The dc series motor develops a very heavy torque during start-up. The motor relies on the connected load to restrain the speed.

Hence, if the dc series motor is run without any load. it may result in overspeeding which can cause serious damage to the motor .

Single Phase Motors Three Phase Motors
Can be operated anywhere as Single phase supply is Widely AvailableCan be used only when three phase supply is available.
Efficiency is lower as a single winding has to carry all the current.Efficiency is Higher as Three windings carry the current
Torque is not uniformTorque is uniform
Larger in Size for the same kW ratingSmaller in Size
Cost is cheaper at the fractional kW range due to high volume of production Cost is higher at low kW range.
Cost is higher at high kW ratings Cost is cheaper at lower kW range.

When a current passes through a human body, it can cause damage in the form of burns.  The burns can also happen in the internal organs as the current passes through the body.  The resistance offered by the body is only by the skin.  Blood, being an electrolyte,  is a good conductor of electricity.

The normal limits of current and the effects they produce are as follows.

1mA - Slight Tingling Sensation

1mA -6mA- This is the let-go current when it is possible to let going of a live object.

9mA to 25 mA - Loss of Muscular control.  In this range, it is not possible to let go of the live object

>25mA - Muscles controlling breathing are affected.

50 - 100mA - Fibrillation.  The electric rhythm of the heart is disturbed. The heart stops.  This is a life threatening condition.

Semiconductor fuses serve to product semiconductor components such as diodes and triacs. These fuses are made specially to act quickly within milliseconds to isolate the electronic component.  

Semiconductor fuses are made of silver strips.  The cross section of these strips determines the fuse limits.  A number of strips may be connected in parallel to provide the desired fuse rating. The strips are enclosed in a tubing containing quartz which serves to quench the arc. 

Semiconductor fuses are designed to act at overloads of 5 to six times the rated capacity.   These fuses are available at MV ratings with a capacity of several thousand amperes.  

Arc Blow in Welding refers to the phenomenon in which the arc does not trace the shortest path between the electrode and the workpiece.  The arc gets deflected away from the desired path.  This is due to the interaction of the magnetic field of the welding current with residual magnetic field which may be present in the metal.

Arc Blow occurs only in DC welding.  In AC welding, since the current and the magnetic field of the arc reverse direction many times a second, there is no net deflection of the arc.

The Stacking factor of the core refers to the thickness of the laminations in the core.  In Electric Machines, particularly AC machines, the core of the machine is made of a number of laminated steel sheets.  This is to prevent losses due to eddy current.

The stacking factor depends on the thickness of the lamination of the steel sheets which comprise the core.

The stacking factor is usually 0.9.  The stacking factor is always less than 1.

The Net Area of the core which forms the magnetic path can be calculated from the gross area by the following relation

The Net Area of the Core = Stacking factor x Gross Area of the Core

In core type transformers, The LV winding of the Transformer is placed near the core in order to reduce the cost of insulation and the size of the Transformer.  If the HV winding of the Transformer is placed near the core, the insulation would have to be thicker leading to higher cost.

Placing the HV winding after the LV winding enables a much lesser thickness of insulation for the HV winding.

In Shell type transformers, the winding is interleaved.  That is, the LV and the HV windings are placed alternately to reduce the leakage flux.

The RoHS is the Restriction on the use of Hazardous Substances in Electrical and Electronic Equipment.  

This is an official directive by the European Union.   The RoHS bans any electrial or electronic equipment which contains more than the stipulated levels of lead, cadmium, mercury, hexavalent chromium, polybrominated biphenyl (PBB) and PBDE (Polybrominated diphenyl ether) flame retardants. 

related resources

A Transient voltage suppression diode  is a diode which is used to protect a circuit against spikes and transient overvoltages.

Spikes can be caused due to switching, lightning strikes and faults in the circuit.  A spike lasts for a small period of time of the order of microseconds.  However, the energy dissipated in a spike is such that it can damage components in a circuit.  

Hence,it is necessary to protect spikes from reaching the components by shunting through a transient voltage suppression diode.  A transient voltage suppression diode is connected in the reverse bias.  With the anode to the negative and the cathode to the positive.

During normal supply voltage, the transient voltage suppression diode is in the non-conducting state.  

When the voltage rises and exceeds the breakdown voltage of the diode, it starts conducting and the surge is diverted before reaching the component.  TVS Diodes can be connected with the anodes facing each other to provide bidirectional protection

When the surge subsides and the voltage returns to normal, the Transient voltage suppression diode comes back to its non-conducting state.

The Transient Voltage Suppression diode has a VI characteristic which is similar to the zener diode.  However, the zener is designed to regulate the voltage while the Transient voltage suppression diode is used to protect against overvoltage.

Gallium is a compound which is used as a semiconductor.  It is a combination of the elements gallium and arsenic.  Gallium is an extremely rare element.

Gallium Arsenide is used widely in the manufacturer of diodes, Field Effect Transistors and in the manufacture

Gallium Arsenide is used for fast switching applications and for components functioning at ultra high radio frequencies.

Gallium Arsenide is used in optical communication system and is used in LEDs.

Panasonic has launched a new pin sized battery aimed at the wearable gadget market. Wearable gadgets such as smart watches need more and more power from smaller and smaller sized batteries.

The pin sized battery is 20 mm long with a diameter of 3.5mm. It is about 20 times smaller than an AAA battery. The battery weighs just 0.6 grams and has a voltage of 3.75 volts. It has a capacity of 13 mAh. The battery which is rechargeable is called the CG-320.

