SSC JE Electrical Previous Year Question Paper 2018-SET 5

Ques. 61. The drive motor used in a mixer-grinder is a

  1. DC Motor
  2. Induction Motor
  3. Synchronous Motor
  4. Universal Motor

  • We use universal motors in mixer grinders. Universal motors can run both on a.c. and d.c.
  • The universal motor works on the same principle that the DC series motor works. DC series motor has the characteristics of operating at high speed when there is no load and operating at low speed when the load is applied. It has high starting torque characteristics.
  • So it is used in mixers, where initially we put some load at starting.

 

Ques.62. The capacitors used in single-phase capacitor motors have no

  1. Voltage Rating
  2. Dielectric Medium
  3. Polarity marking
  4. Definite Value

Electrolytic capacitors: Electrolytic capacitors normally have the smallest volume and cost for a given capacitance. These types of capacitors are mainly used in applications where a large value of capacitance in a small volume is required, such as the filter in a power supply. Electrolytic capacitors are very widely used in filters, time constant circuits, bypass. coupling-decoupling, smoothing, starting of A.C motor and power electronic applications.

POLARIZED ELECTROLYTIC CAPACITORS

Polarized capacitors are generally referred to as electrolytic capacitors These capacitors are sensitive to the polarity to which they are connecters and will have one terminal identified as positive or negative. 

Polarized capacitors can be used in DC circuits only. These capacitors must be connected in the circuit as per the polarity marked on the capacitor. If they are connected in opposite polarity, the reversed electrolysis forms gas in the capacitor It becomes hot and may explode. The advantage of electrolytic capacitors is that they can have very high capacitance in a small case.

NON-POLARIZED ELECTROLYTIC CAPACITORS

A non-polar electrolytic capacitor can be thought of as being the same as having two polarized electrolytic capacitors in series, negative to negative, so the polarity of the voltage supplied is not important for the circuit’s continued operation. 

This capacitor is used as the starting capacitor for many small single-phase motors, as the run capacitor in many ceiling fan motors, and for low-power electronic circuits when a nonpolarized capacitor with a high capacitance is required. The AC electrolytic capacitor is made by connecting two wet-type electrolytic capacitors inside the same case, as shown in the figure.

electrolytic capacitor

In the example shown, the two wet-type electrolytic capacitors have their negative terminals connected. When alternating current is applied to the leads, one capacitor will be connected to reverse polarity and become short. The other capacitor will be connected to the correct polarity and will form. During the next half-cycle, the polarity changes: it forms the capacitor that was shorted and shorts the other capacitor.

 

Ques.63. How will the total capacitance change, when two capacitors are connected in parallel?

  1. The total capacitance increases
  2. The total capacitance decreases
  3. The mean value gives the new capacitance
  4. The total capacitance is found by the reciprocal equation

Capacitance in Parallel

When two equal capacitors are connected in parallel, the plates of the individual capacitors, in effect, combine to form one capacitor representing total capacitance. Notice in Figure. that the effective plate area of the equivalent capacitor has doubled.

Since an increase in plate area increases capacitance, it can be concluded that the total capacitance of the two capacitors in parallel is equal to the sum of the two capacitors. This direct relationship between total capacitance and capacitors in parallel is described bit this equation:

CTotal = C1 + C2

Capacitor in parallel

Mathematically,

The charge on a capacitor is given as

Q = CV

Where

C is the capacitance

V is the applied voltage

In the parallel circuit, the voltage remains the same and the charge depends upon the capacitor size.

Q1 = C1V

Q2 = C2V

Q = Q1 + Q2

CeqV = C1V + C2V

Ceq = C1 + C2

Hence in the parallel plate capacitor, the capacitance increases.

 

Ques.64. When the load on a reluctance motor is increased so that it cannot maintain synchronous speed the motor will

  1. Become unstable
  2. Draw excessive armature current and may burn out
  3. Fall out of synchronism and come to standstill
  4. Run as an induction motor

When the load on a reluctance motor is increased so that it cannot maintain synchronous speed the motor will run as an induction motor.

Reluctance Motor

It is a single-phase synchronous motor that does not require d.c. excitation to the rotor. Its operation is based upon the following principle:

Whenever a piece of ferromagnetic material is located in a magnetic field, a force is exerted on the material, tending to align the material so that reluctance of the magnetic path that passes through the material is minimized.

