B. Tech. III (Third) Semester Examination 2013-14
Course Code: EEE303 Paper ID: 0963229
Electrical Machines
Time: 3 Hours Max. Marks: 70 Max Marks: 75
Note: Attempt six questions in all. Q. No. 1 is compulsory.
1. Answer any five of the following (limit your answer in 50 words). (4x5=20)
a) Why is the air gap between the pole pieces and the armature kept very small?
b) Why is it necessary to wound field coils of dc shunt generator with a large number of turns of a fine wire?
c) A dc shunt motor develops 10 bhp at 600 rpm when drawing 18 amp at 500 volt. Calculate efficiency and torque.
d) What will happen if the back emf of dc motor vanishes?
e) It is desired to have a 4.13 mWb maximum core flux in a transformer at 110 volt and 50 Hz. Determine the required no. of turns in the primary.
f) Why a transformer does has iron core?
g) Define voltage regulation of transformer.
h) What are the factors that affect the speed of dc motor?
2. a) Describe the energy stored in magnetic field. (5)
b) Explain the singly excited system. (5)
3. a) Explain (i) period of commutation (ii) reactance voltage during commutation (iii) emf commutation and (iv) resistance commutation (v) Speed control of DC motors. (5)
b) What is critical field resistance and its significance of a dc shunt generator. What are various possible causes for dc shunt generator not building up voltage? (5)
4. a) A 200V dc series motor runs at 1000rpm and takes 20A. Combined resistance of armature and field is 0.4Ω. Calculate the resistance to be inserted in series so as to reduce the speed to 800rpm, assuming torque to vary as square of the speed and linear magnetization curve. (5)
b) Explain with the help of suitable characteristics curves, why (i) A DC shunt motor is also called as a constant speed motor? (ii) A dc series motor should not be started at no load. (5)
5. a) What is meant by 3-phase transformer groups? State factors affecting the choice of 3-phase connections. (5)
b) Write various applications of open delta system. A 400KVA load at 0.7pf lagging is supplied by three 1-phase transformers connected in ∆-∆. Each of the ∆-∆ transformers is rated at 200KVA, 2300/230V. If one defective transformer is removed from servce, calculate for the Y-Y connection: (i) the KVA load carried by each transformer (ii) percent rated load carried by each transform. (5)
6. A Scott connected transformer set supplies two 1-phase furnaces A and B each at 120V from a 3-phase 6.6KV system. If the furnace connected o teaser transformer takes 300KW at UPF nd the other conneceted to main transformer takes 495KW at 0.8 pf lag, determine the line currents taken from supply mains. Neglect losses and magnetizing current. (10)
7. With neat sketch, explain the principle of operation of an Auto-transformer. What are the advantages and disadvantages compared to two winding transformers? (10)
8. A 500KVA transformer with 1% resistance and 5% reactance is connected in parallel with 250KVA transformer with 1.5% resistance and 4% reactance. The secondary voltage of each transformer is 400V on no load. Find how they share a load of 750KVA at 0.8pf lag. (10)