Panasonic says that mass production and shipping will begin by February 2015 with an initial output of a hundred thousand units.

 Check out

A captive power plant is a power plant owned by an factory for its own power consumption.  That is, all the power generated in the captive power plant is consumed by the factory itself.

Many industries such as textile units, cement factory and petrochemical industries require cheap and reliable power.  The utility may not always meet their needs.  Hence, they set up their own power plants.  These plants do not supply power to the grid.  

These plants are mostly less than 50 MW.

The Alternator in a car generates power for all the electrical equipment in the car.  The Alternator is driven 
by the engine by means of a belt known as the serpentine belt.  The belt drives the shaft of the alternator.  

The rotor of the Alternator contains the field windings.  The field windings are excited by a Voltage regulator. 

The exciting supply to the field windings is through slip rings.  This supply is taken from the batteries.  The 
power is generated on the stator of the alternator which makes evacuation easier.  

The output of the Alternator is AC.  This AC supply is converted into DC by means of diodes.  The DC supply is used to charge the battery and to supply power to all the electrical fitments in the car. 

An Anti fuse is a device which fuses when there is a high voltage across it.  Just as a fuse opens the circuit in the case of an abnormality, the antifuse closes the circuit in the event of an abnormality.

An example of an antifuse can be a serial light connected to the domestic supply voltage.  The individual bulbs are not rated for the domestic voltage.  However, as they are connected in series, they are able to withstand and function in the domestic supply voltage.

A series of 48 lamps of a rating of 2.5 volts can withstand 120 volts.  A series of 96 lamps can withstand 240 volts.

When one lamp in the series fails, there is a risk of the other lamps not getting the supply as the circuit is open circuited.

This is avoided by having an antifuse below the filament which fuses when the bulb filament fails.  This happens as the system voltage is applied across the single bulb.

Once the antifuse operates and closes the open circuit, the current flows as usual to the remaining lamps.

In Electronics, antifuses function by modifies the circuit in microchips depending on the logic to be programmed.  An antifuse is typically an insulating layer sandwiched between two conducting layers.  When the chip is to be programmed, a voltage is impressed across the layer which fuses.

Series capacitors are used in transmission line to compensate for the line inductance.  These capacitors are connected in series with the  lines.  Series Capacitors increase transmission capacity and increase system stability.  

Series capacitors also help reduce voltage drops. However care must be taken when designing series capacitors.  Series capacitors can increase the fault levels in the system.  They can also cause overvoltages when the system is lightly loaded.

Ferro Resonance can occur if the value of the system capacitance and the inductance is close to the resonance value.

Power Wheeling is the transfer of power from the point of generation by a power producer using the transmission networks which do not belong to him.

Power wheeling is used by owners of large power plants which do not have their own transmission networks.  These producers usually pay a sum to the utility or company which owns the power transmission network. 
The charges for using the network may vary with the time of day.  As the congestion on the networks is higher during the daytime and less during the off-peak hours, the charges are higher during the peak hours and lower during the off-peak hours.

Hence, it is essential to measure the power and the time of the day in which it was transferred.  Hence, there are special time-of-day energy meters to record the time as well as the amount of power transferred.

A Time of Day Energy meter or a Time of Usage Energy meter is an Energy meter which measures the energy consumed and also the time of day it was consumed.  

The time of day energy meter, is used in many countries where the consumer is charged based on the time of the day the power was consumed.  

The Time of Day Energy meter gives it output in the form of slabs with the energy units and the time.  The utility then applies the cost per unit depending on the time and the customer gets the final bill.  

The Time of day helps encourage customers to use power during the off-peak hours.  It is also used in Power wheeling in which private power producers use the transmission lines of a utility to transfer power. 

A Power Purchase Agreement is an agreement between a party which produces or intends to produce power and the utility which controls the transmission and the distribution.  

The Agreement stipulates the amount of energy, the price and schedule of delivery, payment terms and penalties and the date of commencement of supply.  The Power Purchase agreement has emerged as an important legal document which helps encourage investment in the power industry.

Special power purchase agreements for renewable energy connect the end customer and the renewable energy producer.  This ensures the viability of renewable projects.  

The Power Purchase Agreement is able to formally define the output of the generating asset.  This helps the Independent Power Producers (IPPs - See Article on IPPs) raise capital from banks and other investors for the project.  

Independent Power producers are private investors who invest in a power plant.  The owners of these plants may be individuals, corporations, consortiums.

These IPPs usually have a Power Purchase Agreement with a Utility to supply power for a specific period of time, usually, years.

Independent Power Producers are also known as non-utility generators (NUGs).

A TRIAC is a three terminal semiconductor.  The TRIAC can control the AC power into a circuit by altering its firing angle.  The TRIAC differs from an silicon controlled rectifier in that it is bidirectional i.e it can conduct in both directions.

TRIACs can be used in speed control of motors.  TRIACs are widely used as fan regulators.  TRIACs are also used as light dimmers (See Article on Light Dimmers)

A light Dimmer is a electronic circuit which permits the gradual dimming of lights.  It is similar to a domestic fan regulator.  

Dimmers are usually powered by a TRIAC (See article on TRIAC).  The TRIAC does not conduct for the full cycle.  It switches the light on only for part of the waveform.  The firing angle of the TRIAC is controlled by an external potentiometer.

