SSC JE Electrical question paper with solution 2016 -Set-3

The characteristics of cumulative compound motor lie between those of shunt and series motors. The series field provides a high starting torque and the shunt field prevents overrunning in the no-load condition.

Class D motors are characterized by high starting torque, low starting current, and high operating slip. The rotor cage bars are made of high-resistance material such as brass instead of copper. The torque-speed characteristic is similar to that of a wound-rotor motor with some external resistance connected to the rotor circuit. The maximum torque occurs at a slip of 0.5 or higher. The full-load operating slip is high (8 to 15 percent), and therefore the running efficiency is low. The high losses in the rotor circuit require that the machine be large (and hence expensive) for a given power. These motors are suitable for driving intermittent loads requiring rapid acceleration and high-impact loads such as punch presses or shears. In the case of impact loads, a flywheel is fitted to the system. As the motor speed falls appreciably with load impact, the flywheel delivers some of its Kinetic energy during the impact.

As we know that the in wound Induction motor we can achieve high starting torque by adding some external resistance. There it can be used for press and punches.

These motors are used for drives where intermittent high load torque is required with the probability of the load being totally removed such as punch, press, shears, planning machine, conveyors, crushers, bulldozers, lid haulage gears, mine hoist, power fans, rolling mills, stamping press, and the large printing press.

• Dummy coil is used with wave winding when the requirement of the winding is not met by the standard armature.
• Dummy coil  is not connected to the commutator so they do not influence the electrical characteristics of the winding
• Dummy coil ends are cut short and taped.
• Their main use is to provide mechanical balance for the rotor because rotor having some slots without winding would be out of balance mechanically. 

Feedback is that property of a closed-loop system that permits the output (or some other controlled variable) to be compared with the input to the system (or input to some other internally situated component or subsystem) so that the appropriate control action may be formed as some function of the output and input.

Feedback can be either negative or positive. In negative feedback, a portion of the output signal is subtracted from the input signal. In positive feedback, a portion of the output signal is added to the input signal.

Negative feedback, for example, tends to maintain a constant value of amplifier voltage gain against variations in transistor parameters, supply voltages, and temperature. Positive feedback is used in the design of oscillators and in a number of other applications. 

Advantages of the Feedback System

• Faster response to an input signal
• Elective disturbance rejection
• Better tracking of reference signals
• Low sensitivity to system parameter errors (e.g., errors in plant or controller gains
• Low sensitivity to changes in calibration errors Recalibration is unnecessary)
• More accurate control of plant under disturbances and internal variations
• Effective and flexible control tuning by varying the control gain.
• Used to stabilize systems that are inherently unstable in the open-loop form

Disadvantages of Feedback Systems

• Since feedback decreases the overall gain, this must be made up by an increase in the loop gain of the system, required additional hardware, and increased complexity.
• The components m the feedback path must be made more accurate because feedback does not reduce the sensitivity to variations in these parameters. The result is a further increase in the overall cost.
• The sensors required for the feedback path may introduce a small amount of noise in the system, therefore reducing the overall accuracy.
• The power costs (due to high gains) are high
• Sometimes obtaining the output measurement is either hard or not economically feasible.
• Initial tuning is more difficult, especially if the bandwidth is narrow
• There is always a steady-state error (with proportional controllers)
• The introduction of feedback may lead to instability of the closed-loop system, even though me open-loop system may be stable. This is caused by inherent time lags in the system, With the result that what was intended as negative feedback may turn out to be positive feedback at some higher frequency. This was one of the first features of feedback observed when automatic control was introduced. 

Figure F is the symbol of the battery and the battery is the source of energy.

Note:-

Figure A represents the Earth electrode

Figure B represents Pre-set Potentiometer

Figure C represents Resistor

Figure D represents Variable Resistors

Figure E represent Attenuator 

• DC machines work either as dc generators or as dc motors.
• A d.c. the generator is based on the principle of electromagnetic induction i.e. when the amount of magnetic flux linking a coil changes, an e.m.f. is induced in the coil.
• In a dc generator, a set of conductors or coils placed on a rotating body called the armature, are rotated continuously inside a magnetic field with the help of a prime mover (prime mover is another machine, maybe a diesel engine or a turbine, which rotates the armature).
• When armature conductors are rotated externally in the magnetic field produced by field windings, an emf is induced in it according to Faraday’s laws of electromagnetic induction.
• The magnetic field is created by passing dc current through the windings of a set of field magnets. When conductors pass under alternate North and South poles, alternating EMF is induced in the armature winding.
• The ac generated gets converted into dc when the voltage is collected from the rotating armature through the brush and commutator arrangement. The brush and commutator arrangement, therefore, works like a full-wave rectifier which converts generated ac into dc for the output circuit.
• A dc generator, therefore, converts mechanical energy supplied through the prime mover to electrical energy to be supplied from the generator armature to an electrical load. 

Earthing shall generally be carried out in accordance with the requirement of I.E. rules, 1956, as amended from time to time and the relevant regulation of the electricity supply.

