The Stroboscopic Effect in Fluorescent lamp is a phenomenon which causes running or moving equipment to appear stationary or appear to be operating slower than they actually are.

In an AC supply, the voltage drops 100 times a second to zero volts as the supply frequency is 50 Hz.  When a Fluorescent lamp is operating with an AC supply, the light intensity drops 100 times a second.  This flicker is not noticeable to the human eye due to the persistence of vision. 

When a worker in a factory observes a running machine, say a flywheel under the illumination of a fluorescent light, the flywheel may appear to be stationary or to be operating at reduced speed.  This can result in accidents and is highly dangerous.

A sewing machine whose needle moves up and down may appear to be stationary and the operator can prick the fingers.

These are some examples where the stroboscopic effect in the Fluorescent lamps can prove to be dangerous.

When using fluorescent lamps around rotating or moving machinery, two lamps powered by two different phases should be used.  This ensures that both the lamps do not flicker due to the zero crossing at the same time. 

If another phase is not available, a capacitor can be added in series to one lamp.  This ensures that there is a phase lag between the two lamps. 

The Stroboscopic effect can be eliminated by  using electronic ballasts where the supply to the lamps is of a very high frequency of the order of kiloHertz.

Optical Cables are used extensively in the field of telecommunication.  They have numerous advantages over conventional communication on wires.  They are efficient, quick and secure.  Optical Communication Cables are designed to provide high efficiency of transmission.  They are also designed to withstand the challenges of the external environment such as corrosion, heat and physical stress.
The Optical Cable has the following main components. 
The Core provides the pathway for the light to travel.  It is made of glass or transparent plastic material. 
The cladding is the layer that covers the core.  The function of the Cladding is to reflect the light which may come out of the core back into the core.  This results in Total Internal Reflection which ensures that there is no loss of the light signal. 
The Buffer is a coating which is outside the Cladding.  The Buffer serves to protect the optical fibre. 
Aramid Yarn Protection
The Aramid yarn which surrounds the buffer provides crush protection to the cable.
Protective Jacket
The protective jacket offers mechanical protection to the cable. 

A Ballast in a fluorescent light is necessary to get the light glowing.  The Ballasts generates a high voltage by means of the would coil which acts as an inductance.  When the starter interrupts the supply to the inductance, a powerful voltage is generated.  These are called magnetic ballasts. 

However, magnetic ballasts take time to get the lamps to start.  They also have an undesirable hum.  They also produce a flicker before the tube lights up continually.

Electronic eliminate the problem of initial flicker and hum.  They also reduce power consumption.
Electronic Ballasts work by converting the AC supply into DC first.  This rectified DC is then chopped by a chopper circuit to generate high voltages to start the discharge in the lamp.  The chopped AC waveform is t a very high frequency of the order of kHz.  This ensures that the flicker is reduced to a minimum.

The efficiency of the lamp is also increased.  Electronic Ballasts are 10% more efficient than magnetic ballasts. Electronic Ballasts can generate harmonics.  This, however, is insignificant as the amount is very small.

Conductivity in metals is due to the presence of free electrons in the atomic lattice.  When the metal is heated, the atoms in the lattice vibrate.  This results in reduced movement of the electrons as they hit against the vibrating atoms.

This results in an increase in resistance of the metals. 

Most Metals have positive temperature coefficient of resistance. There are , however, exceptions such as carbon and semiconductor metals such as Silicon and Germanium.  Some Alloys have zero temperature coefficient of temperature which means that the resistance does not change with increase of temperature.  Manganin is an example.

When a semiconductor is heated, the conductance increases and the resistance decreases.  Semiconductors, thus, have a negative temperature coefficient of resistance.
When heat is applied to a semiconductor material, the outermost electrons in the atom gain energy.  These electrons are able to overcome the attraction of the nucleus and leave the atom.  Thus they become free electrons which can conduct. 
The number of electrons increase exponentially and this results in a large drop in the resistance. 
Electronic devices will behave erratically above a certain temperatures.  Hence, all electronic devices such as laptops will have a safe temperature beyond which they cannot function. 
The effect of vibration of the atomic lattice on the mobility of the electrons is offset by the large numbers of electrons which enter the conduction band. 

In many instances, optical cables are needed to be jointed with one another.  Jointing in optical cables is different from jointing in electrical wires and cables.  The connection should be made such that there is minimum loss of the light energy. 
There are different methods of jointing optical cables
Fusion Jointing
In the method, the surface of the two optical fibres to be connected are heated and fused together.  This ensures that the light is conveyed from one fibre to another efficiently
Mechanical jointing
In this method, the surfaces of the ends are held firmly to make proper contact.  An gel or epoxy is used to match the different reflective indices of the materials.  The fibres are held together by mechanical splices. 

A cleaver is a tool used to cleave optical fibres prior to splicing.  Optical fibres should have a plain and clear surface when they are cut.  The cut should be perpendicular to the longitudinal axis of the fibre.  This is essential to avoid loss of light or distortion.    The cleaving tool is used to cleave the fibre. 
Cleaving Tools generally use diamond tips and blades to cleave the fibres.
Certain mechanical cleaving tools use a diamond blade to make a wedge in the fibre and the twist the fibre to produce a clean break.  The cleave angle should be perfectly 90 degrees. 
The blade in the cleaver may have to be replaced after a certain number of splices.  Most manufacturers provide replacement blades. 
The cleave made is examined by the splicing machine prior to fusion

Optical fusion Splicers are used to join two optical fibres using fusion.  The splicing is done by first heating the ends to be joined.

The fibres after being cleaved are fed into the splicer.  The two fibres to be joined are held against each other and the alignment is checked after which the fusion is done.  The heating is done by means of an electric arc or a laser.  The heating is done in about 15 seconds and the fusion is then carried out.  The device is battery operated. 

Splicers can be programmed for different types of fibre optic cables.  Most Splicers are portable and have a rugged construction.  The life of the heating electrodes is specified after which the electrode may have to be replaced. 

An optical time domain reflectometer is a device which is used to check the integrity of an optical fibre system detect and locate faults in optical cables. 
The optical time domain reflector sends out a series of optical pulses from one end of the cable.  The light which is reflected is analyzed.  This gives information about the state of the optical cable and its terminations.  If there is a drop in the quality of the light of if the distorted, it can indicate a problem such as a cut or an improper splice joint. 
The optical time domain reflectometer can also measure the attenuation of signals through the optical fibre.

The Q factor of an inductor is a very important parameter.  The Q factor tell us how close the inductor to an ideal inductor. 

An ideal inductor is an inductor which has no losses.  That is, its series resistance is zero.  It is not possible to construct an ideal inductor as all inductors are made of wires which have resistances. 

Q factor is the ratio of the inductive reactance to the series resistance of the inductor  at a given frequency.

Q= XL/ R

The Q factor is an crucial parameter when designing resonant circuits as it will affect the damping.  Higher the Q factor, higher is the efficiency of the inductor. 