Light dimmers are used widely in domestic and industrial lighting,  in aircraft, etc.  Light Dimmers can be used with incandescent lamps, Light Emitting Diodes, and compact fluorescent lamps.  

A shunt is defined as a low resistance path across a circuit.  Shunts are used commonly in the measurement of electric current.  

For instance, if you have an ammeter which can measure current only up to 5A and you want to measure 10A.  You can connect a resistance in parallel to the ammeter.  If the value of the resistance in parallel is the same as the internal resistance of the ammeter, the current will be evenly divided between the instrument and the shunt resistance.

Thus when the instrument indicates 5A, the value of the current in the circuit is 10A.  Thus, the shunt resistance is used in parallel to the main ammeter to increase its range. 

The Starter motor in Automobiles is a DC motor which draws a high current. When the key is turned in the ignition, a solenoid operates a switch which connects the battery to the starter motor.  This starter solenoid switch is designed to carry the heavy current which flows from the battery to the starter.  The starter motor engages with the a large gear ring around the rim of the engine flywheel.

The motor is designed to generate heavy torque which can turn the engine.  The engine of the car thus rotates.

As it rotates, it draws air and fuel into it cylinders and the engine begins to rotate on its own.  As soon as the engine picks up speed, the starter motor disengages and the ignition is switched off.

The starter motor is usually a permanent magnet DC motor or a DC series-parallel wound motor.

Single Phase motors.
Three phase motors.

Carry more current for the same power resulting in more losses.
Carry less current for the same power resulting in less losses.

Are easy to construct.
Are difficult to construct.

They can be made into small sizes.
Difficult to construct motors of small sizes.

Single phase power is available easily than three phase power.
Three phase power is not available universally.

Low Starting Torque.
High Starting Torque.

Torque pulsations are high due to the pulsating magnetic field.
Torque pulsations are less due to the rotating magnetic field.

Reversing the direction of rotating is difficult.
Easy to reverse the direction of rotating (changing the position of any two phases)

Ferranti effect refers to the phenomenon in transmission lines when the receiving end voltage is higher than the sending end voltage.

A long transmission line draws two types of currents.  One is the current drawn by the load at the receiving 
end.  Another current is the current drawn by the line capacitance.

If a long transmission line is lightly loaded, the charging current of the capacitance can be more than the actual load current.  The voltage drop caused by the charging curent of the capacitance gets added to the 

sending end voltage and causes the receiving end voltage to be higher than the sending end voltage.  

Hence, a transmission line should not be loaded below its surge impedance loading value.

The Ferranti Effect can be compensated by adding series capacitors to compensate for the series inductances and shunt inductances to compensate for shunt inductances.  

Spinning Reserve

Spinning Reserve in a power system refers to the reserve capacity of a source which is already connected to the grid.  For instance, a 150 MW plant running at 80 MW may be considered part of the spinning reserve. 

The spinning reserve in this case is 70 MW which is the reserve capacity.  This can be brought into the system if the demand increases.  The spinning reserve must be able to supply the required power within 10 seconds.

Non-spinning Reserve

Non spinning Reserve in a power system refers to the power sources which are presently not in line by can brought in to the system if the demand rises.  The non spinning reserve should be able to supply the load within 10 minutes

Leakance is otherwise known as conductance.  It is the inverse of resistance.  It is denoted by the symbol G.
The unit of Leakance and conductance is siemens.

Triboelectricity refers to the static electricity generated when two insulating materials brush against each other.  Electrons are stripped from one material by the other material.  The material which loses electrons becomes positively charged while the material which picks up electrons becomes negatively charged.  

The Triboelectric series is a list of materials arranged in a manner which tells us which materials have a tendency to gain positive charge and to loose negative charge.   If a material lower in the series is rubbed with a material higher in the series, the lower material will become negatively charged while the higher material will become positively charged.   For instance, based on the series below, if glass and nylon are rubbed, glass acquires a positive charge while nylong becomes negative.  

If the distance in the series between the two materials is more, the the voltage produced will be more.  This will be valid only if the materials are clean and dry.  If the materials are dirty, the voltage will not be generated.  

Triboelectric series is as follows

Human skin
Rabbit Fur
Human Hair
Steel (neutral)
Hard Rubber
Nickel, Copper
Brass, Silver
Gold, Platinum
Styrene (Styrofoam)
Polyethylene (scotch tape)
Polypropylene Vinyl (PVC)

Speed control is important in any application.  Fans driven by single phase motors need to operate a different speeds.  Industrial drives may need to run at different speeds too.

Speed control in single phase motors can be done using


In this method, resistances are used to reduce the voltage available at the motor terminals.  This does not provide fine speed control as the resistances will be cut out in steps.  It is however a simple method.  Another disadvantage is the loss of energy in the resistances as heat.  This reduces the overall efficiency of the motor.


A Triac is a three terminal semiconductor device.  The current flowing through a thyristor can be controlled by controlling the gate voltage.  Triac controls are used in control of speed of fans.


Gears are also a method of speed controls.  A gear assembly is coupled to the output of the single phase motor.  The gears are engaged and disengaged as per the speed required.


VFDs or Variable Frequency Drives control the speed of the motor by controlling the frequency of the power supply on the input terminals.  VFDs are thus able to control the synchronous speed of the stator.  This, in turn, controls the speed of the rotor.

For an induction motor to be self starting, the stator needs to have a rotating magnetic field.  In a three phase induction motor, the rotating magnetic field is created by the fluxes of the three phase windings  which are displaced by 120 degrees.

