The following are links to articles relating to Transformers which we have carried in our website over the years.  

Hope you find them useful.

Dry Type Transformers

Determining the vector group of transformers

Comparison of Dry Type Transformers and Oil Filled Transformers

Overfluxing in Transformers

Noise in Transformers

Power Transformers - Introduction

Leakage Transformers

Nitrogen in Transformers

What are the reasons for transformer overheating?

Gas formation in Transformers

Transformer Classification based on the Cooling Medium

Zig Zag Transformers

Conditions for Paralleling two Transformers

What is the function of the Conservator in a Transformer

Harmonic Mitigating Transformers

Transformer Oil Deterioration

All Day Efficiency of a Distribution Transformer

Single Phase Pole Mounted Distribution Transformers

Solutions for Noise and Vibration in Transformers

Calculating the percentage impedance of a Transformer

Turns Ratio and Voltage Ratio of Transformers

K-rated Transformers

Grounding Transformers

Amorphous Metal Transformers

Distribution Transformers

Ferrite Core for High Frequency Transformer

Toroidal Transformers

Control Transformers

Lighting Transformer

Zero-Switching of Transformers

Isolation Transformers

Coefficient of coupling in a transformer

Transformer Oils.

Ultra Isolation Transformers

How Autotransformers work

Metals in Transformer Oil

Understanding and Preventing Transformer Explosions

Measuring the exciting current of a Transformer

Water in Transformer Oil

Losses in a Transformer

Surge Arresters are classified based on their rating, the amount of energy to be dissipated and their application. 

Surge Arresters are classified into three types.

Station Class Arresters

Station Type arrestors are used in substations where the equipments are connected to the Transmission lines.  These arrestors are designed to discharge high amounts of energy and have elaborate systems of pressure relief.  They are used for equipments whose rating is above 20 MVA

Intermediate Class Arresters

These are used for Transformers and rotating machines of 1 - 20MVA.  Intermediate Class Arresters are a compromise between reliability and economy.  Typical examples of applications would be dry type transformers, switching and sectionalizing equipment. 

Distribution Class Arresters

Distribution Class arresters are used in transformers and rotating equipment with a powe rating below 1000 kVA.  These arresters are used on exposed lines connected to the equipment. 

It is important that the surge arrestor is selected properly to ensure correct operation and adequate protection to the equipment. 

There are three important steps in surge arrestor selection. 

Deciding the Surge Arrestor Voltage.  
This is decided by the nature of the earthing i.e. whether we use grounded or ungrounded systems.

Selecting the class of arrestor.
Surge Arrestors are classified into three types Station Class, Intermediate Class and Distribution Class.  The class determines the degree of protection, capacity and, ultimately, the price.  The Station Class Surge Arrestor is the most expensive. 

Deciding on the location of the surge arrestor. 
The location of the arrestor is crucial.  For an equipment to be properly protected, the arrestor should be placed as close to the equipment as possible. 

Spark Gap Arresters are the oldest form of Surge arresters.  The spark Gap lightning arrestors have two horned terminals with a distance between them.  One of the terminals is connected to the line while the other is connected to the earth.  
During normal current, the gap prevents leakage of current.  In the event of an surge, the voltage across the gap is sufficient to cause breakdown and start conducting.  This reduces the steepness of the surge. 

The arc which is formed during operation is extinguished as it rises across the airgap.

The downside of the spark gap arresters is that it is difficult to precisely set the trigger voltage.  Another difficulty with Spark Gap lightning arresters is to switch off the follow-on current after the voltage has returned to normal.

Surges in overhead lines affect substation equipments, can cause backflashovers and can affect power quality.

Surges in overhead lines can be categorized into two types
  1. Internal Surges and
  2. External Surges
Internal Surges are surges caused due to switching of capacitors, earth faults, sudden load swings, etc.  External Surges are caused due to lightning strikes on power lines.

There are two methods of protecting Transmission lines from surges. 

  1. Shielding Methods and
  2. Discharge Methods

Shielding methods use a shielding line run along the top of the tower to shield the lines from lightning.  The shielding line offers protection up to an angle of 30 degrees from the vertical on either side.  Sometimes two shielding lines are run in parallel to increase the area under protection. 

