II B.Tech I Semester Supplimentary Examinations, February 2008
ELECTRICAL MACHINES – I
(Electrical & Electronic Engineering
1. (a) Give applications of singly excited system and explain their working.
(b) A U shaped lifting magnet made of cast steel is wound with an exciting coil of 800 turns. It is required to lift a mass of 160 kg in air at a distance of 0.1 mm. The magnetic path length of magnet is 75 cm with a cross sectional area of 6 cm2. Neglect the resistance of coil.. Calculate the minimum current required to lift the weight. The B-H curve data is as follows.
B = 1.81 1.82 1.83 wb/m2
H = 2800 3000 3500 AT/m.
2. (a) What do you understand by retrogressive and progressive windings? Which winding is generally preferred? Why? What are the advantages of retrogres- sive windings?
(b) Give the relation between all pitch(es) for a DC generator with wave winding. Explain the essential condition for Dummy coil requirement.
3. (a) What are the causes of sparking in a d. c. machine? Explain how commutation is improved by use of inter poles.
(b) A 22.38 kW, 400-V, 2-pole wave-wound d.c., shunt motor has 840 armature conductors and 140 commutator segments. Its full-load efficiency is 88% and the shunt field current is 1.57A. If brushes are shifted backward through 1.5 segments from the geometrical neutral axis, find the demagnetizing and distorting amp-turn /pole.
4. (a) Explain the method of build-up of the emf in a shunt generator
(b) What is classification of generators? Explain with relevant connection diagrams.
5. (a) Sketch the internal and external characteristics of DC shunt and series generators. What are their fields of application?
(b) Discuss the need for parallel operation of generators. Explain the load sharing of DC Shunt generators operating in parallel.
6. (a) How may the direction of rotation of the following DC motors be reversed?
(b) Deduce the expression for torque developed in armature of a DC motor. State the factors on which the torque depends.
7. (a) With Neat diagram Explain the construction and working of 4-point starter
(b) A DC shunt motor runs at 750 RPM from 250 V supply and is taking a full load line current of 60 Amps. Its armature and field resistances are 0.4 and 125 respectively and brush voltage drop is 2V, calculate,
i. The no load speed, for a no load line current of 6 Amp.
ii. The value of resistance to be added in series with armature to reduce full load speed to 600 RPM.
8. (a) Explain the various losses taking place in DC machines. With the help of these losses draw the power flow diagram for a DC Motor.
(b) A 440 V DC shunt motor takes a current of 3 A. at no-load. The armature resistance including brushes is 0.3 Ω and the field current is 1 A. Calculate the output and efficiency when the input current is 20 A.