This rotating field induces an emf in the rotor.  This results in the flow of current which results in the magnetic field in the rotor.  This results in the magnetic field of the rotor trying to catch up with the stator and the rotor starts to rotate.

In a single phase motor, the magnetic field in the stator is the result of only one phase.  Thus the magnetic field produced is pulsating.  The field is not rotating.

Hence, the rotor is pulled in opposite directions by the stator magnetic field.  This results in the rotor staying the staying position.  Hence, the single phase motor needs an auxilliary winding which produces two magnetic fields displaced in time which provides a magnetic field.

Brush holders are used to hold the brush in position.  Brush holders are used in DC motors as well as in AC slip ring motors.  The function of a brush holder is to hold the brush at the proper angle and position.
The brush holder should also ensure that the brush rests on the commutator with the correct pressure.  if the contact pressure is too high, the brush can get worn out faster.  If the contact pressure is too low, arcing may result.

Modern brush holders have markings which indicate the extent of wear on the brush.  The height of the brushes can be adjusted.  Quick disconnect terminals in modern brush holders make replacing carbon brushes easier and quicker.

The National Electrical Manufacturers Association (NEMA)  has classified motors design in the following manner based on starting current, rotor torque, breakdown torque and slip.

They are

NEMA design A

maximum slip of 5 %
medium to high starting current
normal locked rotor torque
normal breakdown torque
suited for a broad variety of applications
Typical applications : fans and pumps

NEMA design B

maximum 5% slip
low starting current
high locked rotor torque
normal breakdown torque
Typical Applications
common in HVAC application with fans, blowers and pumps

NEMA design C

maximum 5% slip
low starting current
high locked rotor torque
normal breakdown torque

Typical application : High inertia starts - as positive displacement pumps

NEMA design D

maximum 5-13% slip
low starting current
very high locked rotor torque
Typical application : cranes, hoists etc.

The speed torque curve of an induction motor is a plot of speed on the x-axis and torque on the y-axis.  When the motor is started, the initial torque is about 250% of the rated torque.  This is the torque required by the motor to overcome the inertial of the standstill.  As the motor picks up speed the torque drops to the pull-up torque.  If the pull-up torque of a motor is less than the torque requirement of the load coupled to it, the motor will stall and over heat.

The breakdown torque is the maximum torque which can be developed by the rotor before it overheats.  The breakdown torque needs to be high for loads with high inertia and which are susceptible to overloads such as conveyor belts.

The full load torque is the torque produced by a motor operating at the rated speed and load.  Exceeding the full load torque causes reduction in the life of the motor.

When the motor is run on no load, the rotor speed reaches the synchronous speed.  The slip becomes zero and the motor runs at zero torque

When the induction motor is run above the synchronous speed which is the speed of the rotating magnetic field, it works as an induction generator.  That is, it generates active power kW while it still consumes reactive power kVAr in order to establish the magnetic field.    This usually happens when another prime mover such as a wind turbine is coupled to the motor shaft.

The slip (Ns-Nr) then becomes negative.

Another scenario where the motor can overspeed is when the frequency of the input power is itself increased by means of a Variable frequency drive.  Then the motor is said to be running like an induction motor but at a higher speed.

The torque characteristics may vary with the varied speed.  The rotor, gears and the coupling may experience increased centrifugal force which can cause damage.  Hence, the overspeed limits need to be ascertained from the manufacturer.

NEMA is the acronym for the National Electrical Manufacturers' Association.  It is an association of Electrical Manufacturers and Medical Imaging manufacturers in the United States.  Founded in 1926, it is headquartered in Washington D.C. 

NEMA concerns itself with publishing standards for the Electrical Manufacturing Industry and in lobbying.  It has nearly 450 members.  NEMA publishes standards pertaining to the generation, transmission and utilization of electrical power.  

Official Website :

The Peak voltage is the maximum voltage that the voltage reaches in an ac power supply.  It is the topmost and the lower most point of the sine curve.

The normal value with which we refer to the voltage is called the rms voltage.  It is the root mean square of the instantaneous voltage supply

V rms = Sq. root (

Where v1,  v2 , v3 and vn are the instantaneous voltages.

The relation between the rms voltage and the peak voltage is as follows.

Vrms = 0.707 x V peak

High Voltage UPS systems are used in industries to provide back up power in the event of power outages. These Power back up systems consists of batteries which are charged from the utility or generator power during normal operation.

When there is a power failure, power is drawn from these batteries to feed the loads in the distribution.

High Voltage UPS systems have been designed using the lithium ion batteries or the lead acid batteries. Manufacturers claim that these sytems have an efficiency of about 99.6%.

The batteries are charged by a rectifier-converter unit which converts the AC into DC for charging the batteries when the power supply is available.  When the power supply is not available, it converts the DC supply of the battery and converts that into AC for the supply to the load.

The resistance split phase motor has two windings in parallel.  One of the winding is the main winding.  The other is the auxiliary winding.  There is a centrifugal switch in series with the auxiliary winding such that the winding is disconnected at around 75% of the synchronous speed.

The auxiliary winding is made of thin wire of a few turns such as that it has low reactance and high resistance. This creates a magnetic field with a displacement of 30 degrees.  This produces a torque which is moderate and can be used in motors up to 250 watts.

A permanent Split phase motor consists of a main winding and an auxiliary winding.  The auxiliary winding has a capacitor in series.  The auxiliary is always in circuit.  It produces a flux which is displaced from the main flux by 90 degrees.