Moulded chokes are chokes which are moulded in a polymer or synthetic material.  These chokes are used in applications such as Led lighting, automotive electronic components, mobile  phones etc. 
Moulded chokes are small in size and highly compact. 
These chokes have very small inductance values from 10 microhenries to 1000 micro henries.

A choke is an inductor which is used to block AC voltages from a circuit.  Thus, it "chokes off" the AC currents. 
Chokes usually have fixed values.
Chokes can be classified into
  • Audio Frequency chokes which function at the power and audio frequency and
  • Radio Frequency chokes which function at the
Audio frequency chokes  have toroidal cores made of ferrite material. 
Radio frequency chokes have iron powder or ferrite materials.

Optical Fibre Metallic Wire
Not a lightning hazard as it is non conducting Can attract and transmit lightning
Lighter in weight Heavier in weight
Not affected by Interference Affected by interference
High data bandwidth Lower data bandwidth
Lower data loss Data loss is more
Faster data transmission Relatively slower data transmission
Unauthorised tapping of data is difficult Easier to tap data without authorization.
Difficult to terminate Easier to terminate
High initial cost Lower initial cost
Less affected by chemicals and pollution More prone to effects of pollution
No risk of sparking.  Hence, can be used in petroleum and chemical industries. Risk of sparking and fire.  Hence, cannot be used in hazardous environments. 

Terminating resistors are used in communication cables to prevent reflection of the transmitted signal.  The reflected signal can cause interference which may affect data transmission.  Hence, to prevent this resistances are connected in parallel.

The value of the impedance will match the wave impedance of the line.  Thus a communication line with an impedance of 120 ohms will have a 120 ohm resistor connected across it.  Short cables can function without terminating resistors. 

A Band Stop Filter is a filter which blocks a set of frequencies in a specific band and permits frequencies above and below that range to pass through. 

Functionally, the Band Stop filter does the opposite of the Band Pass Filter. 

The series LC circuit is connected in parallel.  At the resonant frequency of the LC circuit, the reactance is minimum.  Thus a specific frequency is shorted across the input and prevented from reaching the output.

The Band Stop Filter can be made by connecting a Low Pass Filter and a High Pass Filter in parallel. 

A Band pass filter combines the characteristics of the High Pass and Low Pass Filters. 

The Band pass Filter, as the name suggests, allows only signals of a particular band or range of frequencies to pass through.  All other signals are blocked or shorted.  Band Pass Filter

Band pass Filters can be made by connecting a high pass filter in series to a low pass filter or vice versa. 

A Band pass filter can be made by connecting an inductor in parallel to filter the low frequency components which lie below the desired frequency.  Another inductor in series will then block the high frequency components which lie above the desired frequency.

Likewise, the Band pass filter can also be made using capacitors as in the second figure.  Here, the first capacitor filters the high frequency components and the second capacitor in series blocks the low frequency components. 

High Pass FilterA High Pass Filter is a filter which permits high frequency signals to pass through and blocks only low frequency signals.  The high pass filter has a relatively simple construction.  The filter can be constructed by either providing a low impedance path to high frequency signals from the input to the output or by providing a low impedance path to low frequency

If a capacitor is connected in series between the input and the output, it will provide low impedance to high frequency signals and high impedance to low frequency signals.  High frequency signals alone will be able to pass the capacitor.

Alternatively, if an inductor is connected in parallel to the input, it will offer low impedance to low frequency signals which will get shorted across the input.  High frequency signals will alone reach the output.

A Low pass filter is a filter which permits only low frequency signals to pass through.  High frequency signals are blocked or shorted across the input.  The low pass filter offers low impedance to low frequencies and high impedance to high frequencies.Low Pass Filter

There are two ways of constructing a Low Pass Filter. 

The first method is to connect an inductor in series to the output.  The reactance of the inductor is so chosen that it offers high reactance to high frequency signals.  Thus, high frequency voltages are blocked.  At low frequency, the reactance is low and thus low frequency signals are allowed to pass.

Another method is to connect a capacitor in parallel to the input.  The capacitor provides low reactance to high frequency signals which are shorted across the input.  The low frequency signals see a high reactance in the capacitor and they alone reach the output.  The series resistance serves to limit the current.

Air Termination Rods are used to provide lightning protection to parts of a building which protrude from the superstructure.  Air termination rods can fixed to terraces, beams, etc.  Air termination rods consist of a rod or any pointed surface which is connected to the lightning protection system of the building. 

Air Termination Rods are also used for equipment which are kept in exposed flat surfaces such as solar panels and pipelines. 

Air Termination Rods may need to be supported against winds by means of suitable support structures.

Bonded Connections are used to connect a conductor to another body such as a pipe, gate, door or a railing.  All metallic objects in a building or a facility need to be connected to the earth.  This is necessary to prevent accidental voltage getting induced in them due to contact

Bonded Connectors are usually galvanized to prevent corrosion.  They have flat surfaces which increase the contact area with the object to be bonded.  They are made of a material similar to the conductor material such as copper or aluminium. 

Voltage limiting devices are used in Electric Traction systems, to prevent dangerous voltages from appearing the insulated tracks and the earthed components of the installation. 

Overvoltages can occur due to lightning or due to short circuits.  Voltage limiting devices typically use a MOV (metal oxide varistor) and and air gap mechanism to conduct the high voltage impulse to the ground.

If case of minor overvoltages, the MOV operates and diverts the surge.  It then returns to its normal non-conducting state.  In case of severe overvoltages, a permanent short circuit occurs between the protective electrodes and the device has to be replaced.

Recurrent Surge Oscillation is done on the windings of large generators such as turbo alternators.  The Recurrent Surge Oscillation Test (RSO) helps identify shorts in the winding.

Shorts in the winding occur as the insulation between turns deteriorates and fails.  Shorts can cause localised heating and arcing which can further damage the alternators.  Shorts can also become earth faults in course of time.  Multi-turn shorts can also result in a drop in the voltage.

The Recurrent surge oscillation tests is done by sending voltages of low voltage and high frequency through the winding and checking the waveform at the other terminal.  If the waveform has suffered any distortion, it may indicate an abnormality.  The waveform can give information such as the location of the fault and its severity.

Some short circuits may not be obvious when the rotor is at rest.  The conductors will come in contact with each other only during the running condition, due to the centrifugal force.  To identify such faults, the rotor is made to rotate and the test is conducted.

The Half Wave Rectifier functions using a single diode.  It rectifies only half of the sine wave.  Half Wave Rectifier

During the positive half cycle, the diode D1 is in forward bias and current flows to the load.  In the negative half cycle, when the voltage is applied in the opposite direction, the diode is in reverse bias and no current flows.

The Half wave rectifier is a simple device which requires only one diode to be put in series with the load.

The half wave rectifier has a low power output. 

Besides, the ripple content of the rectified DC supply is very high.  This can damage the loads.

It has very few applications and is only used in emergencies as a temporary measure.

Full Wave Rectifier with Centre Tapped Transformer

This Full Wave rectifier has only two diodes.  Each diode conducts during one half cycle.  In the first half cycle, diode D1 is forward bias and the current flows into the load and returns through the centre tap of the transformer. 

