SSC JE Electrical Previous Year Question Paper 2018-SET 6|MES Electrical

Ques.51. The brush contact losses in a d.c. machine is

  1. Inversely proportional to the square of the current
  2. Directly proportional to the square of the current
  3. Inversely proportional to the current
  4. Directly proportional to the current

In the rotating machine whether it is AC or DC machine the type of losses are almost the same 

Cooper- loss(I2R) Losses:- All windings have some resistance (though small) and hence there are capper-losses associated with current flow in them. The copper-loss can again be subdivided into the stator copper-loss, rotor copper-loss, and brush-contact loss. The stator and rotor copper-losses are proportional to the current squared and are computed with the dc resistance of windings at 75°C.

Loss due to Brush Contact Resistance:- Brushes are placed on the commutator surface to supply or collect current to the armature coils through the commutator segments. In dc machines, brushes make a sliding contact with the commutator and the conduction of current is through minute arcs. Hence, a loss occurs at the brush contacts. The contact voltage drop for a particular grade of the brush is more or less a constant, varying from 1 to 2 V for normal carbon brushes. The brush contact loss is equal to the product of the contact voltage drop and the armature current. Thus brush contact loss is directly proportional to the armature current. Strictly speaking, it is not a copper loss, however, it is normally included in the classification of copper losses.

 

Ques.52. In which transformer, the tertiary winding is used

  1. Star – Delta
  2. Star – Star
  3. Delta – Delta
  4. Delta – Star

In addition to primary arid secondary windings, the transformers may be constructed with Me third winding. This winding is called the tertiary winding. The normal two winding transformers can be converted into three winding transformers with an additional secondary winding having the number of turns as per the requirements.

The tertiary winding has the low-voltage rating whereas the primary has the highest voltage rating. The kVA ratings in three-phase transformers are unequal, whereas it is equal in two-winding transformers. The chief advantage of tertiary winding is that it reduces the imbalance of the primary and secondary and hence the secondary load imbalance is distributed more evenly among the primary phases. This is because when the primaries and secondaries are star-connected and the load is unbalanced, this reflects in unbalanced primary currents and the increased circulating current is reduced in tertiary windings. The figure shows the schematic diagram of a three-winding transformer having the primary (N1), secondary (N2) and tertiary (N3)

Stabilization Due to Tertiary Winding

  • For unbalanced single phase load, the star-star connection offers high reactance to flow of current. Any unbalanced load current has three components viz positive, negative and zero sequence components.
  • The zero sequence component on the secondary side cannot be balanced by primary currents as zero sequence currents cannot flow in the isolated neutral of star connected primary.
  • On the secondary side, the zero sequence current sets up the magnetic flux in the core. The iron path is available for this flux and the impedance offered to the zero sequence currents is very high.
  • But the delta connected tertiary winding permits circulation of zero sequence currents in it. So impedance offered to the flow of zero sequence currents is lowered. For this purpose, the tertiary winding is called stabilizing winding.

Advantages of Three winding transformer

  • If a two winding transformer has to supply an additional load which has to be insulated from the secondary windings for some reasons then three winding transformers may be used with additional load carried by tertiary winding.
  • The phase compensating devices can be supplied with three winding transformers which are not operating at either primary or secondary voltage but at some different voltage.
  • The tertiary winding can be used as a voltage coil in a testing transformer
  • To inject the reactive power into the system, synchronous capacitors are connected across the delta-connected output of the tertiary windings.
  • It is possible to interconnect three-phase supply systems with the help of tertiary windings.
  • The three winding transformer can be used to load large split winding generator.
  • It is possible to supply substation auxiliaries at the different voltage with reselect to the primary and secondary with the help of tertiary windings.
  • The impedance offered to the zero-sequence current by the delta connected tertiary is reduced and hence the sufficient earth-fault current can now, which helps the proper operation of the protective devices.
  • Since continuity of supply is more important, it is possible to give the supply of a single load from two sources.

The disadvantage of a three winding transformer is its construction is little complicated as compared to the normal two winding transformers. A separate third winding is required to be placed which requires more copper and hence cost of three winding transformers is obviously more. The core of the transformer has to carry three windings instead of two as in case of normal two winding transformers.

