Tuesday, 31 May 2016

Linear Induction Motor

Linear Induction Motor (LIM) is an asynchronous motor, working on the same principle an Induction Motor works, but is designed to produce the rectilinear motion, unlike the rotary movement produced by a motor; hence the word Linear Induction Motor.

A Linear Induction Motor (LIM) is an advanced version of rotary induction motor which gives a linear translational motion instead of the rotational motion.

Construction of Linear Induction Motor:

The stator is cut axially and spread out flat in Linear Induction Motor (LIM). In this type of motor, the stator and rotor are called primary and secondary respectively. The secondary of the linear induction motor consists of a flat Aluminium conductor with a ferromagnetic core. 

For understanding the construction of Linear Induction Motor, we will first take a look at the construction of Induction Motor as shown in figure below.

If we cut the Stator along the line ab and make it flat then the Stator will look like as shown below.

This is what which makes the Primary of a Linear Induction Motor. So Primary of Linear Induction Motor is flat and three phase winding is wound on it. Now if we make the Rotor of Induction Motor flat then it will be nothing but a sheet of flat Aluminium which is called the Secondary of Linear Induction Motor (LIM) as shown in figure below.

Hope construction part of Linear Induction Motor is clear.

Working of Linear Induction Motor:

If a three phase supply is connected to the stator of an Induction Motor, a rotating flux is produced. This flux rotates at a synchronous speed in the air gap. Similarly, if the primary of the Linear Induction Motor is connected to the three phase supply, a flux is produced which will travel across the length of the primary. Because of the travelling magnetic flux, a current will be generated in the conductor which is made of the aluminium material in the secondary of Linear Induction Motor.

This current, which is induced in the Linear Induction Motor secondary interacts with the travelling flux and produces a linear force from i(dL×B). If secondary of the linear induction motor is fixed and the primary is free to move, the force will move the primary in the direction of the travelling wave. The working of Linear Induction Motor is depicted in figure below using Double Linear Induction Motor.

Performance of the Linear Induction Motor:

As we know that for a P pole Induction Motor, synchronous speed is given as

Ns = 2f/P rps  where f = Frequency of Supply

Therefore, for Linear Induction Motor the speed of travelling flux will be

V = 2f (Pole Pitch) m/s where V = Velocity

The Linear Force is given as,

Power = Force×Velocity

Here Power will be Air Gap Power and velocity will be the velocity of travelling flux.

Force = (Air gap Power) / 2f(Pole Pitch) Newton

Interesting point which shall be noted that,

  • If a rotary induction motor is compared with the linear induction motor, the LIM requires a larger air gap and hence, the magnetising current is greater and the power factor and efficiency of the motor are lower.
  • In the rotary induction motor the stator and the rotor area are same whereas in the LIM the one of the two is shorter than the other. At the steady speed, the shorter part will be passing continuously over a new part of the other member.

Applications of the Linear Induction Motor:

  • The main application of the LIM is in transportation and in Electric Traction System. The primary is mounted on the vehicle and the secondary is laid on the track as in Maglav. I will post on use of LIM in magnetic Levitation so be there.
  • It is used in the cranes
  • Actuators for the movement of doors
  • Used in High voltage circuit breakers and also in accelerators.

Thank you!

Monday, 30 May 2016

Basics of Servomechanism and Servo Motor

Servomechanism is a powered mechanism producing motion or forces at a higher level of energy than the input level, e.g. in the brakes and steering of large motor vehicles, especially where feedback is employed to make the control automatic.

A servomechanism, sometimes also called Servo, is an automatic device that uses error-sensing negative feedback to correct the performance of a mechanism and is defined by its function.

Components of Servomechanism:

A servo system mainly consists of three basic components

  • A controlled device
  • A output sensor
  • A feedback system

Working of Servomechanism:

Servomechanism is an automatic closed loop control system. Here instead of controlling a device by applying variable input signal, the device is controlled by a feedback signal generated by comparing output signal and reference input signal.