During the negative half cycle, diode D2 is in conduction.  The current flows through the load in the same direction and returns through the centre tap.  Thus the current is in the same direction through the load.

This means of rectifier has generally been replaced by the bridge rectifier as the centre tap is not always available.

Controlled Rectifiers are rectifiers which have thyristors in place of a diode.  A thyristor is a three terminal device which can be switched on by applying a suitable gate voltage.  Controlled Bridge Rectifier

In a Controlled Rectifier, generally two of the diodes are replaced with a thyristor.  The thyristor enables the enables switching on the output of the rectifier at the desired time.  This allows the output voltage and current to be controlled.

A rectifier with two thyristors is called a half controlled rectifier while a rectifier where all the diodes have been replaced with thyristors is called a fully controlled rectifier.

Controlled Rectifiers are also known as converters.

A Bridge Rectifier is a very popular and widely used circuit.  The Rectifier converts an AC supply into a DC supply.  The circuit requires only four diodes. 

The four diodes operate two at a time.  That is, during the positive half cycle, diodes D1 and D4 are in forward bias and conduct.   Diodes D2 and D3 are in reverse bias. Circuit Diagram Bridge Rectifier

In the negative half cycle of the AC supply, diodes D2 and D3 are in conduction while D1 and D4 are in reverse bias. 

Thus the current from the rectifier flows in only one direction.

Bridge rectifiers are used in Alternators for excitation of the field.  They are also used in welding machines for DC welding and in battery Chargers

Bridge Rectifiers are also used in measurement circuits to measure the amplitude of an alternating signal. 

Controlled rectifiers contain thyristors instead of diodes.  This enables control of the output supply.

The Class B Chopper is typically used in applications which require transfer of power from the load to the source.  An example would be regenerative braking in trains, where the power from the driving motor is sent to the power mains.  This is also known as inverting operation. Class B Chopper_Circuit_Diagram

In the Class B chopper, the output voltage is positive while the output current is negative.

In a class B chopper, a diode, in reverse bias,  blocks power from the source to the load.  The chopper is connected parallel to the load and the source.  The load voltage is the back-emf of the winding of a DC motor.  When the chopper is in the ON condition, the current due to the back-emf flows through the inductance and the resistance through the chopper.  The diode does not conduct as the voltage across the chopper is zero as the chopper is 'ON'. No Current flows into the source.Chopper B Graph

When the chopper is switched OFF, the voltage across the chopper increases and this biases the diode in the forward direction.  The diode conducts and the power reaches the source.  The source may be a battery or any other power source. 

There are wide range of choppers which are used for different applications.  These circuits differ in the voltage level, method of functioning and the output waveform.

Choppers can be classified into the following types

Step Up or Step Down Choppers

Step up Choppers, as the name suggests, step up the voltage.  These choppers are used when the voltage has to increased to a higher level.

AC and DC Choppers

Choppers can be classified into AC and DC choppers depending on the supply

Circuit Operation

On the basis of Circuit Operation, Choppers can be classified into

  • First Quadrant
  • Second Quadrant and
  • Fourth Quadrant

On the Basis of Commutation

  • Impulse Commutated Choppers
  • Voltage Commutated Choppers
  • Current Commutated Choppers
  • Load Commutated Choppers

Depending on the Direction of Current

Class A

Class B

Class C

Class D and

Class E

In recent times, Choppers are usually classified based on their application such as switched mode power supplies, Class D Electronic Amplfiers, etc.

A Chopper is an electronic circuit which controls or reduces a dc supply.  Their function can be compared to an ac transformer.  In an AC transformer, voltage is controlled by changing the turns ratio of the transformer.  In a chopper, voltage is varied by connecting and disconnecting the load from the source many times in a second. 

The chopper is essentially a switching circuit which switches off and on many times.  The output of a chopper is a square wave form while the input is a unidirectional dc waveform. 

Choppers can be used in motor speed controls.  They are increasingly being used in electric automobile technology. 

Choppers are used widely in Electronics in circuits in solar power conversion, speed control of motors in the industry.  They are used to reduce DC voltage to different levels in machines and other electronic equipments

Choppers have high efficiency and can be designed to have very fine control.

Electrical conduction in materials occurs due to the free electrons which drift about the atomic lattice.  In an atom, the electrons in the outer most orbit are called the valence electrons.  If the electrons have sufficient energy , they can break free of the atom and flow through the lattice when a voltage is applied.

If the energy levels are graphically represented, we will get a band diagram.

In the Band Diagram, there is the box representing the Conduction band and the box representing the valence band. 

Valence Band

The Valence band is the range of energy levels of the electrons in the outermost orbit of the atom. 

Conduction Band

The conduction band is the range of energy levels all electrons which are involved in conduction. 

In conductors, the valence and the conduction bands overlap.  In  Insulators, the valence and conduction bands are far apart. 

In semiconductors, the distance between the valance and the conduction bands are small.  When external energy in the form of heat or light is applied to the semiconductors, the electrons get excited and jump from the valence to the conduction band. 

The difference between the valence and the conduction band is called the energy gap.

Germanium is used in specific applications such as communication, spectroscopy, etc.  They have largely been replaced with silicon.  Germanium diodes are more expensive compared to silicon.

Germanium diodes have a lower forward bias voltage compared to silicon 0.15 volts.  This enables the use of the Germanium diode at low voltages where silicon cannot be used.

Germanium is also used in photoelectronics application.  Germanium diodes have a smaller band gap  0.66 eV.  This means that the electrons can be excited even by near-infrared radiation. 

Germanium is used in solar cells to capture the energy in near-infrared regions of the light spectrum.  Germanium based sensors are used spectroscopy to detect light radiation at low frequencies.

Silicon diodes are diodes in which the P and N materials are made of silicon.  Silicon Diode have a a forward bias voltage of 0.7 volts.  That is, the diode conducts when the voltage across the it in the forward bias is 0.7 volts or greater. 

They are the most widely used diodes in the industry.  Other diodes such as Germanium diodes are used at voltages below 0.7 volts.

The diode can withstand a voltage of 50V or more in the reverse direction.  This is known as the peak inverse Voltage.

Silicon is the most popular and widely used of the semiconductors.    There are many factors which have made silicon the material of choice in the world of electronics.

Some of the advantages are

  1. Silicon is abundant.  Hence, it is also economical.  The extraction process form its ore is cheaper when compared to other materials. 
  2. It is strong and easy to handle.
  3. It forms a nice stable oxide.
  4. Doping is easy.  Both P type materials and N type materials can be formed.
  5. It can be easily cut into wafers.
  6. It has good mechanical strength. Hence, designing circuits in silicon is easy.
  7. Silicon has fewer free electrons in room temperature.  This means that the collector cut-off current in transistor is lower than in other semiconductor materials, such as Germanium.

When a P type material and a N type material are brought in contact with each other, some of the holes in the P material migrate to the N region and combine with electrons.  Similarly, some of the electrons of  N material migrate to the P region and combine with holes. 

Thus, at the point of contact of the P and N materials, a layer is formed which has no majority charge carriers such as holes or electrons.  This region is called the depletion region as the region has been depleted of its charge carriers. 

