SSC JE Electrical Previous Year Question Paper 2018-SET2|MES Electrical

Ques.61. Reduction in the capacitance of a capacitor-start motor result in reduced_____

  1. Noise
  2. Speed
  3. Starting Torque
  4. Armature Reaction

Capacitor-Start Motors

The capacitor start motor is identical to the split phase motor in both construction and operation, except that a capacitor is installed in series with the starting winding, as shown in Fig.

 

Also, the starting winding of the capacitor start motor is usually wound with larger wire than that used for the starting winding of the split-phase motor. The use of a capacitor in series with the starting winding causes the current in this winding to lead the voltage, whereas the current in the running winding lags the voltage by virtue of the high inductance of that winding. With this arrangement. the phase displacement between the two windings can be made to approach 90 electrical degrees so that we two-phase starting is achieved. For this reason, the starting torque of the capacitor start motor is very high, which makes it an ideal drive for the small compressor that must be started under full load.

So if the low value of the capacitor is used in the motor the phase angle between the starting and running winding will be reduced and the motor can’t generate enough torque to rotate the motor.

 

Ques.62. A single-phase induction motor with only the main winding excited would exhibit the following response at synchronous speed

  1. Rotor current is zero
  2. Rotor current is non-zero and is at slip frequency
  3. Forward and backward rotating fields are equal
  4. The forward rotating field is more than the backward rotating field

The double-field revolving theory is proposed to explain the phenomenon that a single phase induction motor is not self-starting but once rotated in one direction, it will continue to rotate in that direction. This theory is based on the fact that an alternating sinusoidal flux (φ = φm cos wt) can be represented by two revolving fluxes, each equal to one-half of the maximum value of alternating flux (i.e. φm /2) and each rotating at synchronous speed (Ns = 120f/P; ω = 2πf) in opposite directions as shown

Rotor at standstill. Consider the case that the rotor of the single-phase induction motor is stationary and the stator is connected to a single-phase supply. The rotor e.m.f.s induced by the two revolving fields will be in opposite directions. At standstill, the slip in either direction is the same (s = 1) and so is the rotor impedance. Therefore, the starting currents in the rotor conductors are equal and opposite. This means that starting torque developed by one revolving field is equal and opposite to that developed by the other revolving field. As a result, the starting torque is zero i.e. a single phase induction motor is not self-starting.

The single-phase induction motor is not self-starting and it is undesirable to resort to the mechanical spinning of the shaft or pulling a belt to start it. To make a single-phase induction motor self-starting, we should somehow produce a revolving stator magnetic field. This may be achieved by converting a single-phase supply into two-phase supply through the use of an additional winding. When the motor attains sufficient speed, the starting means (i.e. additional winding) may be removed depending upon the type of the motor. As a matter of fact, single-phase induction motors are classified and named according to the method employed to make them self-starting.

(ii) Rotor running:-

The existence of these two fluxes (forward and backward) rotating in opposite directions can be verified by supplying a fractional horsepower in single phase induction motor with rated voltage. The motor does not start, but if the shaft la turned by hand, say in the clockwise direction, the rotor picks up speed. This means that the rotor conductors are rotating in the direction of that field which rotates in clockwise direction. When the motor is braked and stopped without switching off the supply, the rotor remains at rest. If now the shaft is turned by hand in anti-clockwise Direction, the motor picks up speed in that direction which means that the rotor conductors are now rotating in the direction of the other field.

The flux rotating in the clockwise direction (i.e. the direction of spinning) is called the forward rotating flux (φf) and that in the other direction is called the backward rotating flux (φb). The forward rotating flux has synchronous speed Ns (= 120 f/P) and the synchronous speed of rotating backward flux (anticlockwise) is –Ns.

Slip in forward Direction:- 

Sf =  (Ns – N)/Ns = s

Slip in backward Direction:-

Sb =  (–Ns – N)/–Ns = s

Sb =  (2Ns + N+ N)/Ns

Sb = (2 – s)

At standstill, N = 0 so that Sf = Sb = 1. For forward rotating flux, the slip is s (less than 1) and for backward rotating flux, the slip is 2 – s (greater than 1). We know that in a 3-phase induction motor, the torque developed is directly proportional to effective rotor resistance. The effective rotor resistance is R2/s in the forward direction and R2/2 – s in the backward direction. Therefore, the resultant torque in the single-phase induction motor will be in the direction of the forward rotating flux (clockwise in this case). Thus if the single-phase induction motor is once rotated, it will develop the torque in the direction in which it has been rotated and will function as a motor.