Carefully observe the figure above and think. When reference input signal or command signal is applied to the system, it is compared with output reference signal of the system produced by output sensor, and a third signal produced by feedback system. This third signal acts as input signal of controlled device. This input signal to the device presents as long as there is a logical difference between reference input signal and output signal of the system. After the device achieves its desired output, there will be no longer logical difference between reference input signal and reference output signal of the system. Then, third signal produced by comparing theses above said signals will not remain enough to operate the device further and to produce further output of the system until the next reference input signal or command signal is applied to the system.

Hence the primary task of a servomechanism is to maintain the output of a system at the desired value in the presence of disturbances.

Now we will discuss Servo Motor.

Servo Motor:

A servo motor is a DC, AC, or Brushless DC Motor combined with a position sensing device e.g. a digital decoder. Thus any motor controlled using Servo Mechanism is Servo Motor.

Servos are extremely useful in robotics. The motors are small and are extremely powerful for their size.

Now we will discuss Servo Motor Control Methodology using DC Motor and we assume that speed of DC Motor is controlled by controlling the Armature Terminal Voltage.

Servo Motor Control:

For understanding servo motor control let us consider an example of servomotor that we have given a signal to rotate by an angle of 45° and then stop and wait for further instruction.

The shaft of the DC motor is coupled with another shaft called output shaft, with help of gear assembly. This gear assembly is used to step down the high rpm of the motor's shaft to low rpm at output shaft of the servo system.

The voltage adjusting knob of a potentiometer is so arranged with the output shaft by means of another gear assembly, that during rotation of the shaft, the knob also rotates and creates an varying electrical potential according to the principle of potentiometer.

This signal i.e. electrical potential is increased with angular movement of potentiometer knob along with the system shaft from 0° to 45°. This electrical potential or voltage is taken to the error detector feedback amplifier along with the input reference commends i.e. input signal voltage.

As the angle of rotation of the shaft increases from 0° to 45° the voltage from potentiometer increases. At 45° this voltage reaches to a value which is equal to the given input command voltage to the system. As at this position of the shaft, there is no difference between the signal voltage coming from the potentiometer and reference input voltage (command signal) to the system, the output voltage of the amplifier becomes zero.

As per the picture given above the output electrical voltage signal of the amplifier, acts as input voltage of the DC motor. Hence the motor will stop rotating after the shaft rotates by 45°. The motor will be at this rest position until another command is given to the system for further movement of the shaft in desired direction.

From this example we can understand the most basic servo motor theory and how servo motor control is achieved.

Thus we can conclude that, the shaft of the servo is connected to a potentiometer. The circuitry inside the servo, to which the potentiometer is connected, knows the position of the servo. The current position will be compared with the desired position continuously with the help of an Error Detection Amplifier. If a mismatch is found, then an error signal is provided at the output of the error amplifier and the shaft will rotate to go the exact location required. Once the desired location is reached, it stops and waits. Some of the applications of Servomechanism are Position Control, Speed Control etc.

That is what I wanted to share. Thank you!

Difference between Depletion-mode MOSFET and Enhancement-mode MOSFET

Enhancement Mode MOSFET:

For an enhancement MOSFET, the channel does not initially exist. It only comes into existence once a voltage greater than Vth, threshold voltage is applied. For example, in an n-channel MOSFET, the substrate is made of p-type material. Consider the source to be at a reference ground potential of 0 Volts. For a gate to source voltage of 0 Volts, there is no channel in the p type substrate. Once, the voltage starts to increase, holes are pushed away from the region near the gate due to the increasing positive potential and thus leave behind a region of excess electrons. This region of excess electrons forms the channel for the nMOSFET. That is the reason it is called an n-channel MOSFET.

For a p-channel enhancement MOSFET, since the substrate is of n type, to form a p-type channel, we need to push electrons away from near the gate region which effectively means that we have to apply a negative gate to source voltage/potential. Thus, for an enhancement MOSFET, the channel does not exist at Vgs=0V and comes into play only when the threshold voltage, Vth is exceeded. This is the reason why it is called an Enhancement type MOSFET as the application of a voltage enhances the channel from a state of almost non-existence to a state of existence.