The depletion region behaves almost like an insulator.  When a voltage exceeding the barrier potential is applied across the PN junction, current starts to flow.

Barrier Potential in a PN junction refers to the potential required to overcome the barrier at the PN junction.

When a P material and N material are brought in contact in a junction, some of the electrons of the N material near the junction cross over to the P material.  These electrons combine with the holes in the P material.  Similarly, the some of the holes of the P material near the Junction cross over to the N material and combine with the electrons.

The region in the contact area is thus depleted of holes and electrons.  This region is called the Depletion Layer.    The majority charge carriers are absent in this region.  This region almost becomes like an insulator.  Thus, there is no conduction after the depletion layer is formed.

For current to flow through this layer, a specific voltage has to be exceeded.  This is known as the barrier potential.  When an external voltage greater than the barrier potential is applied, the PN junction conducts.  

P type Materials

The P type material is obtained when a semiconductor is doped with a trivalent impurity such as Aluminium or Boron. P type material is a material which has holes as its majority carriers.  Electrons are the minority Charge Carriers in P type materials.  When a trivalent impurity is added to the crystal lattice of a semiconductor, there is a vacancy for every impurity atom added.  This vacancy is called a hole.

N type Materials

N type materials are made when a semiconductor is doped with a pentavalent impurity.  A pentavalent impurity is one whose atom has five electrons in its outermost orbit (valence electrons).   Examples of pentavalent impurities are Phosphorous, Antimony, Bismuth.  In an N type Material, electrons are the majority charge carriers while holes are the minority charge carriers.

When a semiconductor is doped with a pentavalent impurity for every impurity atom added, there is a free electron.  These electrons are responsible for conduction.

When the PN junction is formed, there is movement of the charge carriers across the junction.  The electrons move from the N material across the junction into the P material.  The holes from the P material cross into the N junction.

This movement of charge carriers results in a current across the junction. 

This current is known as diffusion current.  This current occurs in the absence of potential.

Cable Sleeves are used to enclose a single wire or a group of wires.  Cable sleeves are used to arrange a set of wires going through a panel or to an equipment like a motor.  Cable sleeves are made of different materials from braided metals to rubber and even kevlar. 

Sleeves serve to protect the wires and their insulation from sharp edges.  They can protect wires from UV radiation, moisture, oil and temperature.  cable sleeve

In automobiles, special heat resistant sleeves are used to protect the wires from hot surfaces. 

Teflon sleeves have great cut resistance.  Nylon sleeves have great abrasion resistance. 

Sleeves are available in two broad categories depending on the method of installation. 

The first is the slit sleeve which is cut and the wires are inserted into the sleeve.  The second is the wrap-around, side entry type of sleeve. 

Heat Shrinkable Sleeves

Some sleeves are heat shrinkable.  Heat shrinkable sleeves are made of a polymer which contracts when heat is applied. A stream of hot air from a blower is passed on the sleeve.  This results in the sleeves shrinking and wrapping the wires.  

Electrical Safety Mats are important safety equipment.  Safety mats protect personnel from electric shock by providing an insulated surface to work on.  If the worker comes in contact with a live conductor by mistake, he will not get an electric shock as his feet are insulated from the ground by the safety mat.Electrical Insulation Mats

Safety mats come in different colours.  They are usually made of rubber.  The surface of the mats is ribbed to provide an anti-slip surface to workers. 

Safety mats are also designed to resist aging and ozone. 

Safety mats should also be age and fire resistant.

Safety mats come in different voltage ratings.   They should also be resistant to acids and oils. 

The following is the list of mats and their working voltages


Class of Mats Working Voltage Colour
0 1000 Red
1 7500 White
2 17000 Yellow
3 26500 Green
4 36000 Orange


The most common class of safety mats is the type 0 which is rated for a working voltage of 1000 V.

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A synchronous phase modifier is a synchronous motor which is not connected to any load.  The motor can be made to behave like an inductor or a capacitor by decreasing or increasing the excitation. 

Synchronous Phase modifiers are used to regulate the voltage in transmission lines.

Series Capacitors Shunt Capacitors
Used to reduce the line reactance Has no impact on the line reactance
Voltage rise only occurs across terminals.  Can be used to raise the line voltage
No effect on system stability Improves system stability
Does not affect the power factor Used to improve power factor

The ratio of capacitive reactance to the inductive reactance in a transmission line is called the percentage compensation of the Transmission Line.  The inductive reactance is mainly caused by the nature of the load.  The capacitive reactance is caused by the line capacitances and by series capacitors, if any.

Submarine Power Cables are used to carry power underwater.  They are used to connect small islands with the mainland.  Sometimes, the route through the water is shorter than through land.  Submarine Cables are used for both AC as well as DC transmission. 

AC is used up to 80 km while DC is used for long distance transmission.  Submarine cables are mostly gas filled cables.  The cable laying is done by special ships.  XLPE cables fpr submarine transmission are also available.

AC cables can be laid as three separate cables or as a single 3 phase cable.

Offshore wind farms are connected to the mainland by submarine cables.   

Damage to these cables by shipping is a matter of concern.  The routes which contain submarine cables are specially marked in nautical maps.

Gas filled cables are used in many applications.  In these cables, the insulation is filled with gas, usually nitrogen.  The inert gas fills up the voids which may form in the insulation.  There are low pressure and high pressure variants in this type of cable. 

The gas is filled in the insulation and is enclosed by means of a sheath.  Since Nitrogen is easily available, any losses can be easily replaced during installation. 

However, the gas has a low dielectric strength.  It cannot be used at very high voltages.  Gas filled cables can be used up to 275 kV.

Oil Filled cables have almost replaced Gas filled cables in recent times.  However, there are situations where oil filled cables cannot be used such as undersea applications and in hilly terrain.

High Voltage Dielectrics are used for insulation of cables at high voltages.  These dielectrics are subjected to very high stresses and temperatures.  They also have very demanding reliability requirements. 

Some of the common properties of High Voltage Dielectrics are

  1. Dielectrics should be flexible as this reduces the bending radius.  A lower bending radius enables cables to take turns in small spaces. 
  2. The Permittivity should be low so that the charging current is as low as possible
  3. The power factor of the dielectric should be low so that the heat generated is minimum.
  4. The Impulse strength of the insulation should be high.

Impregnated Cables are cables which have insulation which is impregnated with an insulating material.  For instance Paper insulation can be impregnated with oil or synthetic fluid with good dielectric properties.  The impregnated insulation is enclosed in a lead sheath to prevent the ingress of water. 

Some  cables have insulation which requires the insulating compound to be maintained at a particular pressure.  This may require the installation of pumps at periodic locations. 

Oil impregnated cables have special channels for the flow of oil.  The oil which is pressurized fills up any void which may develop in the insulation. 

Modern Impregnated Cables do not require pumps or any pressurizing devices. 