The permanent split phase motor suffers from torque pulsations at main speed.  The permanent split phase motor is used up to 1/4 horse power.  The starting torque is relatively lower.  Hence, the permanent split phase motor is not used for applications which require high torque.

Ground enhancement materials are materials which are used to enclosing the grounding object such as a rod, plate, etc to increase the ground resistance.  The ground resistance in an area depends on the soil conditions.  Areas which have rocky ground, sandy soil or frozen ground will have high ground resistance.  

In order to reduce the ground resistance, ground enhancement materials may need to be used.  These materials have high conductivity.  They increase the ground resistance by increasing the surface area in contact with the ground.  

Common ground enhancement materials are bentonite clay. petroleum coke powder, earthing cement (marconite)  and salts such as sodium chloride, copper sulphate and magnesium sulphate.

Ionised air which has been charged either positively on negatively.  The positive charge is given by removing an electron from the oxygen atoms.  When an electron is added to the oxygen atom, the atom acquires a negative charge.    Ionized air is produced by making air to pass through an electric field.

Most ionized air is negatively charged.  This helps the air to attract dust and other floating particles such as germs and pathogens.  For this reason, ionized air is used in hospitals as a disinfectant.  Thus air ionization has emerged as an effective air cleaning technology.

You would have noticed how the sky appears clearer after a rain.  This is because the air is charged after a rain due to the electric effects of the rainfall.  This attracts the dust particles giving a clearer appearance.

Ionized air also carries an inherent risk.  During the process of ionization, ozone (O3) is also produced.  Ozone can be toxic to humans at high levels.  Hence, ionized air used in medical applications should have low ozone generation.  

Ionized air is also used in electronic to prevent accumulation of static charge.  They are also used in industries to prevent charge accumulation in fabrics, paper, plastic, etc.  

The mesh method of lightning protection is a method of protecting buildings.  The mesh method involves enclosing the building inside a metallic cage.  The cage is also known as a faraday's cage.  The mesh method
works by ensuring that the lightning which strikes the building always flows on the outside through the metal cage.  Hence, no damage is done to the building.

The mesh method cannot be used in circular surfaces.  The number and distance between the conductors in the mesh method needs to be calculated. 

Electrostatic charge builds up when there is rubbing between different materials.  This charge is undesirable in many cases.  Electrostatic charge accumulation can result in sparking which has a risk of fire.  It can also result in jamming of papers in printers.  In the textile industry, the fabric which passes through the machines can get stuck due to charge accumulation.  Charge accumulation also results in dust accumulation on surfaces which is an area of concern in the painting industry.  Charge accumulation and electrostatic discharge can destroy electronic components.

Hence, control of electrostatic charge accumulation is vital in many industries.  Humidity plays an important role in electrostatic control.  When the air is humid, a thin layer of moisture appears on the surface of the material.  This increases the surface conductivity.  This ensures that the charge which accumulates on the surface is dissipated through the conductive layer of moisture.

When dry air blows over insulating materials, it can itself cause charge build up.  

A humidity of 40 to 60 percent can help reduce charge build up.  Relative humidity lower than 30% can result in high levels of charge.  However, if the relative humidity is too high it can result in corrosion.  Hence, any humidity control system should be closely monitored.  

When an aircraft flies, air in the atmosphere rubs against the body.  The air also contains particles such as dust, water droplets, ice crystals, etc.    This results in the aircraft accumulating charge over a period of time.  This potential can reach thousands of volts.  In aircrafts, this can cause interference to the radio equipmen as this charge will bleed off other edges in the aircraft wings and tail.    It can also result in sparks and flashes, when the aircraft touches land.  Hence, this charge has to be safely discharged or bled off into the atmosphere.

Helicopters when used to rescue stranded people should be discharged to the ground before they are lifted in order to avoid an electric shock.  

This is done by  means of wicks in the aircraft wings.  The wicks are pointed metallic rods about 8 inches long place in the airflow.  The wicks have thin rods made of carbon which will keep discharging even as the aircraft builds up charge.  This ensures that the potential of the aircraft is always low.  

The wicks can be replaced periodically.

An electret is an interesting material which is analogous to a magnet. An Electret generates a permanent electrical Field.  It can be called an "Electrostatic Magnet".

Just as  a magnet is able to generate a magnetic field because of dipole polarization, the Electret is able to generate a permanent electric field. The Electret was first described by Oliver Heaviside in 1885.

The name Electret is a combination of the words Electron and Magnet. 

The Electret is made of special materials such as Quartz, PolyethyleneTerephthalate (PET) and polypropylene.  Modern electrets are made using Polytetrafluoroethylene material.

An Electret is made by melting the electret material, then placing the molten material in an electric field and then allowing the material to solidify.  The solidified material, retains the electric charge as the dipoles are polarized.  

Electrets are used in applications such as printing where a uniform electrostatic field is necessary.  They are also used in microphones, gas filters and even in the pharmaceutical industry.  

Regenerative braking refers to the braking process in which the kinetic energy of the vehicle is recovered during the braking operating and supplied back to the mains or stored in a battery.  Regenerative braking thus improves efficiency by recovering the energy which would be lost in a braking action.  

In conventional forms of braking, the kinetic energy of a moving vehicle is converted into heat by the braking pads or the linings.  Regenerative braking is usually done in vehicles where the final drive is electric.  Electric trains and Trams and electric cars are examples where regenerative braking can be readily applied.  In regenerative braking, the motor which drives the wheels can be made to work as a generator which feeds power to the mains.  

