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Wednesday, March 25, 2009

Earth Hour




On Saturday 28 March at 8.30pm residents and visitors to the UK will see key buildings and landmarks going dark for 60 minutes in a graphic demonstration of support for determined international action on climate change.




The people of the UK will join those in over 930 cities and towns in 80 countries, across 25 time zones which will power down for WWF’s Earth Hour. If this sounds of interest to you and you want to know what you can do to join in visit the link.
http://earthhour.wwf.org.uk/


Sunday, March 22, 2009

Tidal Power





If there is one thing we can safely predict is the coming and going of the tide, it is a reliable and predictable source of energy. This gives this form of renewable energy a distinct advantage over other sources that are not as predictable and reliable, such as wind or solar. The Department of Trade and Industry has stated that almost 10% of the United Kingdom’s electricity needs could be met by tidal power. Geographically the UK is in a prime location not only does the sea move due to tide but prevailing winds also help reinforce the movement of water.
A question you may be asking is why do the tides come and go? It is all to do with the gravitational force of the Moon and Sun, and also the rotation of the Earth.
The gravitational attraction of the moon and sun affect the tides and the magnitude of this attraction depends on the mass of the object and its distance away. The moon has the greater effect on earth despite having less mass than the sun because it is so much closer. The gravitational force of the moon causes the oceans to bulge along an axis pointing directly at the moon. The rotation of the earth causes the rise and fall of the tides. When the sun and moon are in line their gravitational attraction on the earth combine and cause a “spring” tide. When they are as positioned in the first diagram above, 90° from each other, their gravitational attraction each pulls water in different directions, causing a “neap” tide.

The rotational period of the moon is around four weeks, while one rotation of the earth takes twenty hours; this results in a tidal cycle of around twelve and a half hours. This tidal behaviour is easily predictable and this means that if harnessed, tidal energy could generate power for defined periods of time. These periods of generation could be used to offset generation from other forms such as fossil or nuclear which have environmental consequences. Although this means that supply will never match demand, offsetting harmful forms of generation it is an important starting point for renewable energy.
Current Technologies
There are two options for getting energy from the tide, a tidal barrage or utilising tidal streams.
The Tidal Barrage System
This is where a dam or barrage is built across an estuary or bay that experiences an adequate tidal range. This tidal range has to be in excess of five metres for the barrage to be feasible. The purpose of this dam or barrage is to let water flow through it into the basin as the tide comes in. The barrage has gates in it that allow the water to pass through. The gates are closed when the tide has stopped coming in, trapping the water within the basin or estuary and creating a hydrostatic head, the larger the tidal range the larger the hydrostatic head and so the larger the potential energy that can be used. As the tide recedes outwith the barrage, gates in the barrage that contain turbines are opened, the hydrostatic head causes the water to come through these gates, driving the turbines and generating power. Power can be generated in both directions through the barrage but this can affect efficiency and the economics of the project.

This technology is similar to Hydro-power or wave-power (which will be a topic for discussion at another time), something that there is a lot of experience with in Scotland. There is potential for a project of this kind in Scotland, one place in particular which has been looked at is the Solway Firth in south west Scotland, where there is a tidal range of five and a half metres.
The construction of a barrage requires a very long civil engineering project. The barrage will have environmental and ecological impacts not only during construction but will change the area affected forever. Just what these impacts will be is very hard to measure as they are site specific, and each barrage is different. Damage to local habitats will have to be reviewed and considered.

The following diagram is a simplified version of a tidal barrage.







There are different types of turbines that are available for use in a tidal barrage. A bulb turbine is one in which water flows around the turbine. If maintenance is required then the water must be stopped which causes a problem and is time consuming with possible loss of generation. When rim turbines are used, the generator is mounted at right angles to the to the turbine blades, making access easier. But this type of turbine is not suitable for pumping and it is difficult to regulate its performance. Tubular turbines have been proposed for the UK’s most promising site, The Severn Estuary, the blades of this turbine are connected to a long shaft and are orientated at an angle so that the generator is sitting on top of the barrage. The environmental and ecological effects of tidal barrages have halted any progress with this technology and there are only a few commercially operating plants in the world, one of these is the La Rance barrage in France.
Pumping
The turbines in the barrage can be used to pump extra water into the basin at periods of low demand. This usually coincides with cheap electricity prices, generally at night when demand is low. The company therefore buys the electricity to pump the extra water in, and then generates power at times of high demand when prices are high so as to make a profit. This has been used in Hydro Power, and in that context is known as pumped storage.

