JNTU II B.Tech I Semester Regular Examinations, November 2008

JNTU II B.Tech I Semester Regular Examinations, November 2008

ELECTRICAL ENGINEERING

( Common to Chemical Engineering and Metallurgy & Material Technology)

SET-1

 

1. (a) State the logic supporting the Kirchhoff‘s Laws of Electrical Circuit?

(b) A coil has a resistance of 22 ohms when its mean temperature is 20 degree

centigrade and 26 ohms when its mean temperature is 70 degree centigrade.

Find its mean temperature rise, when its resistance is 28 ohms and the ambient

temperature is 15 degree centigrade.

 

2. (a) Explain the behaviour of a Series R-L Circuit excited by a sinusoidal source

under steady state with relevant waveforms and phasor diagram.

(b) A resistance of 6 ohm and an inductance of 0.5 ohms and a variable capacitance

are connected in series across a 110 Volt, 50 Hz supply. Calculate:

i. the value of capacitance at resonance, current in the circuit, voltage across

capacitor and inductance.

 

3. (a) Do we use laminations for all iron parts of a DC machine? If not, why?

(b) Which losses are fixed and which losses vary with load in a DC series generator?

(c) A 4 pole lap wound armature has 144 slots with two coil sides per slot, each coil is having two turns. If the flux per pole is 20 mwb and the armature

rotates at 720 RPM, what is the induced e.m.f?

 

4. Explain the speed control methods of a DC shunt and series motors with the help

of neat circuits and also mention the limitations of each method.

 

5. (a) Derive the emf equation of an ideal transformer.

(b) Explain the procedure to conduct OC test on a 1- ? transformer.

 

6. For a 3- ? Induction motor, derive the expressions for maximum torque under standstill and running conditions.

 

7. Discuss the following giving reasons:

(a) cylindrical rotor alternators have small diameter and large core length

(b) salient pole alternators have large diameter and small core length.

 

8. (a) The law of deflection of a moving iron ammeter is given by I = 4?n amperes

where ? is the deflection in radians and n is constant. The self inductance is

10 mH while the meter current is zero. The spring constant is 0.16 N-m/rad.

Determine an expression for inductance of the meter as a function of ? and n.

 (b) For the above problem, with n = 0.75, calculate the meter current and the

deflection that corresponds to a self inductance of 60 mH.

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