Magnetic Pick-up sensors

Magnetic pickup sensors are used to measure the speed of rotating objects such as shafts, flywheels, etc.  These sensors generate a sinusoidal waveform with a variable frequency.  The frequency of the signal depends on the speed of the rotating object.

The magnetic pickup sensor works by the changing magnetic field when a metallic gear passes through it. The sensor consists of a permanent magnet and a coil mounted on the same axis.  Under static condition, the flux from the magnet flows from the north pole to the south pole.  The flux passes through the coil.  However, no voltage is produced as there is no relative motion between the magnet and the coil as they are fixed.  When the tooth of a gear to be measured passes through the sensor.

The magnetic flux lines are distorted as the tooth comes in front of the sensor. And when the tooth passes, the flux lines return to their original position.  They change again when the next tooth comes in front of the sensor.  This change in the flux induces a voltage in the coil placed in the sensor.  The frequency of this emf is dependent on the speed of the gear teeth.

Thus by measuring the frequency, the speed of the rotating object is measured.   Magnetic pickups can

Why can't we use aluminium or copper vessels on an induction stove?

Induction stoves work on the principle of induction.  The stove contains a coil which is excited by a high frequency AC supply.  Above this coil is a plate.  The vessel to be heated is placed on this plate.  The alternating magnetic field set up by the coil induces currents in the vessel surface.  This current generates heat as it circulates in the vessel.

In theory, this principle should work for all metals.  However, the heating is not efficient in non-magnetic materials such as aluminium, copper, etc.  This is because non-magnetic materials such as aluminium and copper have a lesser skin effect.  It means that the current can circulate for a greater depth in the vessel walls.  This reduces the surface resistance which is important for heat to be generated.  Hence, induction heating is not efficient in non-magnetic materials such as copper and aluminium.

All metal induction stoves are in the process of development which can heat vessels of all  metals.  Non-magnetic metals can also be heated, though, at a lesser efficiency.

Anti-reflective coating in solar cells

The light which falls on a solar cell first makes contact with the shiny surface of the cell.  This results in the light getting reflected.  Thus, this the energy of the light rays are also lost along with the reflected rays.  Hence, it is essential that all photovoltaic cell have an effective anti-reflective coating which prevents the light from getting reflected.

A reflective coating ensures that the energy from all the rays incident on the photo voltaic cell is made available for conversion into electricity thereby increasing the efficiency of the cell.

Anti reflective coatings are similar to the coatings used on camera lenses.  They interfere with the process of reflection by creating another light wave which is out of phase with the reflected wave.  This causes the reflected waves to cancel each other out and no energy is lost through reflection. This phenomenon is called destructive interference.

The thickness of the anti-reflective coating is critical.  It should be perfectly selected so that the light from the surface of the anti-reflective coating is out of phase with the light reflected from the surface of the photocell.

Another method of preventing light from getting reflected off the surface is by texturing the surface of the photocell.  This causes the light to get reflected multiple times by the textured surface and a greater amount of the energy in light is captured by the photo cell.  This is called "light trapping"

The Electrolyte in a lead acid battery is sulphuric acid mixed in water.  During the discharging process, the sulphuric acid dissociates into water and sulphate which is deposited in the electrodes.

When the battery is charged, the sulphate recombines with the water to form sulphuric acid.  The specific gravity of the electrolyte thus varies from 1.1 to 1.3 depending on the level of charge. Over a period of time, there is a chance of the water in the electrolyte evaporating.

This may lead to a change in the level of the electrolyte along with a variation in the specific gravity.   This may result in a deterioration in the battery performance.   In such situations, it may be necessary to add water to the electrolyte.

Procedure
Only distilled and demineralized water should be used, as ordinary water contains minerals such as calcium, magnesium, etc which can cause deposits in the electrodes and affect the battery's life and performance. The vent caps of the batteries need to be removed.  Once, the vent caps are removed, the plates will be visible.  See if the plates are completely covered by the electrolyte.  Add water if the level of the electrolyte is low and the plates are exposed.  Take care that excess water is not added this will result in an "overflow".  The acid can overflow and cause damage.

Safety Precautions
Follow the manufacturer's instructions before topping the electrolyte with water.  Always wear protection goggles for your eyes.  The acidic electrolyte is corrosive.  Take care that you do not accidentally come in contact with it.  The sulphurous gases which may escape the electrolyte are explosive.  Do not smoke near the battery as a spark can trigger an explosion.

Product Review - Fluke CNX 3000 Wireless Test Tools

Fluke CNX 3000 Wireless Test Tools is a set of measuring instruments which can be used for making remote measurements of electrical parameters.  The set consists of a central multimeter which can communicate with measuring probes wirelessly.  Fluke says that this enables simultaneous measurements of currents and voltages.  The multimeter can collect signals within a radius of 20 metres.

The data can be read on a computer screen using a wireless PC adapter.  The data is also recordable.  This measuring tool set would be ideal for process industries which require simultaneous measurement of current at different points in the system.

Video from Fluke

Solar Power - PhotoVoltaic Cells

Photo-Voltaic energy is an ideal choice of power for locations which are far from the electric grid.

In many countries, it is not feasible to lay power lines to certain far-flung areas as there may be a small no of consumers.  Investing huge amounts of money in laying powerlines and maintaining them may not be economically viable.  Providing power by means of diesel generators may be expensive.

A practical and environmentally friendly source of power are Solar Cells.  Solar cells convert the sunlight into electricity.

Solar Cells work on the principle of the photo-voltaic effect.  When light falls on certain materials, the atoms absorb the photons and release an electron.  These electrons create an electric potential.  When an external circuit is connected, a current flows.  The direct conversion of sunlight to electricity makes the modules compact without any emissions or residue.  Photovoltaic cells are thus popular for powering small electronic devices  and lighting.

Photovoltaic modules have been used for powering aircraft, cars and even the international space station.

Photovoltaic modules are made of materials such as silicon, cadmium telluride, gallium arsenide, etc.  The basic silicon cell consists of a NP junction. When light is incident on the PN junction, electron-hole pairs are produced.  This creates a voltage across the junction.  When an external load is connected, current flows.