The capital required to start construction of a barrage has been the main stumbling block to its deployment. It is not an attractive proposition to an investor due to long payback periods. This problem could be solved by government funding or large organisations getting involved with tidal power. In terms of long term costs, once the construction of the barrage is complete, there are very small maintenance and running costs and the turbines only need replacing once around every thirty years. The life of the plant is indefinite and for its entire life it will receive free fuel from the tide. The increase in the costs of fossil fuels is making tidal power a more attractive proposition. The economics of a tidal barrage are very complicated. The optimum design would be the one that produced the most power but also had the smallest barrage possible.

Social Implications
The building of a tidal barrage can have many social consequences on the surrounding area. During the construction of the barrage, the amount of traffic and people in the area will increase dramatically and will last for a number of years. The La Rance tidal barrage in France took over five years to build. This will also bring revenue to the area from the tourism and hospitality industry that will accommodate all the different types of visitors that the barrage will bring. This will give a boost to the local economy.

The barrage can be used as a road or rail link, providing a time saving method of crossing the bay or estuary. There is also the possibility of incorporating wind turbines into the barrage to generate extra power. The barrage would affect shipping and navigation and provision would have to be made to allow ships to pass through.
The bay would become available for recreation as the the waters would be calmer not immediately after the barrage but further in towards the land. This would be another tourist attraction and become a feature of the area.

Environmental Aspects
Perhaps the largest disadvantages of tidal barrages are the environmental and ecological affects on the local area. This is very difficult to predict, each site is different and there are not many projects that are available for comparison. The change in water level and possible flooding would affect the vegetation around the coast, having an impact on the aquatic and shoreline cost systems. The quality of the water in the basin or estuary would also be affected, the sediment levels would change, affecting the turbidity of the water and therefore affecting the animals that live in it and depend upon it such as fish and birds. Fish would undoubtedly be affected unless provision was made for them to pass through the barrage without being killed by turbines. All these changes would affect the types of birds that are in the area, as they will migrate to other areas with more favourable conditions for them.
These effects are not all bad, and may allow different species of plant and creature to flourish in an area where they are not normally found. But these issues are very delicate, and need to be independently assessed for the area in question.
Tidal power is perhaps one of the most reliable forms of green energy available but with the impacts on local economy for the short term and impacts to the local ecology there are far more impacts to be considered when compared to a wind farm or solar farm. Wave power has less generating ability but the impacts are reduced compared to tidal but the level of reliability of power is high, This form of energy generation shall be considered in the next entry.

Friday, March 20, 2009

KW, KVA, Kvar, Explained




The explaination of the difference between KW, KVA and KVAR are always difficult concepts to get across, but these values are very important to understand as designers. upwards of 90% of all the worlds energy is used by motors, think about your own home, in the kitchen you will have fans in the oven, motors in the microwave, motors in the dishwasher, extract fans etc, they are all around us, and it is only by understanding the influance of load and power can we understand how to best maximise the effeciency not only of a single unit of equipment but an entire building as a whole. It all comes down to your power factor, if your building has a large number of inducive loads e.g. motors then you may require PF correction, some utilities companies will charge additional for a meter point having a poor PF.



Apart from a small variety of electrical equipment such as heating apparatus, the majority of
domestic and industrial equipment require reactive power to operate. Capacitors can be
used to provide the reactive power that electrical devices need.
Capacitors can be installed on the network called a capacitor bank at the clients side of the meter point or at the device requiring the reactive power.



