Download em 1 model set 1

LOYOLA – ICAM
COLLEGE OF ENGINEERING AND TECHNOLOGY (LICET)
Loyola College Campus, Nungambakkam , Chennai – 34
Branch: EEE
Date: 06-04-2015
Semester: IV
Time: 1.00 – 05.00 pm (3 Hrs)
MODEL EXAM
ELECTRICAL MACHINES I (EE6401)
Part A (Answer all questions)
(10 X 2 =20)
1. Write equation for energy density for a magnetic circuit.
2. What are the factors on which hysteresis loss depends?
3. What are the conditions for parallel operation of transformers?
4. How does change in frequency affect the operation of a given Transformer?
5. Why the field in rotating machines should be quasi-static in nature?
6. What is the basic feature of an electromagnetic energy conversion devices?
7. Define critical field resistance of dc shunt generator.
8. Define armature reaction
9. What are the different methods of speed control in dc motor?
10. What is a regenerative test?
PART B (Answer all the questions)
11. (a) Discuss in detail the following:
(i) B-H relationship
(ii) Leakage flux
(iii) Fringing
(iv) Stacking factor.
(5 X 16 = 80)
(4X4 = 16)
(or)
(b) (i) Derive an expression for energy density in the magnetic field.
(6)
(ii) Explain in detail ‘‘Eddy – current loss’’.
(5)
(iii) The total core loss of a specimen of silicon steel is found to be 1500 W at 50 Hz. Keeping
the flux density constant the loss becomes 3000 W when the frequency is raised to 75 Hz.
Calculate separately the hysteresis and eddy current loss at each of there frequencies.
(5)
12. (a) (i) Explain in detail the various types of three phase transformer connection.
(10)
(ii) Prove that amount of copper saved in auto transformer is (1 – K) times that of ordinary
transformer.
(6)
(or)
(b) (i) (a) The following data were obtained on a 20 kVA, 50 Hz, 2000/200 V distribution
transformer:
Voltage (V)
Current (A)
Power (W)
OC test with HV open-circuited
200
4
120
SC test with LV short-circuited
60
10
300
Draw the approximate equivalent circuit of the transformer referred to the HV and LV sides
respectively.
13. (a) Obtain an expression for the mechanical force of field origin in a typical attracted
armature relay.
(or)
(b) Find an expression for the magnetic force developed in a doubly excited magnetic systems.
14.(a). Explain the effect of armature reaction in a DC shunt generator. How are its
demagnetizing and cross-magnetizing ampere turns calculated?
(or)
(b).4-pole, 50 kW, 250 V, wave wound shunt generator has 400 armature conductors.
Brushes are given a lead of 4 commutator segments. Calculate the demagnetization
ampere-turns per pole if shunt field resistance is 50 ohm. Also calculate extra shunt
field turns per pole to neutralize the demagnetization.
15. (a) (i) With the help of neat circuit diagram, explain Swinburne’s test and derive the
relations for efficiency (both for generator and motor)also state the merits and
demerits of this method
(or)
(b) Explain the Ward-Leonard system of controlling the speed of a DC shunt motor with help
of neat diagram.