Department of Electrical and Electronics Engineering

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MANIPAL INSTITUTE OF TECHNOLOGY, MANIPAL (A Constituent Institute of Manipal University, Manipal)

SECOND SEMESTER B.E. DEGREE MAKEUP EXAMINATION (REVISED CREDIT SYSTEM: 2007)

17 July 2009

BASIC ELECTRICAL TECHNOLOGY (ELE 101/102)

Time: 3 hours Max. Marks: 50

Note: Answer any FIVE full questions.

Missing data, if any, may be suitably assumed.

1A. For the network shown in Fig. Q1 A, determine the equivalent resistance between terminals A and B by star-delta conversion.

(04)

1B. A coil having a resistance of 20 Ω and an inductance of 0.8 H is connected to a 400 V DC supply through a switch. If the switch is closed at time t = 0, find

(i) Time constant of the circuit.

(ii) Time taken for the current to reach half of its final value.

(iii) Energy stored in the inductance at that instant as in case (ii) above

(03)

1C. In the circuit shown in Fig. Q1 C, determine the voltages at nodes A and B using Node voltage analysis. (03)

2A. Determine the currents in 5 Ω and 8 Ω resistances in the network shown in Fig. Q2 A by mesh analysis. (04)

2B. A cicular iron ring having a mean circumference of 120 cm and a cross sectional area of 3.14 cm2 has a saw cut equivalent to an air gap of 1.5 mm across its cross section. It is uniformly wound with 2000 turns of copper wire. Calculate the exciting current required to produce a useful flux of 1.2 mWb in the air gap. Assume the relative permeability of iron as 1200 and leakage factor of 1.14. (06)

3A. For a pure capacitance circuit connected to a single phase supply, sketch the wave forms of voltage, current and active power. Hence analytically prove that the average power consumed by pure capacitor is zero. (03)

3B. A resistance of 35 Ω is connectrd in series with an inductive coil having an internal resistance ‘r’ and inductance ‘L’. When connected across a 230 V, 50 Hz single phase supply, the voltage across the coil is 100 V and the current drawn is 4 A. Find the unknowns ‘r’ and ‘L’. (04)

3C. Two magnetically coupled coils have a coefficient of coupling of 0.832 with self inductances of 65 mH and 5 mH respectively Determine

(i) Mutual inductance between them (ii) Equivalent inductance when the two coils are connected in (a) series addtion and (b) series opposition (03)

4A. Three inductive coils, each with an internal resistance of 15Ω and an inductance of 31.83 mH are connected in delta to a 3-phase, 3 wire, 400V, 50Hz supply. Two wattmeters are connected in any two lines to measure the total power. Calculate

(i) Magnitude of the line current, total power absorbed and readings of Wattmeters

(ii) Same parameters if the connection is changed to star.

(06)

4B. A circuit having a resistance of 5Ω, an inductance of 0.4H and a variable capacitance in series is connected across a 110V, 50Hz supply. Calculate

(i) The value of capacitance for the circuit to be resonant at the supply frequency

(ii) Voltage across the inductance

(iii) Upper and lower half power frequency in Hz (04)

Reg. No. :

Department of Electrical and Electronics Engineering

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Fig. Q2 A

2

15

V

6 A

10 A

8 5

12

V

6

6

6

3

3

3

1

1 1

A

B

Fig. Q1 A

Fig. Q1 C

18 V

6 20

8

10

20 V 4

A B

5A. With the aid of phasor diagram, show that a constant magnitude rotating magnetic field at synchronous speed is produced when a three phase balanced winding is supplied from a three phase voltage.

(04)

5B. The approximate equivalent circuit for a 200/400V, step up transformer has the following parameters referred to the low-voltage side:

(i) equivalent winding resistance and reactance = 0.15 Ω and 0.37 Ω respectively,

(ii) core loss component = 600 Ω,

(iii) Magnetizing reactance = 300 Ω.

When the transformer supplying a primary current of 20A, 200 V at a pf of 0.8 lag,

calculate

(a) The secondary load current (b) Total losses (c) Secondary terminal voltage and

(d) efficiency (06)

6A. Explain the Double Field Revolving theory applied to single phase Induction Motor and hence show that the starting torque is zero. (04)

6B. A 3 phase slip ring Induction motor has 80 volts between slip rings at stand still conditions. The rotor resistance and stand still reactance are 1.5 Ω and 5 Ω per phase respectively. Determine the rotor current and power factor

(i) at starting with slip rings connected to an external star connected impedance of (5+j2) Ω per phase.

(ii) at 5 % slip when the slip rings short circuited (04)

6C. Explain how the current range of Measuring Instruments can be extended. (02)