1

A

PROJECT REPORT ON

“Paper Mill”

In accordance with 15 days Training

At Ballarpur Industries Limited (BILT).

Submitted By:-

Uday Wankar Akhil Wankhede

Vaibhav Komerwar Amitkumar Singh

Uday Randive

2

INDEX 1. INTRODUCTION ………………………………..……………………………………..4

1.1 About BILT…………..…………..……………..………………………...………4

2. Ballarpur Industries Limited (BILT)……..…..………………………………………………5

2.1 An Overview ………………..……..………………………………….………..5

3. Paper machines in BILT………………………………..……………………….………6

4. Paper Production………………………….……………………………………………..8

4.1 Step by step paper production………….………………………………………...9

5.Pulp types and their preparations……………….………………….………………..…11

5.1Stock (pulp) preparation…………………………………………..……………….12

6. Paper machine sections..………………………….…………………………………….13

6.1 Press section……………………………………………………………………….16

6.2 Dryer section………………………………………………………………………17

6.3 Calender section………….………………………………………………………..18

7. Power plants in BILT…………………………………………………………………...19

7.1 40 MW Power station (PP4)………………………………………………………19 7.1.1 Circulating Fluidized Bed Boiler…………………………………………….19

8. Direct On Line starter…………………………………………………………………..26

8.1 Principle of DOL starter…………………………………………………………...26

8.2 Parts of DOL Starters……………………………………………………………...27

9. variable-frequency drive (VFD)………………………………………………………..30

9.1Controller …………………………………………………………………………..31

9.2 Operator interface………………………………………………………………….32

9.3 Drive operation…………………………………………………………………….33

9.4 Energy savings…………………………………………………………………….33

9.5 Control platforms………………………………………………………………….34

3

LIST OF FIGURE

Figure No. Name of Figures Page No. 4.1 6(a)

Paper production process Paper machine

9 13

6(b) Block diagram of paper ma-chine

15

6.1 Press section 16 6.2(a) Dryer section 17 6.2(b) Paper leaving the machine is

rolled onto a reel for further processing

18

7.1.1 Block diagram of CFBC boiler

20

7.1.2 Block diagram of recovery boiler

22

7.2.1 Brushless Excitation System 25 8.1 Direct Online Motor Starter -

Square D 26

8.2.1 Contactor 27 8.2.2 Thermal Overload Relay 28

9 variable-frequency drive for small motor

30

9.1 SPWM carrier-sine input & 2-level PWM output

32

9.3 Electric motor speed-torque chart

33

4

1. INTRODUCTION: About BILT

Ballarpur Industries Limited (BILT) is a flagship of the US$ 4 bnAvantha Group and India's largest manufacturer of writing and printing (W&P) paper. The current chairman of the company is GautamThapar, who succeeded his late uncle L.M. Thapar.

BILT's subsidiaries include Sabah Forest Industries (SFI), Malaysia's largest pulp and paper company, and BILT Tree Tech Limited (BTTL), which runs BILT's farm forestry pro-gramme in several states in India.

BILT has six manufacturing units across India, which give the company geographic cover-age over most of the domestic market. BILT has a dominant share of the high-end coated paper segment in India. The company accounts for over 50% of the coated wood-free paper market, an impressive 85% of the bond paper market and nearly 45% of the hi-bright Maplitho market, besides being India's largest exporter of coated paper.

BILT’s acquisition of SFI in 2007 was a watershed event – it was the first overseas acquisi-tion by an Indian paper company. This acquisition transformed BILT into a major regional player, and elevated the company's ranking among the global top 100.

5

2. Ballarpur Industries Limited (BILT): An Overview

Ballarpur Industries Limited (BILT) is India’s largest manufacturer of writing and printing (W&P) paper. BILT’s subsidiaries include Ballarpur International Graphic Paper Holdings B.V. (BIGPH); BILT Graphic Paper Products Limited (BGPPL); Sabah Forest Industries (SFI), Malaysia’s largest pulp and paper company; and Bilt Tree Tech Limited (BTTL), which runs BILT’s farm forestry programme in several states in India. Mr. R.R. Vederah is the Managing Director and Executive Vice Chairman.

In India, the company has six manufacturing units, giving it geographic coverage over most of the domestic market. The company has a dominant share of the high-end coated paper segment in India. It accounts for over 53% of the coated wood-free paper market, an impressive 80% of the bond paper market and nearly 35% of the hi-bright Maplitho market, besides being India's largest exporter of coated and uncoated paper.

