JNTU Exam Papers I Semester Electrical Technology Nov 2008
1. (a) Develop from first principles an expression for the emf of a dc generator.
(b) The wave connected armature of a four pole dc generator is required to generate an emf of 520V when driven at 600 rev/min. Calculate the flux per pole required if the armature has 144 slots with two coil sides per slot, each coil consisting of 3 turns.
2. (a) Explain the internal and external characteristics of dc shunt motor.
(b) Why starter is necessary for an operation of dc motor? Also explain the different protective devices used in 3 – point starter.
3. (a) Explain with a neat sketches the constructional details of:
i. core type transformer and
ii. shell type transformer
(b) The primary winding of a 50Hz single phase transformer has 500 turns and is supplied from 3300V supply. The secondary winding has 50 turns. Find peak value of the flux in the core and the secondary voltage.
4. (a) Describe the method of calculating regulation and efficiency of a 1- φ trans- former by open circuit and short circuit tests.
(b) A 200/400V, 50Hz, 1- φ transformer has the following test data
O.C. Test: 200V 0.7A 70 W -on L.V.Side
S.C Test: 15V 10A 85 W -on H.V.Side
Calculate the secondary voltage when delivering 5kW at 0.8lag. V1=200V
5. (a) Prove that in a 3 phase induction motor
Starting torque / Maximum torque = 2α / 1+α2 Where α is the ratio of resistance to rotor stand still reactance per phase.
(b) A 3300V, 24 poles, 50Hz 3phase star connected induction motor runs at 247 rpm, while supplying full load torque. Rotor resistance and stand still reactance per phases are 0.016 and 0.265 ohms respectively. Find out:
i. Speed at maximum torque and
ii. Ratio of full load torque to maximum torque.
6. (a) What are the effects of harmonics on pitch and distribution factors?
(b) Find the value of Kd for an alternator with 9 slots per pole for the following
i. One winding in all the slots
ii. One winding using only the first 2/3 of the slots /pole
iii. Three equal windings placed sequentially in 60o group.
7. (a) Explain the operation of single phase induction motor on the basis of :
i. double revolving field theory
ii. Cross field theory
(b) The following test results were obtained in case of 220V, 1-φ induction motor
i. No load test: 220V, 5.8A, and 310watts
ii. Locked rotor test: 120V, 13.8A, 530watts
iii. Stator winding resistance is 1.4 ohms. Determine the approximate equivalent circuit of the motor.
8. (a) Describe the construction and working of PMMC instrument.
(b) Derive the equation for deflection if the instrument is controlled.
(c) Describe the method of damping used in these instruments