The presence of the backward rotating field in a purely single-phase induction motor has a lot of disadvantages:

  • It acts in opposition to the forward rotating field, it reduces the total torque produced.
  • The starting torque is zero, and therefore, it needs auxiliary winding for starting purposes; this increases the size and the cost of the motor.
  • The single-phase motor needs twice the magnetizing current. The large magnetizing current gives the motor a poor power factor.
  • Copper losses in a single-phase induction motor are greater than in a polyphase induction motor.
  • The space occupied by the starting winding in a single motor gives only about two-thirds the output; that is possible from a polyphase motor in the same frame size.
  • The direction of rotation of a single-phase induction motor cannot be changed by interchanging the supply terminals.

It is observed that with only main field energized, the single-phase induction motor will not start. However, if an external torque moves the motor in any direction, the motor will begin to rotate. The absence of a rotating magnetic field is due to only one winding, in order to produce a rotating magnetic field, the motor should have at least two windings with currents which differ in phase. So what is needed is a second (auxiliary) winding, with currents out of phase with the original (main) winding, to produce a net rotating magnetic field.  So if the single phase induction motor is made to rotate at synchronous speed and the starting winding is disconnected and only the main winding is excited then the forward rotating field is more than the backward rotating field and the motor will start in the forward direction

 

Ques.63. The electric motor used in portable drills is_______

  1. Capacitor Run Motor
  2. Hysteresis Motor
  3. Universal Motor
  4. Repulsion Motor

Universal motors are essentially DC motors with series windings. The motors can be operated from either a DC or an AC source. Since the torque in a DC motor is proportional to both the armature and the field currents, connecting the windings in series ensures that the polarity of both the armature and field windings reverses simultaneously producing unidirectional torque.

Universal motors can operate at up to 20,000 rpm and are widely used for vacuum cleaners, portable drills, food mixers, and fans. These motors tend to be noisy when supplied from an AC source with high torque ripples and are also not very efficient. A triac connected in series with the supply can be used to regulate the supply voltage, thereby enabling the use of these machines for low-cost variable-speed applications.

 

Ques.64. In which single-Phase motor, the rotor has no teeth or winding?

  1. Split phase Motor
  2. Reluctance Motor
  3. Hysteresis Motor
  4. Universal Motor

Teeth are usually stator or rotor laminated steel projections, usually with wire wrapped around the base of each projection. The purpose is to strengthen the magnetic field of the wire. Unfortunately, while this strengthens the available torque of the motor, it produces cogging. Cogging is the uneven torque produced between the stator permanent magnets and the rotor projections (teeth) due to the attraction of the magnets to the laminated steel of the projections on the rotor.

Hysteresis motors are single-phase small size synchronous motors.

 

The stator windings are similar to the stator windings of single-phase induction motors. In the auxiliary winding, a permanent value capacitor is connected. Like the main winding, the auxiliary winding is always connected to the supply. When the stator windings are connected to a single-phase supply a rotating field is produced which is rotating at synchronous speed. There is no winding provided on the rotor. The rotor is simply made of aluminum or other non-magnetic material having a ring of a special magnetic material such as cobalt or chromium mounted on it.

The rotating field produced by the stator will induce eddy currents in the rotor. The rotor will get magnetized. But the magnetization of the rotor will lag the inducing revolving field by some angle due to the hysteresis effect. The rotating magnetic field will pull the rotor along with it and the rotor will rotate at synchronous speed. A constant torque will be developed upto the synchronous speed as shown in Fig. The performance of a single-phase hysteresis motor is silent (no noise) because there is no slot on the rotor and the rotor surface is smooth.