• The earth wire and earth electrode will be of the same material.
• The earth wire shall be taken through a G.I. pipe of 13 mm diameter for at least 30 cm length above
  and below the ground surface to the earth electrode to protect it against mechanical damage.
• It is not necessary that an earth wire connected to an earth electrode should be run along with the whole
  wiring system. All the earth wires run along the various sub-circuits shall be terminated and
  looped firmly at the mainboard and from the mainboard, the main earth shall be taken to the earth
  electrode. The loop earth wires used shall not be either less than 2.9 mm2 (14 SWG) or half of the size of the subcircuit conductor.
• The earthing electrode shall always be placed in a vertical position inside the earth or pit so that it
  may not be in contact with all the different earth layers.

Thermocouple

When two dissimilar metal conductors are connected together to form a closed circuit and the two junctions are kept in different temperatures, thermal electromotive force (EMF) is generated in the circuit (Seebeck’s effect). Thermocouples make use of this so-called Peltier-Seebeck effect. Thus, when one end (cold junction) is kept constant at a certain temperature, normally at 0°C and the other end (measuring junction) is exposed to unknown temperature, the temperature at the latter end can be determined by measurement of EMF so generated. This combination of two dissimilar metal conductors is called ‘thermocouple’.

Simply stated, a thermocouple is a device that converts thermal energy to electric energy. The amount of electric energy produced can be used to measure temperature.

Thermistor

Like RTD, Thermistor is also a temperature-sensitive resistor. A thermistor is an electronic component that exhibits a large change in resistance with a change in body temperature. Thermistors are highly sensitive to temperature variation; hence they are also called temperature-sensitive resistors. Thermistors are manufactured from metal oxide semiconductor material, which is encapsulated in a glass or epoxy bead. Thermistors also have a low thermal mass that results in fast response times but is limited by a small temperature range. 

If its resistance increases with temperature, it is said to have a positive temperature coefficient (PTC). If its resistance decreases with temperature, it is said to have a negative temperature coefficient (NTC).

Strain Gauge

Strain gauge is a passive type resistance pressure transducer whose electrical resistance changes when it is stretched or compressed. It can be attached to pressure sensed diaphragm.

LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT)

This is the most widely used inductive transducer for translating linear motion into an electrical signal. As we know that displacement is a vector quantity representing a change in position of a body or a point with respect to a reference. It can be linear or angular (rotational) motion. With the help of the displacement transducer, many other quantities, such as force, stress, pressure, velocity, and acceleration can be found. LVDT gives modulated output. The LVDT gives reasonably high output and hence requires less amplification.

The main electrical displacement transducers work on the principle of

Variable resistance: transducer is a strain gauge.

Variable inductance: transducer is a linear variable differential transformer

Variable capacitance: transducer is a parallel plate capacitor with a variable gap

Synchros and resolvers: used to measure angular displacement

A filter is a circuit that is designed to pass a specified band of frequencies while attenuating all the signals outside that band. It is a frequency selective circuit.

Low Pass Filter:- The low pass filter passes the low-frequency signal from input to output while it blocks high-frequency signals from the input. A low pass filter (even or odd orders based on a ladder network consists of shunt capacitors and series inductors that interconnect these shunt capacitors. No inductor is grounded. The load impedance/resistance is always grounded. For an even order low pass filter, the last reactive element is always an inductor, whereas, for an odd order filter, the least reactive element is always a capacitor.

High Pass Filter:- In high pass filter, the filter rejects the frequencies which are less than cut-off frequencies ω_o and it allows to pass the frequencies which are greater than ω_o. The topology of a ladder network-based high pass filter is complementary to that of a ladder network-based low pass filter. Now all inductors are shunted, and series capacitors interconnect these shunt inductors. The load impedance/resistance is always grounded.

Band Pass Filter:- A band-pass filter is one designed to pass signals with frequencies between two specified cut-off frequencies. A ladder network-based bandpass filter consists of alternating pairs of series and parallels connected capacitors and inductors. Each pair of parallel-connected capacitors and inductors is grounded. Each series-connected pair of capacitor and inductor interconnects two pairs of parallel-connected capacitor and inductor.

Kirchhoff’s Voltage Law (KVL,) or Kirchhoff’s Loop Rule. This law is based on the conservation of energy and may be stated as under:

In any closed electrical circuit or loop, the algebraic sum of all the electromotive force (e.m.f s) and voltage drops in resistors is equal to zero, i.e., in any closed circuit or loop.

The algebraic sum of e.m.f s + Algebraic sum of the voltage drops = 0

The validity of Kirchhoff’s voltage law can be easily established by referring to the loop ABCDA shown in Fig.

If we start from any point (say point A) in this closed circuit and go back to this point (i.e., point A) after going around the circuit, then there is no increase or decrease in potential. This means that the algebraic sum of the e.m.f.s of all the sources (here only one e.m.f. source is considered) met on the way plus the algebraic sum of the voltage drops in the resistances must be zero. Kirchhoff’s voltage law is based on the law of conservation of energy, i.e., the net change in the energy of a charge alter completing the closed path is zero.

V₁ + V₂ − V = 0

or

Kirchhoff’s voltage law is also called as loop rule.

KVL and KCL and apply to any lumped electric circuit; it does not matter whether the circuit elements are linear, nonlinear, active, passive, time-varying, time-variant, etc. In other words, KVL and KCL are independent of the nature of the elements.

When four capacitors of  Capacitance C is connected in series the total capacitance 

1/Ceq = 1/C + 1/C + 1/C + 1/C = 4/C

C_eq = C/4