The other method of protection is the discharge method of protection.  The discharge method of protection uses surge arrestors which discharge the surge through themselves.  These arrestors are mostly non-linear resistors or spark gap based arrestors

Insulated Tools are an essential part of the maintenance equipment in all Electrical Installations.  Insulated are tools such as spanners, pliers, screw drivers which are insulated with special insulating material. 

This is necessary when working with live equipment.  In many cases, there are chances of tools getting accidentally dropped on to live bus bars or terminals. A screw driver making contact to a live circuit can get in contact with an earthed panel.    These can result in a flash over and an explosion with serious consequences. 

All insulated tools are labelled with a voltage rating.  Insulated Tools should never be used as the primary method of protection.  For instance, all insulated tools should be used with an Electrical Glove of appropriate rating

Dry type transformers find use in locations where it is not possible to use oil-filled transformers such as shopping malls. Hospitals, residential complexes, etc due to the risk of fire. In dry type transformers air is used as the cooling medium instead of oil. 

The insulation used in dry type transformers is designed to withstand higher temperatures. Dry type transformers are thus more expensive than conventional transformers Vacuum Pressure Impregnation, Epoxy Resin cast are some of the methods of Insulation adopted in dry-type transformer construction. 

The downside of the dry type transformers is that the efficiency of these transformers is lower than that of conventional transformers. Dry type transformers cannot be designed for very high voltages and are designed only upto the MV range.

Transformer explosions can be catastrophic.  They can cause loss of lives and severe damage to the substation and surrounding area and long interruption to the power supply.

Transformer explosion occur due to low impedance faults within the transformer.  Such low impedance faults result in extremely high currents.  The arcs created during the faults result in very high temperatures which vaporizes the oil.  This heated and vaporised oil results in a sharp spike in pressure within the transformer.

Improper contact between the terminals of the online tape changer has also resulted in Transformer explosions. Hence, it is necessary that all Power Transformers be equipped with a reliable Explosion Protection System.

This pressure waves bounce off the transformer walls and this results in multiple waves due to reflection.  The superimposition of these reflected waves can result in extremely high pressures which result in the explosion.

Conventional methods of pressure relief such as the safety valve and devices such as the Buccholz relay take too long to respond and may not be sufficient to relieve the pressure or to activate a shutdown.  Relay Protection such as Differential Relays will not be adequate to protect the transfer as they take more than 60ms to operate including the time taken for the breaker to trip. 

Transformer Explosion prevention systems work by quickly depressurizing the transformer by activating a valve.   A sensor detects the initial pressure wave and opens a valve leading to a separate chamber to facilitate depressurization.  Thus the pressure is relieved quickly before it can build up.

The explosive gases which are produced by the arc need to be expelled into a separate chamber by pumping inert gas into the system.  This is usually taken care of by the Transformer Explosion Protection System. Failure to do this can result in a severe explosion when the transformer  is opened by technicians resulting in death or injury.

Below is a video on a transformer explosion captured on video

AAAC or All Aluminium Alloy Conductors are increasingly used in transmission linesinstead of ACSR Conductors. AAAC conductors have better mechanical properties and improved resistance to Corrosion as compared to ACSR conductor. 

AAAC Conductors are made from an alloy of Aluminium, Magnesium  and Silicon.  The absence of the steel core which is prone to corrosion makes AAAC conductors as ideal for use in coastal and industrial area. 

AAAC conductors have increased current carrying capacity, lower power loss and higher thermal stability limit.   Besides, AAAC conductors have higher strength-to-weight ratio Which reduce the cost of supports. 

In areas which have poor security, ACSR Conductors are prone to theft, particularly low voltage lines. AAAC conductors are not attractive to thieves as there are no alternative applications for the alloy which is made of aluminium, magnesium and silicon.      

This conductor is manufactured from Aluminium-Magnesium-Silicon alloy of high electrical conductivity containing enough magnesium silicate to give it better mechanical properties. AAAC conductors have better corrosion resistance and better strength to weight ratio and improved electrical conductivity than ACSR Conductors on equal Diameter basis.

The testing of the vector group of the Transformer is a very important check.  It is crucial to know the vector group of the transformer when trying to parallel the transformer.

Even when purchasing a transformer, it is necessary to confirm whether the vector group of the transformer is the same as that mentioned in the nameplate.

