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2003 Course Industrial Drives & Control

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Total No. of Questions : 11] [Total No. of Pages : 4 [3864] - 223 P 1061 B.E. (Electrical) INDUSTRIAL DRIVES AND CONTROL Time : 3 Hours] [Max. Marks : 100 Instructions to the candidates: 1) Answer three questions from Section I and three 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) Figures to the right indicate full marks. 5) Use of logarithmic tables, slide rule, Mollier charts, electronic pocket calculator and steam tables is allowed. 6) Assume suitable data, if necessary. SECTION - I Q1) a) b) c) Q2) a) b) What is an Electric drive? Discuss, essential parts of Electric Drive with the help block diagram. [6] Write a brief note on, speed-torque characteristics of drive operated in all four quadrants. Use the example of hoist load. [6] State the reasons for using load equalisation in Electrical Drives. Also discuss the role of Fly wheel in load equalisation method. [6] OR What is steady state stability of an electric drive? Discuss how we arrive dT L dT M >0 at the criteria for steady state stability dw dw where TL - Load torque TM - Motor torque W - Speed of the drive [8] As shown in fig 2b, for different motor-load combinations (Ex : A & B), discuss wheather drive system is unstable or stable. [4] P.T.O. c) A motor develops a torque given by the relationship TM = aw + b, where a and b are positive constants. This motor is used to drive a load whose torque is expressed as TL = cw2 + d, where c and d are some other positive constants. The total inertia of the rotating masses is J. i) Calculate the equilibrium operating speed. ii) Find wheather the drive is stable at this speed. [6] Q3) a) Draw motor connection circuits and characteristics to explain Regenerative braking principle for dc shunt and separately excited motors. Write all the necessary equations for motoring and braking actions. [8] A 220V dc shunt motor has an armature resistance of 0.062 ohm and with full field, has an emf of 215 V at a speed of 960 rpm. The motor is driving an overhauling load with a torque of 172 N-m. Calculate the minimum speed at which motor can hold the load by means of regenerative braking. [8] b) OR Q4) a) b) Q5) a) b) Explain with neat circuit diagram and characteristics (both motoring & braking), the following braking methods used for 3 phase induction motor. i) Regenerative braking. ii) Plugging. [12] iii) D.C. Dynamic braking. Discuss merits and disadvantages associated with three braking methods mentioned in part (a) of this question. [4] Explain with necessary diagram, full converter fed dc separately excited motor. How speed change is achieved by varying firing angle. Write also the output voltage equations with a sketch of voltage and current waveforms available at the armature terminals of dc motor. [8] A 230 V, 650 rpm 100 A separately excited dc motor has armature circuit resistance and inductance of 0.08 and 8 mH respectively. Motor is controlled by a single-phase half-controlled rectifier with source voltage of 230 V, 50 Hz. Identify the modes and calculate speeds for = 60o and torque = 1000 N-m i) [8] ii) = 120o and torque = 1000 N-m. OR [3864] - 223 -2- Q6) a) b) Explain chopper controlled speed control of dc series motor. Draw the motor voltage and current waveforms. [8] A dc motor is controlled by a dual converter. Draw the necessary diagrams and characteristics and explain speed control technique using a dual converter. [8] SECTION - II Q7) a) b) What is the principle behind V/f speed control strategy? Explain with necessary diagram of a scheme to implement V/f control method using power control devices. [8] Draw circuit diagram for transistorised stator control method for 3-phase induction motor. Explain working principle and voltage & current waveforms at stator of induction motor. [8] OR Q8) a) b) Q9) a) b) c) Sketch a closed loop control scheme for speed control of 3 phase induction motor using a current source inverter. Explain the speed control strategy. [8] A 3 phase, 400 V, 50 Hz, six pole, 925 rpm star-connected, induction motor has the following parameters. r1 = 0.2 r 2 = 0.3 , x1 = 0.5 x 2 = 1.0 The motor is fed from a voltage source inverter with a constant voltage to frequency, V/f ratio. Calculate the maximum torque Tm and the corresponding speed Nm for 50 Hz and 20 Hz. [8] What are the possible reasons for the energy loss in Electric drive system? [4] Explain the factors on which the energy loss in induction motor during starting depends and hence derive an expression relating a moment of inertia of rotor with the energy lost during starting of induction motor. [10] What are the reasons for energy loss during running of a induction motor. How energy can be saved? [4] OR [3864] - 223 -3- Q10)a) b) c) Discuss different methods used to calculate the power rating of motor used for continuous duty with variable load applications. [6] Calculate the power rating of a motor used to drive a load at 730 rpm. Motor duty is as mentioned in fig shown in fig. 10 b. [8] Write a brief note on thermal model of motor and hence explain heating [4] & cooling curves. Q11)Write short notes on : a) Drives used in sugar mills. b) [16] Drives used in Machine tools. In the short notes, mention the load requirements like power rating, speed, duty cycle etc. [3864] - 223 -4-

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