This done by disconnecting the power supply to the motor driving the wheels and reconnecting it either to the power through diodes or to capacitors.  The kinetic energy in the wheels, rotates the motor which works as a generator.  The power at the terminals of the generator is connected to the power supply or alternatively to a capacitor.  

Video on Regenerative braking

Electrostatic Discharge refers to the transfer of charge between two objects.  Electrostatic Discharge is an issue of concern particularly in electronics where sensitive components can be destroyed  by the transfer of charges.  The transfer of charges can be caused due to dielectric breakdown or physical contact.  While the spark may appear small, the voltages generated are of the order of kilovolts.

Electrostatic Discharge Protection is a key aspect of electronics design.  Electrostatic Discharge protection by measures such as the use of anti-static packaging, controlling humidity and is achieved by painting the the use of specially designed pointed features called dischargers.  This is used in Aircrafts

In Petrochemical installations, where the risk of fire is present, electrostatic discharge can result in fire.

The modern car has a sophisticated electrical system.  Electricity is used for starting the engine.  Petrol engines need a spark for ignition which is provided by the spark plug.  Power is required for lighting.  Other equipments such as the engine starter, windshield wiper, the heater and the engine fan all require power for operation.

The power for the electrical system in a car comes from the battery.  The battery in turn is charged by the alternator which is driven by the engine.  The power developed by the Alternator is AC which is rectified and stored in the battery.  

The electrical system in the car has the negative grounded.  Each equipment, thus, requires only one wire which is the positive.  The negative of the battery is grounded to the car body.  The supply reaches the equipment through the wire and returns to the battery through the car body.  

The chief components of the electric system in a car are 
  1. Alternator
  2. Battery
  3. Starter
  4. Lights
  5. Wiper Motor
  6. Wiper liquid pump motor
  7. Windscreen wiper motor
  8. Fan Motor

Jumper cables in cars are used in cars to start a car whose battery is dead.  Car batteries can get drained due to a variety of reasons such as forgetting to switch of a light, Keeping the car unused for  long periods of time, ageing of battery, etc.  

Jumper cables are used to start the engine of a car by using the battery of another car.  Once started, the car's alternator will charge the dead battery.  The jumper cables is made of copper.  It is thick with special heavy duty clamps which can withstand currents of nearly 500 amperes.  

The positive of the live battery is connected to the positive of the dead battery.  The negative of the good battery should be connected to any metal bracket or any other metallic part of  the car to be jump started.  (Do not connected the negative of the good battery to the negative of the dead battery as this may result in sparks or a battery explosion.)

Keep both cars in neutral gear.  Start the good car and let it run for a few minutes.   Now start the car with the dead battery.  

The ignition key is turned and the starter motor runs with the power from the live battery.  Once the engine is started and runs for a few minutes, disconnect the cables in the reverse sequence.  The car with the dead battery should be kept running for at least 30 minutess to charge the dead battery.  

Snubber Circuits are circuits which are placed across thyristors to protect it from transient high voltages which may occur during switching off.  When the current flowing through a thyristor is interrupted, the voltage across the thyristor can increase rapidly according to Faraday's second law.  This high voltage can cause damage to the thyristor or to other components in the circuit.

The snubber circuit works by preventing the voltage from rising too high across the component to be protected.  The snubber is normally used in circuits with inductance.

The Basic Snubber Circuit is a capacitor and a resistor connected in series.  The Snubber works on the principle that the current across a capacitor cannot fall to zero instantly.  Hence, a small current keeps flowing through the capacitor.  This current is enough to prevent the voltage from rising to high levels.

The Flyback resistor is also another example of the snubber circuit.

Shielding in Instrumentation Cables is vital in order to prevent stray coupling from other circuits nearby. 

When an instrumentation cables is laid beside a power cable, there are chances of  electromagnetic coupling taking place which can induce voltages in the instrumentation cable.  This can distort the signal in the instrumentation cable.  To prevent this, instrumentation cables are provided with a shield, usually made of aluminium.  

This shield magnetically shields the wires inside it.  This eliminates the chances of stray coupling.  The shielding needs to be grounded at one end.  Grounding the shield at two ends will cause grounding loops which can cause interference.

A Backplane is a system of parallel electrical conductors in a PCB which are used to connect the connectors.  Backplanes are advantageous over traditional cable-type connections as they are mechanically stronger.  The cards are mounted on backplanes on slots.  This facilitates easy replacement of the cards.  

Backplanes are widely used in computers and in industrial automation such as PLCs, DECS systems, etc.

Backplanes can be classified into Active and Passive Backplanes

Active backplanes are backplanes which have some computing circuitry in built into them.  Passive backplanes are backplanes with no computing circuits.  

A mid-plane is a backplane which has got slots on both sides.  

A ribbon cable is a cable with multiple insulated wires on a single flat plane.  It is called a ribbon cable as it resembles a ribbon.  Ribbon cables are used in computer peripherals such as printers, hard drives.
Ribbon cables are flexible and hence are suited for application such as dot-matrix printers where the printing head has to move back and forth across the page.  Ribbon cables also take up very little space as they are flat and can be accommodated even in cramped locations.  

By convention, one end of the cable is marked red.  This is to be connected to terminal no. 1 in the connector.  One downside of the ribbon cables is that it does not allow flexing in the sideways.  The cable can only bend up or down.

