Introduction

Established in 1962, as Cement Research Institute of India and redesignated as National Council for Cement and Building Materials in April 1985, NCB is an apex body dedicated to continuous research, technology development and transfer, education and industrial services for the cement and building material industries. The entire range of services of NCB is delivered by eight Corporate Centers through its units in Ballabhgarh and Hyderabad. The main laboratories of the Council are located at Ballabhgarh, about 35 kms south of New Delhi.

National Council for Cement and Building Materials (NCB) is the largest Industrial Support Organization of its kind in India, with units in the various regions of the country and in the field of Cement, Building Materials and Allied Areas and covers:

Research Technology Development and Transfer Education Industrial Services

NCB has an over 300 strong team of highly qualified and experienced engineers, scientists and other professionals.

Manpower Training and continued efforts to upgrade knowledge base is always given highest priority in NCB.

NCB’s Services at a glance:

  Turnkey Consultancy for Greenfield

Projects Basic and Detailed Engineering Plant Construction Supervision Geological Exploration Computer Aided Deposit Evaluation Mine Planning EIA & EMP. ISO-14000/EMS Raw Materials Investigation Limestone Consumption Factor Raw Mix Design Establishing Causes of Coating and

Build-up Product Development Refractory Engineering and

Management Wastes Utilisation Industry Oriented Training

Productivity Enhancement Energy Conservation Kiln Shell Ovality Studies Kiln Alignment Environmental Management Plan Maintenance Management Securing Project Funds TQM and Quality Systems

(ISO 9000) Supply of SRMs Testing Services Calibration Services Laboratory Certification Training in Plant Operation Concrete Mix Design Diagnostic Studies for Distressed

Structures Structural Design, Detailing &

Simulator based Training Industrial Information Services Organising Seminars & Workshops

Evaluation Architectural Planning

What is cement?

In the most general sense of the word, a cement is a binder, a substance which sets and hardens independently, and can bind other materials together. The word "cement" traces to the Romans, who used the term "opus caementicium" to describe masonry which resembled concrete and was made from crushed rock with burnt lime as binder. The volcanic ash and pulverized brick additives which were added to the burnt lime to obtain a hydraulic binder were later referred to as cementum, cimentum, cäment and cement.

Cement used in construction is characterized as hydraulic or non-hydraulic. Hydraulic cements (e.g. Portland cement) harden because of chemical reactions that occur independently of the admixture's water content; they can harden even underwater or when constantly exposed to wet weather. The chemical reaction that results when the dry cement powder is mixed with water produces hydrates that are not water-soluble. Non-hydraulic cements (e.g. lime and gypsum plaster) must be kept dry in order to gain strength.

The most important use of cement is the production of mortar and concrete—the bonding of natural or artificial aggregates to form a strong building material which is durable in the face of normal environmental effects.

Concrete should not be confused with cement because the term cement refers only to the dry powder substance used to bind the aggregate materials of concrete. Upon the addition of water and/or additives the cement mixture is referred to as concrete, especially if aggregates have been added.

Cement industry

In 2002 the world production of hydraulic cement was 1,800 million metric tons. The top three producers were China with 704, India with 100, and the United States with 91 million metric tons for a combined total of about half the world total by the world's three most populous states.[21]

Varieties of Cement in India

There are some varieties in cement that always find good demand in the market. To know their characteristics and in which area they are most required, it will be better to take a look at some of the details given below.

Portland Blast Furnace slag cement (PBFSC): The rate of hydration heat is found lower in this cement type in comparison to PPC. It is most useful in massive construction projects, for example - dams.

Sulphate Resisting Portland Cement: This cement is beneficial in the areas where concrete has an exposure to seacoast or sea water or soil or ground water. Under any such instances, the concrete is vulnerable to sulphates attack in large amounts and can cause damage to the structure. Hence, by using this cement one can reduce the impact of damage to the structure. This cement has high demand in India.

Rapid Hardening Portland Cement: The texture of this cement type is quite similar to that of OPC. But, it is bit more fine than OPC and possesses immense compressible strength, which makes casting work easy.

Ordinary Portland Cement (OPC): Also referred to as grey cement or OPC, it is of much use in ordinary concrete construction. In the production of this type of cement in India, Iron (Fe2O3), Magnesium (MgO), Silica (SiO2), Alumina (AL2O3), and Sulphur trioxide (SO3) components are used.

Portland Pozolona Cement (PPC): As it prevents cracks, it is useful in the casting work of huge volumes of concrete. The rate of hydration heat is lower in this cement type. Fly ash, coal waste or burnt clay is used in the production of this category of cement. It can be availed at low cost in comparison to OPC.

Oil Well Cement: Made of iron, coke, limestone and iron scrap, Oil Well Cement is used in constructing or fixing oil wells. This is applied on both the off-shore and on-shore of the wells.

Clinker Cement: Produced at the temperature of about 1400 to1450 degree Celsius, clinker cement is needed in the construction work of complexes, houses and bridges. The ingredients for this cement comprise iron, quartz, clay, limestone and bauxite.

White cement: It is a kind of Ordinary Portland Cement. The ingredients of this cement are inclusive of clinker, fuel oil and iron oxide. The content of iron oxide is maintained below 0.4% to secure whiteness. White cement is largely used to increase the aesthetic value of a construction. It is preferred for tiles and flooring works. This cement costs more than grey cement.