Depletion Mode MOSFET:

For a Depletion type MOSFET, everything is the same except only that the channel is already implanted in the substrate through diffusion. Hence, a current can flow between the source and drain even at Vgs=0 Volt since charge carriers are already present and there is no need to apply a bias voltage to create a region of excess carriers near the gate region.

For example, for the n-channel depletion type MOSFET shown above, the channel has already been implanted into the substrate by diffusion and this is how the device looks without any bias. Compare that to how an enhancement MOSFET looks WITH BIAS as shown in above figure. For the above device, if we start to increase the gate-source voltage, more and more holes will move away from the channel and thus the channel will be deepened, enhanced, as more and more excess uncovered electrons will be left behind by the leaving holes. 

If we now, start decreasing the gate-source voltage, the holes will be attracted towards the gate due to the decreasing potential and thus the channel will start to become more and shallower i.e. depleted. In other words, we can say that the channel is getting Depleted of free carriers i.e. electrons. This is the reason why it is called a Depletion type nMOSFET. It should now become clear that the depletion nMOSFET should have a negative threshold voltage Vth while an n-channel enhancement type MOSFET has a positive threshold voltage.

In short,

Enhancement Mode MOSFET:

Channel doesn't exist initially. When we apply some input voltage known as threshold voltage channel gets created by repulsion of majority carriers in the bulk region between SOURCE and DRAIN nearby GATE. Default state is OFF i.e. no current flows without applying voltage.

Depletion Mode MOSFET:

Channel already exists. We have to apply some input voltage to collapse the channel so as to stop current flow between SOURCE and DRAIN. Default state is ON i.e. current flows without applying voltage.

Thank you!

Resistance of Semiconductors and Conductors as a Function of Temperature

In conductors i.e. metals the electrons that conduct current are called Itinerant electrons. They are essentially free to move around the metal, not bound to any particular atomic core. Resistivity can be understood as Itinerant electrons scattering off of Phonons, or thermal lattice vibrations, in a conductor. As the temperature of the metal increases, the time between Phonon scattering events decreases, leading to increased resistivity. Temperature dependence of copper’s resistivity as a function of temperature is shown in figure below.

The nonlinear region below about 50 K is where the phonons are suppressed to the point that the resistivity is dominated by impurities in the metal. It is clear from the figure that temperature coefficient of Resistivity of a Conductor is positive.

For some materials, like copper, the resistivity is essentially linear with temperature. This fact is exploited to create sensitive thermometers in which a constant current is passed through a resistor with known temperature dependence, and the voltage across the resistor is measured. For other materials, a power law better describes the temperature dependence.

In semiconductors, the resistivity generally decreases with increasing temperature. In the case of intrinsic semiconductors e.g. silicon, one might expect the resistivity to be very high: the valence band in filled and there are no conduction electrons to carry current. However, electrons can be thermally excited to the conduction band, creating electron-hole pairs which can carry current. As one might expect, the production of thermally-excited electron-hole pairs increases with increasing temperature, so the resistivity decreases with increasing temperature. Thus temperature coefficient of Resistivity of Semiconductor is negative.

The resistivity in intrinsic superconductors is still pretty high. As a result most semiconductors are doped, so that there are either more conduction electrons than there are holes (n-type), or there are more holes than there are conduction electrons (p-type). This allows for decreased resistivity and for the fine-tuning of resistivity. Thermal production of electron-hole pairs plays an important role in doped semiconductors.

Thank you!

Sunday, 29 May 2016

Insulator Failures – An Overview

It is generally considered that 500 kV is the most economical voltage level at which to transmit large quantities of electrical energy over long distances. The best material for insulating overhead lines has been found to be porcelain, as its insulating qualities remain practically the same when exposed to all weather conditions. It has low tensile strength but considerable compressive strength, and so most types of insulator are designed to utilize the porcelain in compression.

We will first discuss Puncture Voltage, Creepage Distance and Flash Over Voltage to understand the causes of failure of Insulators.

Flashover Distance:

It is the shortest distance through air between the electrodes of the insulator. For a pin type insulator as shown in Figure below, the double headed red arrow line is flashover distance.