The motor runs up to slip speed as an induction motor, pulls into step with the rotating field by means of the reluctance torque, and then operates as a synchronous reluctance motor. 

reluctance motor

Working Principle

The stator consists of a single winding called the main winding. But single winding cannot produce a rotating magnetic field. So for the production of a rotating magnetic field, the must be at least two windings separated by a certain phase angle.

Hence staler consists of an additional winding called auxiliary winding which consists of the capacitor in series with it. Thus there exists a phase difference between the currents carried by the two winding and corresponding fluxes. Such two fluxes react to produce the rotating magnetic field.

The technique is called the split-phase technique of the production of a rotating magnetic field. The speed of this field is the synchronous speed which is decided by the number of poles for which stator winding is wound.

reluctance motor

The rotor carries the short-circuited copper or aluminum bars and acts as a squirrel cage rotor of an induction motor. If an iron piece is placed in a magnetic field, it aligns itself in a minimum reluctance position and gets locked magnetically.

Similarly, in the reluctance motor, the rotor tries to align itself with the axis of the rotating magnetic field in a minimum reluctance position. But due to rotor inertia, it is not possible when the rotor is at standstill. So rotor starts rotating near synchronous speed as a squirrel cage induction motor.

When the rotor speed is about synchronous, stator magnetic field pulls the rotor into synchronism i.e. minimum reluctance position, and keeps it magnetically locked. Then the rotor continues to rotate with a speed equal to synchronous speed. Such a torque exerted on the rotor is called the reluctance torque. Thus finally the reluctance motor runs as a synchronous motor. The resistance of the rotor must be very small and the combined inertia of the rotor and the load should be small to run the motor as a synchronous motor.

Note:- When the load on the synchronous motor increases then the speed of the reluctance motor decreases working as an induction motor

Advantages

The reluctance motor has the following advantages

  1. No d.c. supply is necessary for the rotor
  2. Constant speed characteristics
  3. Robust construction
  4. Less maintenance

Limitations

The reluctance motor has the following limitation

  1. Less efficiency.
  2. Poor power factor.
  3. The Need for Very low inertia rotor.
  4. Less capacity to drive the loads.

 

Ques.65. A universal motor can run on

  1. A.C. only
  2. D.C. only
  3. Either A.C. or D.C
  4. None of these

The universal motor is one that operates both on A.C and D.C supply. Its principle of operation is the same as that of a de series motor, i.e., force is created on the armature conductors due to the interaction between the main field flux and the flux created by the current-carrying armature conductors. The series wound DC motor is the only type of DC motor that works on AC although the efficiency of the motor is very poor.

Universal Motor

However, the Universal motor is constructed with few series fields turns, laminated armature and field circuits, low reluctance magnetic path, increased armature conductors and commutator segments by using low flux densities. This is done to minimize the adverse effects caused by high field reactance, eddy current, and Hysteresis Losses.

We may consider a typical DC series motor or a DC shunt motor for operation on AC power supplies. It appears that such an operation is possible because reversing the line terminals to a DC motor reverses the current and magnetic flux in both the field and armature circuits. As a result, the net torque of the motor operating from an AC source is in the same direction.

However, the operation of a DC shunt motor from an AC source is impractical because the high inductance of the shunt field causes the field current and the field flux to lag the line voltage by almost 90°. The resulting torque is very low.

A DC series motor also fails to operate satisfactorily from an AC source because of the excessive heat developed by eddy currents in the field poles. In addition, an excessive voltage drop occurs across the series field windings due to high reactance.

To reduce the eddy currents, the field poles can be laminated. To reduce the voltage loss across the field poles to a minimum, a small number of field turns can be used on a low reactance core operated at low flux density. A motor with these revisions operates on either AC or DC and is known as a universal motor. Universal motors in small fractional horsepower sizes are used in household appliances and portable power tools.

Universal motors may be either compensated (distributed field) or uncompensated (concentrated field) type, the latter type being used for the higher speeds anal smaller output ratings (usually not exceeding 200 Watt only.

CONCENTRATED-FIELD UNIVERSAL MOTORS

A concentrated-field universal motor is usually a salient-pole motor with two poles and a winding of relatively few turns. The poles and winding are connected to give opposite magnetic polarity.

DISTRIBUTED-FIELD UNIVERSAL MOTORS

The two types of distributed-field universal motors are the single-field compensated motor and the two-field compensated motor. The field windings of a two-pole, single-field compensated motor resemble the stator winding of a two-pole, split-phase AC motor.

