectifier with an AC source voltage of230V, 50Hz. Rq = 2 Q. Conduction can be assumed to be continuous Calculate firing angles for rated motor torque and (-1000) rpm.                                                                                                                       [6]

b)            Draw circuit diagram of three phase fully controlled rectifier control of separately excited DC motor. Draw waveform for a =30°.                                                        [6]

c)            Draw circuit diagram of chopper control of series excited DC motor and explain its operation with equation.                                                                                   [6]

SECTION – II

Q7) a) How speed control of IM is achieved using stator voltage control. Explain draw backs associated with it.                                                                                               [8]

b) A 2.8 kW, 400V, 50Hz, 4 pole, 1370 rpm 3 phase delta connected squirrel cage induction motor has the following parameters Rs = 2 Q, Rr’ = 5 Q, Xs = Xr’ = 5 Q, Xm = 80 Q. Motor speed is controlled by stator voltage control. When driving a fan load it runs at rated speed at rated voltage. Calculate.                                                                                [8]

i)             Motor terminal voltage, current and torque at 1200 rpm

ii)           Motor speed, current and torque at terminal voltage of 300V

OR

Q8) a) Explain variable frequency control used for 3 phase induction motor.

Explain with power circuit VSI fed Induction motor drive and steady state equations.                                                                                                                               [8]

b) A 3 phase delta connected 6 pole 50 Hz 400V 925 rpm, squirrel cage induction motor has following parameters Rs = 0.2 ohm, Rr’ = 0.3 ohm, Xs = 0.5 ohm, Xr’ = 1ohm Calculate.                                                                                                    [8]

i)              Speed for frequency of 35 Hz and slip speed of 60 rpm

ii)           Frequency and motor current for a speed of 600 rpm and 80% full load torque.

Q9) a) Explain principle of static scherbius drive. What are its advantages over rotor resistance control?                                                                                                                [8]

b)            What are different classes of duties of motor. Explain with one example each.           [8]

OR

QIO) a) How variable speed drives can be useful in saving energy of pumps. [8]

b)            A motor operates on a periodic duty cycle consisting of a loaded period of 20 mins and a no load period of 10 min. The maximum temperature rise is 60°C. Heating and cooling time constant are 50 and 70 min respectively. When operating continuously on no load the temperature rise is 10°C. Determine                                                        [8]

i)             Minimum temperature during duty cycle.

ii)           Temperature when the motor is loaded continuously.

QII) a) Explain basics of vector control induction motor drive with block diagram only.                                                                                                                      [6]

b)            Explain application of ASD in rolling mills.                                                 [6]

c)            Explain working and advantages of commutatorless DC motor.               [6]

OR

Q12)a) Write short note on ASD used in textile mills.                                                    [6]

b)            Explain application of drive in sugar mills.                                                   [6]

Explain how ASD can be used in machine tool applications. [6]