| 1. | (a) |
Explain the terms active power, reactive power and power factor. Also describe the series resonance of RLC circuit and list its important properties?
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| (b) |
A 60Hz sinusoidal voltage v=100sinwt is applied to a series R-L circuit. Assuming R=10 ohm, L=0.01 H. Find the steady state current and realtive phase angle. Also compute the effective magnitude and phase of voltage drops across each circuit elements?
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| 2. | (a) |
Establish relationship between line and phase voltages and currents in a balanced 3-phase star connection? Draw complete phasor diagram of voltages and currents?
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| (b) |
A delta connected load has the following impedence’s:
ZRY = j10 ohm, ZYB = 10L0 ohm and ZBR = -j10 ohm, If the load is connected across 100 V balanced 3-phase supply, obtain the line currents. |
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| 3. | (a) |
Explain what are hysteresis and Eddy current losses. Discuss how these losses are reduced in transformer cores?
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| (b) | A cast steel ring has a circular cross-section of 3 cm in diameter and a mean circumference of 80 cm. A 1 mm air gap is cut out in the ring which is woundwith a coil of 600 turns. Estimate the current required to establish a flux of 0.75 mWb in the air gap. Neglect fringing and leakage. The magnetization data of the material is under: | ||
| HT (At/m) B (T) | |||
| 200 0.10 | |||
| 400 0.32 | |||
| 600 0.60 | |||
| 800 0.90 | |||
| 1000 1.08 | |||
| 1200 1.18 | |||
| 1400 1.27 | |||
| 1600 1.32 | |||
| 1800 1.36 | |||
| 4. | (a) |
Draw the phasor diagram of single phase transformer at lagging power factor laod. Give step by step development of the euivalent circuit of transformer?
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| (b) | A 10 KVA, 2500/250 V, single phase transformer has resistance & leakage reactance’s as follows: | ||
| H.V. winding – r1=4.8 ohm, x1=11.2 ohm | |||
| H.V. winding – r2 = 0.048 ohm, x2 = 0.112 ohm | |||
| With supply voltage held constant at 2500 V, Calculate the secondary terminal voltage when the transformer delivers its rated current at 0.8 pf lagging on the L.V. side. |
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| 5. | (a) |
What is the purpose of performing open circuit and short circuit tests on
transformer? Explain why open circuit and short circuit tests should be performed on L.V and H.V. sides respectively. |
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| (b) | A transformer has its maximum efficiency of 0.98 at 15 kVA at unity power factor. During the day it is loaded as follows: 13 hours: 2 KW at power factor 0.5 6 hours : 12 KW at power factor 0.8 6 hours : 18 KW at power factor 0.9 Find the all day (energy) efficiency. |
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| 6. | (a) |
Define field energy and coenergy. What is the significance of coenergy in the torque/force derivation of an electromechanical energy conversion device?
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| (b) |
Draw the sketch of an attracted armature type relay with its exciting coli fed from a constant voltage source. If the armature is allowed to move extremely slow, would the coil current change? Explain how the mechanical work is done?
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| 7. | (a) | Draw a diagram of 2-pole DC machine showing its magnetic circuit and main components. Discuss the function of field circuit, brushes and commutator? |
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| (b) |
Describe the construction of the stator and the rotor of 3-phase induction machine?
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| 8. | (a) |
Derive an expression for rotating magnetic field produced in a 3-phase induction
motor when it is connected to 3-phase supply. Comment on the speed of the magnetic field? |
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| (b) |
Describe the construction & working of single phase induction motors. Give various
applications of single-phase induction motors? |
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