Oil impregnated cables can have their own cooling systems if used at voltages above 525 kV

Dust in electrical Equipment can affect the operation of the equipment.  Dust can result in tracking which causes current to flow over the insulation eventually leading to failure.  Dust on the surfaces of conductive parts such as breaker poles or relay contacts can affect the conductivity and cause heating and voltage drops

Dust when combined with oil or water in the atmosphere can form acids which can corrode and weaken the insulation. 

A layer of dust on the surface of an equipment can affect the heat transfer and prevent adequate cooling. 

Hence, equipment which are to be installed in dust prone environments should be adequately dust proof.  Panels should have filters which prevent the entry of dust. 

Tracking is a phenomenon in Electrical Insulation.  Tracking refers to the flow of current over the surface of the insulation.  Tracking causes heating which results in damage to the insulation.  As current gets a path to the ground or another energized conductor, tracking can lead to a flash over. 

Tracking is caused by many factors.  One principal reason is dust. Dust forms a layer over the insulation, this when combined with moisture in the atmosphere provides a conductive layer for the current to flow.  Other factors which can cause tracking are humidity which can cause condensation, temperature, air pressure, etc. 

Tracking can be prevented by ensuring proper creepage distance.  Equipment should be kept at the right temperature and humidity. 

The aging of electrical equipment is caused principally by the aging of the insulation.  The insulation made of materials such as rubber, paper or polymers deteriorates over a period of time.  The molecules of the material break down.  The material loses its mechanical and electrical properties.  It finally gives in, leading to failure. 

Aging of insulation in Electric machines has a number of reasons.  Some of them are

  1. Partial Discharges
  2. Tracking
  3. Treeing
  4. Electrolysis
  5. Dielectric Heating
  6. Space Charges


Aging of Electrical Insulation can be monitored by periodically testing the insulation such as Insulation Resistance, Polarization Index, Tan Delta test, Surge Test, etc.

V/Hz ratio is the ratio of voltage to Frequency in Electric Machines.  In Electric Machines such as motors, generators and transformers, the drop in frequency results in a decrease in the reactance of the stator winding and the core.
In a motor, when the frequency drops, the stator reactance reduces.  This causes a higher current to flow for the same voltage.  This can result in increased flux and increased torque. 
In the machine core, the reduction in reactance can cause more eddy currents to flow.  This can result in over heating and damage to the laminations in the core of the stator.  This can permanently damage to the machine. 
This is a condition known as overfluxing. 
Overfluxing protection is an important part of any protection scheme. 
V/Hz is a mode of control in AC drives.  For a motor to run at constant torque, its flux should be constant.  This is achieved by the V/Hz control.  Since the flux can change when the frequency drops, the voltage also reduces during reduced frequency.  This ensures that the flux in the stator field of the motor is always constant. 

Constant Torque Operation refers to the operation of the motors at a fixed torque value.  The torque supplied by a motor is dependent on the load.  However, a motor which has a constant flux is considered to be running at constant torque.

For a motor to run at constant torque it has to be driven by an AC drive.  AC drives are able to vary the frequency and the voltage such that a constant V/Hz value is obtained.  The V/Hz is the ratio of voltage to the frequency in an electric machine.

When the frequency of the motor is adjusted, the stator reactance changes.  This reduces or increases the stator current.  To correct this, the voltage has to be adjusted. 

The conveyor is an example of an application which requires a constant load. 

Derating refers to the operation of equipment at reduced capacity or speed.  Derating in motors can be caused due to the following reasons.


When frequency increases, the speed increases and the torque decreases.  If the frequency increases by 5%, the speed increases by 5% while the torque decreases by 10%. 


Voltage has a direct relation with torque.  When the voltage falls the torque also reduces.  Equipment has suddenly rotate faster or move faster due to voltage fluctuations.


As the altitude increases, the density of air decreases.  This reduces the ability of air to transfer heat and cool the motor.  Thus if the motor is to be operated above 1000 metres above sea level, it has to be derated.

Ambient Temperature

The ambient temperature is also a factor in derating.  If the ambient temperature is high, the insulation may reach its maximum temperature limit quickly.  Hence, the motor may have to be derated.

The Locked Rotor Amperage is a specification which specifies the current in the rotor of a motor when it is locked. 

When the motor is a standstill and the power is given to the terminals, the current is maximum in the rotor. As the speed increases, the current drops.  The Locked Rotor Amperage is typically 3 to 8 times of the Running Current Amperes.

The LRA is also dependent on the input voltage during start up.  As such, the Locked rotor Amperage value is used to select the type of starter for the motor.

The Totally Enclosed Fan Cooled Motor is the most widely used type of motor.  It consists of frame which totally encloses the motor.  Thus there is no airflow between the inside of the motor and the outside.  However, the frame is not airtight.

The term "Totally Enclosed" means that the motor is dust proof.  It is however not submersible.

Cooling occurs by means of a fan which blows air over the frame.  The air which passes over the frame removes the heat created by the motor.  The enclosure has fins to maximize the surface area to enhance cooling. 

Open Drip Proof Motors(ODP)  are motors which are used in applications where there may be dripping water.  These motors are covered by a metal enclosure which ensures that any water which drips will not flow into the motor. 

Drops of water which fall at an angle of 15% will be caught by this enclosure.  Open drip proof motors usually have a fan for cooling the motor.  These motors are relatively inexpensive and used widely in the industry.

Open Drip Proof motors are used in clean, dry environments.  Sometimes, the air from the blower is made to pass through a filter before entering the motor

Puncture Voltage

The voltage at which the insulator punctures is called the puncture voltage.  When the puncture voltage is exceeded, the insulator is permanently damaged as the current flows through the inside of the insulator.

Creepage distance

The creepage distance refers to the distance over the surface between the conductor and the ground.  The creepage distance in insulators is increased by corrugating the body.  The creepage distance depends on the voltage.  Higher voltages will require higher creepage distances. 

Flashover Distance

The Flashover distance is the minimum distance below which a flash over can occur. 

Flashover Voltage

The voltage above which a Flashover can occur is called the Flashover voltage.

AC Servomotors DC Servomotors
Suitable for Low Power applications Used for High Power Applications
They have low efficiency They have high efficiency
The operation is stable and smooth Noise is produced during operation
It has low Maintenance as there is no commutator Relatively more maintenance is required due to the presence of the commutator.


Creepage refers to the minimum distance between two conductive parts through the surface of the insulation.  Creepage is an important characteristic as reduced creepage will result in tracking along the surface of the insulation. 

Tracking refers to the flow of current along the surface of the insulation.  Tracking causes localised heating and carbonization of the surface.  This, eventually, leads to failure of the insulation. 

The Comparative Tracking Index (CTI).  The comparative tracking index gives the minimum voltage which can cause tracking across the surface of an insulation.  Creepage also depends on contamination of the surface, humidity, corrosive chemicals and the altitude in which the equipment is installed. 


Clearance refers to the shortest distance through air between two conductive parts.   If the Clearance is less than required, it will result in flashover due to dielectric breakdown in air.  Dielectric breakdown is dependent on other factors as well such as humidity, contamination, altitude and temperature.

The creepage is calculated based on the value of the rms voltage while the Clearance is calculated based on the peak voltage.

