Mumbai University Previous year question papers
IV Sem Electronics Examination Dec 2007
Electronic and Electrical Measuring Instruments and Machine
N.S. : (1) Question NO.1 is compulsory.
(2) Attempt any four questions’ out of remaining six questions.
(3) Assume suitable data wherever required and state clearly.
(4) Figures to the right indicate full marks.
(5) Illustrate answers with sketches wherever required.
1. Answer any four:
(a) Explain the methods of excitation of D.C. Motors
(b) Discuss why a Movjng Iron Instrument is suitable for both d.c. and a.c. measurements.
(c) Fromthe point of view of measurements,how are the resistancesclassifiedas ?
(d) Explain the principle of operation of a three phase Induction Motor.
(e) What are the advantages of InstrumentTransformersas compared with shunts and multipliers?
2.(a) Differentiate between three point and four point starters by showing proper diagrams.
(b) A series motor with an unsaturated field and negligible resistance when running at a certain speed on a given load takes-50A at 460V. If the load torque varies as the cube of the speed, calculate the resistanc~ required to reduce the speed by 25%. ‘
3.(a) Draw the circuit diagram of a schering bridge and derive the condition for balance. 12
Drawthe phasor diagram.
(b) Explain the errors which occur in Induction type Watthour meters. 8
4. (a) Explain the starting methods of a three phase Induction Motor. 10
(b) . The power input to the rotor of 440V, 50Hz, 6 pole, 3 phase induction motor is 80kW. The rotor emf makes 100 complete alternations per minute.
(ii) Rotor speed
(iii) Mechanical power developed
(iv) Rotor copper loss per phase
(v) Rotor resistance per phase, if the rotor current is 65 A.
5.(a) Discuss the construction and operation of an electrodynamometer type of instrument. Show the connections of an electrodynamometer wattmeter. ~
(b) A moving coil voltmeter with a resistance of 10 Q gives a full scale deflection with a potential difference of 45 mY. The coil ha~ 100 turns, an effective depth of 3 cm and a width of 2.5 cm. The controlling torque exerted by the spring is 49 x 10-6 Nm for full scale deflection. Calculate the flux density in the gap.
6. (a) What is a potentiometer? How can it be used for –
(i) measurement of unknown voltage
(ii) measurement ‘of unknown resistance
(iii) Calibration of ammeter.
(b) Draw the characteristics of a Stepper Motor and explain the working of any one type of Stepper Motor.
7.Write short notes on :
(a) Magnetic materials and their classification
(b) Powerfactor meter
(c) Principle of operation of an energymeter
(d) Rectifiertype Instruments.