Power factor is a measure of the relationship between the real and reactive power taken by
your electricity installation. Your electricity power supply is an alternating current (AC) electrical system. AC systems have two relevant components, real power and reactive power.
Real power is the power required to do the actual work, for example, the power that produces the actual heat, and is measured in kilowatts (kW).
Reactive power is the power required to energise electric and magnetic fields that are ancillary to the production of real power. Reactive power is measured in kilovolt Amps
reactive (kVAr).


In terms of power, real power may be considered the horizontal force because it does
the actual work. Most electrical apparatus (loads) in a plant also require a magnetic field to
operate. Reactive power provides this field, although it does not do any useful work.
These two components exist together in all AC systems.


Power factor is important because transmission and distribution systems must be designed and
built to manage the need for real power as well as its associated reactive component – the total
power. The total power can be greater than 50 percent more than real power alone if the power
factor is poor.
Ideally, power factor should be as close to unity
as possible, Unity is = to 1 if a system adds more capacitance to there incoming power and if not configured correctly it could cause a leading power factor. The further the power factor moves
away from unity, the more network capacity is needed to supply the same amount of usable
power.


Power calculations.

P = Power electrical

V = Voltage

I = Current

PF = Power Factor.

Square root of 3


If we have a new piece of plant being installed, three phase 400v, 50Hz (UK system) the frequency and voltage is different in the US etc. we know the power of the plant and the voltage and the current it will draw. to calculate the PF we do as follows. the measured value of I = 20A, P = power 10 Kw load.



PF = P/(VxIxsq root of 3)


PF = 10,000w / 400 x 20 x sq root 3


this gives us a PF of 0.72 this is quite low PF and it would be advisable to add capacitance to bring it closer to unity (1).


Now we know the PF we can calculate the KVA.


Kva = KW/PF


10 kw/0.72pf = 13.88 KVA.


This is the apparent power value.


we can also get the KVAr which is the reactive power value as follows.


we get the inverse sin and then cos of the PF.


which gives us a value of 0.69, we take this figure.


0.69 x KVA = Kvar


0.69 x 13.88 = gives us a value of 9.57 Kvar.

Tuesday, March 17, 2009

Autocad Tool Pallets Explained


Tool pallets are a way to hold and manage items to be used like symbols for layouts or schematics. It’s a list that can be displayed along the side bar of the autocad screen. It’s a list that isn’t that complicated to create. The tool pallet is divided into tabs. On each tab is a list of whatever you want to be on it. Click on that item and it is inserted into your drawing or use drag and drop to locate the symbol in a certain location on your drawing.
Tool Pallets can also be set up to drop notes or instructions that you may regularly use. e.g. reference a standard or requirement. All these can be set up on a tab for easy retrival.
Tool Pallets can hold a large selection of objects either, a block, a line, a circle, a hatch pattern, text, or even a command. By using Tool pallets on autocad you will spend less doing and increase prodoctivity.
The obvious way is that users do not have to hunt for standard objects that they use on a regular basis. they can also be used like a checklist. Create a Tool Pallet that has everything in it that belongs in a typical drawing. For example; labels, symbols, blocks, callouts, dimensions, design criteria, etc. Users can start at the top of the pallet and work their way down. Once at the bottom, the drawing is complete with the typical needs of the drawing met. This method also ensures your company’s drafting standards are met and that all drawings look the same making revisions and inter office workings much easier.
Another great feature about tool pallets is ease of maintenance. Tool Pallets can reference a block or file from a central location, like a network folder or file. If a particular block needs to be updated, do it one time and each user will now be current. They won’t ever know they were out of compliance! Using Tool Pallets to insert standard blocks and styles is a great way to do more with less effort. It creates a common place to go to get your work done.
Go to the help section on autocad with gives detailed guides on how to set up a tool pallet.
If you have any questions or queries please leave a comment.

Lighting Design The Lumen Method Explained





When Designing and calculating lighting design for internal areas, the quickest system to use is the lumen method. I have used this method for many years and the installed results were always accurate to the designed and required lighting level.





In design it is important to keep the balance between the minimum requirements as called up by relevant standards and by local authorities and balance it against the cost of installed and running the lighting for the development, if you over design the client wont thank you for an non green design and if you under design you could have serious problems with dark areas, or dim lighting.