Apart from these, some of the other types of cement that are available in India can be classified as:

Low heat cement High early strength cement Hydrophobic cement High aluminium cement masonry cement

The setting of cement

Cement sets when mixed with water by way of a complex series of chemical reactions still only partly understood. The different constituents slowly crystallise and the interlocking of their crystals gives to cement its strength. Carbon dioxide is slowly absorbed to convert the portlandite (Ca(OH)2) into insoluble calcium carbonate. After the initial setting, immersion in warm water will speed up setting. In Portland cement, gypsum is added as a compound preventing cement flash setting.

Environmental impacts

Cement manufacture causes environmental impacts at all stages of the process. These include emissions of airborne pollution in the form of dust, gases, noise and vibration when operating machinery and during blasting in quarries, and damage to countryside from quarrying. Equipment to reduce dust emissions during quarrying and manufacture of cement is widely used, and equipment to trap and separate exhaust gases are coming into increased use. Environmental protection also includes the re-integration of quarries into the countryside after they have been closed down by returning them to nature or re-cultivating them.

CO2 emissions

Cement manufacturing releases CO2 in the atmosphere both directly when calcium carbonate is heated, producing lime and carbon dioxide,[14] and also indirectly through the use of energy, particularly if the energy is sourced from fossil fuels. The cement industry produces about 5% of global man-made CO2 emissions, of which 50% is from the chemical process, and 40% from burning fuel.[15] The amount of CO2 emitted by the cement industry is nearly 900 kg of CO2 for every 1000 kg of cement produced. [16]

In certain applications, lime mortar, reabsorbs the CO2 chemically released in its manufacture, and has a lower energy requirement in production. Newly developed cement types from Novacem[17] and Eco-cement can absorb carbon dioxide from ambient air during hardening.[18]

Heavy metal emissions in the air

In some circumstances, mainly depending on the origin and the composition of the raw materials used, the high-temperature calcination process of limestone and clay minerals can release in the atmosphere gases and dust rich in volatile heavy metals, a.o, thallium,[19] cadmium and mercury are the most toxic. Heavy metals (Tl, Cd, Hg, ...) are often found as trace elements in common metal sulfides (pyrite (FeS2), zinc blende (ZnS), galena (PbS), ...) present as secondary minerals in most of the raw materials. Environmental regulations exist in many countries to limit these emissions.

Heavy metals present in the clinker

The presence of heavy metals in the clinker arises both from the natural raw materials and from the use of recycled by-products or alternative fuels. The high pH prevailing in the cement porewater (12.5 < pH < 13.5) limits the mobility of many heavy metals by decreasing their solubility and increasing their sorption onto the cement mineral phases. Nickel, zinc and lead are commonly found in cement in non-negligible concentrations.

Use of alternative fuels and by-products materials

A cement plant consumes 3 to 6 GJ of fuel per tonne of clinker produced, depending on the raw materials and the process used. Most cement kilns today use coal and petroleum coke as primary fuels, and to a lesser extent natural gas and fuel oil. Selected waste and by-products with recoverable calorific value can be used as fuels in a cement kiln, replacing a portion of conventional fossil fuels, like coal, if they meet strict specifications. Selected waste and by-products containing useful minerals such as calcium, silica, alumina, and iron can be used as raw materials in the kiln, replacing raw materials such as clay, shale, and limestone. Because some materials have both useful mineral content and recoverable calorific value, the distinction between alternative fuels and raw materials is not always clear. For example, sewage sludge has a low but significant calorific value, and burns to give ash containing minerals useful in the clinker matrix.[20]

Different Types of Processes

Raw materials which already possess correct composition in their natural state are found in very few places. Hence in the vast majority of cases, cement is made from an artificially proportioned mixture of raw materials.The manufacturing process over the years of development may be classified under the following categories:

- Wet process

- Semi-wet / semi-dry process

- dry process

a) Wet Process

In the wet process raw mix is fed into the kiln in the form of slurry which may have water content of 30 to 40%. The slurry which is easy to blend and homogenise is directly fed into the kiln which in the case of wet process is a relatively long tube. The wet process becomes indispensable in those cases where the naturally occurring raw materials have high moisture content of more than 12% like chalk and marl. This is also essential where relatively poor grade limestone have to be enriched through the process of beneficiation requiring use of water as a process media. In fact, in the earlier times i.e. before 1950 most of the kilns were wet process kilns due to the fact that in the form of slurry it is easy to blend and homogenise the various components of the raw mix. In this process the fuel consumption is the highest (in the region of 1300 to 1600 K.cal/Kg of clinker) the power consumption is lower at 110-115 Kwh/tonne of cement.

b) Semi-wet/Semi-dry Process

This process was evolved to counter the main drawback of the wet process which is high fuel consumption. In this process powdered raw meal is either converted into nodules by adding controlled quantity of water in a nodulising pan or by dewatering slurry in a filter press to form filter cake of the raw material. These nodules or the cake thus formed are fed on to a moving grate where the raw meal gets partially calcined. This partially calcined raw mix in the form of nodules/cake is further charged into a rotary kiln for complete calcining and sintering in the form of clinker. However, this process poses a number of operational problems and capacity problems. The fuel consumption however, improves reasonably to about 900-1100 K.cal/Kg of clinker but the power consumption increases to 115-120 Kwh/tonne of cement.