Photovoltaic cells are usually covered by an anti-reflective coating to prevent the incident light from being reflected away.

Current Photovoltaic cells can achieve effiency of around 30%.  Cells with concentrated sunlight focused on them can achieve still higher efficiencies.

It would be interesting to note that a solar cells is just an LED in reverse.  An LED (A light emitting diode) functions by emiting light when a voltage is applied, while the solar cell generates a voltage when light is incident.  A solar cell is specially designed to have a wide PN junction which can collect more light.

The solar cell technology needs further development before it can be widely used for base power generation.  It is hoped that further research with new materials will improve efficiency and lower the high initial capital costs.

df/dt Protection for Generators

The df/dt protection is used to identify abnormal changes in system frequency and take remedial actions in order to prevent generator overload and the resulting blackout.  The df/dt operates faster than ordinary under frequency relays as it is able to predict the under frequency much earlier.

df/dt relays are also known as ROCOF relays (Rate Of Change of Frequency Relays) The df/dt is usually wired to a load shedding system which trips select breakers to isolate loads.

The df/dt functions by measuring the rate of change of frequency.  When the frequency changes too fast, it is an indicator of a forthcoming under frequency.  The setting of the df/dt relay is in Frequency/Time in seconds. e.g. 0.3Hz/second or 0.4 Hz/.5 seconds.

Some manufacturers provide a more reliable setting involves specifying two frequency set points and the time taken for the frequency to cross the two limits.  For instance, a df/dt relays can be programmed to operate if the system frequency crosses 48.5 Hz and 48 Hz in 0.4 seconds

Factor of Earthing

The Factor of Earthing in a three phase system is defined as the ratio of the phase voltage (phase to earth voltage) on a healthy phase during the fault at the rated frequency to the phase voltage of the healthy phase during normal situation.

When an earth fault occurs in one of the phases of a three phase system, the voltage vectors are distorted.  The neutral shifts in the direction of the fault phase.  This causes the voltage in the other two healthy phases to rise.

The factor of earthing determines the rise of voltage in the healthy phases when one of the phases has a earth fault.

The factor of safety is 100% for an isolated neutral (floating neutral) system.  It is 80% for an effectively earthed system while it is 57.7% for a solidly earthed system.

The factor of safety is an important parameter while performing an insulation coordination in a system.

Thermistors - an Overview

Thermistors are temperature sensors which have a sensing element usually made of polymers or ceramics.  Thermistors function by changing their resistance when the temperature increases.

Thermistors find wide application in the industry, in automobiles and in electric appliances.  Their small size makes them ideal for use in electronic circuit boards and digital thermostats.

The principle of the Thermistor was first discovered by Michael Faraday in 1833.  However, the first practical thermistor was constructed by Samuel Ruben in the year 1930.

When the temperature of the Thermistor changes, the resistance of the Thermistor also changes.  The change can be either positive or negative.  Thus, we have PTC Thermistors (Positive Temperature Coefficient) and NTC Thermistors (Negative Temperature Coefficient).

NTC thermistors are used in temperature measurement while PTC thermistors are used in Electric current control.

Thermistors are generally formed into a disc or bead and sealed in an enclosure made of plastic or gas.

Thermistors are highly accurate and have a quick response.  However, they have a limited range of measurement.  Another downside is that they do not have a linear response.

Thermistors have high stability and are not affected by ageing.  This means that they need not be calibrated for long periods of time.    They are cheaper, rugged and are easy to produce.

Pressure Transducers

Pressure is an important physical quantity to be measured in industrial systems.

Transducers are one of the popular means of measuring Pressure.   Pressure Transducers work by converting the pressure signal into an analog electric signal usually a 4...20 mA signal.

Pressure Transducers can be designed using many principles.  The most widely used of these are the capacitive and the Piezo-resistive transducer.

Capacitive Pressure Transducers
The capacitive transducer consists of a diaphragm which works as one of the plates of a capacitor.  A fixed conductive surface acts as the other plate.  The permittivity of the space in between these plates varies as the diaphragm moves in response to the measured pressure.

This change in capacitance is measured as the process pressure.

The capacitive transducer is used to measure very low pressure values.  Very Accurate measurements are possible using the capacitive pressure Transducers.

Piezo Resistive Pressure Transducers
Piezo Resistive Transducers work on the principle of the piezoresistive effect.  The piezo resistive effect refers to the change in the resistivity of a material in response to force or pressure.  The piezo resistive sensor is used widely in biomedical applications as well as in the automobile industry.

 Piezo Resistive Pressure Transducers
These sensors are low in cost and have high sensitivity.  They can be manufactured for a wide range of pressure measurement.

Piezo Resistive Pressure transducers consist of a diaphragm which is made of silicon.  The diaphragm bends due to the pressure of the system to be measured.

Mounted on the diaphragm are four piezo-resistors which are usually arranged in the form of a Wheatstone bridge.  When the diaphragm bends due to the pressure, the piezoresistors are subject to either tensile or compressive stress.  This results in a change in resistance values which is measured through the Wheatstone bridge formation and is scaled as a pressure measurement

Inductive Proximity Sensors

Inductive Proximity sensors find wide application in the field of industrial instrumentation.  These sensors are extremely popular as they are reliable, robust and have a simple construction.  Inductive Proximity sensors are used to measure speed, detect motion and sense the position of objects.

The inductive proximity sensor consists of an oscillator, a coil and a detector.  The oscillator develops a high frequency signal which is fed to the coil.

The high frequency signal develops a corresponding high frequency magnetic field at
the tip of the sensor.  When a metallic object comes in front of the sensor, eddy currents are induced in the object.  This acts as a load on the oscillator and the amplitude of the high frequency output drops.  This drop in the voltage is detected by the detector unit which causes the switching on or off of a transistor.  This results in a change of voltage level which is interpreted as a digital signal 0 or 1.

The inductive proximity principle can also be applied to speed sensors.  In speed measurement, the inductive proximity sensor is placed near the rim of a rotating object. The rotating object has a number of teeth along its rim.  When a tooth passes near the inductive proximity sensor, a pulse is produced.

This sequence of pulses can be converted into an analog signal can be measured as the speed of the device.

