.........................step4success..............................: ELECTROMECHANICAL ENERGY CONVERSION-I (EMEC-I)

ELECTROMECHANICAL ENERGY

Time: 3 Hours

Total Marks :100

SECTION A

Attempt any 4 question. Each question carries 5 marks :

Show that the energy stored in a magnetic field is equal to the area between the Ψ-i curve for the system and the flux linkage (Ψ) axis.
Show that the field energy in a magnetic system is given by:
W_f = 1/2Li² = 1/2 Ψi = (1/2L)Ψ².
Distinguish between singly excited and doubly excited systems.
For a singly excited linear magnetic system, derive an expression for the electromagnetic torque.
Define field energy and co-energy.
Why most practical energy conversion devices use magnetic field as the coupling medium between electrical and mechanical systems?

SECTION B

Attempt any 4 question. Each question carries 5 marks :

What do you understand by demagnetizing and cross magnetizing effects of armature reaction in a d.c machine?\
Derive the e.m.f and torque equations for dc machines.
Explain clearly the functions of the following in d.c machines:
(i) Interpoles
(ii) compensating winding
What are the various possible cause for dc shunt generator not building up voltage?\
What are the different types of dc generators according to the ways in which fields are excited? Show the connection diagram of each type.
A 4 pole generator with wave wound armature has 51 slots each having 48 conductors. The flux per pole is 7.5 m Wb. At what speed must the armature be driven to give an induced emf of 440 V.

SECTION C

Attempt any 2 question. Each question carries 10 marks :

A 4 pole, d.c shunt motor has a flux per pole of 0.04 Wb and the armature is lap wound with 720 conductors. The shunt field resistance is 240 ohm and the armature resistance is 0.2 ohm. Brush contact drop is 1 V per brush. Determine the speed of the machine when running as:
(i) A motor taking 60 A
(ii) A generator supplying 120 A. The terminal voltage in each case is 480 V.
What are the drawbacks of 3 point starter? Describe a 4 point starter with a neat sketch.
Describe the test conducted on d.c motors in which the temperature rise and the commutation conditions can be checked under rated load conditions such that only losses are being supplied by the power supply.

SECTION D

Attempt any 2 question. Each question carries 10 marks :

Explain Sumpner's test for testing two phase transformers. What is the main advantage of this test over Open circuit and Short circuit tests conducted on single phase transformers?
The HV terminals of a 3 phase bank of three single phase transformers are connected to a 3 wire, 3 phase, 33 kV (LL) system. The LV terminals are connected to a 3 wire, 3 phase load rated 1800 kVA and 2500 V (LL). Specify the voltage, current and kVA ratings of each transformer (both HV and LV windings) for the following connections.
(a) Y - Y
(b) Δ - Δ

(c) Y – Δ
(d) Δ - Y

Derive an expression for saving in conductor material in an autotransformer over a two winding transformer of equal rating. State its advantages and disadvantages over two winding transformer.

SECTION E

Attempt any 2 question. Each question carries 10 marks :

In open delta transformers, show that the secondary line voltages form a balanced three phase system of voltages, in case the supply voltages are balanced.
Two 1 phase Scott connected transformers supply a 3 phase, 4 wire system with 231 V between the lines and the neutral. The HV winding are connected to a 2 phase system with a phase voltage of 6600 V. Determine the number of turns in each selection of HV and LV winding and the position of the neutral point if the induced voltage per turn is 8 V.
What are the conditions for satisfactory operation of 1 phase transformer? Deduced an expression for the load shared by two transformers in parallel when no-load voltages of these transformers are not equal. What will be the load distribution if the voltage ratio is exactly equal?

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