c) Dry Process

In the dry process, the raw materials are dried in a combined drying and grinding plant to reduce the moisture content below 1%. The drying of materials is achieved by using kiln exhaust gases which may be supplemented by auxiliary hot furnaces during rainy season. The ground raw mix is homogenised in large silos. In fact, development of suitable homogenising and blending systems are mainly responsible for making the dry process popular and practicable. The blended and homogenised raw is fed into either a long dry kiln or a short kiln with air suspension preheater in which partial calcination of the raw mix takes place. In fact, long dry kilns have now practically gone out of use and the dry process is mainly confined to the use of air suspension preheater. This process gives the maximum benefit as far as the heat consumption figures are concerned. As a further refinement and development of the dry process, the air suspension preheaters are now being fitted with Precalcinators which ensure complete calcining of the raw mix before it enters the kiln. Fuel consumption is lowest in this process and is in the range of 750-950 Kcal/Kg of clinker. The power consumption is in the range of 120-125 Kwh/tonne of cement. A flow process sheet of all the cement production processes including Pre-calcinator system is indicated in Fig.

CEMENT PROCESS FLOW DIAGRAM

CT & PT

Current Transformers

A current transformer (CT) is a type of instrument transformer designed to provide a current in its secondary winding proportional to the alternating current flowing in its primary. They are commonly used in metering and protective relaying in the electrical power industry where they facilitate the safe measurement of large currents, often in the presence of high voltages. The current transformer safely isolates measurement and control circuitry from the high voltages typically present on the circuit being measured. Current transformers are used extensively for measuring current and monitoring the operation of the power grid. The CT is typically described by its current ratio from primary to secondary. Often, multiple CTs are installed as a "stack" for various uses (for example, protection devices and revenue metering may use separate CTs). Similarly potential transformers are used for measuring voltage and monitoring the operation of the power grid.

  The accuracy of a CT is directly related to a number of factors including:

Burden Burden Class /Saturation Class Rating factor Load External electromagnetic fields  temperature and Physical configuration.

Potential Transformers

Protective Current Transformers are designed to measure the actual currents in power systems and to produce proportional currents in their secondary windings that are isolated from the main power circuit. These replica currents are used as inputs to protective relays which will automatically isolate part of a power circuit In the event of an abnormal or fault condition therein, yet permit other parts of the plant to continue in operation.

Satisfactory operation of protective relays can depend on accurate representation of currents ranging from small leakage currents to very high over currents, requiring the protective current transformer to be linear, and therefore below magnetic saturation at values up to perhaps 30 times full load current. This wide operating range means that protective current transformers require to be constructed with larger cross-sections resulting in heavier cores than equivalent current transformers used for measuring duties only. For space and economy reasons, equipment designers should however avoid over specifying protective current transformers ITL technical staff are always prepared to assist in specifying protective CT's but require some or all of the following information:-

Protected equipment and type of protection. Maximum fault level for stability. Sensitivity required. Type of relay and likely setting. Pilot wire resistance, or length of run and pilot wire used. Primary conductor diameter or bus bar dimensions System voltage level.

Battery Charger

Battery Chargers are used to charge rechargeable batteries time and again. These batteries are available at various current levels. Designed and fabricated with precision, our battery chargers provide a constant voltage and current to charge batteries.

Features :-

1. Accurate float charge to eliminate the possibility of batteries getting overcharged.2. A Change over contactor of emergency lighting and an under voltage alarm is provided in

the charger unit.3. Step less control for charging is being provided to ensure quick charging of batteries if

discharged during emergency.4. Indication lamp LED type for mains on charging on float/trickle/boost with Voltmeter

and Ammeter provided to indicate charging voltage and current.5. Degree of Protection :-IP-52 to IP-65.6. Painting: - Powder Coating, Spray Paint, Epoxy Paint.7. Compartmentalized design eliminates risk to maintenance personal.8. Rubber mat provided for protection against acid spillage.9. The sealed construction, incorporation of the filter float plug ensures that the atmosphere

is absolutely acid free.

Types :-

1. Float cum boost battery charger.2. Float cum boost with Trickle battery charger.3. Manually operated battery charger with multiple voltage & current.

Applications :

1. To charging a battery, specially dry cell unit and to ensure accurate voltage of an electrical system.

2. Uninterrupted working of machines & equipments.

Junction Box

Features :-

1. Available in Floor Mounting & Wall Mounting.2. Degree of Protection :-IP-52 to IP-65.3. Painting: - Powder Coating / Spray Paint / Epoxy Paint.4. Junction Box shall have hinged door with lockable arrangement5. Available with Aluninium / copper bus bar.6. Fabricated with 2/2.5/3mm CRCA sheet steel.

Application :- These are widely used for :-

1. Joining / Connection of various electrical wires coming from field and panel side.2. Junction Box ensures connectivity of electrical & electronics of a system.

Power Distribution Board

power distribution boards that not only save installation costs but also cost of equipment storage, handling and field testing. Following are the features & application of our product:

Features :-

1. Degree of Protection :-IP-52 to IP-65.2. Painting: - Powder Coating / Spray Paint / Epoxy Paint.3. Compartmentalized design eliminates risk to maintenance personal.4. Sufficient space for incoming and outgoing cables.5. Provided with buzzer & hooter during emergency.6. Separate Bus bar and cable chambers.7. Power Distribution Board shall have hinged door with lockable arrangement8. Available with Aluninium / copper bus bar.9. Fabricated with 2/2.5/3mm CRCA sheet steel.10. Power Distribution Board is suitable for use on 415V, 3Phase, 4 wire, 50Hz, AC supply

system.11. Available in Cubicle and Pedestal type.

