SSC JE 2016 Electrical question paper (Set-2)

An overhead transmission line has groups of conductors running parallel to each other, carried on line supports. An electric transmission line conductor has four parameters: which are the series combination of resistance, inductance, shunt combination of capacitance and conductance.

The material used as the conductor for power transmission and distribution lines must possess the following characteristics:

• Low specific resistance leading to less resistance and high conductivity.
• High tensile strength to withstand mechanical stresses.
• Low specific gravity in order to give low weight per unit volume.
• Low cost in order to use over long distances.

Copper and aluminum conductors are used for overhead transmission of electrical power. In case of high voltage transmission, aluminum with a steel core is generally used. Sometimes cadmium, copper phosphor, bronze, copper weld, and galvanized steel are also used as transmission conductors. The choice of the conductor used for transmission purely depends upon the cost, as well as required electrical and mechanical properties.

Interpoles winding or Commutating Pole Winding In DC Machine

•  The commutating poles, or interpole, as they are sometimes called because of their position relative to the main poles, consist of a series of small poles similar to the main field poles in construction and method of fastening, but having a winding that consists of a few turns of heavy copper bus bar of high current capacity and low resistance.
• In DC machine One way to reduce the effects of armature reaction is to place small auxiliary poles called “interpoles” between the main field poles. The interpoles have a few turns of large wire and are connected in series with the armature.
• One of the disadvantages of armature reaction is brush shifting, therefore a person is always required to adjust the brush position in the machine at every load change. We observe that sparking in the brushes can be avoided if the voltage in the coils undergoing commutation is made zero.
• This method tries to do just the same. Small poles called commutating poles or interpoles are introduced in between the main poles along the geometrical neutral axis. Brushes are also set on this axis and kept fixed at this position for all the loads. The interpole winding has fewer turns of heavy copper conductors.

• Interpoles are connected in series with the armature winding so that they carry full armature current, as shown in Fig. As the load on the machine is increased, the current passing through the interpoles also increases, hence the flux produced by the interpoles is very large. Consequently, the large voltage is induced in the conductor that opposes the voltage due to the neutral plane shift and the net result is that they neutralize each other.
• Note that the interpoles can be used equally effectively in motors as well as in generators. When the mode of operation of the machine changes from the motor to the generator, the currents in the armature and the interpoles are reversed in direction. Therefore, their voltage effects cancel each other out.

Thus, we can conclude that:

1. In a generator, interpoles must have the same polarity as the next upcoming pole.
2. In a motor, interpoles must have the same polarity as the previous main pole.

The MMF induced on the interpoles must be sufficient enough to neutralize the effect of armature reaction and to produce enough field in the interpole winding to overcome the reactance voltage due to commutation.

Another important function of the interpole is to neutralize the cross-magnetizing effect of the armature reaction, as shown in Fig. Here, vector F_M represents the MMF due to main poles, F_A represents the cross-magnetizing mmf due to the armature reaction and F_C represents the interpole mmf which is directly opposite to the F_A so that they cancel each other out.

It is important to note here that the interpoles do not affect the flux distribution under the pole faces. So, even by using the interpoles in the machine, the flux weakening problem is not completely eliminated. Most medium-sized general-purpose motors correct the sparking problems with the interpoles and just live with the flux weakening problems.

Main Functions of the Interpole

1. The interpole neutralizes the reactance voltage and gives a spark-free commutation.
2. It neutralizes the cross-magnetizing effect of armature reaction so that the brushes are not required to be shifted from its original position for any load.

• Standards agencies such as the National Electrical Manufacturers Association (NEMA) and International Electrotechnical Commission (IEC) specify certain information that describes the physical and operating characteristics of each type of motor design, which is listed on the motor’s nameplate.
• The purpose of standardizing this information is to ensure the correct interchangeability of motors between different motor manufacturers.
• Both NEMA and IEC motors and motor control components are common in the electrical industry today because the electrical equipment is manufactured in the global economy under both standards.
• NEMA rates motor power in horsepower, and IEC rates motor power in kilowatts.
• The rated voltage of a motor listed on the nameplate is called the terminal voltage because it is the actual voltage on the motor’s terminals at which the manufacturer designed the motor to operate.
• Nominal voltage is the design or configuration voltage of the electrical distribution system.
• Motor terminal voltages are rated slightly less than nominal system voltages to compensate for electrical distribution system voltage drops
• NEMA standard motors are designed to operate within a 10% voltage deviation tolerance from the nameplate rated voltage.
• The nameplate current rating of a motor is measured when the motor is loaded to its full rated horsepower.
• The nameplate current rating of a motor is measured when the motor is loaded to its full rated horsepower.
• The nameplate RPM rating of a motor is measured when the motor is loaded to its full rated horsepower.
• Motors below 1 horsepower are referred to as fractional horsepower motors, and motors 1 horsepower or more are called integral horsepower motors.
• The locked rotor kVA per horsepower multiplier is used to determine the maximum locked rotor amperes a motor design can draw.
• The NEMA duty ratings for motors are intermittent, continuous duty, and special duty.

An electrical conduit is a tube used to protect and route electrical wiring in a building or structure. The electrical conduit may be made of metal, plastic, fiber, or fired clay. Most conduit is rigid, but flexible conduit is used for some purposes.

A motor connection is the means by which the power leads are terminated to a motor. From the estimator’s perspective, the motor terminations must also account for connecting the flexible conduit and fittings from the rigid conduit to the motor terminal box at its use point.

Flexible conduit to motors serves four functions.