Building on its unmatched paper quality, BILT ventured into the paper-based office stationery segment. The company markets its stationery through a well-established network of 350 retail distributors spread over 270 locations. BILT has mega brands such as BILT Royal Executive Bond, BILT Copy Power, BILT Image Copier and BILT Matrix that have now become an integral part of office stationery. BILT Ten on Ten notebooks are targeted at students and are also available with licensed characters such as Barbie, Spiderman, Winnie the Pooh, Hotwheels, Jungle King and Hannah Montana. BILT Student Stationery has won ‘Product of the Year’ award for the last three consecutive years. In 2008, BILT forayed into organised retail through P3 – Paper, Print and Pens – serving both B2C and B2B clients across India.

In 2005, BILT entered into the tissue and hygiene business with two brands: Etiquette and Spruce-up. Since then, the company has acquired Premier Tissues India Limited, the leading player in hygiene tissue products in the domestic retail market.

BILT’s acquisition of SFI, Malaysia, in 2007 was a watershed event – it was the first overseas acquisition by an Indian paper company, it transformed BILT into a major regional player, and elevated BILT’s ranking among the global top 100.

6

3. Paper machines in BILT

Paper machine-1

Make : John Inglis, Canada. Vintage : 1950. Speed : 280 meters per minuit. Deckle width :3.2 meter. Production : 50-60 Megatonne/day. GSM : 44-85. Paper machine-2

Make :Voith , Germany. Vintage :1962. Speed :145 meters per minuit. Deckle widt0h :3.2 meter. Production : 30 Megatonne/day. GSM : 26-60. Paper machine-3

Make : Voith , Germany. Vintage : 1962. Renovated in : 1990. Speed : 480 meters per minuit. Deckle width :3.5 meter. Production : 160-170 Megatonne/day. GSM :68-120

Paper machine-4

Make :Allimand , France. Vintage : 1965. Speed : 220 meters per minuit. Deckle widt0h : 2.84 meter. Production : 45-47 Megatonne/day. GSM : 58-100.

7

Paper machine-6

Make :BertranScinnes UK. Vintage : 1962. Speed : 250 meters per minuit. Deckle widt0h : 2.9 meter. Production : 72-75 Megatonne/day. GSM : 68-120. Paper machine-7

Make :Allimand , France. Vintage : 2009. Speed : 1100 meters per minuit. Deckle widt0h : 5046 meter. Production : 520 Megatonne/day. GSM :54-90.

8

4. Paper production

The base ingredient of the majority of papers is wood, so the first step in paper manufactur-ing is the harvesting of trees. Once the trees have been felled and shred down to their trunks, they are transported to a paper mill. Once they arrive the trunks are fed through a bark-skimming drum in which they are forced to collide and rub together. This process removes the trunks of all their bark, which while useful in other applications – such as being burned as boiler fuel -is detrimental to creating clean white paper.

In the next stage of the process, the de-barked logs are sent to a massive chipping unit, which breaks them down into small pieces. The chippings of wood are then fed into large pressure boilers called digesters. These reduce the wood chippings to a gloopy oatmeal-like pulp, which when extracted from the digester rests at a composition of one part fibre to 200 parts water.

The pulp is then deposited onto a high-speed, mesh screen loop, which removes most of the water content and leaves a thin layer of raw paper. This raw paper is pressed and heated in a series of drying cylinders where any remaining traces of moisture are removed. Finally the paper is treated with a starch solution that seals the surface and helps avoid excessive ink ab-sorption during printing.

Historically paper production has transitioned through three main phases, ranging from the manual and bespoke creation of single small sheets from plant and rag fibres, through larger-scale, water-powered paper mills and on to current fully automated and continuous pa-permaking facilities. Today, many new hardback titles are produced from wood-free paper, which is created exclusively from chemical pulp (a process where the lignin is totally sepa-rated from the cellulose fibres during processing) as it is not as prone to yellowing as tradi-tional, wood-based pulp paper.

9

4.1 Step by step paper production

Fig.4.1 Paper production process.

10

1. Logging – First, wood in industrial quantities is needed, with tree trunks and logs harvest-ed and shorn of their branches.

2. Stripping – The trunks/logs are then sent through a stripping machine, which quickly and efficiently removes their bark.