Application :- These are widely used for :-

1. Protection against electrical Short Circuit, overload and earth fault of the system.2. Control & monitoring of the entire electrical system of an industry / plant.3. Systematic power catering to various load of an electrical system.

4. Normally used in Hospitals, Multi-storied buildings, steel industries, cement industry, open –cast mines, shopping mall etc.

Motor Control Centers

The motor control center offered to customers is capable to feed larger loads as it comes with multiple power stabs that support high capacity industrial motor. Followign are the features & applications of our product:

Features :-

1. Simplex & Duplex pattern.2. Degree of Protection :-IP-52 to IP-65.3. Painting: - Powder Coating / Spray Paint / Epoxy Paint.4. Compartmentalized design eliminates risk to maintenance personal.5. Sufficient space for incoming and outgoing cables.6. Provided with buzzer & hooter during emergency.7. Separate Bus bar and cable chambers.8. Motor Control Center shall have hinged door with lockable arrangement9. Motor Control Center shall have detachable gland plates of 2-3mm thick10. Motor Control Center Available with Aluninium / copper bus bar.11. Fabricated with 2/2.5/3mm CRCA sheet steel.12. Motor Control Center is suitable for use on 415V, 3Phase, 4 wire, 50H.z

Application :- These are widely used for :-

1. Protection against electrical Short Circuit, overload and earth fault of the system.2. Control & monitoring of the entire electrical system of an industry / plant.3. Systematic power catering to various load of an electrical system.4. Normally used in Hospitals, Multi-storied buildings, steel industries, cement industry,

open –cast mines, shopping mall etc.

Control & Relay Panel

Our range of control & relay panel improves power factor to increase energy efficiency. All the specifications and requirements of the clients are taken into account and well complimented.

Features :-

1. Siplex & Duplex pattern.2. Degree of Protection :-IP-52 to IP-65.3. Electromechanical Relay / Numerical Relay.4. Painting: - Powder Coating, Spray Paint, Epoxy Paint.5. Compartmentalized design eliminates risk to maintenance personal.6. Sufficient space for incoming and outgoing cables.7. Separate Busbar and Cable chambers.8. Servicing & maintenance of Relay available.9. Fabricated with 2/2.5/3mm CRCA sheet steel.

Application :- These are widely used for:

1. Protection and Control for Transformers.2. Protection and Control for Transmission and distribution lines

3. Protection and Control for Bus Section and Bus Coupler

Drum Controller & Master Controller

Air break drum controllers are manufactured in accordance with customer specification. They are suitable for controlling A.C & D.C motors used in E.O.T cranes, haulages in mines, winches, steel works. Mainly three sizes are manufactured, 40 Amps, 60 Amps, 150 Amps. Provided with stator reversing contacts.A star wheel and roller arm give definite step location. The crank type-operating handle is provided with off position trigger to prevent accidental starting or unwanted reversing. Operating handle is provided with deadman handle if required by the client Auxiliary contacts are provided for Electrical inter locking.

Arc Shields:Arc shields are provided on both D.C and A.C. type controllers.Cable Entry:Standard cable entry is through a base plate, other type of fittings, can also be provided as per clients desire.Construction:Robust construction, top & bottom made of M.S. Plate. The fingertips are made of Hard drawn copper, grinding finish. Removable front cover made of sheet steel.Drum: The contact segments are of Hard drawn Electrolytic copper, turned after assembly to maintain concentricity. The contact assembly of controllers fixed on to insulated M.S. Shaft.

 

Concepts in switching technology with miniaturization & sophistication for used in automatic control circuit like master control switch where mechanical positions is translated into electrical signals for controlling remote starters, contactors and work in highly contaminated atmosphere and extremely high shock and vibrating condition. Every Limit Switch goes under rigorous

testing before it is supplied. A wide variety to afford a high degree of versatility is available. The range includes:-

Grab differential limit switch. Spindle type rotary geared limit switch. Heavy duty lever type limit switch. Roller lever. Counter weight. Forked lever. V-shaped lever. Double Roller lever Foot Pedal Heavy duty cam operated Rotary Geared Limit Switch. Push rod limit switch. Heavy duty pull cord switch. Explosion proof limit switch for Hazardous atmosphere. Heavy duty snap action limit switch.

Technical Specification & Features

Utilisation category: AC 11 & DC 11 as per IEC IS 6875. Thermal current 10 amp to 40 amps. Insulation voltage 600 V AC & 240 DC. Operating temperature – 20ºC to 110ºC. Mechanical life – 20 million cycle. Contact life: 10 million cycles. Terminal capacity: 2.5 mm2. Operating Torques: 6.804 to 14.968 KLS depending on switch size and cam selected. Enclosure: IP 67 Characteristics Oil tight, Water tight & Dust proof. Time tested, extremely reliable design

Resistance Boxes

Resistance boxes & starting resistors are used for both AC and DC applications. It is widely used for controlling and developing higher torque and have extensively applicable in: Cement Plants.

The grids of starting resistors are made from stainless steel and punched steel sheet and are suitable for maximum temperature rise of 375 degree C as per BSS standards.