Sympathetic Tripping

Sympathetic Tripping refers to the phenomenon in Electrical Systems when a protective device in a healthy section of the system operates for a fault in another section of the system.  Sympathetic tripping results in unnecessary loss of power for healthy equipment.

There are many causes for sympathetic tripping.  The most common reason is undervoltage which occurs across the system when there is a heavy current due to a short-circuit or an earth fault.

Another reason for sympathetic tripping can be the flow of capacitive currents in the healthy feeders when one of the feeders gets grounded.

In Transformers and Generators the Differential relay sometimes operates for an overcurrent which is outside its zone.  This is due to the dc component of the earth fault current.

Preventing Sympathetic Tripping

Sympathetic Tripping can be prevented by designing smaller feeders with the total loads equally balanced across the different feeders

Reducing the fault level can result in lesser currents in the event of faults.  The fault level can be reduced by the use of current limiting reactors which increase the impedance.

Extreme Inverse settings in IDMT relays can also help the relays discriminate between sympathetic overcurrents and genuine faults.

Increasing fault clearing times in the faulty feeders reduces the duration of the undervoltage across the system.

Modern Differential relays have an inbuilt dc filter which prevent sympathetic tripping due to dc components during earth faults.

Videos on Power Transformer Testing

Useful Videos on Power Transformer Testing by OMICRON

Your Guide to Energy Saving Lighting

Lighting within the home currently accounts for about 8% of energy bills in the UK. Incandescent and tungsten bulbs have been a standard feature of electrical use since the 19th century, but are gradually being phased out in favour of more energy saving options that save consumers money and benefit the environment. A number of different options are available from online electrical wholesalers or high street retailers, from energy saving bulbs to compact fluorescents, LEDs and dimmers that can help to cut costs and generate more efficient energy. Moreover, these bulbs can be combined with a number of simple energy saving practices that can be followed within the home.

Types of Bulbs and their Benefits

Early incandescent and halogen bulbs relied on a tungsten filament, and remain the norm for most homes. Halogen bulbs are more efficient than incandescent forms, but still lag behind energy saving bulbs in terms of efficiency. The UK Government have promoted schemes to gradually phase out the use of older bulbs, while encouraging a switchover to energy saving lighting options. Energy saving bulbs and lights remains fairly expensive compared to older bulbs, but have the benefit of lasting longer, and reduce electricity bills.

A basic energy saving bulb is 5 times brighter than a standard bulb, and uses 80% less power. If used responsibly, some energy saving bulbs can have a 10 year life span. This length is based on using certain bulbs for three hours a day in parts of the home, and can be an ideal solution for rooms that are not used very often. Energy saving bulbs can result in 75-80% energy savings, and use 4 times less of the wattage of standard bulbs.

Other energy saving bulb options include compact fluorescents. These bulbs use an alternative gas charging method to standard bulbs, and use 20 to 25% less electricity. Again, higher costs for initial purchases can be offset by their long lasting potential and greater energy efficiency.

Another option is to invest in LEDs, or light emitting diodes. These represent strips of about 36 to 48 lights, which when installed can generate 50,000 hours of capacity. Representing 50 times as much capacity as a standard incandescent light, small LED arrays also act as a stylish alternative to hanging fittings, bulbs and lamps within rooms, and are particularly recommended for kitchens.

These lights emit less carbon dioxide than standard incandescent lights, and can consequently help conserve energy and the environment. When looking for energy saving bulbs always check for an Energy Saving Trust Recommended label, or an Energy Related A tag. Energy saving bulbs can also be recycled, and form part of the EU’s Waste Electrical and Electronic Equipment Initiative.

Other Tips

As well as investing in energy saving bulbs, you can also follow some simple steps when using lights in the home. The most basic solution remains turning lights off when they are not being used. Moreover, try to use lights for particular roles, with bulbs being turned off when watching a brightly lit television or computer. If reading, a single lamp is more efficient than keeping a whole room’s lights on. Dimmer switches are also useful in this regard for regulating the amount of light in a room, and can result in 4-9% of electricity savings.

Serena is a copywriter for a leading supplier of energy saving discount electrical supplies at Discount Electrical. In her spare time she writes various other blogs online on numerous other subjects such as automotive, health and the theatre.

Capacitor Trip Modules.

Capacitor Trip modules are used in breaker circuits to provide a source of back up power for trip operations in the event of the failure of the breaker control supply.  The capacitor Trip device is usually used in switchgear systems which use an AC control supply.

If the control supply of a breaker panel fails during operation, the operator will not be able to operate the breakers from a remote location.  This would also compromise the protection scheme as the breaker will not trip even if a command is sent by a protection relay.

The capacitor trip module consists of a capacitor which stores charge.  This charge is enough to trip the breaker for a certain number of times, say 5 times.  This ensures that the breaker can trip even if the control supply fails during operation.  Capacitor Trip modules are available for both AC and DC control systems.

Grounding Transformers

Grounding Transformers are used in Ungrounded systems to provide a earth point.  Grounding Transformers are classified into two types

1) Zig Zag Transformers and

2) The Star-Delta Grounding Transformer with secondary unloaded.

Let us now look at the Star-Delta Grounding Transformer

The Star-delta grounding Transformer has a primary which is star connected and a delta secondary.  The phases of the star primary are connected to the busbar while the neutral is grounded.  The secondary of the transformer which is delta connected is usually left unloaded, though it can also be used to supply power.  The delta serves to provide a return flux path for unbalanced loads.

During an earth fault, the zero sequence currents can flow through the grounded neutral of the transformer.  If the current is to be limited, a resistor can be added in series to the neutral of the transformer primary.

Braking Resistors in Variable Frequency Drives

Braking Resistors are used in Variable Frequency Drives to dissipate the energy released by the motor into the power system.  The Braking Resistors perform the duty of absorbing the power from the rotor when the VFD reduces the speed to zero and preventing rotor heating.  They also prevent the rotor from exceeding the synchronous speed set by the Variable frequency drive.

A variable Frequency drive consists of three main components - the rectfier which converts the AC supply into DC, the DC busbars and the inverter which converts the DC supply into a variable AC supply.  The VFD varies the speed of the motor by varying the frequency of the AC supply applied at the motor terminals.