By following this guide you will over come most of this pit falls and reach the required balance.

Step One.
Take the measurement of the room you are designing for in Meters and find the area. Then you must determine the working plane, if the room is to be an office then this will be at desk height (0.75m) from the floor, if the room is to be an office then this will be zero.

Take the height from the floor to the ceiling and take the height of the working plane from it.

Example: you have a room with the following.
length 23.8m
Width 14.5m
gives and area of 345.1m

the room is a function room so we take the working plane as 0.75m
The mounting height is 3m (floor to ceiling) - 0.75m gives 2.25m

use the following formula to calculate the room index.

Room index (RI) = A/h(a+b) 345.1/2.25(23.8+14.5) = 4.0

Step 2.
When the lights are installed at first they will give out 100% of there illumination but over time dust and grime will reduce this value so we must make allowances for that now. If the maintenance on the lighting is regular then a high value can be attributed, if the maintenance factor is unknown at design stage then a MF of 0.85 is acceptable.


Step 3.
When designing lighting systems you have to select exact fittings with known values to work to, So select a lighting catalogue that contains detailed data on the fitting and lamp types available and not catalogues that a focused towards architects and interior designers as these are generally just full of pretty pictures, I would recommend using Thorn or Philips are they have a good range of medium to high quality fittings. I would also recommend visiting there web sites to check for updates to there selection or for further technical information.

For our calculation example above the Coefficient of Utilisation is 0.46, which was obtained from the manufactuers catalogue with the use of the RI and the reflectances. For office areas or hotels expect a high reflectance values which are based on the colour of the ceilings walls and floor. Areas such as stores or back of house areas would be expected to have lower relectances values and will have an effect on the coefficient of utilisation fugure you get.

Step 4.
Note the number of lamps (tubes, bulbs) in the selected fitting and the luminour flux per tube, this figure will come from the manufacturers catalogue. Also note the wattage per lamp.

In the case of our example from above the following values resulted:

Luminour Flux per tube 20000

No of Tubes per Fitting 1

Watts per Tube 250

Step 5.
It is important to know for the application the required lux levels to be achieved in the room, for calculations in the UK we refer to the British Standard for minimum levels. For our example the require illumination value was 250.

Step 6.

Riv = Required Illumination Value
A = Area
MF = Maintenance Factor
CU = Coefficient of Utilisation
L = Luminous Flux per tube
T = No of Tubes

Fixtures Required = RIV x A
MF x CU x L x T

250 x 345.1 =
1 x 0.46 x 20000 x 1

86275 = 11.03
7820

11.03 is rounded off to 11 No. fixtures required to meet the lux levels in this room with this type of fitting and this type of lamp installed.

For asthehics this figure was increase to 12 No fittings with 4 rows of 3 No fittings installed.
We use the following Formula to determine the lux output achieved for 12 No fittings installed.

where F = No of Fitting Installed.

Average Illuminance =

F x CU x MF x L x T
A

12 x 0.46 x 0.85 x 20000 x 1 = 271.9 Lux

345.1

There are lighting software packages available that can calculate far more complex rooms and areas which I will discuss in a latter issue.

If you have any questions or queries please leave a comment.

http://www.engineerstoolbox.blogspot.com/

Monday, March 16, 2009

Power Factor Explained

Power factor

All electrical installations will have a power-factor, a relationship between the power (Kw) and the apparent power (Kva). This relationship will either have a lagging power factor, unity power factor or a leading power factor. Except in very specific instances the power factor will not be leading.

A lagging power factor.
This is where the current in a circuit reaches the maximum and minimum values later than the applied voltage has reached its maximum and minimum values. this circumstances is brought about by the presents of an inductance in the circuit. In industrial and commercial applications the most common inductive device is the AC motor. This is because by its construction it is built up of coils, it retards the growth and decay of current in the circuit, which causes a current to be out of phase with that applied voltage.

Power factor correction

Power factor is the ratio of watts to voltage amperes in a circuit, or power to apparent power.