Altitude is very important in the selection of Electrical Equipment.  Electrical Equipment in General are designed for an altitude up to 1000 m above sea level.  At higher altitudes, the density of air decreases.  This results in a decrease in the dielectric properties of air.  The dielectric property of air depends on the density, temperature as well as pressure. 

At high altitudes, all three parameters decrease.  Thus the insulation provided by air decreases.  The insulation coordination should be reviewed.

Hence, there is a risk of flashovers and arcs.  Circuit breakers and other current interrupting devices should be derated at high altitudes. 

The operating altitude should be kept in mind when designing equipment. 

Another limitation caused by altitude is in heat dissipation.  As the air grows less dense, the heat carrying capacity of air decreases.  This results in overheating of the equipment.  Thus equipment and components may require larger heat sinks or cooling equipment with increased capacity. 

  Solid State Relays Electromechanical Relays
Noise No Noise Noise due to contact switching
EMI No EMI as no switching is involved EMI due to arcing during switching
Zero Voltage switching Possible Not Possible
Resistance to shock They are resistant to shock and vibration These relays can be affected by shock and vibration.
Temperature sensitivity Sensitive to Temperature less sensitive to temperature
Heat Generation Heat is generated due to higher resistance Low resistance in the contacts. Hence, less heat is produced
Heat dissipation Heat Sink is required Heat sink is not required.
Leakage Current Solid State Relays do not completely turn off.  There is a slight leakage Current.  This can affect downstream circuits. No leakage current.  The relays can be completely switched on or off.
Lifetime very long lifetimes. Relatively short lifetime

Reduced Voltage Starting in Motors refers to the practice of reducing the motor at a reduced voltage.  This is done to limit the heavy inrush current during starting.  The voltage is reduced by connecting resistors or reactors in series. 

Soft Starters can also be used to reduce the voltage during starting. 

The disadvantage of using reduced voltage starting in motors is that as the voltage is low during startup, the torque is also low at starting.  This method of starting may not be suitable for applications such as conveyors, lifts, etc which require the motor to start with the load. 

Types of Reduced Voltage Starters are

  1. Resistance Starters
  2. Reactance Starters
  3. Autotransformer Starters
  4. Soft Starters
  5. Star Delta Starters

Oil Directed Air Forced Cooling is an updated version of Oil Forced Air Forced Cooling (OFAF).  IN ODAF, the oil is directed through special channels in the core and close to the windings.  This makes the cooling more effective.  The hot oil is then passed through the fins which are cooled by fans. 

Oil Directed Air Forced Cooling is used in transformers with large capacities. 

Oil Forced Water Forced Cooling of Transformers is a method of cooling Transformers in which oil is forced through the transformer by means of pumps.  The oil which is thus forced through the transformer collects the heat from the transformer.  This oil is then passed through a cooler (heat exchanger) which has water as the other medium.  The water which collects the heat from the oil is pumped to a cooling tower where the heat is dissipated.

The OFWF method of cooling is similar to OFAF (Oil Forced Air Forced) method of cooling. 

When a temperature is at no load, its temperature is slightly greater than the ambient temperature.  When the transformer is loaded the temperature rises.  The temperature rise rating of a transformer gives the maximum value to which the temperature of the transformer would rise. 

Dry type transformers are usually available in three standard temperature rises, 80C, 115C or 150C.  Liquid filled transformers have ratings of 55C and 65C.  These values are based on a reference value of 40C.

For instance, a transformer with a temperature rise rating of 80C will reach a maximum temperature of 120 C (40+80) during operation. 

The lower the temperature rise rating, the better is the ability of the transformer to withstand momentary overloads.  Thus a transformer with a temperature rise rating of 80C will have a better overloading capacity than a transformer with a temperature rating of 120C.

The temperature rise rating gives and idea of amount of heat produced and the amount of heat removed.  Transformers with lower temperature rise ratings use windings with lower resistivity. 

Transformers with low temperature rise are used in special applications such as in underground installations, air conditioned buildings

Asynchronous motors are motors which run below the synchronous speed.  The term "asynchronous motors" is usually used to refer to induction motors.  

Motors which run at super synchronous speed are also asynchronous motors.  The doubly fed induction motor is an example of a super synchronous motor.

By "asynchronous" we mean that the speed of the rotor is not equal to the synchronous speed of the rotating magnetic field of the stator.  The difference between the speed of the rotor and the speed of the stator is called slip.  In asynchronous motors, the slip is not equal to zero.  In synchronous motors, the slip is zero.

  Induction Motors Synchronous Motors
Speed The speed varies below the synchronous speed and can be adjusted The speed is constant and rotates at the synchronous speed
Construction Simple and Rugged Construction A slightly complex construction
Starting It is self starting Special Starting Procedures are required
PF control Cannot be used for PF improvement Can be used for PF improvement
Excitation Does not require external excitation Requires DC excitation for rotor
Efficient Less Efficient More Efficient
Cost Cheaper More expensive

Feature Open Loop Control System Closed Loop Control System
Effect of Output on Input No effect on input The input signal affects the controller output into the system
Stability Very Stable The response changes as the input signal changes.
Response to external disturbances No reaction to disturbances.  The Open Loop control works on fixed output The output of the controller adjusts itself in response to the input signal
Ease of Construction The controller is easy to construct Controller is difficult to construct as it is complex
Cost Cheap Expensive
Bandwidth Small Bandwidth Large Bandwidth
Maintenance Low Maintenance More Maintenance is required. 
Feedback There is no Feedback Feedback is always present. 

The requirements of an effective control system are


The control system's response should be swift.  This is necessary to ensure that the process variables do not stray away from the set point which can affect the process. 


The number of oscillations should be as low as possible.  Oscillations can cause disturbances in the system.  The damping of the system should be optimum.


Accuracy refers to the ability of the system to detect even the smallest deviation and initiate a corrective response.   The sensors and the error sensing algorithms should have a high resolution.


Noise refers to undesirable interferences in the signal of the transducer.  There are many reasons for noise.  Electromagnetic Interference (EMI) is one of the reasons.  A good Control system should be able to filter the noise and process only the principal signal. 


Any control system should be stable about the set point.  It should maintain the process variable at the set point.  It should not react to changes in the surroundings, temperature or any other external parameter.  It should react only to the principal input signal. 


Bandwidth is dependent on the frequency of operation.  The Bandwidth should be as large as possible for a good frequency response of the system.

Open Loop and Closed Loop Control Systems

Open Loop Control

Open Loop Control is a type of control in which the output is not given as feedback to the controller.  The Input is totally independent of the output of the controller.   Thus the control regulates the process without taking into account the state of the output.  Examples of Open Loop controls are Traffic signals at intersections, Bread toaster, Volume control in a Television set, etc

Closed Loop Control

Closed Loop control is one in which the output is given as feedback to the input. Thus the input is increased or decreased depending on the output.  This ensures that a set point is maintained.  Examples of Closed Loop Controls are Automatic temperature control in an air conditioner, level controller in boiler, a voltage stabilizer, etc. 