3. Chipping – The de-barked wood is then thrown into a chipping unit, which shreds them down into small strips.

4. Pulping – The small strips are deposited into a large pressure boiler (digester), where they are mixed with large quantities of water.

5. De-mulching – The boiler produces paper pulp, which is one part fibre to 200 parts water. Most of the water is removed via a mesh screen loop.

6. Drying – The remaining raw fibrous paper layer is then passed through numerous drying cylinders in order to solidify its structure.

7. Pressing – Pen ultimately, the paper is fed through a pressing unit, which equalizes its surface texture and form.

8. Treating – Finally, the paper is treated with a starch solution that seals the paper’s surface and helps to avoid excessive ink absorption during the printing process.

11

5. Pulp types and their preparations

The plant fibres used for pulp are composed mostly of cellulose and hemi-cellulose, which have a tendency to form molecular linkages between fibres in the presence of water. After the water evaporates the fibres remain bonded. It is not necessary to add additional binders for most paper grades, although both wet and dry strength additives may be added.

Rags of cotton and linen were the major source of pulp for paper before wood pulp. Today almost all pulp is of wood fibre. Cotton fibre is used in speciality grades, usually in printing paper for such things as resumes and currency.

Sources of rags often appear as waste from other manufacturing such as denim fragments or glove cuts. Fibres from clothing come from the cotton boll. The fibres can range from 3 to 7 cm in length as they exist in the cotton field. Bleach and other chemicals remove the colour from the fabric in a process of cooking, usually with steam. The cloth fragments mechanical-ly abrade into fibres, and the fibres get shortened to a length appropriate for manufacturing paper with a cutting process. Rags and water dump into a trough forming a closed loop. A cylinder with cutting edges, or knives, and a knife bed is part of the loop. The spinning cylin-der pushes the contents of the trough around repeatedly. As it lowers slowly over a period of hours, it breaks the rags up into fibres, and cuts the fibres to the desired length. The cutting process terminates when the mix has passed the cylinder enough times at the programmed final clearance of the knives and bed.

Another source of cotton fibre comes from the cotton ginning process. The seeds remain, sur-rounded by short fibres known as linters for their short length and resemblance to lint. Linters are too short for successful use in fabric. Linters removed from the cotton seeds are available as first and second cuts. The first cuts are longer.

The two major classifications of pulp are chemical and mechanical. Chemical pulps formerly used a sulphiteprocess, but the kraft process is now predominant. Kraft pulp has superior strength to sulphite and mechanical pulps. Both chemical pulps and mechanical pulps may be bleached to a high brightness.

Chemical pulping dissolves the lignin that bonds fibres to one another, and binds the outer fibrils that compose individual fibres to the fibre core. Lignin, like most other substances that can separate fibres from one another, acts as a debonding agent, lowering strength. Strength also depends on maintaining long cellulose molecule chains. The kraft process, due to the al-kali and sulphur compounds used, tends to minimize attack on the cellulose and the non-crystalline hemicelluloses, which promotes bonding, while dissolving the lignin. Acidic pulp-ing processes shorten the cellulose chains.

Kraft pulp makes superior linerboard and excellent printing and writing papers.

Groundwood, the main ingredient used in newsprint and a principal component of magazine papers (coated publications), is literally ground wood produced by a grinder. Therefore it contains a lot of lignin, which lowers its strength. The grinding produces very short fibres that drain slowly.

Thermomechanical pulp (TMP) is a variation of groundwood where fibres are separated me-chanically while at high enough temperatures to soften the lignin.

Between chemical and mechanical pulps there are semi-chemical pulps that use a mild chem-ical treatment followed by refining. Semi-chemical pulp is often used for corrugating medi-um.

12

Bales of recycled paper (normally old corrugated containers) for unbleached (brown) pack-aging grades may be simply pulped, screened and cleaned. Recycling to make white papers is usually done in a deinking plant, which employs screening, cleaning, washing, bleaching and flotation. Deinked pulp is used in printing and writing papers and in tissue, napkins and paper towels. It is often blended with virgin pulp.

At integrated pulp and paper mills, pulp is usually stored in high density towers before being pumped to stock preparation. Non integrated mills use either dry pulp or wet lap (pressed) pulp, usually received in bales. The pulp bales are slushed in a [re]pulper.

