MET-Mar-2021

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 online start squirrel cage induction motor.

Q2. Sketch a circuit diagram for an automatic voltage regulator illustrating how the AVR utilizes a silicon-controlled rectifier to control the excitation system for an alternator. Describe how AVR monitors the output and controls the excitation system.

Q3. Tank liquid level sensors are an integral part of ships. Describe with aid of suitable sketches the working principle of,
(a) capacitive type level sensor;
(b) Ultrasonic level sensor;
(c) Float.

Q4. A. What are the causes of overheating of an induction motor?
B. What preventive measures are provided against damage to an induction motor in installed condition?
C. What is the purpose of ‘fuse back up protection’ provided to an induction motor?
D. How does an induction motor develop torque?
E. What is the condition to be satisfied for achieving maximum running torque in an induction motor?

Q5. With reference to alkaline batteries used onboard ships.
A. Describe the operation of a battery cell and state the material used;
B. Describe how the cells are mounted to form a battery;
C. State the advantages and disadvantages compared with lead-acid batteries.

Q6. A. Describe the normal criteria used for setting thermal protection relays and their advantage compared to magnetic types. 

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 ohm. When connected to a 220 V, 50 Hz, a.c. 2supply the current taken is at first 2A, 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.

Q7. With reference to Synchronous Motors. A. Draw and explain the principle of operation Synchronous Motors. B. Find the synchronous impedance reactance of an alternator in which a given field current produces an armature current of 200 A on a short circuit and a generated e.m.f. of 50V on an open circuit. The armature resistance is 0.1 ohm. To what induced voltage must the alternator be excited if it is to deliver a load of 100A at a p.f of 0.8 lagging, with a terminal voltage of 200 V.

Q8. A. By means of a schematic circuit diagram illustrate the peak rectifier. If the supply voltage is v(t) = Vm Sinωt, what is the voltage across the load resistor?

B. A battery-charging circuit is shown below in Fig. The forward resistance of the diode can be considered negligible and the reverse resistance infinite. The internal resistance of the battery is negligible. Calculate the necessary value of the variable resistance R so that the battery charging current

is 1.0 A.

 

Q9. A. Explain the significance of the root mean square value of an alternating current or voltage waveform; Define the form factor of such a waveform.

B. A total load of 8000 kW at 0.8 power factor is supplied by two alternators in parallel. One alternator supplies 6000 kW at 0.9 power factor. Find the KVA rating of the other alternator and the power factor.