A two-field compensated motor has a stator containing the main winding and a compensating winding spaced 90 electrical degrees apart. The compensating winding reduces the reactance voltage developed in the armature by the alternating flux when the motor operates from an AC source.

img.12

The armature of a typical universal motor resembles the armature of a typical D.C motor except that a universal motor armature is slightly larger for the same horsepower output.

Operation

In a series wound motor, the same current flows through field windings and armature, being connected in series with each other when the motor is connected to either DC or AC supply. The magnetic fields developed by the series field winding and armature currents react with each other and hence develop unidirectional torque.

Since the armature and field are in series, the current is the same through them, and there is no time lag or phase displacement in the magnetic fields. The attraction and repulsion forces will nearly be the same if operated on DC or AC. Because these motors are series-wound, they operate at excessive speed at the no-load condition. As a result. they are usually permanently connected by gears to the devices being driven.

 

Ques.66. Which of the following motors has two separate windings on the motor?

  1. Repulsion motor
  2. Repulsion induction motor
  3. Repulsion start induction run motor
  4. None of these

The repulsion induction motor has two separate windings on the motor.

A repulsion motor operates on the principle that like magnetic poles repel each other, not on the principle of a rotating magnetic field. The stator of a repulsion motor contains only a run winding very similar to that used in the split-phase motor. Start windings are not necessary.

The rotor is actually called an armature because it contains a slotted metal core with windings placed in the slots. The windings are connected to a commutator. A set of brushes makes contact with the surface of the commutator bars. The entire assembly looks very much like a DC armature and brush assembly.

One difference, however, is that the brushes of the repulsion motor are shorted together. Their function is to provide a current path through certain parts of the armature, not to provide power to the armature from an external source.

Operation

Although the repulsion motor does not operate on the principle of a rotating magnetic field, it is an induction motor. When AC power is connected to the stator winding, a magnetic field with alternating polarities is produced in the poles. This alternating field induces a voltage into the windings of the armature.

When the brushes are placed in the proper position, current Hows through the armature windings, producing a magnetic field of the same polarity in the armature. The armature magnetic field is repelled by the stator magnetic field, causing the armature to rotate. Repulsion motors will contain the same number of brushes as there are stator poles. Repulsion motors are commonly wound for four, six, or eight poles.

Repulsion Motor

Repulsion Induction Motor

It works on the combined principle of repulsion and induction. The construction of the stator of this type of motor is similar to that of a repulsion motor, i.e., the main stator winding of a single-phase induction motor. In the rotor, there are two separate windings.

One winding is similar to the rotor winding of a repulsion motor i.e usual d.c winding connected to the commutator. The other winding is of squirrel-cage type, placed below the repulsion-motor type winding. The behavior of the repulsion induction motor is, therefore, the combination of the behavior of the repulsion motor and an induction motor.

Under starting conditions, very little current will flow through the inner squirrel-cage winding since the reactance of the squirrel-cage winding which is placed deep into the rotor slot is very high. As the rotor picks up speed, the frequency of the rotor induced emf and hence the rotor reactance will decrease. More current will flow in the rotor squirrel-cage winding. The motor will work as a combination of repulsion and induction motor.

Repulsion motor diagram

 

Ques.67. In high voltage transmission lines, the topmost conductor is

  1. R-phase conductor
  2. Y-phase conductor
  3. B-Phase conductor
  4. Earth conductor

In EHV transmission Line the topmost conductor is the Earth conductor. Earth wire is also called as the guard wire and it mainly protects lines from lightning.

Earth wire

If in case of lightning strikes then it carries the excessive current inrush to the ground. Its radius is much smaller than the actual transmission wire because as the resistance is inversely proportional to an area of cross-section and as the cross-section decreases, the resistance increases.

The increased resistance of earth wire is able to withstand high inrush of current caused due to lightning and it will safely guide this inrush into the ground. It is placed above all the conductors mainly because if lightning strikes then it will strike at the uppermost point in the line configuration and protect the actual conductors.