 

Ques.53. The secondary winding of an auto-transformer is also called

  1. Compensating winding
  2. Common Winding
  3. Tertiary winding
  4. Damping Winding

Normal transformers have two winding placed on two different sides i.e. primary and secondary. In Auto Transformer, one single winding is used as primary winding as well as secondary winding i.e primary and secondary shares the common single winding. The primary is electrically connected to the secondary, as well as magnetically coupled to it. Auto transformers are often used to step up or step down voltages up to 240 V range.

Auto transformer Working

An auto transformer has a single winding with two end terminals and always having one terminal in common with the primary voltage. The primary voltage is applied across two of the terminals, and the secondary voltage is taken from two terminals. The auto transformer develops a voltage in proportion to its number of turns since the volts-per-turn is the same in both windings. In an auto-transformer part of the current flows directly from the input to the output and the remaining part is obtained by transformer action, therefore, an auto transformer work a the voltage regulator.

 

Ques.54. The maximum allowable voltage between adjacent segment is

  1. 10 – 20V
  2. 20 – 30V
  3. 40 – 50V
  4. 30 – 40V

When the load is connected to the DC motor, the armature winding of the DC motor carries a current. Every current carrying conductor produces its own flux so armature of the DC motor also produces its own flux when carrying a current. So there are two fluxes present in the air gap, one due to armature current while second is produced by the field winding called main flux. The flux produced by the armature is called armature flux

So the effect of the armature flux on the main flux affecting its value and the distribution called armature reaction.

For machines subjected to heavy overloads, rapidly changing loads, or operation with a weak main field, there is the possibility of trouble other than simply sparking at the brushes.

The armature reaction causes the flux density wave to be so badly distorted that when a coil is passing through the region of peak flux densities, the emf induced in it far exceeds the average coil voltage. If this emf is higher than the breakdown voltage across adjacent segments, a sparkover could result which can easily spread over and envelop the whole commutator as the environment near the commutator is always somewhat ionized and conditions are favorable for flashover. The result is the complete short circuit of the armature. The maximum allowable voltage between adjacent segments is 30 – 40 V. The choice of the average coil voltage determines the minimum number of commutator segments for its design.

Compensating winding in addition to interpoles reduces the armature reaction.

 

Ques.55. The field control of a DC shunt motor gives

  1. Constant torque drive
  2. Constant kW drive
  3. Constant speed Drive
  4. Variable load speed drive

Flux Control Method or Field control method

In field control operation, if the armature current is not allowed to change beyond rated value, the reduction in field flux reduces the torque developed with the increase of speed. In field control, armature voltage is kept constant and field voltage is varied. Thus, the product of torque and speed i.e power remains approximately constant. Hence, the field control operation is called a constant power (Constant kW) operation.

The speed of DC Shunt Motor is inversely proportional to the flux and the flux is directly proportional to the Torque. The flux is dependent on the current through the shunt field winding. Thus flux can be controlled by adding a rheostat (variable resistance) in series with the shunt field winding. At Starting the rheostat R is kept at the minimum. The supply voltage is at its rated value. So the current through shunt field winding is also at its rated value. Hence the speed is also rated speed i.e at normal speed. When the resistance R is increased due to which shunt field current Ish decreases, hence decreasing the flux produced. As N =(I/Φ), the speed of the motor increases beyond its rated value. Now as we know that the speed of DC motor is directly proportional to the flux hence when the speed is increased the back EMF also increased. Thus by this method, the speed control above the rated value is possible.

Field Control F is controlled by tapping series field turns or by a diverter resistance across the
series field.

The highest speed achieved in this method is limited by the following reasons:

1. The increase in armature current causes overheating of the armature.

2. Due to high speed, centrifugal stresses are set up in the armature.

3. Because of an increase in the armature current, the increased armature reaction, under the field condition, gives rise to instability of motor at high speed, coupled with poorer commutation.

NOTE:-  Armature voltage control is constant torque drive. This method is employed for speeds above base speed The full load speed at full rated current and rated armature voltage is called the base speed. For operating below base speed, the field flux is kept at full excitation and the applied voltage to the armature is reduced. As the speed decreases the back emf reduces and so the armature current remains practically constant. Since the field flux and the armature current are constant in the armature control method, this method of speed control is called constant torque operation.

