SSC JE Electrical Previous Year Question Paper 2018-SET 4|MES Electrical

The centrifugal switch is used to disconnect the start winding from the circuit when the motor reaches approximately 75% of the rated speed. 

A centrifugal switch is a mechanical device attached to the end of the shaft with weights that will sling outward when the motor reaches approximately 75% speed. For example, if the motor has a rated speed of 1725 rpm, the centrifugal weights will change position at 1294 r.p.m (1725 x 0.75) and open a switch to remove the start winding from the circuit. This switch is under a fairly large current load, so a spark will occur.

If the switch fails to open its contacts and remove the start winding, the motor will draw too much current, and the overload device will cause it to stop. When the motor is de-energized, it will slow down and the centrifugal switch will close its contacts in preparation for the next motor starting attempt. The more the switch is used, the more its contacts will burn from the arc. If this type of motor is started many times, the first thing that will likely fail is the centrifugal switch. This switch makes an audible sound when the motor starts and stops.

Hence if the starting switch fails to open when needed, the starting winding mill almost always overheats and burns out.

Note:-  Centrifigual switch is not used in reluctance motor and hysteresis Motor.

• In order to self-start a single-phase induction motor one must create a rotating magnetic field on the stator. Therefore split phase motor has two winding one is starting winding and the other is main winding or running winding.
• The starting winding has fewer turns and a small diameter, therefore, it has high resistance and low inductance
• The running winding has a heavier wire of more turns, therefore, it has low resistance and high inductance.
• At the time of starting, both the windings draw currents from the supply
• The result is that the currents in the two windings are out of phase with each other.
• For maximum starting torque to be produced, the two currents should be 90 degrees out of phase.
• This condition is not possible with the arrangement shown. Because the current in the start winding lags the applied voltage because of its inductance. However, the current in the run windings lags by a greater amount, because of its higher inductance.

• If the current in the starting winding lags the voltage by 25 degrees and the current in the running winding lags the voltage by 70 degrees, the phase displacement is 45 degrees. This difference in phase relationship is enough to provide a weak starting torque, which is adequate for some applications.
• The auxiliary winding is generally designed to remain in the circuit for only a short time. If energized too long, overheating will result and will damage the insulation. A centrifugal switch connected in series with this winding is designed to open when the rotor reaches approximately 75% of synchronous speed. The motor then operates as a single-phase 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.

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 stator 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.

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, the 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.

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.

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.

Since the universal motor is a series-wound, it has performance characteristics similar to a

• It has high starting torque.
• The no-load speed is very high, but not high enough to break apart.
• It has poor speed regulation when the load to which it is connected changes. To minimize the effect of the load, most universal motors are designed to overcome this by operating at a very high speed of 3500 rpm or greater. In a router application, for example, the universal motor runs at 18,000 rpm.

In a universal motor, the third set of coils called compensating windings is connected in series with the rotor and stator. These windings perform two functions: They correct the neutral plane position that is distorted by armature reaction, and they compensate for undesirable reactive voltages attributed to the inductance of the armature.

The speed of a universal motor is determined by the following factors:

• A change in the applied voltage. As the voltage increases, the current rises to create a stronger magnetic field, which causes the speed to increase.
• A change in the load. As the physical load, the motor is driving becomes harder to turn, the speed decreases.
• A change in frequency of the power supply. As the frequency increases, the inductive reactance of the series coil rises, which causes current flow through both the field and armature windings to decrease. The result is that the magnetic fields around them become weaker, creating a weaker torque, and the motor slows down.

Resistance Method for  Speed control of Universal Motor

Universal motors are speed-regulated by inserting resistance in series with the motor. The resistance may be tapped resistors, rheostats, or tapped nichrome with coils wound over a single field pole. The resistance is connected before the motor, therefore the current to the motor is reduced, which in turn reduces the speed of the motor according to the setting of that variable resistance. 