The vector group of a transformer may be found by a simple test using a 3 phase power supply of about 440 volts and a multimeter.

The 'R' terminal of the HV winding and the 'r' terminal of the LV winding. are shorted together.  The 3 phase supply is connected to the HV winding (It should never be connected to the LV winding as that word cause dangerously high Voltages to appear on the HV side).

Let us assume for a moment that we have a Ynd11 transformer. when one of the terminal on both the HV and the LV winding are shorted and a LV Voltage applied, we will get the following vector notation.

 The connection is verified by taking voltage measurements between each terminal and every other terminal. 

The following table would be helpful

A vector diagram can be drawn graphically based on the data in the table.  This helps to verify the vector group.

High Voltage Cables are protected by many layers such as the insulation, armouring, shielding, etc.  When the cable is to be terminated at a busbar or at an equipment, the insulating and protective layers around it are removed.  This leaves the conductor exposed and there is a risk of flash overs and arcing at these spots.

Hence, cables need to be properly terminated to provide electrical and mechanical protection to the conductors at the point of connection.

There are different types of cable terminations such as straight through joints, cold shrinkable types, Heat shrinkable types, etc.  Cable joints need to be flexible, have good mechanical strength and chemical and thermal stability.

Cable Termination have frills to increase tracking resistance.

Electrical Contact Cleaners are used for cleaning electrical contacts in panels, motors and other fittings. 

The contact cleaner consists of a cleaner such as isopropyl alcohol contained in a spray-container.  The Electrical Contact Cleaner can remove impurities such as oil, grease, etc.  The jet of air is used to remove deposits.

Contact Cleaners using Carbon Tetra Chloride  were widely used until a few years back.  However, today, contact cleaners no longer use Carbon Tetra Chloride as it is considered to be detrimental to the ozone layer. 

Remote Operated MCBs are MCBs which have a provision for remote operation.  Conventional MCBs are used to trip the circuit in the event of an overcurrent.  They are not,however, sensitive to variations in voltage. 

The remote operated MCBs can be connected to a voltage monitoring relay which can trip the MCB in the event of an overvoltage or an undervoltage.  This can be ideal for equipment which are voltage-sensitive.

Remote operated MCBs can also be used in applications where it is practically difficult to operate the MCBs.  An example would be a motor which is located at a height along with the MCB. 

The remote operated MCBs also permit the remote resetting of the MCB.  MCBs of equipments which are located at a distance can be reset remotely from the control room. 

Ordinary MCBs can be fitted with actuators which trip them in response to a remote command.

Illuminated Switches are used to indicate the position of the switch or the condition of a circuit.  Illuminated switches find wide application in control panel and switching equipment.  The illuminated switch consists of a lamp which is placed inside the switch.  The lamp is powered on or off from an external circuit. 

Illuminated switches were made of filament lamps, however, now they are being increasingly made from LEDs. 

The switches can be fitted with one or more contacts.  Illuminated switches are available both as push buttons as well as well as buttons with latch facility. 

Switches are usually maintenance free.  A periodic cleaning of the contacts with a contact cleaner would be sufficient.

The function of a Yoke in a DC machine is twofold.  The yoke acts as path for the magnetic flux.  It also provides mechanical support and shape to the dc machine.

As it is a path for the magnetic flux, the yoke should be made only of a magnetic material. 

The Yoke is usually made of cast iron, forged steel or cast steel.  In certain large machines, the yoke is made of two pieces which are bolted together.

PVC is a material which is widely used for insulating cables, busbars and wires.  Thermal and chemical Stability and flexibility have made PVC the insulation of choice for many applications.  They have smaller bending radius.    PVC insulation is used for cables operating at up to 70 degrees C.

Special heat resistant PVC can with stand temperature upto 90 degree C.  The plastic strength of PVC can be increased by the addition of plasticizers.

Another advantage of PVC is the ease of making joints when connecting two cables. 

PVC insulation tapes are also widely used.

A magnet wire is a wire of a small cross section which is covered with a thin layer of insulation.  Magnet wires are used extensively to construct the windings in motors, small transformers, inductors, etc. 

Magnet wires are made of copper which is electrolytically refined or aluminium. 

The small cross section of the magnet wire enables close winding in small spaces.  Magnet wires are usually made of aluminium, zinc, copper, etc.  They are provided with an insulating films made of polymers.  Different types of polymers are chosen for insulation based on the operating temperature.  

