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2003 Course Industrial Drives & Control

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Total No. of Questions : 11] P940 [Total No. of Pages : 4 [3664]-180 B.E. (Electrical) INDUSTRIAL DRIVES AND CONTROL (2003 Course) 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 answer 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. 5) Assume suitable data, if necessary. SECTION - I Q1) a) Explain speed torque conventions and multiquadrant operation of drives. [8] b) A drive has following parameters : J = 1 kg - m2, T = 15 - 0.01 N, N-m and passive load torque, Te = 0.005 N, N-m, where N is the speed in rpm. Initially the drive is operating in steady - state. Now it is to be reversed. For this, motor characteristic is altered such that T = 15-0.01N. N-m for positive as well as negative values of N. Calculate the reversal time. [8] OR Q2) a) State essential parts of electric drives and function of each parts. [8] b) A weight of 500 kg is being lifted up at a uniform speed of 1.5 m/s by a winch driven by motor running at speed of 1000 rpm. The moments of inertia of the motor and winch are 0.5 and 0.3 kg-m2 respectively. Calculate the motor torque and the equivalent moment of inertia referred to the motor shaft. In the absence of weight motor develops a torque of 100 N-m when running at 1000 rpm. [8] Q3) a) What do you mean by braking while lowering the load? What is the objective of such braking. [8] P.T.O. b) A 400 V, 750 rpm, 70A dc shunt motor has an armature resistance of 0.3 . When running under rated conditions, the motor is to be braked by plugging with armature current limited to 90A. What external resistance should be connected in series with the armature? Calculate the initial braking torque and its value when the speed has fallen to 300 rpm. Neglect saturation. [10] OR Q4) a) Explain d.c. dynamic braking of 3 phase induction motor along with speed torque curves. [8] b) A 3-phase, 440V, 50Hz, 10 pole star connected induction motor has the following parameters : R1 = 0.15 , R2 = 0.45 , X1 = 0.6 , X2 = 1.8 , Sf = 0.05 and the ratio of effective stator to rotor turns 1 3 . The motor is to be braked at rated speed and an external resistance of 1.75 / phase (referred to stator) has been inserted into the rotor circuit. Determine the initial braking torque for the following two cases of braking. [10] i) dc rheostatic braking. ii) reverse current braking. Q5) a) Draw the circuit diagram of 3 phase half controlled converter. Explain switching sequence of the devices. Draw switching pulses output voltage waveform. [8] b) A 200V, 875rpm, 150A separately excited dc motor has an armature resistance of 0.06 . It is fed from a single phase fully controlled rectifier with an ac source voltage of 220V, 50Hz. Assuming continuous conduction, calculate i) firing angle for rated motor torque and 750 rpm. ii) firing angle for rated motor torque and ( 500) rpm. iii) motor speed for = 160 and rated torque. OR [3664] - 180 2 [8] Q6) a) b) Explain closed loop control of d.c. series motor. [8] A 3-phase dual converter feeds a 500V, 60A, dc motor with separate excitation. Ra = 1.5 . The converter is fed from a 420V, 50Hz supply. Assuming a voltage drop of 20V in the converter, determine the firing angle and back emf for i) Motoring operation at full load current with motor terminal voltage of 450V. ii) Regeneration operation at full load current with terminal voltage of 450V. iii) The motor is plugged at a terminal voltage of 400V with a current limiting resistor of 5 . [8] SECTION - II Q7) a) Why is it that the variable frequency requirement is always accompanied by a variable voltage requirement in a.c. machines. [8] b) A three phase, 400V, 50Hz four pole, delta connected squirrel cage induction motor is fed from a six pulse bridge inverter supplied from a dc source, such that the fundamental of the motor voltage is the same as the motor rated voltage at 50Hz. The equivalent circuit parameters of the motor are as follows: Stator impedance (Hj 2.6) rotor impedance referred to stator (0.5 + j 2.4) full load speed is 1425 rpm at 50 Hz calculate (1) speed for frequency of 25 Hz and 80% full-load torque (2) torque for frequency of 25Hz (3) frequency for a speed of 1000 rpm & full load. [8] OR Q8) a) Compare CSI and VSI for induction motor drive. [8] b) Justify variable frequency control of induction motor is more efficient than stator voltage control. [8] Q9) a) Explain different classes of motor duty and how it affects the choice of selection of motor rating. [8] [3664] - 180 3 b) A motor has a heating time constant of 60 min and cooling time constant of 90 min. When run continuously on full load of 20kW, the final temperature rise is 40 C. i) What load motor can deliver for 10 min if this is followed by a shut down period long enough for it to cool. ii) If it is on an intermittent load of 10 min followed by 10 min shut down, what is the maximum value of load it can supply during the on load period. [8] OR Q10)a) Explain how the variable speed drives allows saving of energy in pump drives. [8] b) The temperature rise of an electric motor is 40 C after hour and 60 C after 2 hours. The motor current is 100A. Determine approximately its final temperature rise when it works on load cycle of 4 minutes working, 8 minutes rest with a current of 125A. Neglect the effects of iron losses. [8] Q11)Write short notes on any three: [18] a) Drives used in Sugar mill. b) Drives used in Textile mill. c) Drives used in machine tool application. d) Commutator less D.C. motor. e) Drives used in Rolling mills. Y [3664] - 180 4

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