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Q5. a) Describe the circuit breaker for an a.c. generator using a sketch to show how arcing is controlled. (6)

b) Explain the sequence of events that might occur if the breaker opens on a short circuit and state the check you would require following such event. (5)

c) Give a safe procedure to follow should a main circuit breaker fail to open under fault Condition. (5)

Q2. A. With respect to measuring instruments what is the difference between analogue and digital measuring instruments. Explain the working principle of each type.

B. Describe with the aid of simple sketches one analogue and one digital measuring instrument you have used onboard.

Q3. With respect to power transformers kindly explain the following protections

1. Overload protection 2. Overcurrent protection for phase faults 3. Earth Fault protection 4. Differential protection 5. Directional protection

Q4. A. In a.c. generators, voltage dip occurs in two stages.

I. Sketch a voltage-time graph showing the pattern of voltage dip.

II. Referring to this graph, state with reasons the effect on the electrical system of a small power installation when a large load is suddenly switched on.

B. Explain EACH of the following categories of voltage control:

(i) Error operated; (ii) Functional.

Q5. In some circumstances electrical current may be induced into the shafting of rotating machinery.

a) state the problem that may be caused by this current.

b) Explain with aid of sketches, how currents may be avoided or reduced in the following instances:

(i) D.C machines

(ii) main shafting fitted with a bronze propeller

Q6. (a). Explain the significance of the root- mean- square value of an alternating current or voltage waveform; Define the form factor of such a wave form.

B. Draw the circuit of Half-wave rectifier and its output waveform. A diode whose internal resistance is 20𝛀 is to supply power to 1000 𝛀 load from 110 V (RMS) source. Calculate

(i) peak load current, (ii) DC load current, (iii) AC load current.

Q7.A. By means of a schematic circuit diagram illustrate the peak rectifier. If the supply voltage is v(t) = Vra Sin wt, what is the voltage across the load resistor?

B. A d.c. motor takes an armature current of 110 A at 480 V. The resistance of the armature circuit is 0.2?. The machine has six poles and the armature is lap-connected with 864 conductors. The flux per pole is 0.05 Wb. Calculate;

(i) The speed;

(ii) The gross torque developed by the armature.

Q8. A. List the factors that determine the starting torque of the three-phase induction motor. How does this torque generally compare with the value of the rated torque?

B. A three phase induction motor is wound for four poles and is supplied from a 50 Hz system. Calculate.

i. The synchronous speed;

ii. The speed of the rotor when the slip is 4 per cent;

iii. The motor frequency when the speed of the rotor is 600 r.p.m

Q9. A. What is leakage flux as it applies to the iron-core transformer? How is it considered in the analysis of the transformer?

B. Three conductors fitted side by side in the stator of a salient-pole alternator. Each generates maximum voltage of 200V (sinusoidal). The angle subtended at the centre of the stator between adjacent conductors is 20 electrical degrees. If the three conductors are connected in series, find

(i) the r.m.s. value of the effective voltage and

(ii) the ‘breadth factor’. Using the theory that is the basis of this problem, give one reason why three-phase current has been introduced.

Q10. A. What are the factors which determine the synchronous speed of a motor?

B. A twelve-pole, three-phase, delta-connected alternator runs at 600 rev/min and supplies a balanced star-connected load. Each phase of the load is a coil of resistance 35 ohm and inductive reactance 25 ohm. The line terminal voltage of the alternator is 440V. Determine (a) frequency of supply, (b) current in each coil, (c) current in each phase of the alternator, (d) total power supplied to the load.