 

Ques.65. The range of efficiency for shaded pole Motors is

  1. 95% to 99%
  2. 80% to 90%
  3. 50% to 75%
  4. 5% to 35%

Shaded-pole motor has a salient pole staler similar to the stator of dc machine. The pole is laminated to reduce the core losses. The pole is physically divided into two sections as shown in  A heavy, short-circuited copper ring, called the shading coil, is placed on the smaller section of the pole. This section covers around one-third of the pole arc and is called the shaded portion of the pole. The remaining two-thirds section of the pole are is referred to as the unshaded portion. The main single-phase winding is wound on the entire pole section. The rotor used is similar to the rotor of any other single phase induction motor.

When single phase supply is fed to the main winding, an alternating flux is produced in the pole. A portion of this flux links with the shading coil and induces a voltage in it. As shading coil is a short-circuited coil, a large current flows in it. The current in the shading band causes the flux in the shaded portion of the pole to lag the flux in the unshaded portion of the pole. Thus the flux in the shaded portion reaches its maximum value after the flux in the unshaded portion reaches its maximum. This is equivalent to a progressive shift of the flux from the unshaded to the shaded portion of the pole, that is it is similar to a rotating field moving from the unshaded to the shaded portion of the Pole. Hence. the motor reproduces a starting torque.

Because of the small phase of displacement of the currents in the mail and the auxiliary winding and because of the winding misalignment lower than π/2 the start torque is very low.

The shaded pole motor efficiency suffers greatly due to the presence of winding harmonic content, particularly the third harmonic which produces a dip in the speed torque curve at approximately 1/3 synchronous speed. In addition, there are losses in the shading coils. These factors combine to make the shaded pole the least efficient and noisiest of the single-phase designs. It is used mostly in air moving applications where its low starting torque and the third harmonic dip can be tolerated.

Some important Points of shaded Poles Motor

  • The peak efficiency of the shaded pole motor is about 20% to 40%.
  • The power factor of the shaded pole motor is around 50% to 60%
  • The starting torque of the shaded pole motor is 40% to 50% of the full load torque.
  • The shaded pole motor is the cheapest of all single-phase motor.

Therefore shaded-pole motors are especially suited for small fans and pumps. Other applications are juice presses, clothe dryers, grills, clocks, simple butterfly control waves, massage apparatus, hot-air stoves and cabinet fans. Drives for reversing duties can be built with two motors assembled homologously. Shaded-pole motors are low-cost motors. Because of their low efficiency, they mostly need intensive cooling.

 

Ques.66. The direction of rotation of universal Motor can be reversed by reversing the flow of current through________

  1. Armature Winding
  2. Field Winding
  3. Either armature winding or field winding
  4. None of these

The direction of rotation of the universal motor can be changed in the same manner as changing the direction of rotation of a DC series motor.

The direction of rotation of a salient pole type universal motor can be reversed by reversing the flow of current through either the armature or the field w iodine This can be easily done by interchanging the leads of the brush holders as shown in Fig.

 

Ques.67. Which of the following statement is incorrect?

  1. As the temperature rises, the tension in the transmission line decreases.
  2. As temperature rises, the sag in transmission lines reduces
  3. Tension and sag in transmission lines are complementary to each other
  4. None of these

Sag and Tension

The overhead lines are mounted on the mechanical structure which includes insulators, cross arms and towers, etc. Hence, the structure must be mechanically strong so that there is no mechanical failure of the line, even in abnormal weather conditions. Various forces act on the conductor such as its weight, wind force, and tension, etc. Any conductor has an ultimate limit up to which it can be stretched and that is determined by the conducting material.

The tension in the conductor depends on many factors such as the diameter of the conductor, span, conducting material, sag, wind pressure, and temperature. The relationship between the tension and sag depends on the loading conditions and variations in temperature. If the temperature is reduced, tension is increased and sag is decreased correspondingly. When the line conductor is suspended between two line supports, it looks like a centenary or parabola. So, the difference in level between the lowest point on the conductor and the points of line supports is called as sag. Here one thing is noted that the sag is very small compared to the span.

The various factors that affect the sag in the overhead line are discussed as below.

(i) Conductor Weight: The sag is directly affected by the weight of the conductor. If the conductor is heavier, the sag will be more. In locations where there is snowfall, the sag is high.

(ii) Span: The sag is directly proportional to the square of the span length. If the other factors such as the type of conductor, working tension, temperature, etc. are kept constant, the longer span will increase the sag.