Power Factor = Watt / Volts x Amperes = Cos q

Every load will require a specified power input and the general aim is to supply this while keeping the volt-ampere demand to a minimum, i.e. the power factor to a maximum of as near unity as possible.
The main cause of a low power factor in industrial plant is the induction motor which is the most widely used industrial drive. Particular note should be taken for the values at reduced load. For this reason as well as others, motor should not be oversized on installation, and motors under loaded due to cyclic loads will have a similar effect. The reason for this is that the cause of lagging power, is the magnetizing current drawn by the motor, which is constant, irrespective of load. On light load, therefore, the proportion of lagging current to the total current is increased with a consequent reduction in power factor.

Methods of Correction

Compensation may be effected by either rotary or static equipment and the following is a list of the general methods available. However, for the vast majority of installations static capacitors would be the automatic choice. For that reason while giving the full list the discussion is confined to static capacitors.

Static capacitors,
Synchronous induction motors,
Salient pole synchronous induction motors,
Induction motors with phase advancers
Synchronous condensers.

Starting of asynchronous motors

Starting of asynchronous motors

Star delta starters are specially suited to machines which do not present a high load torque at start up or which normally start off-loads. The relatively high peak current during the star to delta transition, which is characteristic of these starters, means that where large motors above a certain rating are to be used, some form of current limiting techniques may be necessary.
Introducing a delay of one to two seconds during the star to delta transition. This will effectively reduce the level of peak transient current. It should be noted however, that this solution may only be applied to low inertia loads, to avoid too large a drop in speed during the transition.
Three stage starting, star – delta plus resistance stage in star. The transient peak current still exits but its value is limited by the resistance being left in circuit for about three seconds after the star delta transition.
Star-delta starting plus resistance stage and a closed transition. The resistance bank is put into the circuit just before the star contactor opens. In this way, the motor current is never actuarially broken and so the transient peak is eliminated. The adoption of the above techniques will obviously mean that more components are needed and so for a given starter, the cost could increase quite considerably.
For difficult applications the use of a suitable electronic “soft start” such as the telemecanique “altistart” for example, could well be the best solution.

Starting Methods for Induction Motors.

Starting Methods for Induction Motors.

The desire at all Times would be to start on full voltage, as this would give simplicity of starting equipment, a good starting torque and hence acceleration. However, other factors must be considered and the starting method chosen will depend
a. The starting torque required, which is determined by the load
being driven.
b. The starting current, which is a function of the driven load and the design characteristics of the motor.
c. The requirement or otherwise by the load, of very gradual and smooth acceleration.
d. The mains capacity to supply the load.

The starting methods available form two, distinct groups.

[a] Starting on full voltage. i.e. Direct on-Line and Rotor resistance.


[b] Starting on reduced voltage; je. Star-Delta. Auto Transformer, Stator Resistance.

Direct on Line Starter:-This is a single stage starting method applied to standard
or special three phase squirrel cage induction motors. which 'connects the motor to the supply without a change in voltage. It gives a good starting torque, having alterative torque characteristics for different rotor designs. By the choice of ratio of rotor resistance to reactance, the maximum. torque position in the torque slip characteristic of an induction, motor can be determined. For the non specialist rotor design the starting torque is typically 1. 5 Tn. However this method of starting is not suited to low capacity mains, as the motor so started, draws five to eight times its normal running current, when starting. Because of this high starting current demand, the' supply authority places a restriction on the kW. rating, which may be started without notification and permission. This is usually three kW
The Direct on Line starter uses the minimum of components ensuring low capital and maintenance costs with high reliability. Typical applications are: shaping machines, milling machines, drills, hoists, low power pumps etc.
Rotor Resistance Starter:.
The torque of an induction motor is a maximum when the rotor resistance is equal to the rotor reactance. At the instance of start the rotor reactance is at its maximum value. Because the frequency of the rotor induced current is at its maximum 50 Hz . As the motor accelerates the frequency of the rotor current decelerates progressively until at full speed it will be approximately 1.5 Hz. Therefore the reactance has gone from a maximum value to a minimum value over the period of start. While the resistance has remained relatively constant. Hence maximum or pull out torque, only occurred at one speed and only for an instant, as the rotor accelerated through that speed.
where optimum torque is required through the starting period it is therefore necessary to vary the resistance of the rotor circuit in step with the reducing reactance. This can. only be done by constructing a special rotor-a wound rotor-and connecting external resistances in its circuit. These resistances, together with their s\4itching, and ancillary equipment form the rotor resistance starter. The function of this starter is to insert the appropriate external resistance in the rotor circuit at the instant of start and then progressively reduce that resistance Value as the rotor accelerates. with the motor operating at full speed, all the external resistance is removed- and the slip rings short circuited, it is effectively operating as a squirrel cage machine.

