Bhavnagar University 2007 B.Sc Computer Science ELECTRICAL ENGINEERING - Question Paper

ELECTRICAL ENGINEERING



Time: three hrs
Max Marks: 70

First ques. is Compulsory

ans any 4 from the remaining ques.

All ques. carry equal marks

ans all parts of any ques. at 1 place

1. (a) discuss the terms
(i) Magnetomotive force
(ii) Reluctance
(iii) Flux density.

(b) discuss
(i) Statically induced e.m.f.
(Give 1 application of every generator based on the above characteristics)

(c) discuss the significance of back e.m.f. provide its formula for a shunt motor in terms of supply voltage and armature resistance drop.

(d) In a 3-phase delta connected load, the phase current is 20 Amperes. What is the line current drawn from the supply? provide the power formula in a 3-phase circuit interms of line values.

(e) Draw the phasor diagram of 1-phase transformer when it supplies lagging power factor load.

(f) Draw the torque ship characteristic of a 3-phase induction
(i) the starting torque
(ii) pull out torque and
(iii) full load torque
(iv) describe distribution factor

2. (a) State and discuss Faradays legal regulations of electromagnetic induction.

(b) A steel ring has a circular cross-section of two cm in diameter and mean radius of 20 cms. The ring is uniformaly wound with a coil of 500 turns. If an air of two mm width is made in the ring, estimate the current taken by the coil if a flux of 3500 mwb is to be produced in the gap.
The magnetization curve of the sample is found from the readings below:

Amp. Turns/m 200 400 600 800 1200 1400
Fluxdensity wb/m2 0.66 0.94 1.07 1.14 1.2 1.243. (a) Derive the e.m.f formula of a D.C. generator.

(b) A four pole long shunt compound generator with wave connected annature having shunt field, series field and armature resistances of 50, 0.05 and 0.1 ohm respectively, supplies sixty 100 volt, 40 watt lamps. compute the total armature current. the current per armature path and the generated e.rn.f presume one volt drop per brush.

4. (a) explain the different methods of speed control of a D.C. motor.

(b) A2-pole. D.C. shunt motor operating from a 200 volts supply takes a hill load current of 35 Amps drawn from the supply. the no-load current being two Amps. The field resistance is 500 ohms and the annature resistance is 0.6 ohm Calcuate the efficiency of the motor on full load.

5. (a) A circuit takes a current of three Amps at a power factor of 0.6 lagging when connected to a 150 volts, 50 Hz supply. a different circuit takes a current of five Amps at a power factor of 0.8 leading when connected to the identical supply. If these 2 circuits connected to the identical supply. If these 2 circuits connected in series across 230 volts. 50 Hz supply. compute the current and the power consumed.

b)Three equal impedances are delta connected to a a3-phase, 400 volts, 50 Hz supply. If the resistance and inductive reactances of every phase are 20 ohms and 30 ohms respectively. compute the line current drawn from the supply and the power consumed.

6. (a) discuss how is the approximate regulation of a1-phase transformer at various power factors can be predetermined by using the data found from short circuit test

(b) A 10 KVA, 500/250 volts. 1-phase transformer has its maximum efficiency of 94% when delivering 90% of its rated output at unity power factor. Estimate its efficiency when delivering its full load output at 0.8 p.f. lagging.

7. (a) Derive the expression for torque developed by the 3-phase induction motor from fundamentals. Hence find the condition for maximum torque.

(b) A3-phase, four pole volts, 50 Hz induction motor develops 15 H.P (Metric) at 1425 r.p.m. with a power factor of 0.8 lagging. The mechanical losses amount to 0.75 H.P. (Metric). Determine for this load, the slip, rotor copper losses and line current. Take stator losses as 1200 watts.

8. (a) define briefly the different methods of starting synchronous motor.

(b) A 3-phase, 16 pole alternator has a star connected winding with 144 slots and 10 conductors per slot. The flux per pole is 30 mwb. sinusoidally distributed and the speed 375 RPM. obtain the frequency and the phase and line e.m.fs of the alternator.

Earning:   Approval pending.