When the motor is required to be stopped suddenly, the Variable frequency drive reduces the supply frequency to 0 HZ.  In this condition, the rotor is rotating at speed higher than the synchronous speed.  This causes the motor to behave like a generator and send power in the reverse direction, into the DC bus bars.  During this time, the voltage across the DC busbars can rise to very highlevels.  The braking resistors absorb power in this situation and prevent the voltage from rising beyond limit and damaging the Drive.

The value of the resistances determines the rate of fall of the motor speed (braking).

Special provision is made for cooling the resistors which can generate a huge amount of heat when in operation.

Resistance Temperature Detectors

Resistance Temperature Detectors or RTDs are sensors which measure the temperature by altering their resistance. >The Resistance temperature detector consists of an element made of a metal such as platinum  located in a metallic casing.

When the temperature increases, the resistance of the sensor increases (positive temperature coefficient of resistance) This increase in the resistance is measured through a wheatstone bridge. The relationship between temperature and the resistance is linear.  Thus, the temperature can be deduced from the measured resistance.

Platinum and Nickel are two metals used to construct the sensing elements.

Some common types of RTDs are the Pt-100 and Pt-1000.

The Pt stands for Platinum while the number 100 stands for the ohmic value at 0 degrees Celsius.

The resistance increases linearly with temperature.

For example, the Pt100 has an ohmic value of 100 ohms at 0 °C and a value of 161 ohms at 160°C

Accuracy
Long Term Stability
Ability to withstand shock and vibration

Slow response
Internal Self heating

2 Wire, 3 wire and 4 wire RTDs

One of the disadvantages of the RTD is the error caused by the lead resistance.  That is, the indicating device which measures the sensor resistance to calculate resistance also measures the resistance of the leads connecting the sensor to the device.  This is unavoidable, though the error can be minimized by running a wire in parallel to one or both the leads. (Refer diagram)

4 - 20 mA current signal in Instrumentation

The 4 - 20 mA signal is an extremely popular signal specification in instruments.  The 4 to 20 mA signal provides "live zero" function.  i.e. in the event of a wire break, the signal drops to zero mA.  This can be used to detect wire break or sensor failure.  This is a crucial advantage of the 4 to 20 mA format.

Being a Current signal, it is also less susceptible to external interference.

The 4 to 20 mA format works by varying the resistance to a constant source voltage.  The sensor consists of a transistor which regulates the current passing through it in accordance with the measured value.  Sensors which use this format can be categorized into active and passive transducers.  Active transducers are devices which can provide the system voltage as well as regulate the current.  Passive devices require an external voltage and only regulate the current.

Another advantage of this 4 to 20 mA format is that it can be easily converted into a voltage signal by means of a resistor. (1 - 5 volts for a 250 ohm resistor).

Electrical Software List

List of Electrical Software

ETAP
Popular design software used across the industry for a wide range of design applications  in areas such as power management, substation automation, load shedding, etc..  Demo Version Available.
www.etap.com

Popular Software for Electrical Design.  Offers Trial Version.

Elecdes
Electrical Design Software with a free demo version. Suitable for Electrical Panel Design, Plant Instrumentation, Wiring Diagram, Plant Raceway Design, Cable Routing,
http://www.elecdes.com/

PCschematic
CAD software with free Trial version.
www.pcschematic.com

SmartDraw
Electrical Software for designing electronic Circuits, Automotive Wiring, Circuit Schematics and designing digital circuits.
www.smartdraw.com

Electrical Estimating Software
Software for Estimating Electrical projects. Contains online demo.
www.electricalestimatingsoftware.com

EasyPower
Software for Arc Flash Analysis and short circuit Calculations, Relay setting coordination and Equipment Sizing.  Offers Trial Version.
http://www.easypower.com/

Software for designing electrical installations in industrial and tertiary buildings.  Trial version available.
http://www.soft.schneider-electric.com/

What do we mean by MTOE

MTOE is the acronym for Million tonnes of Oil Equivalent.  It is a unit to quantify the amount of energy which is released by the burning of a million tonnes of crude oil.

Different fuels have different MTOE values.  The OECD defines one tonne of energy equivalent for crude oil to be 11,630 kWh.  That is, 11630 units can be produced from one tonne of crude oil.

MTOE is important as it helps one understand how many units of electricity can be from an given fuel.  This depends on the calorific value of the fuel and the efficiency of the generating process.

Hydroelectricity can also be quantified in terms of MTOE (million tonnes of Energy Equivalent).  When we say that Brazil has 89.6 tonnes of oil equivalent in hydroelectricity.  It means that the power generated by the total hydroelectric resources and infrastructure is equal to the power which could be generated by 89.6 tonnes of oil.

MTOE is a unit which helps compare the potential and contribution of different sources of power such as hydroelectricty, wind electricity, etc.

Economy Resistor in DC Coils

When a dc coil connected to a contactor or a solenoid is energized, it draws a high current. This high current is necessary to generate enough magnetism to pull the plunger or the contactor. This is known as the pull in current.

After the the contactor has been pulled in, less current will be sufficient to hold the coil. This is called the holding current.

Hence, to limit the current after the contactor has operated, a series resistance is connected. This is known as the economy resistor. The economy resistor enables the design of contactors which can draw heavy initial current and less holding current (resulting in lower power consumption during normal operation).

In the case of AC coils this is not required as the current is limited after the initial inrush due to the back-emf.

Amorphous Metal Transformers

The chief losses in a transformer are the losses that occur in the core.  These are the the eddy current loss and the hysteresis losses.  About 1 to 4 percent of the power which passes through a transformer is lost due to these losses.

The losses in the distribution transformers constitute nearly 20% of the total losses in the distribution system. Since transformers are online continually, the no load losses of the transformers is constant throughout the day regardless of the load.

The Amorphous Metal Transformer is fast emerging as an efficient alternative to the conventional transformer.  The Amorphous Metal transformer has a core which is made of ferromagnetic materials such as Iron or Cobalt in a glass former such as phosphorous, silicon or boron.

Metglas, as this substance is known, has high susceptibility, low coercivity and high resistance.  The low coercivity reduces the hysteresis losses while the high resistance greatly reduces the eddy current losses.