Manual and Automatic Systems

Manual Control System

A Manual Control System is one in which the process is manually controlled to achieve the set point.  An example would be a tank with a pump.  If a person switches on the pump every day to fill the tank to a particular level and stops the pump once the level in the tank has been reached, it is called manual control

Automatic Control System

In Automatic control, the process is automatically controlled.  If the pump has a timer which switches on the pump for 15 minutes every day to reach a certain level in the tank, the control system can be said to be automatic. 

Both open loop and closed loop control systems can be manual or automatic.

Protection Relays serve to protect equipment and circuits from abnormalities such as overcurrent, overvoltage, overloading and under reactance.  The input to the protection Relays are the current and the voltage of the system.  Using these two basic parameters, the relays are able to calculate a host of values such as kW, kVAr, pf, etc.

Thus, every protection relays needs an input from the Potential Transformer or the current transformer or both.  When the relay is connected to the circuit of an instrument transformer, it becomes a load.  The Potential or Current Transformer acts as the source.  

When designing the Protection Scheme, we must ensure that the Potential Transformer or the Current Transformer does not get overloaded.  This is done by adding the VA burden of each protection relay in the system.  Every Relay will have the VA burden mentioned in the manual. The total VA burden imposed by all the relays should be calculated.  The VA capacity of the instrument transformer should be greater than this. 

While designing a system, the instrument transformer should have an excess capacity of 10% of the present load.  This is an allowance for future relays which may be added to the system. 

If the transformer is overloaded, the voltage and current signals will not be accurate. 

The wires used in the protection system should be of sufficient thickness so as not to added unnecessary burden on the transformers.

Heat Sequencers are switching devices used in Electric Furnaces.  Sequencers work by sequentially switching the heating elements on or off.  electric furnace sequencer

If all the heating elements in a furnace were to be switched on at the same time, they would trip the circuit breaker. 

The sequencer avoids this by switching the elements in sequence.  The sequencer consists of a thermostat.  The thermostat senses the furnace Temperature and activates the elements. 

The thermostat sends a current to a small element in the sequencer.  This elements heats up a bimetallic strip which closes and supplies power to the heater element.  When the supply to the heater coil is cut off, the bimetallic strip cools and open the circuit.  The heating element is thus switched off.

The sequencer is thus able to switch on and off the heater by means of bimetallic strips. 

Microwave ovens have become and indispensable part of our kitchen.  These useful appliances are able to heat and cook food quickly and efficiently.  They are compact and clean. 

Microwaves work by heating the food with microwaves.  Microwaves are waves which are situated between the infrared and radar waves in the electromagnetic spectrum.  microwave

The microwaves are generated by a device called a magnetron.  The magnetron generates the microwaves at a frequency of 2.45 GHz and a wavelength of 12.2 cms.  The waves generated by the magnetron are focused at the food by means of a waveguide. 

The magnetron is powered by a high voltage transformer.  A microprocessor controls the entire process while a fan cools the components.

The Microwave oven works on the principle of dielectric heating.  A dielectric is a material which can be polarized.  All food materials contain water.  The water molecule has charge.  When it is placed in an alternating electric field, the molecule moves back and forth as the direction of the electric field changes. 

This produces heat which cooks the food. 

The food is placed in a cooking chamber,  The chamber is enclosed in a metallic cage which forms a faraday cage.  This is to ensure that the radiation does not come out. 

Microwave cooking is efficient as the food alone gets heated.  The heating is uniform, unlike conventional cooking where the heat travels from the outer surface of the food to the inner parts.  This eliminate localized overcooking. 

Microwave cooks at a lower temperature.  This prevents the formation of tars and char in the food. 

Incoloy is an special alloy.  It is used in high temperature applications and is known for its corrosion resistance.  Electric Heaters are sheathed with Incoloy as it offers corrosion protection at high temperatures.  The Incoloy sheath encloses the heating element such as nichrome.

Incoloy is composed of Nickel, Iron and Chromium.  There are small additions of Molybdenum and copper as well. 

Incoloy has excellent corrosion resistance properties.  It resists stress corrosion as well as localised pitting.  It is resistant to both reducing and oxidising acids. 

Such chemical resistance is required for electric heaters as the medium they will be heating can be corrosive.  Incoloy protects the heating elements from damage from corrosive media.

Electric Ovens are used in industries to dry products.  They are an important part of many industrial processes.   industrial oven

Ovens are used for curing, for heat treatment, for drying and for many other industrial processes.

There are different types of ovens used in the industry such as Cabinet Ovens, Conveyor Ovens and Truck-In Ovens.

Ovens are categorized into Gravity Convection and Forced Air Circulation Models.  In Gravity Convection Ovens, air circulates within the ovens by means of convection.  In Forced Air circulation models, Air is drawn from the outside and circulated inside the furnace. 

Furnaces are available in a wide range of temperature ranges.  Furnaces are generally heated by an electric heating element.  The elements are made of materials such as nichrome.  Temperature Controllers regulate the temperature in the oven.

Rotating Diodes are diodes fitted in the rotor of the synchronous machine between the excitor and the main field winding.  The function of the Rotating Diodes is to rectify the AC output of the excitor into DC which rotating diodecan be used to magnetise the main field windings and poles.

These diodes are connected in the shape of a normal three phase rectifier with six diodes (sometimes twelve diodes are used).  The Diodes are called Rotating diodes as they are mounted on the rotor which is rotating.  The diodes do not rotate by themselves. 

The DC output fromrotating diode block the rectifier assembly is usually collected from two rings in the assembly.  The Diodes are protected by a Varistor against voltage spikes during sudden load fluctuations.

The synchronous machine is used widely in industry both as a generator for power generation as well as a motor for driving industrial equipment such as conveyors, pumps, crushers etc.

The synchronous machine has the following parts


The Stator of the Synchronous Machine consists of the Core.  The core is made of electrical steel.  It is made in the form of insulated laminations which are stacked together.  This prevents the flow of eddy currents in the core.  The core has slots.  The windings of the core are placed in the slots. The windings are made of copper.  The winding are preformed and placed in the slots.


The rotor of the Synchronous Generator consists of a number of poles.  The number of poles depends on the speed and frequency of the machine.  The relation between the number of poles and speed or the frequency is N=120 x f /p ( where N is the speed in rpm, f is the frequency and p is the number of poles).

The rotor of the synchronous machine can be either salient or non-salient in construction.  In salient pole rotors, the poles are protruding from the rotor while in non-salient pole construction, the rotor windings are placed in slots machined in the rotor (See article on Salient and Non-salient Rotor Construction).Alternator Block Diagram


The excitor can be imagined to be a small generator placed in the rotor.  The excitor provides the excitation power for the the excitation.  The excitor consists of a field winding in its stator.  The armature winding of the excitor is placed in the the rotor of the machine. 

Rotating Rectifiers

The output of the excitor is a 3 phase AC supply.  This supply is rectified by means of the rotating rectifiers which are fitted on the rotor shaft.  This rectifier assembly converts the AC power into DC.  The rectified DC is then supplied to the main rotor windings. 