AC / DC TachogeneratorsThe AC Tacho generators provide the AC voltage output proportional to the speed of its mover. Their main application areas include indication & display purposes. On basis of simple calculations, the frequency of 2 pole, 8 pole and 48 pole will produce 16.66 Hz, 66.66 Hz and 400 Hz, respectively.  Some of the salient attributes of our AC/DC Tacho generators include:

Compact Light weight High linearity High stability and extreme reliability in hostile industrial environments

Its technical specifications are tabulated below: Current Up to 50 mASpeeds Up to 4000 rpmVoltages 4V to 40 V / 1000 rpmPole Design 2, 8 and 48

Stepper Motors with Controller

stepper motor with controller or controlled stepper motor having high torque density, rugged design and long life bearings. Available in different models to meet diversified requirements of our customers, these durable steeper motors possess following attributes:

Thermal protection Short circuit and wrong polarity protection Optoisolated signal input Low vibration, high speed and high torque Potentiometer LEC for adjustable current reduction Potentiometer SA for adjustable precision of microstep High Torque to inertia for quicker start and stops Higher Power, rare earth magnets

These steeper motors have following technical specifications: Supply voltage 20 .. 80 V DCOutput Current 1.9 .. 4.0 AStep Modes 1.8Current Adjustments via DIP switchesClock Frequency 0.. 400 kHz max.Pulse Width (Clock) Min 1.25 high / lowAmblent Temperature 0oC .. 100oCConnection Type Screw Type TerminalsHolding Torque Up to 30Nm

Frame Size 42 mm square to 110mm square

Static Eliminators & Electrodes

That is mainly used to achieve full output in the synthetic material processing. The importance of these products is evident from the fact that during the aforesaid process, when material is dry and speed is high, then at those operating speeds the voltage of charges increases so much that the entire process is completely disturbed. This results in prevention of full output and often sparking becomes troublesome, causing extreme dangers to the machine operators.

Some of its salient attributes include:

Design: -High Voltage transformer with 3 Piano Switches achieve Low, Medium and High stage i.e. 5KV, 7.5 KV and 10KV / 12 KV respectively.

Three OR Four Connectors to ionators OR Electrodes Consumption 230/25 VA Ionator / Electrode: - As per the width of Fabric / Cloth

Encoders

Encoders measure the transduction angle speed and distance. Popular for their low resolution, these have mechanical precision component & assembly. The output of these efficient shaft encoders is available in the form of pulses, incremental step by step in counter or processor. The direction is sorted out with the assistance of 2 trains of pulses. In quadrature one index position pulse is available. As per the special demands of our customers, we can also offer our encoders in the heavy duty versions. Technical Specifications:

Type of Output RS 422 (TTL) or Pulse-Pull or HTL or PNP or NPNResolution 10-500 PPRPrecision Range 250-10000 PPRShaft Size 6-10 or 6-19 or 11-30 mmNumber of Channel 3 or 6Type of mounting Servo with clamp or face mountingProtection class IP 66/IP 55

Relays

A relay is an electrically operated switch. Current flowing through the coil of the relay creates a magnetic field which attracts a lever and changes the switch contacts. The coil current can be on or off so relays have two switch positions and most have double throw (changeover) switch contacts as shown in the diagram.

Relays allow one circuit to switch a second circuit which can be completely separate from the first. For example a low voltage battery circuit can use a relay to switch a 230V AC mains circuit. There is no electrical connection inside the relay between the two circuits, the link is magnetic and mechanical.

The coil of a relay passes a relatively large current, typically 30mA for a 12V relay, but it can be as much as 100mA for relays designed to operate from lower voltages. Most ICs (chips) cannot provide this current and a transistor is usually used to amplify the small IC current to the larger value required for the relay coil. The maximum output current for the popular 555 timer IC is 200mA so these devices can supply relay coils directly without amplification

Relay showing coil and switch contacts

Relays are usually SPDT or DPDT but they can have many more sets of switch contacts,

Most relays are designed for PCB mounting but you can solder wires directly to the pins providing you take care to avoid melting the plastic case of the relay.

The supplier's catalogue should show you the relay's connections. The coil will be obvious and it may be connected either way round. Relay coils produce brief high voltage 'spikes' when they are switched off and this can destroy transistors and ICs in the circuit. To prevent damage you must connect a protection diode across the relay coil.

The animated picture shows a working relay with its coil and switch contacts. You can see a lever on the left being attracted by magnetism when the coil is switched on. This lever moves the switch contacts. There is one set of contacts (SPDT) in the foreground and another behind them, making the relay DPDT.

The relay's switch connections are usually labeled COM, NC and NO:

COM = Common, always connect to this, it is the moving part of the switch. NC = Normally Closed, COM is connected to this when the relay coil is off. NO = Normally Open, COM is connected to this when the relay coil is on. Connect to COM and NO if you want the switched circuit to be on when the relay coil is

on. Connect to COM and NC if you want the switched circuit to be on when the relay coil is

off.

Choosing a relayYou need to consider several features when choosing a relay:

1. Physical size and pin arrangement If you are choosing a relay for an existing PCB you will need to ensure that its dimensions and pin arrangement are suitable. You should find this information in the supplier's catalogue.

2. Coil voltage The relay's coil voltage rating and resistance must suit the circuit powering the relay coil. Many relays have a coil rated for a 12V supply but 5V and 24V relays are also readily available. Some relays operate perfectly well with a supply voltage which is a little lower than their rated value.

3. Coil resistance The circuit must be able to supply the current required by the relay coil. You can use Ohm's law to calculate the current:

Relay coil current   =   supply voltage 

  coil resistance

4. For example: A 12V supply relay with a coil resistance of 400 passes a current of 30mA. This is OK for a 555 timer IC (maximum output current 200mA), but it is too much for most ICs and they will require a transistor to amplify the current.

