SSC JE Electrical Previous Year Question Paper 2018-SET 6

The working principle of an ac machine is primarily “one field following another field”. In the case of a multiphase induction motor, there will be a virtual rotating magnetic field. But considering the case of a single-phase induction motor, it’s only a pulsating field that is produced and not a rotating one. This can also be explained on the basis of ‘DOUBLE REVOLVING FIELD THEORY‘, which is based on Ferraris Principle

As per Ferrari’s principle, the alternating magnetic field produced by the stator can be split into two rotating magnetic fields of half the magnitude and rotating at synchronous speed in opposite directions. When the alternating supply is fed to the stator winding, an alternating flux is developed This flux rotates and cuts the rotor conductors. Due to this, an EMF is induced. As the rotor circuit is closed the current flows through the rotor conductor. This rotor current will cause rotor flux and at any instant, its magnitude is given by,

φ_s = φ_m COS ωt

where φ_m is the maximum flux developed in the motor. According to this theory, the alternating flux φ_s can be resolved into two components of and φ_f  & φ_b such that the magnitude of by φ_f  & φ_b is equal to half the magnitude of φ_s . Let us assume that φ_f rotates in a clockwise direction and φ_b rotates in the anti-clockwise direction.

An emf is induced in the rotor circuit due to each rotating field. If the polarity of the induced emf in the rotor due to φ_f  is taken as positive then emf induced in the rotor due to φ_b is negative (i.e. in phase opposition). As, at standstill, the slip in either direction is the same (i.e. s = 1), the rotor impedance will also be the same. Thus, the rotor currents are equal, but opposite in phase that is the starting torque developed by each revolving field is the same, with one acting in forwarding direction and the other acting in the backward direction. Thus, the net torque developed by the motor is zero.

Based on double-revolving field theory, a single-phase induction motor can be visualized as having two rotors revolving in opposite directions with a common stator winding. At standstill, the two rotors develop equal torques in opposite directions and hence the net torque developed is zero.

The Shaded-pole motor has a salient pole stator 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 the figure.  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 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 a 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 a 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 main 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, 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.

A repulsion motor operates on the principle that 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-Start Induction-Run Motor

The repulsion-start induction motor, exactly like a repulsion motor, is capable of developing high starting torque At about 75 percent of synchronous speed, a centrifugally operated device short-circuits the entire commutator. From this speed onwards, the motor behaves like an induction motor. After the commutator is short-circuited, brushes do not carry any current, hence they may also be lifted from the commutator, in order to avoid unnecessary wear and tear and friction losses.

Repulsion-start motors are of two different designs:

1. Brush-lifting type in which the brushes are automatically lifted from the commutator when it is short-circuited. These motors generally employ the radial form of the commutator and are built both in small and large sizes.

2. Brush-riding type in which brushes ride on the commutator at all times. These motors use an axial form of the commutator and are always built-in small sizes.

The torque developed is proportional to the product of the stator flux and rotor current. Rotor current depends on the stator flux and stator flux is proportional to the stator current. Therefore, the torque developed by a repulsion motor is proportional to the square o the staler current. Since the torque developed is proportional to the square of the stator current, the torque-speed characteristics of a repulsion motor are high. The magnitude of starting torque of a repulsion motor is high. The magnitude of starting torque depends on the position of the brush axis. Its speed regulation, like a series motor, is poor.

The starting torque of such a motor is in excess of 350 percent with a moderate starting current. It is particularly useful where the starting period is of comparatively long duration, because of high inertia loads. Applications of such motors include machine tools, commercial refrigerators, compressors, pumps, hoists, floor-polishing and grinding devices etc.

The disadvantage is:-  noisy performance, poor speed regulation, and periodic commutator maintenance refinement. 

Because of the disadvantages mentioned above, repulsion motors have largely been replaced by capacitor-type motors and very few repulsion motors are manufactured.

The rotor bars are not exactly parallel to the shaft but are given a slight skew. This skew helps to make the motor run quietly by reducing the magnetic hum. Another advantage is that this skew reduces the locking tendency of the rotor.

Rotor conductors are skewed because of the following main reasons

• Cogging:- Cogging is magnetic locking. When an induction motor refuses to start even if the full voltage is applied to it, this is called cogging. This happens when the rotor slots and stator slots are the same in number or they are integer multiples of each other. due to this the opposite poles of the stator and rotor come in front of each other and get locked. Skewing the rotor bars prevents the locking thus preventing Cogging.
• Starting torque of an induction motor depends on the product of the magnitude of stator and rotor current and sine of the angle between both.
• If the conductors remain linear, the angle between stator and rotor current will be 180 degrees. As sin(180)=0, the starting resultant torque will be zero and thus motor will fail to start. This phenomenon is called cogging.
• Skewing helps the motor run more quietly because the magnetic fields are slightly skewed to offset alignment with the rotor field coils. This feature tends to reduce vibration or magnetic hum as the rotor speed changes slightly every time the conductor bars align with the rotor magnetic field. As the speed is the function of frequency so the induction motor is unable to attend the resonance frequency, therefore, the magnetic vibration is reduced.
• Skewing makes the rotor conductor longer with the reduced cross area. This increases the rotor conductor resistance hence starting performance and the torque of an induction motor is improved.