When the machine is first started, residual magnetism in the excitor field winding induces a voltage in the excitor stator.  This 3 phase supply is rectified and fed to the field windings of the main rotor poles.  These poles get magnetized and induce the output voltage in the stator.

The control of the machine output voltage is done by a device called the AVR ( Automatic Voltage Regulator).  The output voltage of the machine is connected through a potential transformer to the AVR.  The AVR then regulates the excitation input based on the feedback received.  This a closed loop control.

Electromechanical Relays are the oldest type of relays.  Electromechanical, have been gradually been replaced by Static and then by Numerical Relays. 

However, Electromechanical Relays are still being used.

The Advantages of Electromechanical relays are-

  1. They are quick acting and can be reset fast.
  2. They are simple in construction.
  3. They are reliable. 
  4. The values can be easily set.  No special programming device is required.
  5. People can be trained on these relays easily

The disadvantages of Electro-mechanical relays are

  1. The VA burden of these relays is higher than static and numerical relays.  Hence, the Potential and the Current Transformers should have a higher capacity.
  2. These relays require to be calibrated periodically and tested.
  3. These relays suffer from the effects of age.  As time passes, the springs and the linkages inside the relay grow weak.  This causes the setting values to drift.  This can result in maloperation and false trips.
  4. These relays do not have the directional feature. 
  5. The speed of operation is limited by the mechanical inertia of the moving components. 
  6. Multifunctioning is not possible.  One relay can perform only one function.

Common Terms in Relay Protection are

Pick Up Value

Pick up Value of relay  refers to the value above which the relay will generate the alarm or the trip

Operating Time

The Operating Time is the time which is allowed to elapse after the pick up value has been exceeded before the output is given. 

Reset Value

Once the relay operates, it has to be reset.  The relay resets after the measured value falls beyond a certain value.  This value is below the pick up value in case of functions such as overvoltage or overcurrent.  In case of functions such as undervoltage or underfrequency, this value is above the pick up value

Reset Time delay

Reset Time delay is the time taken by the relay to reset after the reset value has been reached.

Reach of the relay

This is a term used in distance protection.  The distance Relay operates when there is fault in a cable.    Reach of the relay refers to the distance till which the relay can sense the fault. 

Electric Protection Relays are vital components in the protection scheme.  Protection Relays protect an equipment such as a Transformer or a Generator from internal and external faults such as overvoltage, overcurrent, earth fault, etc.

Relays have been in existence since the early years of Electrical Engineering.  Relays can be configured for instantaneous operation or delayed operation. 

There are three broad categories of relays based on the principle of functioning. 

They are Electro-Mechanical Relays, Numerical Relays and Solid State Relays

Electro-mechanical Relays

Electro-mechanical Relays are the oldest type of relays.  These relays are simple in construction.  They are easy to adjust.  These Relays consist of a disc, usually made of aluminium, which rotates when there is a fault.  The rotation occurs due to the presence of eddy currents caused by current and voltage coils. 

When a fault occurs, the rotating disc rotates and closes the alarm contacts which generate the alarm.  If the fault is severe or persistent, it closes the tripping contacts which issue the tripping command to the circuit breaker. 

The disadvantages of these relays is that the values tend to drift over time, due to the effects of heat, vibration and aging on the relay components.  These relays are gradually being replaced by numerical and solid State Relays

Numerical Relays

These relays are electronic relays.  They do not have moving parts.  In the Numerical Relay, the analog values are converted into numbers.  The alarm and the trip values are also fed into the relay and stored as digital values.    Numerical relays are also called digital relays.  The microprocessor monitors the field values and generates the alarm or the trip command.  Numerical Relays are programmable.  The behaviour and the characteristics or these relays can be programmed.  Numerical Relays are also multifunctional which means that the same relay can be used for overvoltage as well as overcurrent protection. 

Modern Numerical Relays can also communicate with protocols such as Ethernet, RS 485, etc.  They can store historical data of trends and events.  This feature will be helpful in analyzing a fault condition or a blackout.  Timestamping also enables sequential registering of events.

Static Relays

Static Relays are analog relays.  In static relays, the voltage or the current from the field is converted into rectified voltages and currents.  These values are then processed by means of op-amps, transistors, etc and the output signal is generated. 

Materials are classified based on their response to a magnetic field as Ferromagnetic, Paramagnetic and Diamagnetic materials.

Ferromagnetic Materials

Ferromagnetic materials are materials which have a large susceptibility to magnetic fields.  These materials can get magnetised when placed in an external magnetic field.  They experience strong attraction to magnetic fields.  This behaviour comes from the large magnetic domains in these materials.  In the demagnetized condition, these domains are aligned in different directions.  However, when they are placed in a magnetic field, these domains get aligned in the same direction.  Thus, the material as a whole is magnetized. 

Examples of Ferromagnetic materials are iron, cobalt and nickel. 

Paramagnetic Materials

Paramagnetic materials experience a weak attraction to magnetic fields.  This magnetic field comes from the magnetic moments of unpaired electrons in the materials.  There are no domains in paramagnetic materials.  Examples of paramagnetic materials are magnesium, lithium and tantalum.

Diamagnetic materials

These materials have negative susceptibility.  These materials are slightly repelled by magnetic field.  There are no unpaired electrons in these materials.  All the electrons have paired and hence the magnetic moment of these electrons cancel each other.  Copper, Gold and Silver are examples of diamagnetic materials. 

A reluctance motor works on the principle of reduced reluctance.  Reluctance motors have high power density.  However, their downsides are low efficiency, low torque and low pullout torque.  However, they are ideal for small power applications such as hard disk drive motor, Analog electric meters, etc.

The reluctance motor works on the principle that a piece of iron placed in a magnetic field will rearrange itself such that the reluctance of the magnetic path is minimal. 

The Reluctance motor consists of a wound stator.  The rotor is made of laminated material in which poles are cut so that a salient pole rotor is produced.  The number of rotor poles is made less than the number of stator poles. 

When a three phase supply is connected to the stator, a rotating magnetic field is set up. The rotor tries to align itself in a minimum reluctance path with reference to the magnetic field of the stator.  As the stator magnetic field keeps rotating, the rotor moves along with it. 

Reluctance Motors are classified into

Switched Reluctance Motors and

Synchronous Reluctance Motors

Synchronous Motors are used in the following situations

  1. Applications which require constant Speed
  2. Applications which require load as well as power factor improvement

Synchronous Motors are used in applications which require constant speed.  Examples can be escalators which need to rotate at a constant speed as people get in and get out.  In industries, synchronous motors are used in systems which have to operate in synchronism such as in bottling plants, in robotics and in conveyors. 

Fractional Synchronous motors are used for small applications such as in microwaves, electric clocks and in data storage devices.  Fractional Synchronous motors do not have wound excitation system.  The rotor contains permanent magnets which run in synchronism with the stator. 

Synchronous Motors for Power Factor Correction.

Synchronous Motor operated in an overexcited condition can be used for power factor correction.  The overexcited synchronous motor draws active power from the grid and supplies reactive power.  This helps improve the power factor of the system.  A motor run in this manner is called a Synchronous Condenser