5. Switch ratings (voltage and current) The relay's switch contacts must be suitable for the circuit they are to control. You will need to check the voltage and current ratings. Note that the voltage rating is usually higher for AC, for example: "5A at 24V DC or 125V AC".

6. Switch contact arrangement (SPDT, DPDT etc) most relays are SPDT or DPDT which are often described as "single pole changeover" (SPCO) or "double pole changeover" (DPCO). For further information please see the page on switches.

Protection diodes for relays

Transistors and ICs must be protected from the brief high voltage produced when a relay coil is switched off. The diagram shows how a signal diode (eg 1N4148) is connected 'backwards' across the relay coil to provide this protection

Current flowing through a relay coil creates a magnetic field which collapses suddenly when the current is switched off. The sudden collapse of the magnetic field induces a brief high voltage across the relay coil which is very likely to damage transistors and ICs. The protection diode allows the induced voltage to drive a brief current through the coil (and diode) so the magnetic field dies away quickly rather than instantly. This prevents the induced voltage becoming high enough to cause damage to transistors and ICs.

Reed relays

Reed relays consist of a coil surrounding a reed switch. Reed switches are normally operated with a magnet, but in a reed relay current flows through the coil to create a magnetic field and close the reed switch.

Reed relays generally have higher coil resistances than standard relays (1000 for example) and a wide range of supply voltages (9-20V for example). They are capable of switching much more rapidly than standard relays, up to several hundred times per second; but they can only switch low currents (500mA maximum for example).

The reed relay shown in the photograph will plug into a standard 14-pin DIL socket ('IC holder').

For further information about reed switches please see the page on switches.

Relays and transistors comparedLike relays, transistors can be used as an electrically operated switch. For switching small DC currents (< 1A) at low voltage they are usually a better choice than a relay. However, transistors cannot switch AC (such as mains electricity) and in simple circuits they are not usually a good choice for switching large currents (> 5A). In these cases a relay will be needed, but note that a low power transistor may still be needed to switch the current for the relay's coil! The main advantages and disadvantages of relays are listed below:

Advantages of relays:

Relays can switch AC and DC, transistors can only switch DC. Relays can switch higher voltages than standard transistors. Relays are often a better choice for switching large currents (> 5A).

Relays can switch many contacts at once.

Disadvantages of relays:

Relays are bulkier than transistors for switching small currents. Relays cannot switch rapidly (except reed relays), transistors can switch many times per

second. Relays use more power due to the current flowing through their coil. Relays require more current than many ICs can provide, so a low power transistor

may be needed to switch the current for the relay's coil.

Switchgear

The term switchgear, used in association with the electric power system, or grid, refers to the combination of electrical disconnects, fuses and/or circuit breakers used to isolate electrical equipment. Switchgear is used both to de-energize equipment to allow work to be done and to clear faults downstream. Switchgear is already a plural, much like the software term code/codes, and is never used as switchgears.

The very earliest central power stations used simple open knife switches, mounted on insulating panels of marble or asbestos. Power levels and voltages rapidly escalated, making open manually-operated switches too dangerous to use for anything other than isolation of a de-energized circuit. Oil-filled equipment allowed arc energy to be contained and safely controlled. By the early 20th century, a switchgear line-up would be a metal-enclosed structure with electrically-operated switching elements, using oil circuit breakers. Today, oil-filled equipment has largely been replaced by air-blast, vacuum, or SF6 equipment, allowing large currents and power levels to be safely controlled by automatic equipment incorporating digital controls, protection, metering and communications.

TypesSwitch Gear   Sheet Metal Deep Drawn Boxes Switch gears are for the use of single phase and three phase power supply as well as for disconnection through fuse. Switch is fitted in elegant deep drawn box.

 

16 x 240 DP

32 x 240 DP

16 x 415 Triple Pole

32 x 415 Triple Pole

63 x 415 Triple Pole

100 x 415 Triple Pole

200 x 415 Triple Pole

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Fuse Units With Silverised Heavy Copper Contacts

16 AMP 32 AMP 63 AMP 100 AMP 200 AMP

A piece of switchgear may be a simple open air isolator switch or it may be insulated by some other substance. An effective although more costly form of switchgear is gas insulated switchgear (GIS), where the conductors and contacts are insulated by pressurized sulfur hexafluoride gas (SF6). Other common types are oil [or vacuum] insulated switchgear.

The combination of equipment within the switchgear enclosure allows them to interrupt fault currents of many hundreds or thousands of amps. A circuit breaker (within a switchgear enclosure) is the primary component that interrupts fault currents. The quenching of the arc when the ciruit breaker pulls apart the contacts open (disconnects the circuit) requires careful design. Circuit breakers fall into these four types:

Oil circuit breakers rely upon vaporization of some of the oil to blast a jet of oil through the arc.

Gas (SF6) circuit breakers sometimes stretch the arc using a magnetic field, and then rely upon the dielectric strength of the SF6 to quench the stretched arc.

Vacuum circuit breakers have minimal arcing (as there is nothing to ionize other than the contact material), so the arc quenches when it is stretched a very small amount (<2–

3 mm). Vacuum circuit breakers are frequently used in modern medium-voltage switchgear to 35,000 volts.

Air circuit breakers may use compressed air (puff) to blow out the arc, or alternatively, the contacts are rapidly swung into a small sealed chamber, the escaping of the displaced air thus blowing out the arc.

