Ques.11. The main function of a transformer is to change
- Power
- Frequency
- Voltage Level
- Power factor
Answer.3. Voltage level Explanation:- Transformers are used in an electric circuits to change the voltage level. We can step up or step down the voltage level according to our needs.
Ques.12. Which of the following is a cold cathode lamp?
- Sodium lamp
- Neon Lamp
- CSL Lamp
- Tubelight
Answer.1,2. Neon Lamp and sodium vapor lamp Explanation:- SODIUM VAPOR LAMP A sodium vapor lamp is low pressure and cold cathode lamp i.e its electrode is not electrically heated by a filament. A sodium vapor discharge lamp consists of a U-shaped tube enclosed in a double-walled vacuum flask, to keep the temperature of the tube within the working region. The inner U-tube consists of two oxide-coated electrodes, which are sealed with the ends. These electrodes are connected to a pin-type base construction of a sodium vapor lamp is shown in Fig. This sodium vapor lamp is a low luminosity lamp so the length of the lamp should be more. In order to get the desired length, it is made in the form of a U-shaped tube. This long U-tube consists of a small amount of neon gas and metallic sodium. At the time of start, the neon gas vaporizes and develops sufficient heat to vaporize metallic sodium in the U-shaped tube. This is a cold cathode lamp, in which no filament is used to heat the electrode for starting. The neon lamp consists of two electrodes placed at the two ends of a long discharge tube. The discharge tube is filled with neon gas. A low voltage of 150 V on DC or 110 V on AC is impressed across the two electrodes; the discharge takes place through the neon gas that emits light or electromagnetic radiation reddish in color. The sizes of electrodes used are equal for both AC and DC supplies. On DC, neon glow appears nearer to the negative electrode; therefore, the negative electrode is made larger in size. The neon lamp electric circuit consists of a transformer with high leakage reactance in order to stabilize the arc. The capacitor is used to improve the power factor. Neon lamp efficiency is approximately 15-40 lumens/W. The power consumption of the neon lamp is 5 W If the helium gas is used instead of neon, pinkish-white light is obtained. These lamps are used as night lamps and as indicator lamps and used for the determination of the polarity of DC mains and for advertising purposes.Neon Lamp
Ques.13. The V.I characteristics of the diode lie in the
- 1st and 2nd Quadrant
- 1st and 3rd Quadrant
- 1st and 4th Quadrant
- Only in 1st Quadrant
Answer.2. 1st and 3rd Quadrant Explanation:- V. I Characteristics of Diodes The combined forward and reverse characteristics is called V-I characteristics of a diode. The potential at which current starts increasing exponentially is also called offset potential, threshold potential, or firing potential of a diode.
Ques.14. Which of the following is used in a semiconductor device?
- Silicon
- Graphite
- Copper
- Aluminum
Answer.1. Silicon Explanation:- Silicon is the most widely used semiconductor material because it has a useful temperature range and is abundant, cheap, and easy to manufacture. Germanium was among the first semiconductor materials to be developed and is less widely used than silicon, but it is useful in very high-speed devices when alloyed with silicon.
Ques.15. Generator transformer is
- Step-up transformer
- Step-down transformer
- Autotransformer
- One-one transformer
Answer.1. Step-up transformer Explanation:- A step-up transformer associated with the turbine generator systems is called a generator transformer. Its role is to raise the voltage of the electric energy generated which can be efficiently transmitted to the substation via buried cables or overhead lines. Generator transformers step the voltage up from the generator level of typically 10-20 kV to the transmission or sub-transmission voltage.
Ques.16. Two electric bulbs designed to operate with a power of 500 watts in 220-volt line are connected in series with a 110-volt line. The power generated by each bulb will be
- 31.25 watts
- 3.125 watts
- 22 watts
- 62.5 watts
Answer.4. 62.5 watts Explanation:- Given P = Power = 500 W Voltage V = 220 volt Resistance R = The power is given as P = V2/R 500 = (220)2/R R = 96.8 ohms Now bulb is connected to 110 V supply. Therefore power will be P = V2/R P = (110)2/96.8 P = 125 watts Hence power generated by each bulb as they are connected in series will be P = 125/2 = 62.5 watts
Ques.17. The resistance of an ideal ammeter is
- Low
- High
- Infinite
- Zero
Answer.4. Zero Explanation:- An ammeter is used to measure the current in a circuit. It is connected in series with the circuit to avoid division of current, but ‘when placed in series with the circuit, it increases the resistance and decreases the current being measured by it. Hence, an ideal ammeter should have zero resistance.
Ques.19. Power factor can be improved by connecting which among the following?
- Static capacitor
- Resistor
- Synchronous condenser
- Both 1 and 3
Answer. 4. Both 1 and 3 Explanation:- Normally, the power factor of the whole load on a large generating station is in the region of 0.8 to 0.9. However, sometimes it is lower and in such cases In order to improve the power factor, some device taking leading power should be connected in parallel with the load. This can be achieved by the following equipment Advantages Disadvantages A synchronous condenser is a synchronous motor running without the mechanical load. The Synchronous motor takes a leading current when over-excited and therefore behaves as a capacitor. When such a machine is connected in parallel with the supply, it takes a leading current which partly neutralizes the lagging reactive component of the load. Thus the power factor is improved. Disadvantages There are special commutator machines, which are used to improve the power factor of the induction motor. When the supply is given to the stator of an induction motor, it takes a lagging current. So, the induction motor has a low lagging power factor. For compensating this lagging current, a phase advancer (mounted on the same shaft) is used. It supplies MMF to the rotor circuit at slip frequency. Advantages Advantages of power factor improvement.
Static Capacitor:-
Synchronous condenser
Phase Advancer:
Ques.20. Which of the following rotor is used in thermal power plants?
- Cylindrical rotor
- Salient pole rotor
- Either of these
- Square cage rotor
Answer.1. Cylindrical rotor Explanation:- The rotor of the alternator is of two types Cylindrical Rotor Rotor and stator are the main parts of a generator. The rotor is placed inside the stator. The air gap between the stator and the rotor is very less. In power generators of thermal power plants, the cylindrical rotor is used because they run at a very high speed. This type of rotor is also called a non-salient or drum-type rotor. The rotor of the generator carries field winding. When DC power supply is applied to this winding, it becomes magnetic. Rotor also carries a fan that circulates the cooling medium inside the generator to cool down the generator. The design of a rotor is critical, as the field winding is put inside the slots provided and it has to rotate at a speed of normally 1500 rpm or 3000 rpm. Also, the mass of a rotor is high. Normally, cylindrical rotors for two or four-magnetic pole designs are used for the generation of power at power plants. The number of poles of a rotor depends upon the speed of the generator.