Cochin University of Science and Techology (CUST) 2006 B.Tech Electrical and Electronics Engineering ELECTRICAL MACHINES1 - Question Paper

BTECH DEGREE IV SEMESTER exam MAY 2006
ELECTRICAL MACHINES I
Time:3 Hrs Max marks:100

1. a) discuss the principle of working of a DC generator. Classify its kind and write the
general emf formula of a generator.

b) A 4-pole shunt generator with lap connected armature having field and armature
resistance of 50ohm and 0.1ohm supplies 60, 100V, 60W lamps. compute the
armature copper loss, current in every armature conductor and the generated emf.
Allow a contact drop of IV per brush.
OR

2. a) What is meant by the term armature reaction? How does it affect the main field
flux? describe GNP and MNP. discuss armature reaction in dc machines and show
how cross magnetizing and demagnetizing m.m.fs are produced?

b) The brushes of a certain lap connected 400KW, six pole generator are provided a lead of
18 degree electrical. From the provided data, compute 1) the demagnetizing ampere
turns 2) the cross magnetizing ampere turns 3) series turns needed to balance the
demagnetizing component. The full load current is 750A and total no. of conductors
are 900 and leakage coefficient is 1.4.

3. a) Draw and discuss the subsequent characteristics of dc series and shunt wound
generators. 1) no load and load magnetization characteristics 2) external and
internal characteristics.

b) The OCC of a dc shunt generator driven at 600rpm is as follows.
A: 0.5 one two three four five
V: 75 225 330 390 400 425

obtain 1) the emf to which the generator will excite when the circuit to resistance is
100ohm 2) the additional resistance needed in the field circuit to decrease the emf
to 350V 3) the critical field resistance for 600rpm.
OR

4. a) define the advantage of using several small dc generators in parallel over the use
of single large generator. discuss how two shunt generators work parallel and how
they share the load?

b) two shunt would generators running in parallel have every an armature resistance of
0.02ohm and a field resistance of 50ohm and the combined external load current is
5000A. The fields are excited so that the emf induced in 1 machine is 600V and in

Earning:   Approval pending.