 

Ques.68. Different types of line support used for transmission lines include

  1. RCC and PCC poles
  2. Steel Towers
  3. Steel Poles
  4. Wooden poles, steel poles, RCC and PCC poles and steel towers

Main components of the overhead lines: The various components of the overhead transmission line are as under:

  • Conductor
  • Line supports
  • Cross arms and clamp
  • Insulators
  • Lightning arrestors
  • Earth wire
  • Fuses and isolating switches

Various conductors used in the transmission lines are as follows:

  • Copper conductor
  • Aluminum conductor
  • Aluminum conductor steel reinforced
  • Cadmium copper
  • Galvanized steel
  • Solid conductor
  • Hollow conductor
  • Stranded conductor
  • Bundle conductor

Various line supports used in the transmission lines are as follows:

  • Wooden Poles
  • Steel poles—tubular, rail pole, and rolled steel joist poles
  • RCC/PCC pole
  • Steel tower—Self-supporting tower and Guyed or stayed tower

Various types of insulators are used depending on the line voltage and requirement which art as under:

  • Pin-type insulator
  • Shackle-type insulator
  • Suspension-type insulator
  • Strain-type insulator
  • Stay insulator

LINE SUPPORTS

The main function of the line support is to support the conductors to keep them at suitable heights from the ground. Hence, in overhead lines, various types of supporting structures are used such as poles and towers which are called as line supports. Generally, line supports have the following properties:

(i) Good mechanical strength

(ii) Lightweight

(iii) Cheap

(iv) Longer life

Various types of line supports are used in overhead transmission and distribution systems. The selection of line support depends on many factors such as cost, the span of the line, line voltage, local conditions, etc. The various types of line supports are as follows:

Wooden Poles:

This is one of the simplest forms of the line support. Generally, wooden poles are used. They are cheap, easily available, and have good insulating properties. These poles must be straight, strong, and free from knots. The poles should be properly seasoned to prevent rapid decay because of crakes. An aluminum, zinc, or cement cap is used at the top of the wooden pole to prevent decay due to snow and rain. They are widely used for distribution purposes up to 22 kV and short spans (up to 60 metros). In cities where timber is easily available and the cost of transportation of steel towers is more, single and double pole structures either A or H type are widely used for overhead lines for 130 kV and for the span of 150 meters as shown in Fig.

Wooden pole

Wooden poles generally rot below the ground level and to prevent this preservative compounds such as creosote oil is used.

Advantages of Wooden Poles

  • They are cheap and readily available.
  • They are light in weight so easy to install.
  • They have a good life, i.e., approximately 20 years.
  • They can be directly buried in the ground or anchor-based and hence no special foundation is required.

Disadvantages of Wooden Poles

  • They start rotting underground.
  • They cannot be used for voltage higher than 22 kV.
  • They require regular inspection.
  • They have less mechanical strength.

Steel Poles

The drawbacks of wooden poles are absent in the steel poles and hence they are generally used as a substitute to the wooden poles. They have better mechanical strength and longer life. They can be used for a long span. These poles are used for distribution purposes in urban areas. These types of poles are generally galvanized or painted to increase their life. These poles are shown in Fig. 

Steel pole

Generally, three types of steel poles are available as follows:

  • Rail poles
  • Tubular poles
  • Rolled steel joist pole

Advantages of Steel Poles

  • They are cheap and available in multiple shapes.
  • They have more good mechanical strength than wooden poles.
  • They have a long life, i.e. approx. 15 to 30 years depending on the environment.
  • They can be directly buried in the ground or anchor-based and no special foundation is required.

Disadvantages of Steel Poles

  • They are subjected to corrosion and rust.
  • Due to heavy-weight, large equipment is required to load, unload and install.
  • They have a high maintenance cost.

 Reinforced or Plain Cement Concrete (RCC/PCC) Pole:

The Reinforced Cement Concrete (RCC) poles give more mechanical strength and have a long life than the wooden pole and can be used for very long spans than steel poles. Further, the outlook of the RCC pole is good. It requires less maintenance and has good insulating properties. These poles are mainly used for secondary transmission and primary distribution systems. This pole is shown in Fig.

RCC pole

The main problems associated with these poles include their high cost and difficulty in transportation due to heavy-weight. To overcome this problem, pre-stress concrete supports are used. They are made in small pieces and then assembled at the site. The pre-stressed concrete poles (PSCP) are less bulky than that of the RCC pole. Further, less material is user in PCC poles and they are more durable than any other pole. They are widely used on 11 kV lines.

Advantages of (RCC/PCC) Pole

  • They have better mechanical strength than wooden poles.
  • They are not subjected to harmonic vibrations.
  • They have a very long life, i.e., 50 years.
  • The maintenance cost is low.
  • They are corrosion-free.

Disadvantages of (RCC/PCC) Pole

  • Due to its heavy-weight, difficult to transport and install.
  • These are difficult to dispose of or recycle.
  • They have limited installation options, i.e., only direct burial.

Steel Towers

The poles discussed earlier are used only for distribution purposes. Fo high voltage and extra high voltage transmission lines, steel towers are mainly used because these lines need large air and ground clearance. Steel towers have strong mechanical strength and can be used for a long span.

