# JNTU Exam Papers II B-Tech-I Sem Regular Exams,Electrical Technology,November 2008

## SET 4

1. (a) With reference to OCC of a dc generator, explain the following:
i. Why is the emf not zero when field current is zero?
ii. Will the residual flux induce a voltage in the machine, if speed of the machine is zero
iii. Why does the slope of the curve change, after a certain value of field current?
(b) A 4 pole generator has 48 slots and 8 conductors per slot. The useful flux per pole is 30mWb and speed is 800 rpm. Find the generated emf, if the machine is wave connected.
2. (a) Explain the principle of operation of dc motor with suitable diagram.
(b) What are the different methods of speed control of dc shunt motor and explain any one of them in detail.
3. (a) What is an ideal transformer? Draw the no load phasor diagram of transformer.
(b) The primary winding of a 50Hz, 1- ? transformer has 500 turns and is supplied from 3300V supply. The secondary winding has 50 turns. Find the peak value of the flux in the core and the secondary voltage.
4. (a) A 25 kVA, 2500/250V, 1- ? transformer gave the following test figures:

O.C Test    (LV Side):    250V 1.4A    105W
S.C Test     (HV Side):    105V 8A        320W

Compute the equivalent circuit parameters referred to LV side and HV side. Also obtain percentage regulation at full load with 0.8 p.f lagging.
(b) Define efficiency of a transformer. Obtain the condition for maximum efficiency.
5. (a) Explain briefly about principle of operation of 3-phase induction motor
(b) A 4 pole, 3 – phase induction motor operates from a supply whose frequency is 50 Hz. Calculate

i. The speed at which the magnetic field of the stator is rotating.
ii. The speed of the rotor when the slip is 0.04.
iii. The frequency of the rotor current when the slip is 0.03.
iv. The frequency of the rotor current at stand still.
6. (a) Draw the vector diagrams of a loaded alternator.
(b) A 60 KVA, 220V, 50Hz, 1-? alternator has effective armature resistance of 0.016 ohms and an armature leakage reactance of 0.07ohms. Compute the voltage induced in the armature when the alternator is delivering rated current at a load power factor of unity.
7. (a) Explain how a capacitor-start split phase induction motor develops a rotating magnetic field.
(b) An experimental 120V, 1/4hp ,60Hz split phase motor has the following locked rotor parameters referred to the main and the auxiliary windings.
Rmax=3.94ohms     Xmw=4.20ohms
Raw=8.42ohms         Xaw=6.28ohms
Determine:
i. Locked rotor current drawn by each winding
ii. Line current at locked rotor.
8. (a) Describe the general requirements for a material to be used for shunts for ammeters and multipliers for voltmeters for PMMC instruments.
(b) What are the commonly used materials and describe their properties as regards to their suitability to be used for shunts or multiplier