The reduction in resistance can be manual or automatic, time or current controlled, whole current or current transformer activated. The external resistance can be wire wound: liquid or liquid converted to vapour. The wire wound is the most widely used and can be air or oil cooled. The oil cooling is the most efficient and where starting frequency is high the tank enclosure itself can be water cooled.
having regard to cost and complexity, it is the most complex and expensive form of starting, especially as a wound rotor is utilized. However the torque requirement will dictate its choice or otherwise.

In general its application would be to any high torque- load which cannot be satisfied using a squirrel cage rotor, of normal or special design. e.g. Elevators, hoisting machinery, cranes, piston type compressors, piston pumps.

Star-delta Starter: -
This is a two stage starter used in conjunction with a three phase, squirrel cage, six terminal induction motor. Its purpose is to-reduce the starting current to the motor, and it does this by reducing the starting ¼oThage to phase voltage value. This is accomplished by initially connecting the stator winding in star formation and subsequently changing to delta, i.e. full voltage, when the rotor has attained it maximum speed at the reduced voltage. A typical value for this speed would be eighty percent of full speed and the change over point is generally a function of time, though it could be current, or speed controlled.

As the torque produced by the stator is proportional to the square of the applied voltage, when the torque available using this method of starting is reduced to one third of that available , under direct on line starting, clearly then it is only suitable for application which have minimum starting torque. Requirements. In the standard form of star delta starter the supply is disconnected from the motor during the change over period. This results in loss of momentum, and the duration of the disconnection is critical in minimising switching peaks.

Compared to the direct on line starter it requires additional components, eg. Contactors, timer, six terminal motor, larger enclosure. In general it is suited to lathes, drilling -machines, saws, metal cutters, grinders, Applications which, can be started off load, and for reasons of mains capacity cannot be started direct-on-line.

Auto-Transformer Starter: -
A single or multi stage starter used in conjunction with a three phase: squirrel cage, three terminal, induction motor. Its function is to supply a reduced voltage to the stator during the starting period, in order to reduce the starting current. This it does, in one or more steps, depending on the requirements of the load. The number of steps in the starting sequence as well as the voltage available at each step can be specified to match the load and supply requirements. eg. Three steps of values, 50%, 60%, and 75% of line voltage.
As the steps are graded it gives smooth acceleration without,­ disconnecting the motor from the supply, which avoids any danger of switching transients or momentary loss of speed. However, because it is a reduced voltage method of starting there is a significant torque reduction which limits its application to low inertia loads. As it is only in circuit for the starting period its windings are short time rated, which reduces cost but makes starting times and transformer temperature protection critical. Compared to direct-on-line starting it requires additional components e.g. contactors timers, auto-transformer. Transformer protection, larger enclosure, etc.

Stator resistance starter;­
A single or multi stage starter applicable to a three phase squirrel cage three terminal induction motor which gives a reduced starting voltage by inserting resistances in the stator circuit during the starting sequence. These resistances are taken out of circuit in one or more steps as the motor accelerates. While this -is usually timed controlled, it can also be current controlled or indeed respond to a signal generated by shaft speed. The single Stage starter usually permits approximately three times full load current as an inrush-current, while the multi stage would restrict this about twice full load value.

As this is a reduced voltage method of starting the torque is severely restricted, therefore its application must be carefully chosen. It does give an extremely smooth acceleration for the following reason:
On the initial application of voltage the inrush current is a maximum, thereby giving maximum voltage at the motor terminals. As the motor accelerates its current demand decreases, thus giving a reduction in voltage drop across the resistance and an increase in voltage at the motor terminals.