 

Ques.56. The no-load current in a transformer is

  1. Sinusoidal
  2. Non-Sinusoidal
  3. Trapezoidal
  4. Stepped

When the ac power source is connected to a transformer, a current flows in the primary winding, even when the secondary winding is open-circuited. This is the current required to produce the flux in the ferromagnetic core. The no-load current lc, taken by the primary consists of two components:

A reactive or magnetizing component, “Im responsible for producing flux in the core

An active or energy component, Ie which supplies the hysteresis and eddy current losses taking place in the core and I2R1 loss taking place in the primary winding. The I2Rloss in the primary winding at no-load is very small and can be neglected for all practical purposes.

The no-load current can thus be broken up into its two components, i.e. (a) the magnetizing current,“Im” and (b) the energy current, Ie.. How magnetizing current varies with the applied voltage can be explained as follows:

At No- load the induced emf in the primary winding is approximately equal and opposite to the applied voltage. The equation for the induced emf in any winding of a transformer is given by the expression:

E = 4.44.φm.f.N V

If the frequency of supply φ and the number of turns N are constant then induced emf E is proportional to flux φ. Now flux φ is produced by the magnetizing current Im (a component of the no-load current). The curve giving the relation between B and Im for a magnetic material is known as the magnetizing characteristic.

Therefore, in this case, the relation between the induced emf (or terminal voltage) and the magnetizing current will be similar to the magnetizing characteristic of the core material.

The core of a transformer is generally made of some ferromagnetic material. In a ferromagnetic material, all the dipoles are aligned parallel. If a small value of the magnetic field is applied, a large value of magnetization is produced. Ferromagnetic material has the permanent dipole moment and the susceptibility is positive. The magnetization in a ferromagnetic material is non-linear and it becomes saturated if a large value of the magnetic field is applied.

When a transformer primary winding is supplied with a sinusoidal alternating voltage with the secondary open-circuited, the current flowing through the primary winding produces an alternating magnetic field which in turn induces an emf in this winding approximately equal and opposite to that of the applied voltage. For this emf to be sinusoidal, the flux must vary sinusoidally with time The magnetic flux is produced by the magnetizing current flowing through the primary. The curve showing the relation between magnetic flux and magnetizing current is called magnetizing characteristics.

 At time t1 the instantaneous value of the flux is Φ1. At the same time, the corresponding value of the mmf loop is H. From this value the corresponding value of magnetizing current I1 can be obtained. At time t2, the flux has the same value of Φ1, but now the flux starts decreasing. The corresponding values of the mmf and current i.e H2 and I2. In this way, the complete waveform of the magnetizing current can be obtained. The wave shape of the magnetizing current would be sinusoidal if the magnetization curve for the core material was linear. However, due to the nonlinear characteristic of the magnetization curve the wave shape of the magnetizing current is nonsinusoidal. Of course, it is symmetric, but not sinusoidal. It contains a series of odd harmonics including the fundamental waveform. If we analyze by using the Fourier-series method, we mostly find third harmonics.

According to the Lenz’s law, these harmonic currents try to neutralize the harmonic flux produced by the harmonic currents and make it sinusoidal. In single-phase transformers, harmonics are limited only in the primary circuit because the source impedance is much smaller than the load impedance. Thus, we can say that if the flux is sinusoidal, the magnetizing current contains the harmonics with predominant third harmonic but if the magnetizing current is sinusoidal, the flux and hence induced emf will be non-sinusoidal and flat-topped. This can be understood clearly if the transformer is connected to the sinusoidal current source. In this case, the harmonic currents cannot be supplied by the source and the induced emf will be non-sinusoidal and having harmonic voltages. So, when the secondary of the transformer is connected to the load, harmonic currents flow through the load.

 

Ques.57. Transformer cooling and insulation oil must be of

  1. Low viscosity
  2. High viscosity
  3. Low BDV
  4. Low resistivity

Transformer oil is used in the oil filled transformer and in some other system such as high voltage capacitors, fluorescent lamp ballasts, circuit breaker etc.

Transformers generate a lot of heat in dropping high voltage to low voltage. The heat has to be removed or the copper will melt and electrical contact will be lost.

Transformer Oil has a very good heat transfer coefficient (removes heat easily), does not conduct electricity at all (so electrical shorts won’t occur), and is selected to have a high boiling point, so it remains a liquid inside the transformer. It is also very chemically stable, so there is no breakdown over time.

