Cochin University of Science and Techology (CUST) 2011-5th Sem B.Tech Electrical and Electronics Engineering BTech Iester ,- EE 403 ELECTRICAL MACHINES I ( 2006 scheme) - Question Paper
BTS (C) - IV -11- 019-J
B. Tech Degree IV Semester Examination April 2011
EE 403 ELECTRICAL MACHINES I
(2006 Scheme)
Time : 3 Hours Maximum Marks : 100
PART-A
(Answer ALL questions)
(8x5= 40)
I. (a) What is Reactance voltage? How does it affect commutation in D.C machines?
(b) What are interpoles? How is it connected in D.C machines?
(c) What are Equalizer rings? Where is it used?
(d) What are compensating windings? How is it connected in D.C machines?
(e) Why series motor is never started on no load? Explain with necessary equations.
(f) Derive the condition for maximum efficiency in a D.C machine.
(g) What is meant by voltage regulation of a transformer? Derive the condition for zero regulation and maximum regulation.
(h) What are the conditions for satisfactory parallel operation of two transformers?
PART-B
(4x15 = 60)
II. (a) Explain the main parts in a D.C machine.
(b) A 6 pole machine has an armature with 90 slots and 8 conductors per slot and runs at 1000 rpm and the flux per pole is 0.05 wb. Determine the induced emf if winding is (i) lap connected (ii) wave connected.
OR
III. (a) What is armature reaction in D.C machines? What are the effects of armature
reaction?
(b) A 250 V, 14.9 KW, 8 pole D.C motor has single turn coils. The armature is wave wound with 94 commutator segments. If the brushes are shifted by 2 commutator segments at full load, calculate (i) total armature reaction ampere turns (ii) demagnetizing ampere turns (iii) cross magnetizing ampere turns. Assume motor efficiency to be 80% and motor input current is equal to armature current.
IV. (a) What are the conditions for voltage build up in D.C generators?
(b) A 4 pole long shunt lap wound generator supplies 25 KW at a terminal voltage of 500 V. The armature resistance is 0.03 Q, series field resistance is
0.04 Q and shunt field resistance is 200 Q. The brush drop may be taken as 1 V. Determine the emf generated.
OR
(P.T.O.)
What is critical resistance and critical speed for a D.C generator? How are they determined from open circuit characteristics?
Two shunt wound generators running in parallel have each an armature resistance of 0.02 Q and a field resistance of 50 Q. The combined external load current is 5000 A. The fields are excited so that the emf induced in one machine is 600 V and in the second machine is 610 V. Calculate the bus bar voltage and the output of each machine.
Draw and explain the following characteristics for D.C series motor,
(i) Speed - Current (ii) Torque-Current (iii) Speed - Torque.
A 250 V d.c shunt motor having an armature resistance of 0.25 Q carries an armature current of 50 A and runs at 750 rpm. If the flux is reduced by 10%, find the speed. Assume that torque remains the same.
OR
Why Hopkonsons test is known as Regenerative test? Explain how efficiency of the d.c machine is determined using this test.
A shunt generator has a full load current of 195 A at 250 V. The stray losses are 720 W and the shunt field resistance is 50 2. It has a full load efficiency of 90%. Find the armature resistance. Also find the current corresponding to maximum efficiency.
Draw the Approximate Equivalent circuit of a transformer. Explain how equivalent circuit parameters are determined from O.C and S.C tests.
The primary and secondary windings of a 40 KVA, 6600/250 V single phase transformer have resistances of 10 Q and 0.02 Q respectively. The total leakage reactance is 35 Q as referred to the primary winding. Find the full load regulation at a pf of 0.8 lagging.
OR
Explain any one method for the transformation of 3 phase power using two single phase transformers.
Two single phase transformers with equal turns have impedances of (0.5 + j3) Q and (0.6 + jlO) Q with respect to the secondary. If they operate in parallel, determine how they will share a load of 100 KW at pf 0.8 lagging?
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| Earning: Approval pending. |