They can also withstand severe climatic conditions. The chances of interruption in service are very less because of the long span. They are fabricated from painted galvanized angle sections so that they can be transported separately and the] are joined at the site. The towers are generally either made of aluminum or steel.

Steel Tower

Advantages of the steel tower

  • These towers have the very good mechanical strength
  • They can be used for HV and EHV transmission lines
  • They have a very long life, almost infinite
  • They can withstand severe climatic conditions

Disadvantages of the steel tower

  • Because of the heavy-weight, transportation and installation are difficult and costlier
  • They require a special type of foundation.
  • Painting or galvanizing is required periodically.

 

Ques.69. Transmission lines and cables

  1. Generate Reactive power at light loads and absorb reactive power at full load
  2. Generate Reactive power at light loads as well as full load
  3. Absorb Reactive power at light loads as well as full load
  4. Absorb Reactive power at light loads and generate reactive power at full load

  • Transmission lines and cables absorb and generate reactive power
  • A heavily loaded transmission line consumes reactive power, decreasing the voltage of the line while a lightly loaded transmission line generates reactive power, increasing the voltage of the line
  • Transmission lines absorb reactive power when fully loaded and generate reactive power on light loads

Electricity is consumed by a wide variety of loads, including lights, heaters, electronic equipment, household appliances, and motors that drive fans, pumps, and compressors. These loads can be classified based on their impedance, which can be resistive, reactive, or a combination of the two. Heaters and incandescent lamps have purely resistive impedance, whereas motors have an impedance that is resistive and inductive.

Purely resistive loads only consume real power. Loads with inductive impedance also draw reactive power. Loads with capacitive impedance supply reactive power. Because of the abundance of motors connected to the network, the power system is dominated by inductive loads. Hence, generating units have to supply both real and reactive power. 

Loads normally absorb reactive power. A typical load bus supplied by a power system is composed of a large number of devices. The composition changes depending on the day, season, and weather conditions. The composite characteristics are normally such that a load bus absorbs reactive power. Both active power and reactive power of the composite loads vary as a function of voltage magnitudes. 

It can be seen that the minimum demand for reactive power occurs at no-load, very close to the synchronous speed. As the load increases, the motor speed decreases and the reactive power consumption increases.

Under light load conditions or no-load conditions, the capacitance associated with the line generate more reactive power than the reactive power which is absorbed hence the voltage at the receiving end is found to be greater than that at sending end. This effect is also called as Ferranti Effect.

The transmission line load can also be explained by surge impedance Loading.

The characteristics impedance of the Transmission line is expressed in terms of surge impedance loading or Natural loading. The surge impedance loading Is given as SIL = V2LL/Zo in which VLL is the line-to-line voltage in volts. Loaded below its SIL, a line supplies lagging reactive power to the system, tending to raise system voltages. When loaded above the SIL, the line absorbs reactive power, tending to reduce the voltage.

The relationship of surge impedance loading as a function of power loading on the line can be seen in another way. As loads draw more power, they are really drawing more current: voltage is held constant. Inductance increases with increasing current; thus, higher loading draws more current and increases inductance and reactive power consumption. Capacitance, on the other hand, is dependent upon the line voltage; at lower loading levels, the current is lower and the capacitance dominates, leading to the injection of reactive power.

Note:-  Terminology is often used in which inductive loads “consume” reactive power and capacitive loads “supply” reactive power. These terms cannot be taken literally; reactive power is not literally consumed or supplied here. Inductive loads cause the lag in current relative to voltage previously discussed; in that sense, reactive power appears to be “needed” and “used” by inductive loads.

Similarly, capacitive loads can counteract the inductive load reactance current lag by shifting the voltage and creating a leading current. Thus, the capacitive load appears to be compensating for the reactive power “used” by the inductive loads and thus “supplying” the required reactive power. In reality, utilities often simply increase real power to compensate for reactive power requirements. 

 

Ques.70. Two condensers of capacity 2F and 3F are connected in series, the third condenser of  1F is connected in parallel to them, the resultant capacity in Farad (F) will be

  1. 6F
  2. 5/11 F
  3. 5/6 F
  4. 11/5 F

Let C1 = 2F , C2 = 3F, C3 = 1F

∴ C3 || (C1 + C2) = 1F || (2F + 3F)

1F || (2 × 3) ⁄ (2 + 3) = 1 + 1.2

Ceq = 2.2 = 11/5 F

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