(iii) Working Tensile Strength: The sag is inversely proportional to the working tensile strength of the conductor if the other factors are kept same. The working tensile strength is found by multiplying the ultimate stress on the conductor and area of cross section and divided by a factor of safety.

(iv) Temperature: We know that all the metallic bodies are expanded if the temperature is increased. Hence, the length of the conductor is also increased as the temperature is increased and sag is also increased.

The sag is inversely proportionate to the tension in a conductor. This can be shown by the following parabolic approximate relationship:

Sag “S” = WL2 ⁄ 8T

Where

W = Weight of the conductor

T = Tension in a conductor

L = Span Length

So if the temperature increases then the tension in the conductor decreases. If the tension decreases the Sag in the conductor increases.

 

Ques.68. Series capacitor in transmission lines are of little use when

  1. The load VAR requirement is small
  2. The load VAR requirement is High
  3. The load VAR requirement is fluctuating
  4. None of these

Series capacitors are connected in series with transmission line conductors in order to offset the inductive reactance of the line. This tends to improve electromechanical and voltage stability, limit voltage dips at network nodes and minimizes the real and reactive power loss. Typically the inductive reactance of a transmission line is compensated to between 25 and 70%. A full 100% compensation is never considered as it would make the line flows extremely sensitive to changes in angle between the voltages at the line

Inductive reactance is the opposition to the flow of current, which results in the continual interchange of energy between the source and the magnetic field of the inductor. In other words, inductive reactance, unlike resistance (which dissipates energy in the form of heat), does not dissipate electrical energy (ignoring the effects of the internal resistance of the inductor.)

The power sent over a transmission line is inversely proportional to the inductive reactance of the system. In that sense, reactance sets a limit on the maximum power that can be transmitted by a line for a given transmission voltage.

Consider the power flow through a transmission line. The amount of power that is transmitted from bus 1 (sending-end bus) to bus 2 (receiving-end bus) can be expressed as

{P_{12}} = \dfrac{{{V_1} \times {V_2}}}{{{X_L}}}\sin \delta

where:

P12 = power transmitted through the transmission system,

V1 = voltage at the sending end of the line

V2 = voltage at the receiving end of the line,

XL = reactance of the transmission line

δ = phase angle between phasors V1 and V2, that is

δ = θ1 − θ2

If the total reactance of a transmission system is reduced by the installation of capacitors in series with the line, the power transmitted through the line can be increased. In that case, the amount of power that is transmitted can be expressed as

\begin{array}{l}{P_{12}} = \dfrac{{{V_1} \times {V_2}}}{{{X_L} - {X_c}}}\sin \delta \\\\{P_{12}} = \dfrac{{{V_1} \times {V_2}}}{{{X_L}(1 - K)}}\sin \delta \end{array}

where K = (XC / XL) is the degree of compensation.

The degree of compensation K is usually expressed in percent. For example, 60% compensation means that the value of the series capacitor in ohms is equal to 60% of the line reactance.

As stated previously, series capacitors are used to compensate for the inductive reactance of the transmission line. They may be installed remote from the load, as an example, at an intermediate point on a long transmission line. Benefits of series capacitor compensation are:

  1. Improved line loadability.
  2. Improved system steady-state stability.
  3. Improved system transient-state stability.
  4. Better load division on parallel circuits.
  5. Reduced voltage drops in load areas during the severe disturbance.
  6. Reduced transmission losses.
  7. Better adjustment of line loadings.

The figure shows a schematic of the typical series capacitor compensation equipment. Series capacitors are almost always in transmission lines, rather than within a substation bus arrangement.

Series capacitor compensation equipment is usually mounted on a platform at line potential and has the necessary amount of capacitors, spark gap protection, metal-oxide varistor (MOV), bypass switch (or breaker), and control and protection, as shown in Figure

Merits of series capacitors

With a series capacitor, the regulation, or reduction in voltage drops achieve depends mainly on the reactive power of the load, and it follows that this type c capacitor is of little use unless the conditions are such that reactive power is consumed by the load. Therefore If the load Var requirement is small, series capacitors are of little use.

If the voltage drop is the limiting factor, series capacitors are effective. Voltage fluctuations due to arc furnaces are also evened out

If the total line reactance is high, series capacitors are very effective and stability is improved. Synchronous Compensators

The major drawback of series capacitors is: high overvoltages are produced when a short circuit current flows through the capacitor and special capacitive devices have to be incorporated (e.g., spark gaps).