Magnet Wires are designed in both AWG and metric sizes.

Lightning Masts are used to protection installations from lightning.  They are different from other forms of lightning protection in that they are made of a single long pole which is raised to a height of a few metres.

Lightning masts are made of aluminium, steel or galvanised iron. 

Masts are advantageous over shielding wires in that they are more robust.  There have been cases where shielding wires have failed due to the high current and causing the installation to be without protection. 

Masts are cheaper that shielding wires.  A mast with a pointed tip will also attract lightning more easily. 

The area around the mast which is protected is called the "cone of protection".  The area on the ground which lies within 30 degrees from the tip of the mast is called the cone of protection. 

More than one lightning mast can be used depending on the size of the area to be protected. 

The supply given to an induction motor may have harmonics present in it.  These harmonics will have their own torques in addition to the synchronous torque.  Let us consider a supply with odd harmonics.  The 3rd harmonic will be absent in 3 phase systems.  Hence, we only have to consider the 5th and 7th harmonics.  The other higher order harmonics can be neglected.

The torque produced by the 5th harmonic rotates in the opposite direction.  Thus, the forward torque is given by the sum of torques produced by the primary frequency and the 7th harmonic. 

The rotating field of the 5th will rotate at one fifth of the synchronous frequency (Ns/5).  However, the torque produced by the 5th harmonic rotates in the reverse direction.  Similarly, the 7th harmonic will rotate at one seventh of the synchronous frequency.  The torque produced by the 7th harmonic is maximum at 1/7th of the supply frequency.

When some poorly designed motors are started with load, the motors may not reach the nominal speed.  The motors will get stuck at 1/7th of the nominal speed. 

This phenomenon is known as crawling.  Crawling can be overcome by properly selecting the number of rotor bars in the rotor of the induction motor   

Potentiometers find wide application in electric circuits. Potentiometers are used for controlling voltage or current in an electric circuit. A common example  for the potentiometer is the volume control knob in the radio set.

The rather strange name for the potentiometer is due to its history. In the early years of electrical technology, the potentiometer was used in circuits designed to measure voltage.

A potentiometer consists of a coil of a wire that is wound over a circular or linear support.

A metallic contact known as the wiper moves over the  coil. The wiper is usually made of graphite.

The potentiometer has three terminals. The wiper is usually the central terminal. The other two terminals belong to the coil.
Preset Potentiometers are very small potentiometers which are enclosed in the equipment. These potentiometers have a small groove to be adjusted with a screwdriver.   These Potentiometers are to be adjusted only on rare occasions when the equipment is to be calibrated. 

While the potentiometer has three terminals, in many applications only two terminals are used. In such an application, the potentiometer is used as a rheostat. When all the three terminals of the potentiometer are and the potentiometer behaves like a Voltage divider.

The linear potentiometer and the logarithmic potentiometer are  types of Specialised potentiometers.

Paper is an excellent insulating material in the dry condition.  The difficulty is in keeping it dry.  This is because paper is extremely hygroscopic (it absorbs moisture).  Hence, paper should be kept out of contact with air.  Paper insulation is, therefore, enclosed in a sheath which protects it from moisture.  One of the common materials used for the sheath is lead.  These cables are known as Paper Insulated Lead Covered (PILC) cables. 

Paper insulation is usually impregnated with mineral oil or with other special compounds.  This prevents the formation of air pockets in the paper insulation.  Paper insulation can be used upto a temperature of 80 deg. C. 

However, the downside of paper insulation is the difficult jointing process.  Special jointing procedures and equipments are needed to ensure that the insulation remains sealed.  This makes it difficult to work with.  Hence, paper insulation has gradually given way to PVC and XLPE Insulation. 

Another form of paper insulation is the pressboard.  Pressboards are formed by pressing paper sheets together to a thickness of a few mm.  An impregnating material is used for binding.  These pressboards provide mechanical support along with electrical insulation. 

The amount of metal in Transformer oil can be used as an index of the condition of different components of a transformer.  Faults which involve high energy cause deterioration of not only the insulation but also the windings and other components.  Therefore, it is not unusual to see particules of metals such as copper, zinc, tin, silver, lead, etc. 