There are two main functions of the transformer oil:-

  1. Coolant: The coils made up of copper are carrying a very high current so they will soon become hot and the transformer oil is a good conductor of heat so to reduce the temperature of the coils, the transformer oil is used. So the transformer oil prevents the burning of the coils of the transformer.
  2. Insulator: Transformer oil has a great dielectric strength so it can withstand a quite high voltage, so is used as an insulator in the transformer.

The transformer oil should possess the following properties:

  1. High dielectric strength.
  2. Low viscosity to provide good heat transfer.
  3. Low Volatility for low vaporization of oil.
  4. Good resistance to the emulsion.
  5. Free from inorganic acid, alkali, and corrosive sulfur.
  6. High flash/fire point.

 

Ques.58. Single Phase transformers can be used in parallel only when their voltages are

  1. Equal
  2. Unequal
  3. Zero
  4. None of these

The need for operation of two or more transformers in parallel often arises due to:

  • Load growth, which exceeds the capacity of an existing transformer
  • Lack of space (height) for one large transformer
  • A measure of security (the probability of two transformers failing at the same time is very small)
  •  The adoption of a standard size of transformer throughout an installation
  •  To maximize the electrical power efficiency, availability, reliability, and flexibility.

When two or more transformers run in parallel, they must satisfy the following conditions.

  1. Same voltage ratio of transformers
  2. Same percentage impedance
  3. Same Polarity
  4. Same phase sequence
  5. Same voltage ratio

Same Voltage Ratio

An equal voltage ratio is necessary to avoid a no-load circulating current. Circulating current may be defined as that current which flows in transformers operating in parallel when they do not supply the load. Consider two transformers connected in parallel on their primary sides only. If the voltage readings on the secondaries are not same, there will be circulating currents between the secondaries when they are connected in parallel. This also creates circulating currents in primaries even when the load is not connected.  As the internal impedance of the transformer is small, a small voltage difference may cause the high circulating current which increases copper losses. These circulating currents produce undesirable effects as under:

  1. They reduce the permissible output of the parallel combined group because circulating current produces unequal load sharing.
  2. They increase the power loss.
  3. They may overload one of the transformers.

Same percentage impedance

The current shared by two transformers running in parallel should be proportional to their MVA rating. And current carried by these transformers are inversely proportional to their internal impedance. Hence the impedance of the transformer running in parallel is inversely proportional to their MVA Rating Each transformer has a particular value of impedance which may be different from the other transformer i.e their resistance to reactance is different.

Consider two transformers whose ratings are in the ratio of 4: 1. It is obvious that the first transformer must have one-fourth of the impedance of the second transformer hence the current drawn by the first transformer will be 4 times to the second transformer.

Due to the difference in the quality of percentage impedance, there will be the divergence of the phase angle of the two currents so that one transformer will be working with a lower and other with a higher power factor than the combined load.

Same Polarity.

The polarity of all transformer running in parallel must be the same. The polarity of transformer refers to the instantaneous direction of Induced EMF in the secondary. If the instantaneous direction of induced secondary EMF in two transformers are opposite to each other when the same input power is fed to both of the transformers, then the transformer is said to be in opposite polarity. If the instantaneous direction of induced secondary EMF in two transformers are sane when the same input power is fed to both of the transformers, then the transformer is said to be in the same polarity.

The difference in the polarity causes the huge circulating current that flows in the transformer and even may lead to the dead short circuit.

 

Ques.59. When the phase sequence of supply currents are reversed, then the direction of rotation of the resultant magnetic field wave

  1. Not unchanged
  2. To and fro
  3. Reversed
  4. None of these

The order in which the individual phase voltages attain their respective maximum values in a three-phase system is called phase sequence. So whether it is the balanced system or an unbalanced system the phase voltage depends on the phase sequence.

The direction of the roof. is always from the alas of the leading phase of the three phase winding towards Me lagging phase of the winding. In a phase sequence of R-Y-B, phase R leads Y by 120° and Y leads B by 120°. So r.m.f. rotates from axis of R to the axis of Y and then to the axis of B and so on. So its direction is clockwise as shown in the figure. This direction can be reversed by interchanging any two terminals of the three phase windings while connecting to the three phase supply. The terminals Y and B are shown interchanged in the Fig.B. In such case the direction of r.m.f. will be anticlockwise.