1. It can absorb the normal motor vibrations that would, over time, fatigue and crack rigid conduit connections.
2. Since the vibration does not loosen flexible conduit as it would rigid parts, the ground path continuity is not disrupted.
3. Most motor installations require that the motor be movable so that its shaft can be aligned with the shaft of the driven device.
4. There is a distinct labor-saving in connecting a flexible conduit to a motor box over the field-bending rigid conduit to fit.

According to the National electric code (NEC), the NEC does restrict the length of the flexible metal conduit of the motor leads between the motor and required junction box to a maximum of 6 ft(1.8 m) to limit the ground return path. 

Strain-Gauge Rosettes

A strain-gauge rosette is, by definition, an arrangement of two or more closely positioned gage grids, separately oriented so as to measure the strains along with different directions in the underlying surface of a test part.

Rosettes are designed to serve a very practical purpose in experimental stress analysis. It can be shown that for the not uncommon case of the general biaxial stress state, with the principal directions unknown, three independent strain measurements (in different directions) are required to determine the principal strains and stresses. Even when the principal directions are known in advance, two independent strain measurements are needed to obtain principal strains and stresses.

For the purpose of meeting these requirements, strain-gage manufacturers typically offer three basic types of strain gage rosettes, each in a variety of forms:

• Tee – two perpendicular grids
• Rectangular- three grids, with the second and third grids angularly displaced from the first grid by 45 and 90°, respectively
• Delta – three grids, with the second and third grids 60 and 120° away, respectively, from the first grid

As illustrated in Fig. rectangular and delta rosettes may appear in any of several geometrical different, but functionally equivalent, forms.

When the directions of the principal strains are unknown, a three-element rectangular or delta rosette is required, and the rosette can be installed without regard to orientation. Functionally, there is little difference between the rectangular and delta rosettes. Because the gage axes in the delta rosette have the maximum possible uniform angular separation (effectively 120°), this rosette is assumed to yield the optimum sampling of the underlying strain distribution.

The induction motor is also known as an asynchronous motor. To start the induction motor, let us assume that the induction motor has been started without any load on it. The motor will come to its no-load speed, which may be at a slip as low as 0.1 percent.

At full load, the motor runs at a speed of N. When mechanical load increases, motor speed decreases till the motor torque again becomes equal to the load torque. As long as the two torques are in balance, the motor will run at constant (but lower) speed. 

The motor may be loaded continuously till pull out (or break down) torque is developed, at which point the motor will Stop if more load is placed on it. 

A current mirror is a circuit designed to copy a current through one active device by controlling the current in another active device of a circuit, keeping the output current constant regardless of loading.

Basically, a current mirror is a circuit that receives a current at the input and provides a copy of it at the output. This function can be theoretically modeled by a dependent current source, as mentioned before. Current mirrors are used to copy both DC and AC currents. In the applications where a current mirror is mainly employed to take part in the AC function of a circuit, its input and output resistances become important. Because the input and output signals are currents, an ideal current mirror is expected to have zero input resistance which helps to keep the input current constant regardless of drive conditions, and infinite output resistance which helps to keep the output current constant regardless of load conditions.

Induction-type Single-phase Energy Meter

An induction-type instrument can be used as an ammeter, voltmeter, or wattmeter, the induction-type energy meters are more popular. Induction-type single-phase energy meter is used invariably to measure the energy consumed in an AC circuit in a prescribed period where supply voltage and frequency are constant. The energy meter is an integrating instrument that measures the total quantity of electrical energy supplied to the circuit in a given period.

A single-phase energy meter has four essential parts:

1. Operating system
2. Moving system
3. Braking system
4. Registering system

Operating System

The operating system consists of two electromagnets. The cores of these electromagnets are made of silicon steel laminations. The coils of one of these electromagnets (series magnet) are connected in series with the load and are called the current coil. The Current coil consists of a few turns of thick wire, connected in series with the load.   It carries full load current which depends upon the angle of lag or lead of the load. Therefore the currents in the pressure coil and current coil have a phase difference of nearly 90 degrees. 

The other electromagnet (shunt magnet) is wound with a coil that is connected across the supply, called the pressure coil. The voltage coil consists of many turns of fine wire encased in plastic, connected in parallel with the load, and is highly inductive. This coil is connected in parallel with the supply or load and carries current proportional to voltage. The current in this coil lags behind the voltage approximately 90 degrees. The pressure coil, thus, carries a current that is proportional to supply voltage.

Shading bands made of copper are provided on the central limb of the shunt magnet. Shading bands as will be described later, are used to bring the flux produced by a shunt magnet exactly in quadrature with the applied voltage.

The two field fluxes produced by the pressure coil and current coil act on the aluminum disc, induce eddy currents in the disc, and hence the disc rotates due to the interaction of the two fluxes developed. The speed of the disc is proportional to the product of voltage, current, and the number of revolutions of the disc (i.e. time). In other words, the disc speed is proportional to the energy consumed by the load. The number of revolutions completed by the disc for one kilowatts hour is called meter constant.

When the load is inductive in nature the voltage leads the current by an angle φ.

When the load is capacitive in nature the current leads the voltage by an angle φ.

If the inductive reactance is greater than the capacitive reactance, i.e X_L > X_C  the net reactance is inductive, then the RLC circuit has a lagging phase angle and if the capacitive reactance is greater than the inductive reactance, i.e X_C > X_L the net reactance is capacitive then the RLC circuit have leading phase angle and if both inductive and capacitive are the same, i.e X_L = X_C then the circuit will behave as the purely resistive circuit. 

When the capacitor is connected in series then the total capacitance

1/C = 1/C₁ + 1/C₂ + 1/C₃

1/C = 1/30 + 1/30 + 1/30

C = 10μF