The quantity of these metals can serve as an indication of specific components.  The levels are calculated using various types of spectroscopy in the laboratory.  The transformer windings are made of copper or aluminium while the core is made of iron.  Metals such as lead, zinc, and silver can be found in components such as bushings, tank walls, etc.

There are no specific limits for the levels.  It would be better to monitor the levels periodically and to investigate any sharp increase in the level of metals.

The Ferranti Surge Absorber is use to minimize the steepness of a travelling wave.  The Ferranti surge absorber is made of a coil which is enclosed in a metallic cylinder which is grounded.   The metallic Cylinder is known as the dissipator.

The construction resembles an air core transformer with a low inductance primary and a single turn secondary short which is short circuited. 

When the wave reaches the Ferranti surge absorber, it induces an emf in the dissipator.  Energy is transmitted from the coil to the dissipator and expended as heat.  This reduces the steepness of the travelling wave.

A solar pond is a device which traps the heat in sunlight and makes it available for power generation.  A solar pond consists of a reservoir of salt water. The salt water in any reservoir has an inherent salinity gradient.  That is, the water at the lower layers will have a higher concentration of salt (around 90%) as compared to water at the top layer (30% salinity).

The water in the reservoir is transparent as to allow sunlight to penetrate to the deeper layers.  When sunlight falls on the bottom of the reservoir.  It heats the bottom layers of the salt water.  When normal water is heated, it tends to rise up due to convection and the water circulates. 

However, since the pond is filled with salt water, the hot water is trapped in the lower layer of the pond as it cannot rise up due to the high density.  This heat is removed by means of a liquid which passes through a pipe which runs through the lower layers of the pond.  The fluid in this pipe is kept in circulation by means of a pump.  this heat is used to heat water in a boiler which can be used to power a turbine and a generator. 

Bushings are insulating equipment which are used to bring out a conductor from a metallic enclosure such as transformer tank or a the enclosure of a breaker.

Bushings, generally use oil or gas as the insulating material.  They consist of a conductor surrounded by an insulating medium and then an non conducting enclosure.

The electrostatic stress is highest on the insulating material closer to the live conductor and gradually decreases towards the periphery.  Because of this, a large quantity of material is needed for bushings used at higher voltages.  This makes the bushing bigger in size and costlier.

The condenser bushing consists of the metallic conductor surrounded by a cylindrical sheath of paper and then by a metallic sheet.  This is again followed by a paper sheet and then by a metallic sheet.  This order continues till the periphery.   This can be visualized as a series of concentric capacitors.

This ensures that the electrostatic stress is evenly distributed across all the insulating material.  Condenser bushings are smaller, use less material and are therefore cheaper.

Condenser bushings are usually used above 52 kV

The ANSI(American National Standards Institute) has standardized the codes to be used for protection relays. Each protective function is indicated by a specific no. such as 50 for instantaneous overcurrent protection and 59 for overvoltage protection.

Following is the list of the functions. The codes are sometimes followed by an alphabet which gives some additional information for instance, the code 51G may indicate an overcurrent ground relay. 50N may indicate a ground sensitive overcurrent relay based on neutral current measurement. 87T may indicate that a differential relay may be used for Transformer protection.