Flashover Voltage:

The voltage at which the air around insulator breaks down and flashover takes place shorting the insulator is called Flash Over Voltage.

Puncture Voltage:

The voltage at which the insulator breaks down and current flows through the inside of insulator is called Puncture Voltage.

Creepage Length:

The creepage length is the shortest distance between two metallic end fittings of insulator along the surface of insulator. In the string of insulators for creepage length calculation the metallic portion between two consecutive insulator discs is not taken into account.
The corrugation below the insulator is for the purpose of obtaining longer creepage path between the pin and cap. The corrugation increases the creepage length so consequently increasing resistance to the insulator leakage current. The leakage current that flows through the surface of insulators should be as little as possible.

The Creepage Distance required in clean air may be 15 mm per kV (line voltage). In the polluted air depending on the level of pollution of air the required creepage distance increases.

To clearly understand the Creepage Length / Distance, suppose we drop a water droplet at the top of insulator, then the path which will be followed by the water droplet to come down at the bottom of Insulator will be a zig zag path through many discs which is nothing but the Creepage Distance.

Electrical failure follows a puncture through the porcelain or by ‘flash-over’ round its surface, which produces an arc short-circuiting the line. As puncture destroys the insulator, it is more serious than flash-over. Therefore Safety Factor is defined for an Insulator. Safety factor of an Insulator is defined as the ration of Puncture Voltage to the Flash Over Voltage.

Safety Factor = Puncture Voltage / Flash Over Voltage

For pin type insulator the value of Safety Factor is about 10 which mean that Puncture Voltage is 10 times that of Flash Over Voltage. It is expected that Flash over to take place first as Insulator Puncture damages the Insulator and after puncture of insulator it needs to be replaced.

Insulators are designed with a puncture voltage of about twelve times and a flash-over voltage of about six times the working voltage. Failures occurring in practice are usually due to lightning or to deposits of soot or sea salt on the insulator surface. 

Lightning affects the design of the transmission line rather than that of the insulators. Often no permanent damage is done by lightning flash-over. The problem of deposits on the surface of the insulators is a serious one and has not yet been completely solved, although many suggestions have been made for improving the standard types. For use near the sea, anti-deposit insulators have long, recessed, protected surfaces. 

For industrial areas, types with open exposed surfaces which can be cleaned by wind and rain have proved the best. For testing purposes, a percentage of the finished insulators are selected at random and tested for flash-over voltage both dry and in rain produced artificially by a watering pot, impulse flash-over voltage, mechanical strength and electrical puncture.

Thank you!

Different Type of Insulators Used in Power System

The purpose of the insulator or insulation is to insulate the electrically charged part of any equipment or machine from another charged part or uncharged metal part. At lower utilization voltage the insulation also completely covers the live conductor and acts as a barrier and keeps the live conductors unreachable from human being or animals. In case of the high voltage overhead transmission and distribution the transmission towers or poles support the lines, and insulators are used to insulate the live conductor from the transmission towers. The insulators used in transmission and distribution system are also required to carry large tensional or compressive load.

The HV/EHV insulators are broadly divided into two types based on the material used. One is ceramic and the other is polymer (composite) insulator. Traditionally ceramic insulators of porcelain are used in both transmission and distribution lines.

Now polymer or composite insulators are increasingly used in high voltage transmission systems. The polymer insulators have a fibre rod surrounded by outer sheath of some polymer. Due to the hydrophobic nature of the polymer insulator surface, dry areas are formed between wet areas resulting in discontinuities in wet creepage path. This phenomenon helps improve the performance of the polymer insulator in polluted and coastal areas. The polymer insulators have one great advantage that it is quite lighter in comparison to porcelain insulators. Polymeric insulator surface degrade faster in comparison to porcelain insulator. One important disadvantage with porcelain insulator is that the porcelain insulators can bear large compressive force but less tensional force. The porcelain insulators surface is hydrophilic in nature, which means affinity for water. Polymer insulators age faster than ceramic insulators.