Non-Polarized plug and socket. 
Polarized Power sockets refer to sockets where the phase and neutral connections are fixed.  That is, the plug can be inserted in any direction.  Polarized connections, on the other hand, are designed that the phase and neutral pins can go into specific holes.  Three pin connectors are in general polarized connectors. An exception is the Italian mains connector with three pins which can be connected either way.

Polarized power sockets are used in applications where interchanging the phase and the neutral conductors can result in shocks as in a toaster.  Since the switch is connected to the phase line, send the phase supply through the neutral would mean that the appliance will have power even when it is switched off.
Observe the difference in pin width

A table lamp needs a polarized socket as interchanging the phase and the neutral lines would result in supply reaching the lamp holder even in the power off condition.

A condenser and a capacitor are essentially the same thing.  They are just two different names.  Capacitors were called condensers earlier.  The term "condenser" is still used in the automotive industry.  A capacitor is a electric component which consists of two conducting plates separated by a dielectric.

Capacitors find wide application in the electric and electronic industry.

Polarized capacitors are capacitors which can be connected only in one polarity.  That is, the positive and negative terminals cannot be changed. Electrolytic capacitors which use capacitors which use rubber, paper or glass are examples of non-polarized capacitors.

Electrolytic capacitors are examples of polarized capacitors.  In these capacitors the polarity is fixed.  The positive and the negative terminals of the capacitor should be connected properly.

Interchanging the positive and negative terminals can cause the capacitor to explode.

Galvanotechnics is the science and technology of Electroplating.  Galvanotechnics deals with the principles and processes of electroplating, electrodeposition and electrorefining.

Insulation displacement connectors are connectors which can be mounted directly over an insulated wire without stripping the insulation.  These connectors come with a sharp blade like arrangement which slices through the insulation and makes contact with the conductor.  

The blade and the conductor are cold welded to form a strong connection.  Insulation displacement connectors are used in telephone and network circuits.  Ribbon cables are generally used with insulation displacement connectors.  Ethernet cables are also mounted to insulation displacement connectors.

Conductive paint is a special type of paint which conducts electricity.  The conduction continues even after the paint has dried.  Conductive paint is a paint which contains micron-sized particles of silver, copper or nickel.  These particles give the paint its conductive property.  

Conductive paint is spray painted on to the surface.  The thickness of the conductive paint can be between 0.125mm to 0.5 mm thick.  

Conductive paint is used in electronic devices to provide shielding from Electromagnetic fields and Radio waves.  

As conductive paint is a suspension with metallic particles, it is necessary to shake the paint container before painting.  If the particles settle down at the bottom of the container and are not uniformly distributed, the conductive properties of the paint will be less.  

This is a question we come across often.  Electroplating involves depositing one metal which is connected to the anode on to another metal which is connected to the cathode.

Non-metals such as glass, plastic can be electroplated after they have been coated with a conductive paint.  The conductive paint provides a surface on which the coating can take place.  Another method of providing a conductive surface is the use of amorphous carbon powder over which the coating can be done.  

More recent techniques involve etching the plastic surface with Chromic acid.  The acid is then neutralized.  The surface of the plastic is activated by with a solution containing tin or palladium. The surface is then coated with a layer of nickel or copper. 

Magnetic Hydraulic Circuit Breakers work on the principle of the magnetic effects of the overcurrents.  However, they differ from standard magnetic circuit breakers as they have a hydraulic time delay mechanism.

The delay is created by forcing the core to move through a cylinder filled with silicone fluid.  When the overcurrent occurs, the magnetic field created pulls the core.  The  core has to pass through a cylinder filled with silicone fluid.  This introduces the time delay.

Thus if the overcurrent is momentary, the core goes back after the current comes back to normal.  If the overcurrent persists, the core travels towards the coil.

When the core reaches the coil, the reluctance of the  magnetic circuit changes.  This creates sufficient flux to attract the armature which causes the protecting device to trip and the contacts to separate.

Once the contacts separate, the current becomes zero.  The magnetic field ceases and the core returns to its original position.

The advantage of the magnetic hydraulic circuit breakers is that they can be reset immediately after tripping unlike thermal overcurrent elements which require a cooling period.

Magnetic Hydraulic Circuit Breakers provide accurate, relatively inexpensive and reliable overcurrent protection.  They are independent of ambient temperature.

Magnetic Hydraulic Circuit Breakers are available both for AC and DC applications.

Electronic Overcurrent Protection is a form of overcurrent protection which is increasingly used in Low voltage Switchgear.  Electronic overcurrent is used as an alternative to the Thermal Magnetic Trip which uses the Thermal and the magnetic effects of overcurrent to effect a trip in the circuit breaker.

The Electronic overcurrent protection works by sensing the current through the circuit using a current transformer.  The output of the current transformer is fed into an electronic circuit.  

The electronic circuit makes decisions relating to overcurrent response.  Electronic overcurrent protection devices have a transistor through which the load current flows.  In an overcurrent scenario, they are able to quickly switch off the transistor and interrupt the fault current.  

This helps in preventing a general undervoltage of the system and very fast response in isolating the fault.

Thermal Magnetic Trip is a method of over current protection which is widely used in LV switch gear such as MCBs and circuit breakers.  The Thermal Magnetic Trip works by sensing the current and tripping the breaker.

The Thermal magnetic element, as the name suggests, has two units viz. the thermal unit and the magnetic unit.  The thermal unit is used to sense the current using the heating effect of current.