By using Amorphous Metal Transformers, it is estimated that many millions of units of electricity can be saved.  Amorphous Metal Transformers are widely used in developing economies such as India and China in an effort to bring down the distribution losses.

Some of the other advantages of Amorphous Metal Transformers are the lower operating temperature, higher overloading capability, slower ageing of the winding insulation and better performance when subjected to harmonics.

Surface Charge in batteries

Surface charge and surface discharge in batteries refers to the superficial charging and discharging which occurs only on the surface of the electrodes.  A lead acid battery consists of lead oxide anode and a lead cathode.  When discharging, the lead oxide in the anode and the lead in the cathode get converted to lead sulphate.  When charging, the reverse happens.

Surface charge refers to a condition when the chemical changes mentioned above occur only in the surface of the electrode.  For instance, if only the lead sulphate in the surface of the anode of a discharged battery gets converted into lead oxide, the battery will indicate a full charge when the open circuit voltage is  measured.  However, the charge in the battery will last only for a short time.

Surface discharge is a condition when the open circuit voltage of the battery wrongly indicates a discharged condition, when the battery is still holding charge.  This too relates to chemical changes which occur superficially on the surface of the electrode.

The surface charge can be removed by applying a slight load on the battery.  In vehicles, switching on the headlights for a while can help remove the surface charge.

Sulfation in batteries

Sulfation in lead acid batteries refers to a condition when the battery is not able to hold any charge.  It occurs when the plates of a battery get hardened with a layer of lead sulphate.

Sulfation occurs when the battery is kept unused for long periods of time in the discharged state.  This makes the lead sulfate in the plates to get hardened.  These hardened plates prevent the battery from charging.

The lead acid battery consists of an anode made of lead oxide and a cathode made of lead.  These two electrodes are placed in an electrolyte of sulphuric acid.  When the battery is discharged, the sulphuric acid reacts with the electrodes which are transformed into lead sulphate.  As a result, the sulphuric acid in the electrolyte becomes dilute and almost becomes water.  The specific gravity of the electrolyte thus drops.

When the battery is recharged, the lead sulphate in the cathode and anode is converted into lead and lead oxide respectively.  The sulphate in the electrodes react with water to again form sulphuric acid.

When the battery is kept unused for long periods of time in the discharged condition.  The lead sulphate which is in the electrodes solidifies into a layer which has high electrical resistance.

When the battery is recharged using an external supply, this hardened layer of lead sulphate prevents charging.  The electrolyte too, does not become sulphuric acid.  Thus the battery is not able to absorb charge.

Another reason for sulfation is incomplete charge and discharge, When the battery is not charged capacity and  not discharged completely, a small amount of the sulphate always remains in the electrodes.  This forms a hardened layer and diminishes battery capacity.
Identifying Sulfation in batteries

If your battery is connected to the charger for a long time and still does not have charge.  Test the specific gravity with a hydrometer.  If the specific gravity is low, the battery probably has the problem of sulfation.

Repairing Sulfated batteries

Sulfated batteries can be repaired by applying a high charging voltage and low charging current from the battery charger.

When batteries are to be kept unused for long periods, they can be connected to a battery minder, an electronic device which continually monitors the voltage level of the battery and prevents sulfation from occuring by firing electronic pulses into the battery to break any sulphate layer which may have formed.

Single Phase Pole Mounted Distribution Transformers

Single Phase Pole Mounted Transformers are usually used in rural areas where three phase power may not be required.  These Transformers reduce the voltage from the line voltage 11kV to a single phase voltage usually 230V.

The secondary of the Pole mounted Distribution is usually connected between the two phases of a MV line.  The secondary voltage is a single phase voltage which is fed to the house.  These transformers can be easily installed and do not require extensive mounting Structures.
Pole mounted Transformers come in sizes up to 500 kVA.

These transformers have a fuse to protect against faults inside the transformer.  They are also equipped with an interrupting device.  These Transformers are tested to withstand the impulse of lightning. In the US, The secondary of these transformers has a centre tapping and therefore has three terminals.

The voltage between the end terminal and the centre tapping will provide 120V while the voltage between two end terminals will be 220 volts. These Single Phase Pole Mounted transformers can also be used to provide three phase LV supply.

Three pole mounted transformers are connected in wye or delta to get the desired connection. Besides, small size three phase transformers which can be mounted on poles are also available. Video Showing the Manufacture of Single Phase Transformers

Wireless Switches

Wireless Switches are used in homes, godowns, offices, etc to remotely control an electric appliance such as a lamp or a fan. Wireless switches work by preventing power from flowing through a receiver into the device.

The switch is mounted to the switchboard of the plug. The appliance to be controlled is connected through the device. When the switch is operated, it emits a radio frequency which activates or deactivates the receiver controlling the supply to the device. Wireless Switches can result in energy savings as it is easy to switch off the lights and fans in the room at the press of a single switch from another room.

Wireless switches can also be used to dim lights or reduce the speed of fans. You can adjust the lighting to suit your moods. You can switch on the lights when opening the front door when you return home in the evening. Wireless switches can be used to save energy. Dimmed lights consume less power. You can control your energy consumption while using optimum lighting.

Wireless switches can also be made part of a lighting program. These lighting programs can be activated when you are on a vacation. Lights can be switched on and off at different times of the day. This gives an impression that your house is occupied and can ward potential burglars and thieves.

Here is a video which describes the wireless switch and its function

Distribution Transformers

Distribution Transformers play a vital role in the system which delivers electricity to the end user.   It is the final part of the transmission system from the power plant to the consumer.  Distribution Transformers step down the MV power, usually 11kv into the domestic LV, 440 V supply.

Distribution Transformers are a critical part of the distribution network.  These transformers are always online throughout the year.  Hence, design of the distribution transformer is made considering the high iron losses.  Besides, the transformer is sized to have high efficiency at 70% of the load as the power output varies through the day as per the load cycle.

Distribution transformer are protected by fuses in the HV side.  They are also designed to withstand unbalanced loading.  They have ONAN cooling (Oil Natural, Air Natural ).