As Y and B of windings are connected to B and Y from the winding point of view the phase sequence becomes R-B-Y. Thus R.M.F. axis follows the direction from R to B to Y which is anticlockwise.

Thus by changing any two terminals of three-phase winding while connecting it to three-phase A.C supply, the direction of rotation of r.m.f. gets reversed.

 

Ques.60. The availability of full -rated torque at starting is obtained from an induction motor is

  1. Rotor resistance control
  2. Stator voltage control
  3. Slip ring control
  4. Line current control

When the load torque is small, the speed control of an induction motor is obtained by variation of stator voltage. In the high torque range, in the case of a wound rotor motor, rotor resistance control is used. For a good dynamic response, external resistance in rotor circuits can be varied statically and steplessly by a high-frequency thyristor chopper circuit. The figure shows such a speed control method for an induction motor.

In the conventional rotor resistance control method, a three-phase variable resistance is inserted in series with the rotor winding as shown in Fig. This way the value of rotor resistance per phase can be controlled. This method is applicable for slip-ring induction motors because we can add external resistance to slip ring rotor and not to squirrel cage rotor. The characteristics such as starting torque and speed can be controlled by varying the rotor resistance per phase. Let us see the effect of the change in rotor resistance on torque produced.

 

The torque develops in the 3−φ induction motor is given as

{T_{\max }} = \dfrac{{E_2^2}}{{2{X_2}}}

T \propto \frac{{sE_2^2R_2'}}{{R_2^{2} + {{(s{X_2})}^2}}}

Where

X2 = Rotor reactance per phase at standstill
R2 = Rotor resistance per phase at the standstill
E2 = Rotor induced E.M.F per phase on standstill condition

Now, external resistance in added in each phase of rotor through slip rings as shown in Fig.

Corresponding torque:-

T \propto \frac{{sE_2^2R_2'}}{{R_2^{'2} + {{(s{X_2})}^2}}}

Similarly, the starting torque at slip = 1 for R2 and R’2 can be written as

{T_{st}} \propto \frac{{sE_2^2R{_2}}}{{R_2^{2} + {{(s{X_2})}^2}}}

and

{T_{st}} \propto \frac{{sE_2^2R{'_2}}}{{R_2^{'2} + {{(s{X_2})}^2}}}

Maximum starting torque is obtained when the slip is equal to the ratio between the rotor resistance (R2) and the rotor inductive reactance (X2).This slip is also known as slip at maximum torque, labeled as Sm.

{T_{\max }} = \dfrac{{E_2^2}}{{2{X_2}}}

It can be observed that Tmax is independent of R2; hence whatever may be the rotor resistance, the maximum torque produced never changes, but the slip and speed at which it occurs depend on R2.

For R2

Sm = R2 ⁄ X2

For R’2

Sm = R’2 ⁄ X2

As R’2 > R2. then S’m > Sm . Due to this, we get a new torque-slip characteristic for rotor resistance R’2.

From the slip-torque characteristics of the induction motor shown in Fig.  it is clear that the starting torque T‘st with rotor resistance R’2 is greater than staring torque Ts, with R2. Therefore, staring torque can be improved by adding external rotor resistance. If now resistance in further added to the rotor to get resistance R”2 and so on, it can be seen that Tmax remains the same but the slip at which it occurs increases to S”m and so on. Similarly, starting torque also increases to T”st and so on as shown in Fig. If maximum torque Tmax is required at the start, then Sm = 1 as at start slip is always unity.

Sm = R2 ⁄ X= 1

R2 = X2 which is the condition for getting Tst = Tmax

Thus, by adding external resistance to the rotor until it becomes equal to X2, the maximum torque can be achieved at the start. It is represented by point A as shown in fig. During running condition, the external resistance is removed in order to increase efficiency by reducing I2R losses. Hence such added resistance is cut off gradually and finally removed from the rotor circuit in the normal running condition of the motor. This method is specially designed to improve the starting torque at the time of staring itself.

Advantages of Rotor resistance Control

  • Reduce the starting current
  • Increase the starting torque
  • The power factor of the line is improved
  • There are no harmonics in the line current
  • Speed control is smooth and of the wide range.

Disadvantages

  1. Efficiency is reduced due to wastage of slip energy in the rotor circuit.
  2. If rotor resistances are not equal, unbalance in currents and voltages.

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