Different types of Insulators used in Power Transmission for supporting the conductors on Tower are as follows:

Pin Type Insulator:

This is the first developed insulators and being used for overhead lines for voltage grade up to 33 kV. The live conductor is place on the top of the insulator and the bottom of the insulator in connected to earth. The insulator has to withstand the potential stress between conductor and earth. When insulator is wet, its outer surface becomes almost conducting. Hence the flash over distance of insulator is decreased. The electrical insulator is designed such that the decrease of flash over distance is minimum when the insulator is wet. That is why the upper most petticoat of a pin insulator has umbrella type designed so that it can protect the rest lower part of the insulator from rain. The upper surface of top most petticoat is inclined as less as possible to maintain maximum flash over voltage during raining.

Post Insulators:

Post insulator is suitable for higher voltage. It has higher numbers of petticoats and has greater height. This type of insulator can be mounted on supporting structure horizontally as well as vertically. The insulator is made of one piece of porcelain and it has fixing clamp arrangement are in both top and bottom end. For higher voltage application Two or more insulators can be fixed together to meet the requirement.

Suspension Insulator:

Using post insulator in higher voltage is not economical and suspension type insulator is evolved. Disc insulators are connected together in series to make a string which is suspension type insulators. As per the voltage grade the no of disc isolators are increased or decreased so that is is suitable for any voltage level. When suspension insulators are used a conductor is always hanging / suspended below the metallic tower level and it is always protected from lightning. On the other hand in order to maintain minimum clearance between conductor and ground/ equipment the tower hight use to be higher. The amplitude of free swing of conductors is larger in suspension insulator system, hence, more spacing between conductors should be provided.

String Insulators:

When suspension string is used to sustain extraordinary tensile load of conductor it is referred as string insulator. When there is a dead end or there is a sharp corner in transmission line, the line has to sustain a great tensile load of conductor or strain. A strain insulator must have considerable mechanical strength as well as the necessary electrical insulating properties. In string Insulator, each porcelain disc is designed for 11 kV. Thus for 132 kV overhead line around 12 disc will be assembled.

Stay Insulators:

For low voltage lines, the stays are to be insulated from ground at a height. The insulator used in the stay wire is called as the stay insulator and is usually of porcelain and is so designed that in case of breakage of the insulator the wire will not fall to the ground.

Shackle insulators:

It is usually used in low voltage distribution network. It can be used both in horizontal and vertical position. The conductor in the groove of shackle insulator is fixed with the help of soft binding wire.

Thank you!

Difference between Current Transformer & Potential Transformer

Current Transformer (CT) and Potential Transformer (PT) are instrument transformers basically used for measurement and protection purpose in power systems. A Current Transformer (CT) is used to obtain reduced current signals for purpose of measurement, control and protection. They reduce the higher current to lower values which are suitable for operation of Relays and other instruments connected to their secondary winding.

While a Potential Transformer (PT) used to reduce the voltage to lower values and to provide isolation between high voltage power network and the relays and the other instruments that are connected to their secondary.

Major differences between the Current Transformer (CT) and Potential Transformer (PT) are listed below.

Current Transformer:

Top Core Type CT is shown in figure below.

  • The secondary of a C.T cannot be open circuited on any circumstance when it is under service.
  • A CT may be considered as a series transformer.
  • The primary current in a C.T is independent of the secondary circuit conditions (burden).
  • The primary winding of the CT is connected in series with the line carrying the current to be measured. Hence it carries of the full line current.
  • With the help of CT, a 5A ammeter can be used measure a high current like 2000A by using 2000/1 CT.

A Current Transformer (CT) is represented as shown below.

Potential Transformer:

  • The secondary of a P.T can be open circuited without any damage being caused either to the operator or the transformer.
  • P.T may be considered as a parallel transformer.
  • The primary current of a P.T depends upon the secondary circuit conditions (burden).
  • The primary winding P.T is connected across the line of voltage to be measured. Hence the full line voltage is impressed across its terminal.
  • With the help of P.T, a 120V voltmeter can be used to measure very high voltages like 11 kV.

A Potential / Voltage Transformer (PT / VT) is represented as shown below.

Connection / Wiring Difference between a Current Transformer (CT) and a Potential Transformer (PT):

Thank you!