Distribution Transformer are usually of the vector Dyn11.  While, designers are not particular about any particular vector group , most systems will standardize on one particular vector group, usually the Dyn11.  Some systems also use the Dyn1.  These vector groups have a difference of 30 degrees between the primary and secondary vectors which is unavoidable in delta to star conversion.

B, C and D types in Miniature Circuit Breakers

MCBs (Miniature Circuit Breakers) are categorized into B, C and D types.  These three ratings are determined by the level of overload which causes the MCB to trip.

B type MCBs operate at an overload of 3 to 5 times the rated current.  Type B MCBs are usually used in domestic installations where the inrush currents and surges are low.

C type MCBs operate at an overload of 5 to 10 times the rated current.  These MCBs are used in commercial and industrial installations where high inrush current are likely due to motor starting or due to large no of fluorescent lighting.

D type MCBs are used in special applications such as x-ray machines and transformers which can draw heavy inrush current.

Plante Batteries

Plante Batteries were invented by the French Scientist Gaston Plante in the year 1859.  The Plante Battery was the first rechargable battery.  It is also the first lead acid battery.

As a  lead acid battery, the Plante Battery uses pure lead plates.  The advantage of the Plante battery is that the capacity remains the same throughout its life.  Its grid design enables it to generate high currents and is thus suitable for applications which require high bursts of currents.  The active material in the positive plate is generated and regenerated throughout its lifetime.  Hence, there is  no loss of capacity.

Each cell typically has a voltage of 2 volts.  Its ampere hour efficiency is 90%.

The downside of this battery is its large size and high cost.

What is a Trivector Meter ? Where is it used ?

The Trivector meter is a measuring instrument which measures the kW, kVAr, the kVA of a power line.  These instruments can measure both power as well as energy.    Trivector meters are normally used in substations and to measure the power flowing through the feeders.  They are used for billing power drawn by industrial customers.  The Trivector enables the simultaneous measurement of different electrical parameters which enables accurate assessment of the power consumed.

Trivector is called so as it measures three vectors representing the active, reactive and apparent power of a line.  Trivector meters come in two quadrant and four quadrant models.  The four quadrant model can measure both the incoming (import) and the outgoing power (export) while the two quandrant trivector meter can measure either imported or exported power.

In earlier days, the Electromechanical trivector meters were used.  Today, though, almost all Trivector meters are of the static type.  Modern Trivector meters can measure many parameters apart from the active, reactive and apparent power.

Here are a few videos

Micro USB Charging

The Micro USB has emerged the medium of choice for charging mobile phone and other hand held electronic devices.  The micro USB is a further step in the miniaturization of the USB.  The Micro USB was developed by the USB Implementers Forum, a body dedicated to the promotion of USB technology.  The standardization of the micro USB as the charging standard will eliminate the many different charging pins devised by different manufacturers.

As mobile phones and other smart devices become ever more smaller, the micro USB appears ideal to suit the sleeker designs.  The micro USB is also more robust.  Present designs made of stainless steel are supposed to withstand 10,000 insertions and extractions.  The USB is designed to withstand stress both in fitment and removal.

In keeping with this development, the International Electrotechnical Commission (IEC) has released a standard the IEC 62684.  This standard had been developed after discussions with the USB Implementers Forum.  Major Manufactures of mobile phones such as Nokia, Samsung, LG, Research in Motion, Huawei had also given specifications which had been incorporated in the standard.

Class of Power Supplies in Nuclear Power Stations

In a nuclear power station, the sources of power are classified on the basis of reliability into four distinct classes.  Each of these four classes is, in turn,  provided with multiple redundant system to ensure maximum reliability.
Class 1 - DC Power from  battery banks

Class 2 - AC Power from UPS

Class 3 - AC Power from Emergency DGs

Class 4 - AC Power from the Grid and the Turbo Generator of the Power Plant. In the event of a major external incident such as an earthquake or a storm, class 4 power can be interrupted.  The Class 3 power is supposed to be activated by starting the emergency DGs.

Class 1E refers to the power source which is essential for the safe shutdown of the reactor, isolation of the containment and removing the residual heat from the reactor in the event of a shutdown. The Class 1E bus can draw power from the Emergency DGs as well as the Grid.

Type Testing of panels

Type testing of panels involves checking whether the panel fulfils specific requirements that are mandatory for operation in a specific location or function.  The standards can be internationally recognized standards such as the ANSI, the IEC, etc or local national standards.

The components of a Type tests include tests for short circuit, temperature rise, electro magnetic compatibility, creepage, clearance, ingress protection, mechanal function, etc.

While Type testing of panels is expensive and time consuming, they are a necessary to ensure safe and reliable operation of the panel.  Installation and operation of panels which have not been type tested can lead to loss of insurance claims and chances of prosecution in the event of accidents.

Load banks are used for testing generators, transformers, discharging batteries and UPS systems.  They are used to prove the capacity of a power source or in tuning the speed and voltage regulating system in generators.  They consist of elements which serve as the load.  Load Banks can be designed to draw both active as well as reactive power.  It is possible to have the load bank draw power at a specific power factor.

The high power consumed by the load bank results in the generation of heat.  This is heat is dissipated usually by forced air cooling involving fans.  Load Banks are usually portable and are enclosed in containers.  Load banks which draw power at different voltages are available.  Load banks have provisions to incrementally increase the loads on the power source.

Cathodic Protection

Electrochemical Corrosion is one of the types of corrosion in metals.  Electrochemical corrosion occurs, chiefly, in submerged metallic structures such as pipelines, storage tanks, water circulating systems, ship hulls and off shore platforms.  Electrochemical Corrosion occurs when the potential on the surface of the metal is not uniform.

This uneven potential on the surface is caused due to impurities in the surface or uneven stress on the surface of metals.  This causes certain parts of the surface to act as an anode while other parts of the surface act like the cathode.  The current which flows between the anodic and the cathodic regions can cause corrosion.

Cathodic protection refers to a method of protection of these metallic structures from electrochemical corrosion.  This is achieved by making the metal to be protected the cathode.  When a material which has a higher electrode potential is kept in the same medium as the object to be protected, it becomes the cathode while the object to be protected becomes the cathode.  A simple electrochemical cell is created.  The electrons move from the anode to the cathode in the medium.  In the process, the anode gets corroded while the cathode is protected from corrosion.  The anode used is called the sacrificial anode.