1 - Master Element
2 - Time Delay Starting or Closing Relay
3 - Checking or Interlocking Relay
4 - Master Contactor
5 - Stopping Device
6 - Starting Circuit Breaker
7 - Anode Circuit Breaker
8 - Control Power Disconnecting Device
9 - Reversing Device
10 - Unit Sequence Switch
11 - Reserved for future application
12 - Overspeed Device
13 - Synchronous-speed Device
14 - Underspeed Device
15 - Speed - or Frequency, Matching Device
16 - Reserved for future application
17 - Shunting or Discharge Switch
18 - Accelerating or Decelerating Device
19 - Starting to Running Transition Contactor
20 - Electrically Operated Valve
21 - Distance Relay
22 - Equalizer Circuit Breaker
23 - Temperature Control Device
24 - Over-Excitation Relay (V/Hz)
25 - Synchronizing or Synchronism-Check Device
26 - Apparatus Thermal Device
27 - Undervoltage Relay
28 - Flame Detector
29 - Isolating Contactor
30 - Annunciator Relay
31 - Separate Excitation Device
32 - Directional Power Relay
33 - Position Switch
34 - Master Sequence Device
35 - Brush-Operating or Slip-Ring Short-Circuiting, Device
36 - Polarity or Polarizing Voltage Devices
37 - Undercurrent or Underpower Relay
38 - Bearing Protective Device
39 - Mechanical Conduction Monitor
40 - Field Relay
41 - Field Circuit Breaker
42 - Running Circuit Breaker
43 - Manual Transfer or Selector Device
44 - Unit Sequence Starting Relay
45 - Atmospheric Condition Monitor
46 - Reverse-phase or Phase-Balance Current Relay
47 - Phase-Sequence Voltage Relay
48 - Incomplete Sequence Relay
49 - Machine or Transformer, Thermal Relay
50 - Instantaneous Overcurrent or Rate of Rise, Relay
51 - AC Time Overcurrent Relay
52 - AC Circuit Breaker
53 - Exciter or DC Generator Relay
54 - High-Speed DC Circuit Breaker
55 - Power Factor Relay
56 - Field Application Relay
57 - Short-Circuiting or Grounding (Earthing) Device
58 - Rectification Failure Relay
59 - Overvoltage Relay
60 - Voltage or Current Balance Relay
61 - Machine Split Phase Current Balance
62 - Time-Delay Stopping or Opening Relay
63 - Pressure Switch
64 - Ground (Earth) Detector Relay
65 - Governor
66 - Notching or Jogging Device
67 - AC Directional Overcurrent Relay
68 - Blocking Relay
69 - Permissive Control Device
70 - Rheostat
71 - Level Switch
72 - DC Circuit Breaker
73 - Load-Resistor Contactor
74 - Alarm Relay
75 - Position Changing Mechanism
76 - DC Overcurrent Relay
77 - Pulse Transmitter
78 - Phase-Angle Measuring or Out-of-Step Protective Relay
79 - AC Reclosing Relay
80 - Flow Switch
81 - Frequency Relay
82 - DC Reclosing Relay
83 - Automatic Selective Control or Transfer Relay
84 - Operating Mechanism
85 - Carrier or Pilot-Wire Receiver Relay
86 - Lockout Relay
87 - Differential Protective Relay
88 - Auxiliary Motor or Motor Generator
89 - Line Switch
90 - Regulating Device
91 - Voltage Directional Relay
92 - Voltage and Power Directional Relay
93 - Field Changing Contactor
94 - Tripping or Trip-Free Relay
95 - Reluctance Torque Synchrocheck
96 - Autoloading Relay

In commercial liquids, which are not pure, the presence of suspended foreign particles has a significant impact on the overall breakdown strength.

When an electric field is applied across such a commercial liquid, the suspended particles align themselves depending on their permittivities.  If the permittivity of the suspended particles is more than the liquid, eg. paper particles, the particle will experience a force towards the area of higher stress.

If the permittivity of the particle is lesser than that of the liquid, eg. gas bubbles, the particle will experience a force towards the area of lower stress.

This results in the particles aligning themselves in a region.  As they accumulate, they may bridge the two electrodes and cause a breakdown.

Liquid Dielectrics find wide application as insulating materials in electric equipment such as transformers, cables and switchgear.  In Transformers, insulating oils are used to provide insulation to the live parts and to transfer heat away from the hot regions.  In Circuit breakers, oil is used to extinguish the arc which occurs when the breaker opens.

Any change in the insulating properties of these dielectrics can cause damage to the equipment and result in breakdowns.  Hence, it is necessary to have a good understanding of the insulating properties of dielectrics.

From an Electrical perspective, the main properties of the insulating oil are the dielectric constant, the dielectric strength and the electrical conductivity.  

The purity of the insulating liquid is vital.  Even a small amount of water, say 0.01% can reduce the dielectric strength of oil by 20%.  The presence of other impurities can also reduce the dielectric strength sharply.  

From the perspective of insulation, liquids are classified into pure liquids and commercial liquids.  Pure liquids are those in which the amount of impurities are less than 1 in 10^9.  

Commercial liquids, on the other hand, are impure liquids which contain impurities such water, other chemical molecules and foreign particles.  These liquids are not homogeneous.  It is common to find that two samples taken from the same transformer having different properties.

Universal Motors are motors which run on both AC as well as DC power.  These motors find wide application in household electric appliances such as mixers, blenders.