 

Ques.69. Stability of a system is not affected by_____

  1. Reactance of Line
  2. Losses
  3. Reactance of generator
  4. Output Torque

The stability of an interconnected power system is its ability to return to normal or stable operation after having been subjected to some form of disturbance. Conversely, instability means a condition denoting loss of synchronism or falling out of step. Stability considerations have been recognized as an essential part of power system planning for a long time.

The stability limit is the maximum power that can be transferred in a network between sources and loads without loss of synchronism.

The steady-state limit is the maximum power that can be transferred without the system becoming unstable when the load is increased gradually under steady-state conditions.

The transient limit is the maximum power that can be transferred without the system becoming unstable when a sudden or large disturbance occurs.

Synchronous stability may be divided into two main categories depending upon the magnitude of the disturbance.

Steady-state stability:- Steady-state stability refers to the ability of the power system to regain synchronism after small and slow disturbance, such as gradual power changes.

Steady-state stability is subdivided into static stability and dynamic stability.

1. Static stability. Static stability refers to inherent stability that prevails without the aid of automatic control devices such as governors and voltage regulators.

2. Dynamic stability. Dynamic stability, on the other hand, denotes artificial stability given to an inherently unstable system by automatic control devices. Dynamic stability is concerned with small disturbances lasting for times of the order of 10-30 s with the inclusion of static control device.

Transient stability:- Transient stability is the ability of the system to regain synchronism after a large disturbance. The large disturbance can occur due to sudden changes in application or removal of large loads, line switching operations, faults on the system, the sudden outage of a line, or loss of excitation. Transient stability studies are needed to ensure that the system can withstand the transient conditions following a major disturbance.

Stability studies are helpful for the following purposes:

  1. Determination of critical clearing time of circuit breakers (CBs)
  2. Investigation of schemes of protective relaying
  3. Determination of voltage levels
  4. Transfer capability between systems

The seven essential factors affecting the stability are broadly classified into two parts

  1. Mechanical factors:
    1. Prime mover input torque
    2. The inertia of prime mover and generator
    3. The Inertia of motor and shaft load
    4. Shaft load output torque
  2. Electrical factors:
    1. Internal voltages of the synchronous generator
    2. The reactance of the system including the generator, line, and motor etc
    3. Internal voltage of the synchronous motor

Suppose In case of the generator connected radially without automatic voltage regulators, the instability will occur which is due to lack of sufficient synchronizing torque.

The Maximum Power transfer without the loss of stability is given by

{P_{12}} = \dfrac{{{V_1} \times {V_2}}}{{{X_L}}}\sin 90

The reactance can be reduced by either increasing the sending end voltage or receiving end voltage Thus increasing the stability of the system.

 

Ques.70. Which of the following is not a constituent making porcelain insulators?

  1. Silica
  2. Kaolin
  3. Feldspar
  4. Quartz

External insulations which are produced in the form of mechanically stable suspension insulators, supporting insulators and housing insulators are predominantly made of mechanically strong porcelain. Moreover, ceramics and porcelain are suitable for all thermally high stressed insulations, such as immersion heaters, spark plugs or thermally conductive insulations for the assembly of power semiconductors. Owing to a closed porous surface (glazing), excellent weathering resistance and impassivity against the effect of chemicals are provided Only hydrofluoric acid, concentrated phosphoric acid and long-acting hot sodium hydroxide and potassium hydroxide can be corrosive for porcelain.

For manufacturing porcelain insulators, to mineral raw materials feldspar (alkali alum num silicate), kaolin (aluminum silicate) an quartz (SiO2) or aluminum oxide are finely ground with water and mixed to give a moldable mass. The composition of the porcelain: paste determines the mechanical, thermal, and electrical properties of the finished insulator. Thus, Aluminum oxide-porcelain has better mechanical properties and can be better processed than quartz-porcelain.

4 thoughts on “SSC JE Electrical Previous Year Question Paper 2018-SET2|MES Electrical”

  1. sir ek request hai kripya aap answer ko hide kar dijiye taki as a practise ise solve iske baad click karne pe answer dikhaye. ye achha huga practise ke liye.

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