Crawling:– Crawling is a phenomenon where harmonic components introduce oscillations in torque. With the bar skewed, the amount of the bar cutting the field line grows continuously and the next bar starts cutting the field lines as the first finishes. Due to this, we get Uniform Torque.

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, 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.

Hysteresis motor is the synchronous motor that does not require any d.c. excitation to the rotor and it uses non-projected poles. Hysteresis motors start by virtue of the hysteresis losses induced in the rotor by the rotating magnetic field produced by the stator windings.

It consists of a stator that carries main and auxiliary windings so as to produce a rotating magnetic field. The stator can also be shaded pole type. The rotor is the smooth cylindrical type made up of hard magnetic material like chrome steel or alnico for high retentivity (it is the capacity of an object to retain magnetism after the action of the magnetizing force has ceased. This requires selecting a material with a high hysteresis loop area. The rotor does not carry any winding.

Generation, Transmission, and Distribution systems are the main components of an electric power system. Generating stations and distribution systems are connected through transmission lines. Normally, transmission lines imply the bulk transfer of power by high-voltage links between main load centers. On the other hand, the distribution system is mainly responsible for the conveyance of this power to the consumers by means of lower voltage networks. Electric power is generated in the range of 11 kV to 25 kV, which is increased by stepped-up transformers to the main transmission voltage.

Electric Power can either be transmitted by means of AC or DC. Each system has its advantages and disadvantages. Therefore it is very crucial to have a comparative study of their merit and demerits and then decide which method should be adapted to transmit power.

ALTERNATING CURRENT (A.C.)

An alternating current is one that periodically changes in magnitude and direction. It increases from zero to a maximum value, then decreases to zero and reverses in direction, increases to a maximum in this direction, and then decreases to zero. The complete set of variations is known as a ‘cycle’. Thus during one-half of the cycle, the current flows in one direction, and in the following Cycle, it flows in the opposite direction.

DIRECT CURRENT (D.C.)

DC current is that current that may or may not change in magnitude but it does not change its direction.

A.C. VERSUS D.C.

It is worthwhile to give advantages of ac. and d.c. Advantages of a.c. over d.c.

ADVANTAGES OF A.C OVER DC

1. Alternating voltages can be stepped up or stepped down efficiently by a transformer. This permits the transmission of electric power at high voltages to achieve economy and distribute the power at utilization voltages.
2. A.C. motors are cheaper and simpler in construction than the d.c. motors.
3. The switchgear (e.g. switches, circuit breakers, etc.) for a.c. the system is cheaper than the d.c. system.
4. A.C. can be easily converted into d.c. by rectifiers.
5.  Alternating current can be regulated by using a choke coil without any significant wastage of electrical energy.
6. For long-distance transmission of power, a higher voltage is best. Higher voltage means a lower current for the same power, and the lower current means fewer losses due to resistance in the wires. AC has the advantage that simple, robust transformers can be used to easily and efficiently step the voltage up at one end, and down at the other end to the voltages used domestically. DC requires much more complex circuitry or inefficient motor/generator sets to convert the voltage.
7. We all know that we got A.C supply in our homes and we got this supply by transmitting ac over long distances. AC can be transmitted using step-up transformers but direct current or dc cannot be transmitted by this method.
8. The maintenance of a.c. sub-stations is easy and cheaper.
9. A wide range of voltage can be obtained by using the step-up or step-down transformer.
10. A.C. can be produced and transmitted more easily and cheaply than D.C.
11. A 3-phase A.C. Dynamo can produce more energy than a single-phase D.C. Dynamo of the same cost. A.C.
12. Dynamo (using slip rings) has less loss of energy and wear and tear than a D.C. Dynamo (using split-ring commutator).
13. Transmission of A.C. at ‘high-voltage’ and ‘low-voltage‘ reduces line losses.

