What is a Captive Power Plant ?

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

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. 

What is an Anti Fuse ? Where is it used

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 in Transmission Lines

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.

What is Power Wheeling ?

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.

What is a Time of Day Energy Meter ?

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. 

What is a Power Purchase Agreement ?

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 (IPP)

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).

What is a TRIAC ?

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)

Light Dimmer

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.  

What is a Shunt and where is it used ?

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. 

Engine Starter in Automobiles

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.

Comparison of Single and Three Phase Motors

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 - An Overview

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.  

What is meant by spinning reserve and non-spinning reserve in a power system ?

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

What is Leakance ?

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.

The TriboElectric series

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

Leather

Rabbit Fur

Glass

Human Hair

Nylon

Wool

Fur

Lead

Silk

Aluminum

Paper

Cotton

Steel (neutral)

Wood

Amber

Hard Rubber

Nickel, Copper

Brass, Silver

Gold, Platinum

Polyester

Styrene (Styrofoam)

Polyurethane

Polyethylene (scotch tape)

Polypropylene Vinyl (PVC)

Silicon

Teflon 

Methods of speed control in single phase motors

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

Resistances

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.

TRIAC 

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

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

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.

Why is the single phase motor not self starting ?

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

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.

NEMA Motor Ratings

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.

Speed Torque Curve of an induction motor

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

What happens when an induction motor is run above the rated speed ?

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 (N_s-N_r) 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 - An Overview

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 : www.nema.org

What is the relation between peak voltage and rms voltage ?

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 (v₁+v₂+v₃+v₄.....v_n)/n

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

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

V_rms = 0.707 x V _peak

High Voltage UPS (Uninteruptible Power Supply)

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.

Resistance split phase motor

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.

Permanent Split Phase motor

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 - An Overview

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.

What is ionized air? What are its uses ?

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.  

How is Mesh Protection for lightning done ?

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 Control and Humidity

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.  

Static Electricity in Aircrafts

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.

What is an Electret ?

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.  

What is Regenerative Braking ?

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