Circuit breakers are usually able to terminate all current flow very quickly: typically between 30 ms and 150 ms depending upon the age and construction of the device.

Several different classifications of switchgear can be made[1]:

By the current rating. By interrupting rating (maximum short circuit current that the device can safely interrupt)

o Circuit breakers can open and close on fault currentso Load-break/Load-make switches can switch normal system load currentso Isolators may only be operated while the circuit is dead, or the load current is very

small. By voltage class:

o Low voltage (less than 1,000 volts AC)o Medium voltage (1,000–35,000 volts AC)o High voltage (more than 35,000 volts AC)

By insulating medium: o Airo Gas (SF6 or mixtures)o Oilo Vacuum

By construction type: o Indoor (further classified by IP (Ingress Protection) class or NEMA enclosure

type)o Outdooro Industrialo Utilityo Marineo Draw-out elements (removable without many tools)o Fixed elements (bolted fasteners)o Live-fronto Dead-fronto Openo Metal-enclosedo Metal-clado Metal enclose & Metal clado Arc-resistanto By IEC degree of internal separation [2]

No Separation (Form 1) Busbars separated from functional units (Form 2a, 2b, 3a, 3b, 4a, 4b)

Terminals for external conductors separated from busbars (Form 2b, 3b, 4a, 4b)

Terminals for external conductors separated from functional units but not from each other (Form 3a, 3b)

Functional units separated from each other (Form 3a, 3b, 4a, 4b) Terminals for external conductors separated from each other (Form 4a, 4b) Terminals for external conductors separate from their associated

functional unit (Form 4b) By interrupting device:

o Fuseso Air Blast Circuit Breakero Minimum Oil Circuit Breakero Oil Circuit Breakero Vacuum Circuit Breakero Gas (SF6) Circuit breaker

By operating method: o Manually-operatedo Motor-operatedo Solenoid/stored energy operated

By type of current: o Alternating currento Direct current

By application: o Transmission systemo Distribution.

A single line-up may incorporate several different types of devices, for example, air-insulated bus, vacuum circuit breakers, and manually-operated switches may all exist in the same row of cubicles.

Ratings, design, specifications and details of switchgear are set by a multitude of standards. In North America mostly IEEE and ANSI standards are used, much of the rest of the world uses IEC standards, sometimes with local national derivatives or variations.

Functions

One of the basic functions of switchgear is protection, which is interruption of short-circuit and overload fault currents while maintaining service to unaffected circuits. Switchgear also provides isolation of circuits from power supplies. Switchgear is also used to enhance system availability by allowing more than one source to feed a load.

Safety

To help ensure safe operation sequences of switchgear, trapped key interlocking provides predefined scenarios of operation. James Harry Castell ([1])invented this technique in 1922. For example, if only one of two sources of supply are permitted to be connected at a given time, the

interlock scheme may require that the first switch must be opened to release a key that will allow closing the second switch. Complex schemes are possible.

Indoor switchgear can also be type tested for internal arc containment. This test is important for user safety as modern switchgear is capable of switching large currents. ([2])

PORTABLE INSTRUMENTS FOR MEASUREMENTS

ELECTRICAL INSTRUMENTS (AMPS, VOLTS, POWER, POWER FACTOR)

GAS ANALYSERS (OXYGEN, CO, CO2)

TEMPERATURE MEASURING INSTRUMENTS (CONTACT & NON-CONTACT)

AIR FLOW MEASURING INSTRUMENTS (PITOT TUBE, MICROMANOMETER, VANE ANEMOMETER)

PRESSURE MEASURING INSTRUMENTS (MICROMANOMETER, DIAL GAUGE)

FLUE GAS ANALYSER

INFRA-RED RADIATION PYROMETER

CLAMP-ON POWER METER

Ultrasonic leak detectors

Light measurements

MOTORS

ELECTRICAL CHARACTERISTICS - RUNNING, STARTING, SPEED CONTROL, BRAKING

MECHANICAL FEATURES - ENCLOSURE, BEARINGS, TYPE OF COUPLING/TRANSMISSION, NOISE

SIZE (RATING AND SERVICE CAPACITY) - CONTINUOUS, INTERMITTENT OR VARIABLE LOAD

COST - CAPITAL COST, RUNNING COST (LOSSES, P.F., MAINTENANCE, DEPRECIATION)

TEMP RISE AND INSULATION STRENGTH

Proper Sizing of Motors

It is important to remember that it is the load that determines how much power the motor draws. The size of the motor does not necessarily relate to the power being drawn. For example, a fan requiring 15 kW could be driven by a 15 kW motor; in which case, it is well matched. It could also be driven by a 30 kW motor, and although it would work, it would not be very efficient.

Motors are often oversized because of:

1. Uncertainty about load;

2. Allowance for load growth,

3. Rounding up to the next size;

4. Availability;

Because motor efficiency curves vary substantially from motor to motor, it is difficult to make a blanket statement as to which motors should be downsized. In general, if the motor operates at 40% of its rated load or less, it is a strong candidate for downsizing. This is especially true in cases where the motor load does not vary much.

It often makes sense to replace oversized motors even if the existing motor has not failed. Remember, energy costs for a motor over the course of a year can be up to five times the cost of a new motor. This is especially true in cases where the motor is operating at a lower efficiency level due to oversizing.

