| 1. | (a) |
A series circuit containing a resistance R and an inductance L is connected to a 200 V, 50 Hz source V. The current drawn is I amperes r.m.s. taking the source voltage as reference representated by S=200 root 2 sin(2pi50t). Write the expressions for:
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| (i) | The current through the circuit, i(t) | ||
| (ii) | Voltage across the resistance, R(t) | ||
| (iii) | Voltage across the inductance, L(t) | ||
| What is the peak vale of the instantaneous power drawn? | |||
| (b) |
A series R-L-C circuit with R = 10 ohm, L = 100/314 h, C = 1/31400 F is excited from a 10 V, 50 Hz (w=314 rad/sec) source. Determine the r.m.s value of voltages across
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| (i) | Resistance | ||
| (ii) | Inductance | ||
| (iii) | Capacitance | ||
| 2. | (a) |
A 3-phase, 4-wire, supply is obtained from a star connected alternator. The output voltage VRN is represented by 250L0 V, Phase sequence is RYB.
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| (i) | What are VYN and VRN ? | ||
| (ii) | Express VRY, VYB and VBR | ||
| (iii) | If the phase sequence is reversed, would your answers change? Explain. | ||
| (b) |
A 3-phase balanced load draws 10 KW power from a 400 V, 3-phase, 50 Hz, 4-wire supply at 0.8 lagging power factor:
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| (i) | Determine the line current. | ||
| (ii) | What would be the line current if power factor were raised to unity, power remaining the same? | ||
| (iii) | How can the power factor be improved to unity ? | ||
| 3. | (a) | Explain the following terms and give their SI units, if any: | |
| (i) | Flux | ||
| (ii) | Flux Density | ||
| (iii) | Intensity of magnetization | ||
| (iv) | Relative permeability | ||
| (v) | Coefficient of fringing | ||
| (b) |
An iron ring of 40 cms mean diameter and a circular cross-sectional area of 1 cm2 is excited by a 100 turn coil carrying 1 ampere. A saw cut of 0.4 pi mm is made radially in the ring. Assuming the relative permeability of the ring as 10’4. Determine the flux through the air gap. What are the assumption you made ?
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| 4. | (a) |
Why does the primary current in a transformer increase when a load is connected across the secondary? Explain the mechanism through which primary draws more current on loading the secondary. Is the ratio of primary current to secondary current exactly equal to the ratio of secondary turns to primary turns ? Explain.
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| (b) |
A transformer rated 200 V/50 V, 10 KVA has a core loss of 100 watts. What is the maximum efficiency of the transformer at 0.8 lagging power factor? Assume full load copper loss as 200 watts. At what load this maximum efficiency is obtained? How and why would your answers be different if the power factor were unity ?
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| 5. |
A singly excited system, like a relay, has the permeability of the magnetic circuit of ferromagnetic part as infinity. Explain the process of energy flow into the magnetic field as the current through the coil increases with relay open. Which part of the magnetic circuit stores the energy ? If the air gap suddenly closes, explain the process of energy adjustment ?
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| 6. | (a) |
Derive an expression for generated e.m.f in a.b.c. machine of P poles with Z armature conductors. Armature speed is N rpm. What is assumption made ?
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| (b) |
What are the ways, the speed of a DC motor can be changed? Explain why does the speed change through these methods a DC shunt motor is running at 1500 rpm when excited from a 200 V DC supply with armature in parallel with the field winding. The voltage is suddenly reduced to 100 V DC. Explain what happens.
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| 7. | (a) |
Explain how a rotating magnetic field with constant magnitude revolving at constant angular velocity is created is a 3-phase induction motor. An alternator (Synchronus generator) also has a 3-phase winding carrying 3-phase currents. Does it also produce a rotating field? Explain.
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| (b) |
The core loss in a 3-phase induction motor is 100 W and equals the mechnaical loss. Stator copper loss is 150 W when developing 2000 watts as the shaft output. What is the efficiency of the machine? Assume the slip as 4% ?
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| 8. | Write short note on any three of the following: | ||
| (i) | Coenergy | ||
| (ii) | Star-Delta Connection | ||
| (iii) | Star or wye (Y) connection | ||
| (iv) | Delta or Mesh connection | ||
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