While there is a power loss in the resistances during starting there is an improvement in the power factor, which is desirable. Compared to the direct on line starter it requires additional components. E.g. stator resistance contactors, timers, larger enclosure.

Programmable Logic Controller Facilities

PLC Facilities.

Timers.
Many control systems will require a time delay or delays between different actions. All PLC’s will have programmable timers available.

One shot. (Fleeting Contact, Pulse)
This is a provision in the PLC, which gives an output only for an instant.
C1 is an internal coil. All PLC’s will have an internal coil, which can be called up, without recourse to an external output. They can be used for internal signals.

It is required to start two motors in sequence. Motor 1 is subject to manual push button control or controlled by a float switch. Motor 2 starts automatically following a 20 second delay and provided pressure is below a certain value and a valve is opened. There is a visual indicator of the status of each output and an alarm when 1000 starts have been made.

Programmable Logic Controllers Basics

Programmable Logic Controllers

A PLC is a microprocessor based controller designed for industrial and commercial applications, they were originally developed to replace relay logic applications. However, because of their flexibility and sophistication they have now developed far beyond there original application. PLC’s are available form very small sizes e.g. 6 to 10 inputs up to controllers capable of handling thousands of inputs. There constituent parts are as follows.

Input modules. The input module receives the signal. I.e. a voltage from the sensor out in the plant. It isolates them from the delicate electronics of the processor and reduces the voltage level to a very low value comparable with the processor. Every sensor or manual control switch in the scheme will have a separate conductor to the terminal of the input module. Naturally the number of input modules required will reflect the complexity of the process to be controlled.

CPU Central Processing Unit. The CPU contains the logic processor, which makes the logical decisions based upon the comparison of the input signals and the stored program. This CPU is protected from external plant voltages by optocupler in both the input and output sides.


Memory. The Memory of the PLC contains the user instructions. I.e the stored programme. This is installed in the memory through a programming device, naturally it must be written in a specific language which is recognised by the memory and the CPU. This can generally take two forms, ladder language or a statement list. These two forms are interchangeable.

Output Module. This is the interface between the CPU and the actuators, (relays, contactors, sounders, valves, indicators). These will normally operate at 24V, 110V, 220V, and 380V. The actuators perform the work.

Programming Unit. This enables the user to input the programme which has been designed to deliver a control solution to the CPU. It can take on of two forms. I.e a dedicated unit specific to the controller or dedicated software operating through a computer.

Simple Programme. Any programme will have inputs and outputs and will operate on the basis of the presents or absents of a signal (voltage). The position and the status of the input is important where the inputs can only have one of two states. Active or inactive. Inputs are designated X1 while outputs are Y1.

Sunday, March 15, 2009

Wind Farm Prospects

Profitability of a wind farm depends on the wind speed in that location, cost and performance of a wind turbine installation, and energy prices.

Large turbines selling power to the grid in recent times have found to be financially viable where the average wind speed is more than about 7 m/s and are compeditive against coal and gas. They are likely to become attractive to more businesses in future, as technology continues to improve and the deregulated energy market develops throughout the UK and Ireland.

Small turbines and wind pumps may be viable with average wind speeds as low as 5 m/s, if the only alternative is a more expensive power source such as a diesel generator, but this is only suited to very remote locations.

De-regulation of the electricity supply industry in the UK has changed the situation, and allowed more traders in energy to enter the marketplace and compete against traditional methods of electricity generation.

A wind turbine owner now has the prospect of supplying power to consumers anywhere in the country, as well as to his or her own business. Needless to say this is subject to a number of constraints such as licence and metering requirements and charges for use of the grid system - cables, poles, pylons, transformers, etc. - but there are also exemptions which can help to make this attractive.
The average power requirement on most farms is quite small in comparison to the output of a large wind turbine, but may be similar to the output of a smaller machine. The attractiveness of wind farm generation in locations that previously were deemed to expensive because of the terrain or the low average wind speeds is increasing due to the increase in fossil fuel costs. Wind power is the future, a future that is fast approaching.