Of course, there are benefits to oversizing motors in certain cases that should not be overlooked when determining what the proper motor is for a given application. In addition to providing capacity for future expansion, oversized motors can accommodate unanticipated high loads and are likely to start and operate more readily in under voltage conditions. These advantages can normally be achieved, however, with a modest oversizing margin.

The efficiency of motors operating at loads below 40% is likely to be poor and energy savings are possible by replacing these with properly sized motors.

Oversized Motors lead to the following problems :

Higher investment cost due to larger size

Higher running cost due to decrease in efficiency

Higher maximum demand due to poor power factor

Higher cable losses and demand charges

Higher switchgear cost

Higher installation cost

Higher rewinding cost (in case of motor burnout)

There are two methods to optimise loading of a running motor :

Connecting motors in STAR

Use of Soft starter with energy saving features

The following suggestions are made:

If a motor is oversized and continuously loaded below 30% of its rated shaft load, the motor can be permanently connected in Star.

If the motor is normally loaded below 30% but has a high starting torque requirement, then the motor can be started with a suitable starter and, after overcoming the starting inertia, be automatically switched from Delta to Star, using timer control or current sensing.

If the load is below 30% most of the time, but if the load exceeds 50% some times, automatic Star-Delta changeover Switches (based on current or load sensing) can be used. However, if the changeover is very frequent the contactors would get worn out and the savings achieved may get neutralised by the cost of frequent contactor replacements.

If the motor is nearly always operating above 30% of the rated load and sometimes runs below 30% load, star-delta changeover will not be economical.

UNDERLOADING OF MOTORS

• DECREASES OPERATING POWER FACTOR

• DECREASES OPERATING EFFICIENCY

SELECTION OF MOTOR SHOULD BE SUCH THAT IT IS NOT OVERSIZED MOREOVER, EXISTING MOTOR SHOULD BE OPERATED NEAR ITS RATED CAPACITY

• PLANT CAPACITY : 2500 TPD (DRY)

• RAW MILL FAN:-- MOTOR RATING : 850 kW- MAX. LOAD : 450 KW- % LOADING : 53 ADDITIONAL ENERGY- BILL : Rs 1.00 LAKH/YR (US$ 0.02 LAKH/YR) (@ RS 4/50 PER KWH) ( 1 US $ = RS 45)

COST BENEFIT ANALYSIS FOR REPLACEMENT OF MOTOR

NAME OF MOTOR : COOLER COMPARTMENT NO 1 FAN

(3 Ǿ SQUIRREL CAGE INDUCTION MOTOR)

EXISTING MOTOR PROPOSED MOTOR

RATED HP : 150 100

FULL LOAD

CURRENT AMPERES : 188 130

CURRENT LOADING

AMPERES : 106(56%) 87(67%)(ESTIMATED

EXPECTED POWER SAVING : 3.19 Kw

REDUCTION IN MAXIMUM DEMAND : 13.5KVA

ANNUAL SAVING : Rs 1.40 LAKH (US $ 0.03LAKH)

@ RS4/50 PER KWH , RS 150/KVA M.D CHARGES

1 US $ = RS 45,330 DAYS OPERATION/YR

ENERGY EFFICIENT MOTORS

DESIGNED SO THAT EFFICIENCY REACHES A PEAK AT ABOUT TWO THIRD OF FULL LOAD AND REMAINS AS HIGH AS FULL LOAD EFFICIENCY EVEN AT HALF LOAD

POWER FACTOR IS EQUAL TO OR SLIGHTLY HIGHER THAN STANDARD MOTORS

CONTAIN HIGHER ACTIVE MATERIAL, HENCE COSTLIER BY UPTO 30% AS COMPARED TO STANDARD MOTORS

RECOMMENDED IN PROJECT STAGE AND FOR REPLACEMENT OF STANDARD MOTORS WHICH HAVE BEEN REWOUND MORE THAN TWICE

PAYBACK PERIOD OF 2-3 YEARS

ENERGY EFFICIENT TRANSFORMERS

TRANSFORMERS

DESIGNED FOR HIGH EFFICIENCIES (98% OR MORE)

IRON LOSSES REMAIN CONSTANT AND Cu LOSSES VARY AS SQUARE OF THE LOAD

EFFICIENCY MAXIMUM AT 60-70% LOAD (UNLIKE MOTORS AND FANS)

LOAD LOSSES REDUCE WITH IMPROVEMENT IN LOAD POWER FACTOR FOR SAME KVA LOAD

LOCATION OF TRANSFORMER IS IMPORTANT TO MINIMISE CU LOSSES (I2R LOSSES) THROUGH OPTIMISED CABLE LENGTHS

Most energy loss in dry-type transformers occurs through heat or vibration from the core. The new high-efficiency transformers minimise these losses. The conventional transformer is made up of a silicon alloyed iron (grain oriented) core. The iron loss of any transformer depends on the type of core used in the transformer. However the latest technology is to use amorphous material – a metallic glass alloy for the core. The expected reduction in energy loss over conventional (Si Fe core) transformers is roughly around 70%, which is quite significant. By using an amorphous core– with unique physical and magnetic properties- these new type of transformers have increased efficiencies even at low loads - 98.5% efficiency at 35% load.

Electrical distribution transformers made with amorphous metal cores provide excellent opportunity to conserve energy right from the installation. Though these transformers are a little costlier than conventional iron core transformers, the overall benefit towards energy savings will compensate for the higher initial investment. At present amorphous metal core transformers are available up to 1600 kVA.