MET-Jul-2021 (1)

Q1. (a) Briefly explain the principle of operation of Induction Motors. (b) What is Slip for an induction motor? (c) Draw a simple ladder logic diagram of the star-delta starting of an Induction Motor. (d) Sketch the torque against the speed curve of the induction motor.

Q2. Explain why it is necessary to have reverse power protection for alternators intended for Operation. 

A. Sketch a reverse power trip; 

B. Explain briefly the principle on which the operation of this power trip is based and How tripping is activated.

Q3. A. Sketch a standby battery charging/discharging circuit; B. Describe the circuit sketched, making special reference to how battery charge is maintained and how it operates upon loss of main power.

Q4. 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.

Q5. What are semiconductor devices? What are its advantages over thermionic devices? With respect to semiconductor devices describe the working principle and application of the following A. Zener Diode B. Transistor, C. Photocell, D. Thyristor

 

Q6. 72 KVA transformer supplies a heating and lighting load of 12 KW at unity power factor and a motor load of 70 kVA at 0.766 (lagging) power factor; Calculate the minimum rating of the power-factor improvement capacitors which must be connected in the circuit the ensure that the transformer does not become overloaded.

 

Q7. A. What is back emf? Derive the relation for the back emf and the supplied voltage in terms of armature resistance.

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

 

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. Compare the effectiveness of a current limiting circuit breaker with that of an HRC fuse.

B. A coil having a resistance of 10 Ohm, and an inductance of 0.15 H is connected in series with a capacitor across a 100V, 50Hz supply. If the current and the voltage are in phase what will be the value of the current in the circuit and the voltage drop across the coil?

 

Q10. 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.