The small size, high power and speed of universal motors make them ideal in these applications.  The Universal Motor is the motor of choice for washing machines as they can be used to agitate the drum by easily reversing direction. 

A universal motor is a dc series motor which usually has some modifications which enable it to run efficiently on AC as well.  Running the motor on AC leads to eddy currents being developed in the core of the motor.  Hence, the core and poles of universal motors are made of laminated sheets.  The windings of the motors are made thicker to reduce the reactance.   

Like all DC machines, universal motors suffer from the effects of commutation such as sparking.  Hence, universal motors are used mainly below 1000W.  The high speed of the universal motor enables it to deliver a higher power even while having a small size.  Universal motors are usually kept coupled to the loads such as gears or blowers as they tend to run at very high speeds on no load conditions. 

Speed Control is usually done by Electronic means.  

Interpoles are additional poles which are placed between the main
poles in a dc machines. These poles are designed to counteract the
effects of armature reaction in the dc machines.

Interpoles are placed at the geometric mean axis. They are wound byconductors which are connected in series to the main armature winding.

The polarity of the interpoles is the same of the next approaching pole.

The chief function of the interpoles is to compensate the reactanceemf and to minimize sparking across the brushes.

Armature Reaction refers to a phenomenon in DC machines where the magnetic field of the poles is distorted by the magnetic field produced by the armature current. 

In a generator operating on no load, the armature current is zero and the only field inside the machine is that produced by the poles.  When this generator is connected to a load, current starts to flow in the armature windings.  This current causes a magnetic field around the armature conductors. 

This magnetic field interacts with the magnetic field caused by the generator poles.  The effect of this magnetic field is both demagnetising and cross-magnetising as reduces the net effect of the main field on the armature conductors while distorting the magnetic field at the same time. 

Effects of the armature reaction. 

The principal effect of armature reaction is related to commutation.  The brushes on a dc machine are placed in the neutral plane.  The neutral plane refers to the plane where the armature brushes move in parallel to the magnetic field of the poles.  At this point the emf of the armature conductor is zero.  This facilitates easy commutation across the commutator segments. 

Due to armature reaction, the natural direction of the magnetic field of the poles is distorted.  Thus the neutral plane is also altered.  Therefore, the armature conductors are not at zero potential when they come in contact with the brushes.  This leads to sparking across the brushes and loss of power.

Overcoming Armature reaction.

There are two methods of addressing the effects caused by armature reaction.  One method is through the use of compensating winding in the field poles. Another method is the use of interpoles between the main poles to prevent distortion of the main field.

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IREDs or InfraRed Emitting Diodes are a type of LEDs which emit light in the infrared range of the electromagnetic spectrum. IREDs are widely used in process control, optical switching circuits and Logic circuits.

Like LEDs, Infrared Emitting Diodes are PN junctions which emit light when connected in a forward bias.  Under forward bias conditions, the carriers are given energy to cross the depletion layer.  On crossing the depletion layer, the carriers, both holes and electrons recombine.  Energy is released in the process of recombination in the form of a photon.

One common application where Infrared Emitting Diodes are used is the TV remote

Infrared Emitting Diodes are made of materials such as Gallium Arsenide (GaAs) and Gallium Aluminium Arsenide (GaAlAs). 

IREDs emit radiation at wavelengths such as 880nm and 940 nm (nanometres).  Thus, they are ideal for switching on phototransistors which are sensitive to electromagnetic radiation at such wavelengths.

A British Company, has developed a simple, low cost light which can be used by remote communities isolated from the main electricity grid.  This simple low cost light source can be an alternative to the kerosene lamp which is polluting and expensive as it consumes kerosene which consumes a significant part of the family's income. Kerosene has also been linked to accidents.

The Gravity lamp is powered by a dynamo which is drive by a weight (a bag of sand) which is raised to a certain level.  As the weight gradually descends, it drives the dynamo which powers the LED.

This gravity powered light can last for half an hour by when the weight comes to its lowest position.  After that, it has to be lifted to its higher position which takes about 3 seconds.  It then starts again.  The power from this dynamo can be used to power LEDs for illumination, to charge batteries or to power a radio. 

This lamp would ideal for households in remote communities.

The company says that with increased research and production, the efficiency and cost would improve further..