5.1 Stock (pulp) preparation

Stock preparation is the area where pulp is usually refined, blended to the appropriate pro-portion of hardwood, softwood or recycled fibre, and diluted to as uniform and constant as possible consistency. The pH is controlled and various fillers, such as whitening agents, size and wet strength or dry strength are added if necessary. Additional fillers such as clay, calci-um carbonate and titanium dioxide increase opacity so printing on reverse side of a sheet will not distract from content on the obverse side of the sheet. Fillers also improve printing quality.

Pulp is pumped through a sequence of tanks that are commonly called chests, which may be either round or more commonly rectangular. Historically these were made of special ceramic tile faced reinforced concrete, but mild and stainless steels are also used. Low consistency pulp slurries are kept agitated in these chests by propeller like agitators near the pump suc-tion at the chest bottom.

In the following process, different types of pulp, if used, are normally treated in separate but similar process lines until combined at a blend chest:

From high density storage or from slusher/pulper the pulp is pumped to a low density stor-age chest (tank). From there it is typically diluted to about 4% consistency before being pumped to an unrefined stock chest. From the unrefined stock chest stock is again pumped, with consistency control, through a refiner. Refining is an operation whereby the pulp slurry passes between a pair of discs, one of which is stationary and the other rotating at speeds of typically 1,000 or 1,200 RPM for 50 and 60 Hz AC, respectively. The discs have raised bars on their faces and pass each other with narrow clearance. This action unravels the outer layer of the fibres, causing the fibrils of the fibres to partially detach and bloom outward, increas-ing the surface area to promoting bonding. Refining thus increases tensile strength. For ex-ample, tissue paper is relatively unrefined whereas packaging paper is more highly refined. Refined stock from the refiner then goes to a refined stock chest, or blend chest, if used as such.

Hardwood fibres are typically 1 mm long and smaller in diameter than the 4 mm length typi-cal of softwood fibres. Refining can cause the softwood fibre tube to collapse resulting in undesirable properties in the sheet.

From the refined stock, or blend chest, stock is again consistency controlled as it is being pumped to a machine chest. It may be refined or additives may be added en route to the ma-chine chest.The machine chest is basically a consistency levelling chest having about 15 minutes retention. This is enough retention time to allow any variations in consistency enter-ing the chest to be levelled out by the action of the basis weight valve receiving feedback from the on line basis weight measuring scanner.

13

6. Paper machine sections

From the machine chest stock is pumped to a head tank, commonly called a stuff box, whose purpose is to maintain a constant head (pressure) on the stock as it feeds the basis weight valve. The stuff box also provides a means allowing air bubbles to escape. The basis weight valve meters the stock to the recirculating stream of water that is pumped, by the fan pump, from a whitewater chest through the headbox. On the way to the headbox the pulp slurry may pass through centrifugal cleaners, which remove heavy contaminants like sand, and screens, which break up fibre clumps and remove over-sized debris.

Fig. 6(a) Paper machine

Wood fibers have a tendency to attract one another, forming clumps, the effect being called flocculation. Flocculation is lessened by lowering consistency and or by agitating the slurry; however, de-flocculation becomes very difficult at much above 0.5% consistency. Minimiz-ing the degree of flocculation when forming is important to physical properties of paper.

The consistency in the headbox is typically under 0.4% for most paper grades, with longer fibres requiring lower consistency than short fibres. Higher consistency causes more fibres to be oriented in the z direction, while lower consistency promotes fibre orientation in the x-y direction. Higher consistency promotes higher calliper (thickness) and stiffness, lower consistency promotes higher tensile and some other strength properties and also improves formation (uniformity). Many sheet properties continue to improve down to below 0.1% consistency; however, this is an impractical amount of water to handle. (Most paper machine run a higher headbox consistency than optimum because they have been sped up over time without replacing the fan pump and headbox. There is also an economic trade off with high pumping costs for lower consistency).

The stock slurry, often called white water at this point, exits the headbox through a rectangu-lar opening of adjustable height called the slice, the white water stream being called the jet and it is pressurized on high speed machines so as to land gently on the moving fabric loop

14

orwire at a speed typically between plus or minus 3% of the wire speed, called rush and drag respectively. Excessive rush or drag causes more orientation of fibres in the machine direction and gives differing physical properties in machine and cross directions; however, this phenomenon is not completely avoidable on Fourdrinier machines.

