Model Question Paper
B.Tech. IVth Semester (EEE)
Electro-Mechanical Energy Conversion-I (TEE-401)
Time: 3Hrs M.Marks:100
Note: Attempt all Questions. Each Question carries equal Marks.
1. Attempt any four parts of the following: (5*4)
(a) What are advantages of analyzing energy conversion devices by field energy concepts?
(b) Show that the reaction of coupling magnetic field on the electrical or mechanical system is essential for the electromechanical energy conversion process.
(c) For a singly excited magnetic system, drive the relation for the magnetic stored energy in terms of reluctance.
(d) Distinguish between singly excited and doubly excited magnetic system.
(e) Define field energy and co energy. Give the significance of co energy in the derivation of torque or force in an electromechanical energy conversion device.
(f) Prove that energy and co energy in a linear magnetic system are given by identical expressions.
(Electrical Machinery by P.S.Bhimbhra, Chapter 2)
2. Attempt any four parts of the following: (5*4)
(a) Drive an expression for electromagnetic torque of a dc motor. Also, show when power developed will be maximum.
(b) Explain the speed-current, torque-current and speed-torque characteristics of a dc shunt motor.
(c) Explain with neat diagram three-point starter for dc motor and explain the function of over load and no-load release coil.
(d) What is meant by speed control? Explain with circuit diagram a speed control method of a dc shunt motor by varying field flux.
(e) A 230V dc shunt motor takes an armature current of 3.33A at rated voltage and at no load speed of 1000rpm. The resistance of armature and field winding are 0.3ohm and 160ohm respectively. The line current at full load and at rated voltage is 40Amp. Calculate the full load speed if armature reaction weaken no load flux by 4%.
(f) Explain in detail Ward Leonard Method of speed control of DC Machines
(Energy Conversion by J.B.Gupta, Chapter 2, 3, 4)
3. Attempt any two parts of the following: (10*2)
(a) Explain the process of commutation in a dc machine and discuss the methods to improve it.
(b) What is meant by the braking of dc motors? Discuss briefly the various methods of braking dc shunt motors.
An 8-pole dc shunt generator has 778 wave connected armature conductors running at 500 rpm supplies a load of 12.5ohm resistance at a terminal voltage of 250V. The armature resistance is 0.24ohm and the field resistance is 250ohm. Find out the armature current, the induced emf and the flux per pole.
(c) What is the armature reaction? What are the effects of armature reaction? How the armature reaction is minimized.
A 4-pole 220-volt shunt motor has 540 lap wound conductors. It takes 32 Amp. From the supply mains and develops output power of 5.595kW. The field windings take 1Amp. The armature reaction is 0.9ohm and the flux per pole is 30mwb.Calculate
(i) The speed and (b) the torque developed in Newton meters.
(Energy Conversion by J.B.Gupta, Chapter 2, 3, 4)
4. Attempt any four parts of the following: (5*4)
(a) Give the merits and demerits of a delta/star connected three phase transformer.
(b) A 3-phase transformer bank consisting of three 1-phase transformers is used to step down the voltage of a 3-phase 6600V transmission line. If the primary line current is 10Amp. Calculate the secondary line voltage, line current and output kVA for the following connection: (a) star/delta and (b) delta/star. The turn’s ratio is 12.Neglecte losses.
(c) Compare the behavior of star/star transformer with star/zigzag transformer.
(d) Describe the merits and demerits of a bank of three single phase transformer as compared to a 3-phase core type transformer.
(e) Explain with the help of connection and phasor diagrams how scott connection are used to obtain two phase supply from the three phase supply mains.
(f) Explain the constructional detail of a 3phase transformer, with diagram.
(Electrical Machines by Nagrath and Kothari, Chapter 3& Generalized Electrical Machines by P.S.Bhimbhra)
5. Attempt any four parts of the following: (5*4)
(a) Discuss briefly the essential and desirable conditions to be fulfilled for parallel operation of 3-phase transformer.
(b) A transformer has its maximum efficiency of 0.98 at 15kVA at unit power factor. Compare its all-day efficiency for the following load cycles:
(a) Full load of 20kVA 12 hours/day and no-load rest of the day.
(b) Full load 4 hours /day and 0.4 full load rest of the day.
Assume the load to operate on unity power factor all day.
(c) What is Autotransformer? Explain how Auto transformer is different from two winding transformer
(d) Drive an expression for the saving of copper in autotransformer as compared with an equivalent two winding transformer.
(e) Explain Sumpner’s (back-to-back) test with suitable diagram. Also, explain how this is beneficial in finding efficiency of a transformer.
(f) Briefly enumerate the application and advantages autotransformer.
(Electrical Machines by Nagrath and Kothari, Chapter 3 & Electrical Machines by J.B.Gupta)