JNTU Previous Exam Papers B.Tech 1st Sem Electrical Technology Nov 2008
B.Tech Semester I November 2008
( Common to Electronics & Communication Engineering, Electronics &
Instrumentation Engineering, Bio-Medical Engineering, Electronics &
Control Engineering, Electronics & Telematics, Electronics & Computer
Engineering and Instrumentation & Control Engineering)
1. (a) Derive the equation for induced emf of a dc machine.
(b) Discuss the process of self excitation in a dc machine. What conditions must be fulfilled for self excitation?
2. (a) Explain the principle of operation of dc motor using suitable diagram.
(b) A 220V series motor runs at 800 rpm, when taking a current of 15 A. The motor has Ra = 0.3 Ω and Rƒ = 0.2 Ω . Find the resistance to be connected in series with armature if it has to take the same current at the same voltage at 600 rpm. Assume flux is proportional to current.
3. (a) What is an ideal transformer. Draw the no load phasor diagram of transformer.
(b) The maximum flux density in the core of 240/2400V, 50Hz, single phase trans-former is 1.0Wb/sq.m. If the emf per turn is 8 Volts, determine:
i. the primary and secondary turns and
ii. area of the core.
4. (a) With neat circuit diagrams, explain the procedure for conducting OC and SC tests on a given single – phase transformer to determine the regulation for lagging and leading power factors.
(b) A 100 kVA transformer has iron losses of 1.2 kW and full load copper losses
of 1.5kW. Find :
i. the kVA for maximum efficiency
ii. maximum efficiency.
5. A 440V, 4pole, 50 Hz, 3-phase stator connected slip ring induction motor has a voltage of 80V between the slip rings when full voltage is applied to the stator and the slip rings are open circuited with the rotor stationary. The stator current is 2A at a power factor of 0.2 lagging. The resistance and stand still reactance per phase of the rotor winding are 0.05 and 0.25 ohms respectively. The rotor winding is star connected. For the motor running with slip rings short circuited and a slip of 5% determine:
(a) The mechanical power developed
(b) Rotor copper losses
(c) Stator current and power factor. (Neglect the stator resistance and reactance)
6. (a) Explain the synchronous impedance method of regulation with vector diagram.
(b) Find the synchronous impedance and reactance of an alternator in which a given field current produces an armature current of 200 A on short circuit and a generated e.m.f. of 50V on open circuit. The armature resistance is 0.1 ohm. To what induced voltage must the alternator be excited if it is to deliver a load of 100 A at a pf of 0.8 lagging, with a terminal voltage of 200 V.
7. (a) Explain why the value of motor time constant is zero on no-load of a servo motor.
(b) A capacitive start motor does not run when the breaker is closed; It hums loudly and starts to smoke. If the breaker is closed while the shaft is rotated by hand however the machine comes up to speed and operates properly. Explain the probable faults.
8. (a) Explain the working of attracted disc type voltmeter with neat sketch.
(b) How will you use a PMMC instrument which gives full scale deflection at 50 mV p.d. and 10 mA current as
i. Ammeter: 0-10A range
ii. Voltmeter: 0-250V range.