The sacrificial anode has to be periodically replaced as it gets eaten away.

Distilled water in batteries

Distilled water needs to be added periodically to the Lead Acid Batteries to compensate for the water lost during electrolysis.  When a discharged battery is charged with the power from an external charger, the current causes an electrolysis of the water in the electrolyte.  The water dissociates into hydrogen and oxygen.  These gases are released through the vents.  This process results in drop in the level of the electrolyte.  To compensate for this, distilled water is added when the level of electrolyte falls below a particular level.

Distilled water alone should be used as it is pure water without any impurity.  Tap water should never be used as the chemicals present in tap water can seriously damage the batteries.  Care should be taken that water is not added in excess known as overwatering.  Overwatering will dilute the electrolyte and affect battery performance.

APC UPS Systems - A Guest Post

UPS or Uninterrupted Power Systems are widely used in offices, hospitals, industries, in telecommunication as a back up against sudden power cuts. Sudden Power cuts can cause disturbances and losses in the form of lost data, interruption to life support systems in hospitals and unpredictable behaviour in industrial systems. A UPS which usually consists of an battery system and an inverter steps in the moment the supply voltage dips beyond a certain value preventing the interruption of supply to critical systems.

The central part of the UPS is the battery which stores power when the supply is available and makes power available when the mains supply is switched off. A UPS system can provide power backup for a time period ranging from a few minutes to many hours. The duration of the back up supply depends on the capacity of the battery. APC is a division of Schneider Electric which deals with UPS and Power back up systems. The company provides a wide range of batteries with different capacities and ratings for the power back up solutions.

Besides their use as sources of back up power, these batteries can also be used to protect critical equipment against sudden dips in the voltage which can electronic equipment to switch off or reset causing unwanted interruption. APC provides a wide range of UPS solutions for Homes, Small Business and for Large Corporations. The UPS provides sine wave output and distinct LED indicators which indicate the state of the battery and the power conditions.

The systems have a Home away switch which charges the batteries while you are away besides a intelligent battery charging system which enables controlled charging of the batteries thus prolonging battery life. All products are backed by a warranty and the wide service network of APC.

You can check out their products in their website http://www.apc.com

Alex from HTBS  is a technology expert who often writes about topics related to batteries and especially regarding the APC Replacement Battery

Stuck breaker protection

Stuck breaker protection is a situation in which a circuit breaker fails to operate even after receiving a tripping signal from a relay or a switch.  Stuck breaker can undermine the protection scheme and can cause damage to machinery and is a danger to personnel.

Common reasons for a circuit breaker not opening are a disconnection in the trip circuit or a mechanical problem with the circuit breaker.  In these conditions, there needs to be a backup protection device which can interrupt the fault and isolate the system.  In some cases, the entire section of the bus to which the breaker is connected is de-energized to interrupt power.

A simple Stuck breaker protection schemes functions by sensing the position of the circuit breaker through the limit switches in the circuit breaker.  The protection system waits for the open status from the circuit breaker after the open signal has been given.  If the signal is not received within a preset time, the scheme assumes that the breaker is stuck and initiates backup measures.

However, this system has its limitations.  The system cannot detect a situation where the current continues to flow despite the breaker having tripped.  This can occur due to situations where the arc has not been quenched (failure of the arc extinction system) and the current flows even though the contacts have mechanically separated.

To ensure proper feedback of the interruption of the current, advanced stuck breaker schemes sense the current as well as the position contacts of the circuit.  This ensures that an accurate feedback of the breaker status.

Voltage Supervision Relays

The Voltage Supervision Relay is an integral part of any  protection system.  The voltage supervision relay protections systems from undervoltage and overvoltage.  Overvoltage in a system can result in serious damage to insulation and equipment while undervoltage can cause motors to draw more current and reduce the speed of the motors, disturbing the process.

Besides protecting against overvoltage, the voltage supervision relay can also be used to detect earth faults as the phase to earth voltage is distorted when there is an earth fault in one of the phases.  Voltage supervision relays can generate alarms when the voltage is low or high in only one phase.  This is also known as phase asymmetry.

In motor circuits, the voltage supervision relay protects against single phasing.  Single phasing can cause serious damage to motors.

A simple auxiliary relay can also be used to generate alarm for undervoltage.  When the voltage drops, the relay can drop off thus generating an alarm or a shutdown.

Trip Circuit Supervision in Circuit Breakers

Trip circuit supervision in Circuit breakers is an vital part of any protection scheme. If the trip relay fails to operate, it may result in upstream tripping or even in damage to equipment.  Trip circuit supervision makes sure that the tripping coil of a circuit breaker is always in the healthy condition.

The Trip circuit supervision is particularly important in breakers which have only one trip coil.    The Trip circuit supervision relay continually measures the resistance of the trip coil of circuit breakers.  It also measures the control voltage of the trip coil and gives and alarm when the control voltage falls to low levels.

The Trip circuit supervision relay injects a constant current through the trip coil of the breaker and measures the voltage drop across the coil.  Thus, the relay is able to measure the resistance of the coil.

The Trip circuit supervision relays can also monitor more than one breaker coil.

If the Trip circuit supervision Relay detects a fault, it activates the breaker failure logic which can activate a backup breaker if installed or cause the tripping of upstream breakers.

What is the difference between sine wave and non-sine wave ups

 Square Wave Output
Inverters are circuits which convert DC into AC.  Inverters are an integral part of UPS (Uninterrupted Power Supplies).  They convert the DC stored in the batteries of the UPS and produce an AC voltage which is provided to connected load in the event of a power failure.

The AC produced by the inverter is not always a sine wave.  Some inverters produce a square wave.  These inverters are known as square wave inverters.   Square waves can be used where the load on the UPS is going to be mostly resistive loads such as heaters, incandescent lamps, etc.
 Modified Sine Wave Output

Another form of inverter output is the modified sine wave or the quasi-sine wave inverter.  These inverter produce a waveform that has an intermediate voltage level which brings it closer to a sine wave.

