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

The function of the economizer is to recover a portion of heat from the exhaust gases before the flue gases enter the chimney and are discharged into the atmosphere.

The economizer is placed in the path of the flue gases in between the boiler exit and entry to the chimney. Feedwater coming from the feed pump, when passed through the economizer tubes, absorbs the heat in the exhaust gases. This increases the temperature of the water entering the boiler.

Due to the high temperature of feed water, fuel consumption reduces, and this increases the overall efficiency of the boiler.

Hard magnetic materials retain a considerable amount of magnetic energy after the magnetizing force has been removed. These materials are difficult to demagnetize, such materials are also called permanent magnetic materials. E.g Carbon and Tungsten steel

Characteristics of Hard Magnetic Materials

1. Low permeability
2. High curie temperature
3. High coercive force
4. Good residual magnetism

Carbon Steel is used for Magnets of toys, compass Needles, latching relays, meters.

Tungsten steel is used to prepare DC motor magnets.

An oscillator operated near class A has good frequency stability and low harmonic. The Colpitts Oscillator has good stability and low harmonic output. It is used at a frequency above 10 MHz. Therefore, the Colpitts oscillator is generally employed to generate signals of fixed frequency.

An electrical measuring instrument depends on its operation on one of the many effects produced by current or voltage. When current flows through a wire, it produces a magnetic field as well a heating effect. When it flows through a battery, it produces a chemical effect in the electrolyte solution.

In a similar way, it produces an electrostatic field and can also induce emf in a nearby circuit due to electromagnetic induction. In a secondary instrument, one of the above effects is used to produce the deflecting torque. For example,

1. The magnetic effect is used in ammeters, voltmeters, wattmeters, and energy meters.
2. The heating effect is used in ammeters and voltmeters.
3. The chemical effect is used in dc ampere-hour meters.
4. An Electrostatic effect is used in voltmeters. electromagnetic induction effect is used in ac ammeters, voltmeters, and wattmeters are energy meters.

Gas Turbine Plant

A gas turbine plant may be defined as one “in which the principal prime-mover is of the turbine type and the working medium is a permanent gas”.A simple gas turbine plant consists of the following

1. Turbine.
2. A compressor mounted on the same shafi: or coupled to the turbine.
3. The combustor.
4. Auxiliaries such as starting device, auxiliary lubrication pump, fuel system, oil system, and duct system, etc.

A modified plant may have in addition to the above an intercooler, regenerator, reheater etc. The working fluid is compressed in a compressor which is generally rotary, multistage type. Heat energy is added to the compressed fluid in the combustion chamber.

This high energy fluid, at high temperature and pressure, then expands in the turbine unit thereby generating power.The gas turbines work on an open cycle, semiclosed cycle or closed cycle. In order to improve efficiency, compression and expansion of working fluid are carried out in multi-stages.

A Schrage motor is a polyphase commutator motor with shunt characteristics. The Schrage motor has its primary winding on the rotor connected to the supply through slip-rings and brushes. The rotor also carries a low-voltage commutator winding with conductors located in the same slots as, and above, those of the primary.

The secondary winding is on the stator. The primary and secondary windings are similar to those of a conventional induction motor. Hence in the Scharge motor, the rotor carries two windings, of which one receives current from the supply by means of collector rings, while the other is connected to the commutator.

A Schrage motor is usually started up by switching directly to the supply when the brushes are in the low-speed position. To ensure that the brushes are in this position a small interlock switch is fitted, connected in series with the low voltage release coil in the main supply switch; thus the main switch cannot be closed unless the brushes are in the low-speed position. Therefore scharge motor can be started by applying the low voltage.

FET is a voltage-controlled device, i.e. voltage at the input terminal controls the output current, whereas BIT is a current-controlled device, i.e. the input current controls the output current.

The input circuit of FET is reverse biased, FET exhibits a much higher input impedance (in the order of 100 MΩ) and lower output impedance and there will be a high degree of isolation between input and output.

So, FET can act as an excellent buffer amplifier but the BJT has a low input impedance (20Ω) because its input circuit is forward biased. The output impedance is comparable for both BJT and FET but the output impedance is slightly more than the BJT.

Advantages of Induction instrument

1. A very large scale Deflection (300°) is possible using the “circular scale” type of instrument.
2. The stray magnetic field has a limited effect on the reading.
3. Good and effective Damping

Susceptibility for Paramagnetic materials is Positive and small.

Some substances when placed in a magnetic field are feebly (weakly) magnetized in the direction of the applied magnetic field. These substances get weakly attracted in a magnetic field and in the absence of a magnetic field they do not retain their magnetic property. Such materials are referred to as paramagnetic materials.

Property of Paramagnetism is found in those substances whose atoms or molecules have an excess of electrons spinning in the same direction. Therefore atoms of paramagnetic material possess permanent magnetic dipole moment (due to orbital and spin magnetic moment.

In electromagnetism, magnetic susceptibility is one measure of the magnetic properties of a material. If the magnetic susceptibility is greater than zero, the substance is said to be “paramagnetic”; the magnetization of the substance is higher than that of empty space. If the magnetic susceptibility is less than zero, the substance is “diamagnetic”.

Paramagnetic materials have a small positive susceptibility of the order of 10-3 at room temperature. Potassium, oxygen, and yttrium oxides are examples of paramagnetic material

Induction wattmeters can be used on AC the circuit only (in contrast with dynamometer wattmeters can be used both on D.C. and A.C. circuits) and are used only when the frequency and supply voltage are constant.

The operation of all induction instruments depends on the production of torque due to the reaction between flux φ (whose magnitude depends on the current or voltage to be measured) and eddy currents induced in a metal due or drum by another flux φ₂ (whose magnitude also depends on the current or voltage to be measured).

Since the magnitude of eddy currents also depends on the flux producing them, the instantaneous value of the deflecting torque is proportional to the square of the current or voltage under measurement and the value of the mean deflecting torque is proportional to the mean square of the current or voltage.