An application example is a sewing machine. Its speed is varied by a foot pedal, Chicle contains a variable resistor. Another example is a variable-speed hand drill. As the trigger is pulled, resistance in series with the motor is changed to vary the RPM.

Field Tapping Method of speed control of universal motor

By tapping a field coil at various points, as shown in Figure, different inductive reactance values are developed. When the switch is set at the low-speed position, the entire winding is used and the maximum inductive reactance forms and causes minimum current to flow.

At the high-speed position setting, the entire coil is bypassed and the inductive reactance is reduced, causing a higher current to flow. An application example of this principle is a blender. A different field pole connection is made for each speed position setting.

The induction motor, of course, takes electrical power and converts most of it to mechanical shaft power; and, when it does, it absorbs the electrical power. Here the current always lags the voltage or is said to have a lagging power factor.

• For loading conditions, the power factor is 0.6 – 0.8 lagging
• For the unloading condition, the power factor is 0.2-0.3 lagging.

The hysteresis motor can be considered as a self-starting synchronous motor. From the time of starting until it reaches synchronous speed, the motor produces a synchronizing torque and an ideally flat speed/torque characteristic. Due to the absence of slots and teeth on the rotor, the operation of this motor is smooth and quiet as there is no mechanical vibration.

The motor behavior is similar to that of a conventional synchronous motor but there is no excitation winding on the rotor or permanent magnets, giving the motor a high efficiency and power factor. These characteristics make it ideal for many industrial applications, such as in electronic and medical equipment, tape and video recorder drives, computer drives, clocks, gyroscope rotors for inertial navigation, robotics, and other special applications where precision and reliability are required.

• In a short transmission line, the most common fault is the line to ground fault hence reactance type relay is used because more of the line can be protected at high speed.
• In the case of the short transmission line, the protection due to arc resistance is the major factor. hence, the relay should be selected which is less affected by arc resistance. and according to R-X diagram of impedance and reactance relay, the area of reactance relay is more and it is less affected by arc resistance fault but impedance relay is moderately affected due to arc resistance

Note:- In the case of short lines, power surges remain for a shorter period, and hence, their effect is unimportant. The predominant factor is, therefore, the effect of arc resistance. The effect of power surges stays for a longer period in the case of long lines and hence, a relay that is least affected by power surges is preferred for the protection of long lines.

The MHO unit is less affected by power surges than the impedance and reactance relays and hence, it is best suited for the protection of long lines against phase

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 lightening loads are supplied using a line and neutral wire. Loads like three-phase motors are supplied using the three-phase line

Transmission lines are classified into three types based on the length of the transmission line and the operating voltage. They are:

1. Short transmission lines
2. Medium transmission lines
3. Long transmission lines

Short transmission lines:- When the length of an overhead transmission line is about 50 km with an operating voltage upto 20 kV, it is considered as the short transmission line. Due to the smaller length and low operating voltage, the charging current is low. So, the effect of capacitance on the performance of short transmission lines is extremely small and therefore, can be neglected. Thus only resistance and inductance is to be taken into account while analyzing short transmission lines.

Medium transmission lines:-  When the length of the transmission line is lying between 50 to 150 km and the line voltage is moderately high i.e. in between 20 kV and 100 kV, it is treated as the medium transmission line. In the analysis of the medium transmission line, the capacitance is taken into account as the line length is appreciable.

Long transmission line: When the length of the transmission line is more than 150 km and the line voltage is very high i.e. above 100 kV, the line is considered a long transmission line. In the analysis of such lines, the line constants are considered uniformly distributed over the whole length of the line.

Cables are used for the 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.

The transmission line is more expensive than overhead lines, especially at high potentials. Besides the increase in temperature is high in the cables. However, there is the limitation of raising the operating voltage. In low and medium voltage distribution in urban areas, cables are more widespread.

In generals 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 (not affected by acid and alkaline) and physical stability throughout.
• It should be non-hygroscopic because the dielectric strength of any material goes very much down with moisture content