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Q5. With reference to electronic control systems:

A. Draw a simple block diagram for temperature control. (8)

B. Describe each component shown in the diagram in (a).  (8)

Q2. A. (i) Describe with the aid of a sketch the operation of a synchro scope; (ii) State the information obtained from it. B. Suggest a substitute in the event of synchro scope and stand by light failure.

Q1. A. Sketch a circuit diagram of a push button direct online contactor starter for a three phase Incorporating overload and short circuit protection.

B. Indicate, on a sketch of the typical characteristic curves of current and torque against Speed, disadvantages of a direct on line start squirrel cage induction motor.

Q4. What are the conditions for producing sustained oscillations? Classify oscillations with respect to frequency range, principle involved, etc. It is possible to produce oscillations with RC networks in phase shift oscillators. Discuss in detail. (16)

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)

Q9. 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? (6)

B. The low-voltage release of an A.C. motor-starter consists of a solenoid into which an iron plunger is drawn against a spring. The resistance of the solenoid is 35 ohms. When connected to a 220 V, 50 Hz, A.C. supply the current taken is at first 2 A, and when the plunger is drawn into the “full-in” position the current falls to 0.7 A. Calculate the inductance of the solenoid for both positions of the plunger and the maximum value of flux-linkages in weber-turns for the “full-in” position of the plunger. (10)

Q7. (a) State briefly, the meaning of the expressions 'star-connected' as applied to three-phase a.c. practice. What is the ratio of the maximum line voltage to the maximum phase voltage in each case. (b) Determine the line current taken by a 440V, three-phase, star-connected motor having an output of 45KW at 0.88(lagging) power factor and an efficiency of 93 percent.

Q9. A 20kVA, 2000/220V, single-phase transformer has a primary resistance of 2.1Ω and a secondary resistance of 0.026Ω. The corresponding leakage reactance’s are 2.5Ω and 0.03Ω. Estimate the regulation at full load under power-factor conditions of.

(a) Unity;

(b) 0.5 (lagging) and

(c) 0.5 (leading).

Q8. A. Electric motors contain a stationary member as well as a rotating member. For each of the following machines, identify in which part of the motor three field winding and the armature winding are located: three phase induction motor, three phase synchronous motor, d.c. motor. B.  An 18.65 KW,4-pole,50HZ, 3 phase induction motor has friction and windage losses of 2.5 percent of the output. The full load slip is 4% compute for full load (a) the rotor Cu loss (b) the rotor input (c) the shaft torque (d) the gross electromagnetic torque.

Q9. (a) Describe an accurate method of comparing the capacities of two condensers. (b) A diode valve, having the following characteristic is connected in series with a resistor of 10,000 ohm to a 240 V d.c. supply. If a resistor of 40,000 ohms is connected between the anode and cathode, determine the current through the diode.