Note:- There’s nothing inherently wrong with using DC for high-voltage long-distance power transmission, but there are some particular characteristics that sometimes make AC transmission more attractive. If the high cost of converter stations is excluded, the dc overhead lines and cables are less expensive than ac overhead lines and cables. Until the eighties, only AC was easy to step up or down as voltages, simply using transformers. Since then, advances in power semiconductors and microprocessor controllers have allowed DC to be easily converted to higher or lower voltages. But most high voltage power lines remain AC because it seems to be difficult to change the whole system

Electric power transmission, a process for the delivery of electricity to consumers, is the bulk transfer of electrical power. All major generating stations are usually interconnected by transmission lines or network that distributes the power from generating stations to the distribution systems, which ultimately supply the load points or load centers.

Primary transmission: The generated electric power (in 132, 220, 500 kV, or greater) is transmitted to load points by the three-phase three-wire overhead transmission system. This type of transmission is called primary transmission, which is also known as extra-high-voltage AC (EHV-AC) transmission. Primary transmission is generally done through overhead transmission lines. The high voltage overhead lines are generally constructed by the aluminum alloy made up of several strands and reinforced with steel strands. The primary transmission uses a three-phase three-wire system.

Secondary transmission: The primary transmission ends at a substation known as Receiving Station (RS), which is far from the city (outskirts). At receiving station, the level of voltage is reduced by step-down transformers up to 132, 66, or 33 kV, and electric power is transmitted by the three-phase three-wire overhead system to different substations located in the city. It is known as secondary transmission. The power is transmitted to various stations using the overhead 3-phase 3 -wire System. The conductor, used for the secondary transmission are called feeders.

Distribution System:- At the substations, transformers are again used to step the voltage down to a lower voltage for distribution to commercial and residential users. This distribution is accomplished with a combination of sub-transmission (33 kV to 115 kV, varying by country and customer requirements) and distribution (3.3 to 25 kV). Finally, at the point of use, the energy is transformed to low voltage with the help of step-down transformers (100 to 600 V, varying by country and customer requirements). Similar to the transmission, the distribution of electric power is also divided broadly into two parts:

(i) Primary Distribution: At the substations, the voltage level is reduced to 6.6 kV, 3.3 kV and 1.1 kV with the help of a step-down transformer. It uses the 3-phase 3-wire underground system. And the power is further transmitted to the local distribution centers. This primary distribution is also called High Voltage (HV) Distribution. For larger consumer-like factories and industries, the power is directly transmitted to such loads from a substation. Such big loads have their own substations.

(ii) Secondary Distribution: At local distribution centers, the voltage level of 6.6 kV, 1.1 kV is further reduced to 440 V using distribution transformers. The power is then transmitted to the user or consumers; this is called a secondary distribution. The 1-phase lightning loads are supplied using a line and neutral wire. Loads like three phase motors are supplied using the three-phase line

Aluminum conductor steel-reinforced cable (ACSR) is a type of high-capacity, high-strength stranded conductor typically used in overhead power lines.

Cables are used for transmission and distribution of electrical energy in highly populated areas of towns and cities. Cables from the artery system for the transmission and distribution of electrical energy. A cable is basically an insulated conductor. External protection against mechanical injury, moisture entry, and chemical reaction is provided on the cable. The conductor is usually aluminum or annealed copper, while the insulation is mostly polyvinyl chloride (PVC) or other chemical compounds.

In general, a cable must fulfill the following necessary requirements:

• The conductor used in cables should be tinned stranded copper or aluminum of high conductivity. Stranding is done so that conductor may become flexible and carry more current.
• The conductor size should be such that the cable carries the desired load current without overheating and causes voltage drop within the permissible limit.
• The cable must have the proper thickness of insulation in order to give a high degree of safety and reliability at the voltage for which it is designed.
• The cable must be provided with suitable mechanical protection so that it may withstand the rough use in laying it.
• The material used in the manufacture of cables should be such that there is complete chemical and Physical stability throughout.

Insulating Materials for Cables

The satisfactory operation of a cable depends to a great extent upon the characteristics of the insulation used. Therefore, the proper choice of insulating material for cables is of considerable importance. In general, the insulating materials used in cables should have the following properties:

1. High insulation resistance to avoid leakage current.
2. High dielectric strength to avoid electrical breakdown of the cable.
3. High mechanical strength to withstand the mechanical handling of cables.
4. Nonhygroscopic, that is, it should not absorb moisture from air or soil. The moisture tends to decrease the insulation resistance and hastens the breakdown of the cable. In case, the insulating material is hygroscopic, it must be enclosed in a waterproof covering like lead sheath.
5. Noninflammable.
6. Low cost so as to make the underground system a viable proposition.
7. Unaffected by acids and alkalies to avoid any chemical action.

None insulating material possesses all the above-mentioned properties. Therefore, the type of insulating material to be used depends upon the purpose for which the cable is required and the quality of insulation to be aimed at. The principal insulating materials used in cables are rubber, vulcanized India rubber (VIR), impregnated paper, varnished cambric, and PVC.