 Sine Wave
Sine wave inverters produce an actual sine wave. Sine wave inverters are more expensive than the square wave and the modified sine wave inverters.  However, they are ideal as electric devices such as motors, Television sets, chargers are designed to use a natural sine waveform.  Using square wave inverters on these devices can produce harmonic distortion, humming.  This leads to reduced efficiency and loss of power.

What are Trip Free and non-Trip Free Circuit Breakers

Trip Free Circuit Breakers are circuit breakers which can trip even if they are held in the "ON" position.  Hence, it is not possible to forcibly keep them in the closed position.  Trip free circuit breakers are used in circuits with equipment which are sensitive to overload and in circuits which are not critical.

Non-Trip Free circuit breakers are used in circuits which are critical for plant operation and for safety.  In these systems, it may be economical to lose a motor or a heater than to interrupt the manufacturing process.    In these circuit breakers, the trip function can be bypassed by forcing the circuit breaker to the on position.

Conditions for Paralleling two Transformers

Transformers may need to be paralleled with other transformers to share loads greater than the capacity of the individual transformer.  When transformers are to be connected in parallel, it is necessary for them to satisfy certain basic conditions.

They are

The Same Voltage Ratio.

Two transformers in Parallel should have the same primary and secondary voltage ratings.  Any error in the voltage ratio would cause heavy circulating currents to flow between the transformers.   This circulating current will result in a corresponding imbalance in the primary currents, and result in overloading of one transformer.  This circulating current will result in increased copper losses.

The Same Percentage impedance

For two transformers of different capacity to share the load proportionally, their impedances should be in the inverse ratio of their  ratingsThat is, the percentage resistance and the percentage reactance of the two transformers should be equal to ensure equal sharing of the active and reactive loads between the transformers.

The Same Polarity

Two Transformers in parallel should have the same polarity.  Connecting transformers with wrong polarity can result in circulating currents or short circuits.  Some transformers may have inherent polarity errors which needs to be detected before connecting them.

The Same Phase Sequence

Only Transformers having the same Phase sequence can be connected in parallel.  The phase sequence can be detected using a phase sequence indicato.

The Same Phase Angle

The difference in the Phase angle between the secondary voltages should be zero.  This is ensured by checking the compatibility of the vector groups of the transformers to be paralleled.

Innovations in DC Technology by ABB

Informative video from ABB on developments in DC Power Transmission and distribution. It also touches on some of the advantages of DC over AC

Deep Cycle Batteries

Deep Cycle Batteries are batteries which can be discharged to very low levels.  These batteries are generally used where continuous use of the battery is required such as  in battery operated vehicles such as golf carts, floor sweepers and forklifts.
The deep cycle battery differs from other batteries in its construction. The plates of the battery  are thicker as compared to the sponge type plates of other batteries.

The use of solid plates with lower surface area  in the deep cycle battery means that the battery cannot give  sudden pulses of high current as may be required for batteries used in applications vehicle starting.  The deep cycle battery is designed to provide a high current over a long period.

Deep Cycle batteries are widely used in the field of renewable energy.  They can be used to store energy from solar panels during the day and can be discharged during the night.  Deep Cycle batteries usually have a higher C rating (refer article on C rating).  This enables them to be discharged over many hours.

Peak Load Shaving refers to operating certain generators specifically to address peak demands in the load cycle of a utility.   The load cycle tends to peak in the mornings and falls during the noon.  It again rises in the evening and the night.

Managing these peaks in the load cycle is an important function of distribution systems. Peak Load Shaving, as this process is known, is done by starting certain power sources specifically to offset this peak demand.  These are usually sources with higher generating costs like diesel generator sets.  These power sources are run only during periods of peak loads or in times of emergency.

Another method of peak load shaving is by providing incentives to customers to reduce consumption during times of peak demands.  This levels the load cycle curve and enables optimum loading of power sources.

Synchronous Compensators

Synchronous Compensators are synchronous motors which are run without any load.  These motors are used to generate or absorb reactive power from the system.  Generally located near large loads, these motors, running in the overexcited condition provide reactive power according the the load demand.

In large transmission lines, the line capacitances generate excessive capacitive line charging current which can lead to a rise of voltage in the system. In such a scenario, the synchronous compensator can run underexcited and absorb the excess reactive power.  This is particularly true in EHV (Extra high voltage systems) where the capacitive line charging is higher than the magnetizing power of the load even during loaded conditions.

While static reactors and capacitors can be used in reactive power regulation in lines, sychronous compensators have the advantage of quick responses and fine control depending on the excitation.

In newer installations, static compensators with thyristor controls have replaced synchronous compensators owing to their low maintenance and running costs.

Thyrite Arrestors

Thyrite is a material obtained by a special type of clay mixed with carborundum (Silicon Carbide).  Thyrite is used widely in lightning arresters.  Thyrite is a non-linear resistor. i.e. it has high resistance at low voltages and low resistance at high voltages.  A two times increase in voltage causes the current to increase by nearly 12 times.  Hence, heavy currents can be discharged during voltage strikes and other surges.  This heavy discharge of current enables quick reduction of the surge voltage preventing flashovers.

The Thyrite arresters are usually arranged in parallel with the primary winding of the Transformer.  The Thyrite inside the arrester is arranged in the form of discs.  The sides of the discs are metal plated to decrease resistance between the discs.

Once the surge has been discharged, the Thyrite quickly returns to its high resistance state.

Slow Blow Fuses

Slow Blow fuses are fuses which have an inherent time delay.  These fuses are widely used in motor protection circuits.  These fuses help discriminate between the high inrush current of motors and a high current caused by a fault.

The time delay in the fuse is achieved by designing the fuse element with a higher mass for the same current rating.  This higher mass causes the element to heat gradually in the event of an overcurrent.  Thus, the fuse element takes time to heat and melt to isolate the current.

Kraft Paper Insulation

Kraft Paper is a special paper used in electrical insulation.  Kraft paper is just plain paper from which the alkaline salts have been removed.  It is usually impregnated with mineral oil, jelly or wax to improve its dielectric properties.
Kraft paper is made thermally resistant by applying a diamond patterned epoxy resin to both sides.  This is known as thermally upgraded Kraft paper which finds wide application in oil filled transformers for coil layer insulation.

Paper capacitors are also made using Kraft paper as the dielectric.