The Thermal unit consists of a bimetallic strip which bends due the heat produced by high current passing through it.  The bimetallic strip takes time to bend.  This enables the element to have a time delay feature.  The time delay depends on the magnitude of the over current protection.

The Magnetic Trip element comes into play in case of severe faults.  When the current is extremely high, say 400 %, the current causes the magnetic trip element to attract a trip element which causes the unit to trip.

Thus, the thermal element is used to trigger a delayed response to minor overcurrents while the magnetic element is used to swiftly respond to high overcurrents.

Air Circuit Breakers are Circuit breakers where air is used as the medium of extinguishing the arc.  The air is usually compressed and kept in a cylinder.  When the breaker operates and the contact separation occurs,
the arc is driven into special arc chutes by means of compressed air which is blown through specially designed nozzles.  

Air Circuit Breakers are mostly used in the LV range.  They can interrupt currents of several thousand amperes.  

Air Circuit breakers are provided with an inherent current sensing mechanism like the thermo-magnetic release.

Arc Chute used in Air Circuit Breakers
The Arc chute is a component which is used to weaken and quench the arc.  The Arc Chute contains a number of splitters which split the arc into a number of sections.  This increases the length of the arc and quenches it.  The arc which is formed during the separation of the fixed and the moving contact is driven into the chute by means of the pressurized air.  The arc chute is usually made of composite refractory materials.  

Air Circuit Breakers are available in both Three pole and Four pole versions.

Air Circuit breakers are used widely in the industry for the protection of facilities and transmission lines.  They are also used for protection of electric equipment such as transformers, motors, etc.   Air circuit breakers are also used in mines and on board ships.  

Twisted pair cables, as the name suggests, are cables where a pair of conductors are twisted.  The pair of wires are twisted in order to cancel out the effects of electromagnetic coupling. 

The twist rate or the pitch of the pair refers to the number of twists per metre.  When two twisted pairs are lying together over large distances, there is a possibility of coupling between the two.  Hence, the twist rates of two pairs laid together should be different.

The number of poles is selected such that the frequency of the armature emf is between 25 Hz and 50 Hz.  A higher frequency can result in hysteresis losses in the armature.

The number of poles is an even number like 2, 4, or 6.

Probable number of poles such as 2,4 or 6 are considered and the prospective frequency for each of the number of poles is calculated using the formula

F = N x P/ 120

where  P is the number of poles
           F is the frequency of the armature emf and 
           N is the speed of the motor in rpm

Capacitors Protection Relays are dedicated relays which are designed to provide a range of protection functions for capacitors.

Capacitors are widely used in power systems for VAr regulation and PF control.  They are also used for filtering harmonics.  

Capacitor banks need to be protected against overload by harmonic currents. They also need to be protected against system overvoltages. 

Capacitor Protection Relays consist of a number of different protection elements such as overcurrent, overvoltage, differential protection, etc.    They also have protective interlocks such as preventing an energised capacitor from being connected to the network.

Hot Conductors are conductors in transmission lines which can high currents through normal cross section. They can thus tolerate high current density.  
Hot Conductors are made of special high grade aluminium to which is added many other elements particularly zirconium.  
On further processing, a microstructure is created which gives the alloy a high recrystallization temperature.  This enables the conductor to withstand the high temperatures generated by the high currents.  

A hot conductor can carry about 50% more than a normal conductor of the same cross section.

By giving a black coating of polyurethane to the conductor, the radiation of the heat is increased.  This permits the conductor to carry current up to 70% more than a conventional conductor.

Liquid Cooled Resistors are used in applications where large amount of heat needs to be dissipated through resistors.  Liquid Cooling helps design resistors which are compact.  Liquid Cooling Resistors use water or a glycol-water mixture.  The resistors are cooled directly with water which has been de-ionized or through water-cooled heat sinks.

The Resistors are made of materials such as copper, stainless steel and aluminium.  Cooling mediums used also vary.  Deionized water, salt water, Glycol and other non-conductive liquids can be used for cooling.

Aluminium Clad Wound resistors are used in applications which require dissipation of a large amount of heat in a confined space.  The resistor used is of a wound type.  The resistor assembly is encased in an aluminium
casing with fins for effective heat dissipation.

Aluminium Clad wound resistors can be used as Charging resistors.  They can also be used in dynamic braking applications to absorb the energy of the braking action.  These resistors are also widely used in Cranes, Hoists and CNC Machines.

In power systems, reactive power stability is extremely important.  Reactive power is dependent on the nature of the load, the line capacitances and the line reactances.  Large power lines which are lightly loaded may need reactors to balance the line capacitances.  Hence accurate control of the reactors in line is one of the requirement for the proper maintenance of reactive power.

This Variable Shunt Reactors are used in applications where the range of the reactive power to be adjusted is large but the regulation is not dynamic, that is, the change in the reactive loading is gradual.

Dynamic regulation will require Static VAr compensators with thyristorised controls.

Variable Shunt Reactors are fitted with reactors whose values can be adjusted through tappings.  These tappings are selected using motorised controls.  These adjustments can be made on load.

In Electronics, a mother board is a board which contains all the other boards.  In computers, the mother board may contain the video card, the network adaptor card, the LAN card and so on.  It will also have a slot for the microprocessor.

The boards which are mounted on the mother board are called the daughter boards.  The daughter boards are mounted on the mother boards using slots.

Mother boards which are used in laptops come with integrated video cards.  They also have USB and HDMI ports.