UPPCL JE 2016 Electrical question paper with Solution (Evening Shift)

A thermal power station is a power station in which heat energy is converted to electric power. In most places in the world, the turbine is steam-driven. Water is heated, turns into steam, and spins a steam turbine which drives an electrical generator. After it passes through the turbine, the steam is condensed in a condenser and recycled to where it was heated; this is known as a Rankine cycle.

The Rankine cycle is a model used to predict the performance of steam turbine systems. It was also used to study the performance of reciprocating steam engines. The Rankine cycle is an idealized thermodynamic cycle of a heat engine that converts heat into mechanical work while undergoing a phase change.

• During starting of the squirrel cage induction motor if the number of stator slots is equal to the number of rotor slots or an integral multiple of the rotor slots then it causes magnetic locking between stator and rotor slot.
• The upper and lower attracting force between stator and rotor slot becomes more prominent than the tangential force, as a result, the rotation of the motor is stopped.

A DOL starter connects the motor terminals directly to the power supply. Hence, the motor is subjected to the full voltage of the power supply.

By connecting the motor in the star configuration the reduction of voltage is 1/√3 = 0.577, therefore, the starting current also gets reduced to 0.577 i.e 57.7%.

DIAMAGNETIC SUBSTANCES, when placed in a magnetic field, acquire feeble magnetism opposite to the direction of the magnetic field. These substances are somewhat repelled when brought near the ends of a powerful magnet. They are called ‘diamagnetic‘ substances, and their magnetism is called ‘diamagnetism.’

Examples of diamagnetic substances are Bismuth, Zinc, Copper, Silver, Gold, Diamond, Salt, Water, Mercury, Nitrogen, hydrogen, etc.

Reverse biasing

In reverse biasing, the positive terminal of the battery is connected to the n-side and the negative terminal to the p-side of the junction.

In the reverse-biased condition, a transient current flows, and both the carriers are pulled away from the junction. The increase in barrier energy is given by eV, where V is the voltage applied across the junction. When the potential formed by the widened depletion layer equals the applied voltage, the current ceases except for the small thermal current generated by the minority carriers. This current l is called reverse saturation current, which is independent of the applied reverse voltage (V) and increases with the temperature of the diode. The p-n junction conducts the current easily when forward biased and practically no current flows when it is reverse biased.

Magnetic flux is the product of the average magnetic field times the perpendicular area that it crosses.

Magnetic flux = Magnetic field × Area × (angle between the planar area and the magnetic flux)

The equation is:

Φ = BA cos(θ)

Where:

Φ: Magnetic Flux

A: Area

B: Magnetic field

θ: angle between a perpendicular vector to the area and the magnetic field

If the surface area is held parallel to electric field lines so that the vector area is perpendicular to electric field lines that is θ=90 degrees between B & A then electric flux is zero.

Φ = B A cos(90)

= B A × 0

Φ = 0

The N-type impurity loses its extra valence electron easily when added to a semiconductor material, and in so doing, increases the conductivity of the material by contributing a free electron. This type of impurity has 5 valence electrons and is called a PENTAVALENT impurity.

Arsenic, antimony, bismuth, and phosphorous are pentavalent impurities. Because these materials give or donate one electron to the doped material, they are also called DONOR impurities. When a pentavalent (donor) impurity, like arsenic, is added to germanium, it will form covalent bonds with the germanium atoms.

Pure germanium may be converted into an n-type semiconductor by “doping” it with any donor impurity having 5 valence electrons in its outer shell. Since this type of semiconductor (N-type) has a surplus of electrons, the electrons are considered MAJORITY carriers, while the holes, being few in number, are the MINORITY carriers.

A null method of measurement is a simple, accurate, and widely used method that depends on an instrument reading being adjusted to read zero current only. The method assumes:

(i) If there is any deflection at all, then some current is flowing;

(ii) If there is no deflection, then no current flows (i.e. a null condition).

E.g of null detection method is DC potentiometer, Kelvin double bridge, AC potentiometer, Wheatstone bridge.

In a Deflection type system, the quantity to be measured produces an effect either in the form of a voltage or a current. This effect is then utilized to produce a torque that causes a mechanical deflection. With the help of a spring system, this torque is countered by a restoring torque that increases with the increase in deflection.

The accuracy of a deflection type instrument depends upon the degree of accuracy of the calibration. The accuracy of null type instruments is higher than deflection type instruments because null indication means zero current flowing through the instrument and hence there is no effect of error due to calibration in measurement. However, null-type instruments are not suitable for the measurement of a quantity that changes with time.

Variable Speed Drives (VSD) are used to control the speed, torque, power, and direction of motors. They are used for induction, synchronous, and DC motors. Drives are available in two basic types

• DC drive used to control DC motors
• AC drive is used to control induction and synchronous motors.
• DC drives are used only in industrial systems, particularly in heavy industries such as metallurgical and paper and pulp. They are used mainly for process or production control.
• AC drives are used fa process production as well as for services/off-site facilities such as heating, ventilation and air conditioning lifts and water treatment. The drives for services are used mainly to control pumps and fans.
• In commercial and residential systems the AC variable speed drives are used for services and are typically less than 200 kW, the low-voltage type. In industrial systems, AC drives are used for processes (production and services.

A Mercury vapor lamp is a discharge lamp. Gas-discharge lamps are a family of artificial light sources that emit light by sending an electrical discharge through an ionized gas, i.e. plasma. Typically, such lamps are filled with a noble gas (argon, neon, krypton, and xenon) or a mixture of these gases. Most lamps are filled with additional materials, such as mercury, sodium, and metal halides.

Mercury vapor discharge at low pressure gives mainly ultraviolet radiation. HPMV lamp consists of two tubes or bulbs. The inner discharge tube is made of tough quartz glass having liquid mercury and argon gas {for starting). The discharge tube is contained in an outer envelope, the inside of which is coated with fluorescent power.

The space between two bulbs is filled with an inert gas. For starting the ignition, an auxiliary electrode is kept inside the discharge tube near one of the main electrodes and connected to another electrode through carbon resistance. When the normal supply voltage is applied, an electric field is set up between starting auxiliary electrode adjacent to the main electrode. This causes the discharge to first take place between them by limiting carbon resistance. This charge to full brilliance requires 2 to 4 minutes.

Once the arc tube is filled with mercury vapor, a low resistance path is created for the current to Bow between the main electrodes due to which arc shifts to the main electrode and the discharge continue. When the applied voltage is removed, the lamps will not restart till mercury vapor pressure has filled low enough to allow for re-discharge. This time is again 4 to 8 minutes.

The discharge from the tube again strikes with the fluorescent coating of the outer bulb from which secondary emission starts which give crisp white light in color corrected type design (fluorescent type).

The HPMV lamps are available at iterates of 80, 125, 250, and 400 watts and for special use 700, 1000 and 2000 Watts, giving light output over 125000 lumens.