Rajasthan Technical University 2009-3rd Sem B.E Electrical Engineering : (- ) Electrical Machines-1 - Question Paper
Rajasthan tech. University(RTU)
B.Tech III sem. Main exam
January 2009
Electrical Machines-1
B.Tech. (Sem.lll) (Main/Back) Examination, January * 2009 (3EE4) Electrical Machines*!
(Electrical Engineering)
[Total WSjjRr 80 [Min. Passingftp/ksf:
Attempt five questions in alL Schematic diagrams must shown wherever necessary. Any data you feel missing may suitably be assumed and stated clearly.
Use of following supporting material is permitted during examination. (Mentioned in form No. 205)
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(a) What is meant by dynamically induced emf and statically induced emf ? On what factors do these depend ? Derive equations for these two emf s.
(b) A circular coil of 200 turns with a mean diameter of 30 cms. is rotated about a vertical axis in the earth's field at 32 revolutions per second. Find the instantaneous value of induced emf in the coil, when its plan is (i) parallel, (ii) perpendicular and (iii) inclined at 30 to the magnetic meridian (H = 14.3 AT/m).
(a) In every electromagnetic conversion device, both generator V and motor action take place simultaneously. Explain.
3E1484] ill [Contd...
flux of 0.002 web/m2, (a) If the coil is rotated at 25 rev. per second, find the voltage induced in the coil (b) what wil be the force on each conductor and torque acting on the coil, if it carries a current of
10 Amps.
6+10
(a) Draw a neat sketch of a d.c. generator with name of each parts. Derive an equation for emf induced in a d.c. generator.
(b) An 8-pole dc generator has 500 armature conductors and has a useful flux per pole of 0.065 web. What will be the emf generated if it is lap connected and runs at 1000 r.p.m. ? What must be the speed at. to be driven to produce the same emf JiJ wound ?
(a) What are the different types of d.c. generators according to the ways in which fields are excited ? Shgw the*connection diagram of each type. ------------
(b) A shunt generator gives full load output of 30 kW at a terminal voltage of 200 V. The armature and field resistances are 0.05 Q and 50 Q respectively. The iron and friction losses are 1000 W. Calculate (a) generated emf (b) copper losses (c) efficiency.
8+8
(a) What is meant by back emf in a d.c. motor ? Derive the equation for torque produced in d.c. motor.
(b) A 250 V d.c. shunt motor having an armature resistance of 0.25 Q carries an armature current of 50 Amp and runs at 750 rpm. If the flux is reduced by 10%, find the speed. Assume that the load torque remains same.
8+8
[Contd...
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torque characteristics of a d.c. shunt, series and compound motors in one figure and compare them. Which characteristic is more suitable for traction purpose and why ?
(b) A 120 V dc shunt motor having an armature circuit resistance of 0.2 n and field circuit resistance of 60 Q, draws a line current of 40 A at full load. Tlffi* brash~voltag_e_ drop is 3 V and rated speed full load speed is 1800 rpm. Calculate the (a) speed at half load (b) speed full load.
4 (a); Develop the exact equivalent circuit of a l-phase*
From this derive the approximate and simplified equiv? ckt. of transformer state the various assumptions made.
(b) A 100 kVA, 50 Hz, 440 /11000 volt 1-phase transformer has ah efficiency of 98.5% when supplying full load current alt 0.8 power factor lagging, and an efficiency of 99% when supplying half of full load current at unity power - factor. Find the core losses and the copper losses corresponding to full-load current. At what value of load current will the efficiency be maximum ?
- 8+8
4 (a) (i) . Develop the phasbr diagram of a 1-phase transformer under load conditions. Assume lagging power factor load.
(ii) Draw and explain the curve of variation of efficiency of transformer with load.
(b) Define voltage regulation of transformer
Derive conditions for (i) Zero regulation, (ii) Maximum regulation.
8+8
3E1484I IIIIHiinillllllE 3 [Oontd...
a 3-phase transformer unit ? What are advantages of a 3-phase unit transformer over three single-phase transformer bank of same kVA rating ?
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(b) What is an open delta system ? What are its applications ?
(c) Explain with the help of connection and phasor diagrams, how Scott connections are used to obtain two-phase supply from 3-phase supply mains.
5 (a) Briefly explain the function of closed delta tertiary winding employed in some three-phase transformers.
(b) Explain how the exciting current (no load current) in transformer contains harmonics even when supply voltage is a sine wave.
(c) Explain with the helpr of neat connection diagrams how to obtain three to six phase conversion.
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