On lower speed machines at 700 feet per minute, gravity and the height of the stock in the headbox creates sufficient pressure to form the jet through the opening of the slice. The height of the stock is the head, which gives the headbox its name. The speed of the jet com-pared to the speed of the wire is known as thejet-to-wire ratio. When the jet-to-wire ratio is less than unity, the fibres in the stock become drawn out in the machine direction. On slower machines where sufficient liquid remains in the stock before draining out, the wire can be driven back and forth with a process known as shake. This provides some measure of ran-domizing the direction of the fibres and gives the sheet more uniform strength in both the machine and cross-machine directions. On fast machines, the stock does not remain on the wire in liquid form long enough and the long fibres line up with the machine. When the jet-to-wire ratio exceeds unity, the fibers tend to pile up in lumps. The resulting variation in pa-per density provides the antique or parchment paper look.

Two large rolls typically form the ends of the drainage section, which is called the drainage table. The breast roll is located under the headbox, the jet being aimed to land on it at about the top centre. At the other end of the drainage table is the suction (couch) roll. The couch roll is a hollow shell, drilled with many thousands of precisely spaced holes of about 4 to 5 mm diameter. The hollow shell roll rotates over a stationary suction box, normally placed at the top centre or rotated just down machine. Vacuum is pulled on the suction box, which draws water from the web into the suction box. From the suction roll the sheet feeds into the press section.

Down machine from the suction roll, and at a lower elevation, is the wire turning roll. This roll is driven and pulls the wire around the loop. The wire turning roll has a considerable an-gle of wrap in order to grip the wire.

Supporting the wire in the drainage table area are a number of drainage elements. In addition to supporting the wire and promoting drainage, the elements de-flocculate the sheet. On low speed machines these table elements are primarily table rolls. As speed increases the suction developed in the nip of a table roll increases and at high enough speed the wire snaps back after leaving the vacuum area and causes stock to jump off the wire, disrupting the for-mation. To prevent this drainage foils are used. The foils are typically sloped between zero and two or three degrees and give a more gentle action. Where rolls and foils are used, rolls are used near the headbox and foils further down machine.

Approaching the dry line on the table are located low vacuum boxes that are drained by a barometric leg under gravity pressure. After the dry line are the suction boxes with applied vacuum. Suction boxes extend up to the couch roll. At the couch the sheet consistency should be about 25%.

15

Variation of the Fourdrinier forming section

Fig 6(b) Block diagram of paper machine

The forming section type is usually based on the grade of paper or paperboard being pro-duced; however, many older machines use a less than optimum design. Older machines can be upgraded to include more appropriate forming sections.

A second headbox may be added to a conventional fourdrinier to put a different fibre blend on top of a base layer. A secondary headbox is normally located at a point where the base sheet is completely drained. This is not considered a separate ply because the water action does a good job of intermixing the fibers of the top and bottom layer. Secondary headboxes are common on linerboard.

A modification to the basic fourdrinier table by adding a second wire on top of the drainage table is known as a top wire former. The bottom and top wires converge and some drainage is up through the top wire. A top wire improves formation and also gives more drainage, which is useful for machines that have been sped up.

The Twin Wire Machine or Gap former uses two vertical wires in the forming section, thereby increasing the de-watering rate of the fibre slurry while also giving uniform two sid-edness.

There are also machines with entire Fourdrinier sections mounted above a traditional Fourdrinier. This allows making multi-layer paper with special characteristics. These are called top Fourdriniers and they make multi-ply paper or paperboard. Commonly this is used for making a top layer of bleached fibre to go over an unbleached layer.

Another type forming section is the cylinder mould machine using a mesh-covered rotating cylinder partially immersed in a tank of fibre slurry in the wet end to form a paper web, giv-ing a more random distribution of the cellulose fibres. Cylinder machines can form a sheet at higher consistency, which gives a more three dimensional fibre orientation than lower con-sistencies, resulting in higher calliper (thickness) and more stiffness in the machine direction (MD). High MD stiffness is useful in food packaging like cereal boxes and other boxes like dry laundry detergent.

Tissue machines typically form the paper web between a wire and a special fabric (felt) as they wrap around a forming roll. The web is pressed from the felt directly onto a large diam-

16

eter dryer called a yankee. The paper sticks to the yankee dryer and is peeled off with a scraping blade called a doctor. Tissue machines operate at speeds of up to 2000 m/min.

6.1Press section

Fig.6.1 Press section.

The second section of the paper machine is the press section, which removes much of the remaining water via a system of nips formed by rolls pressing against each other aided by press felts that support the sheet and absorb the pressed water. The paper web consistency leaving the press section can be above 40%.

Pressing is the most efficient method of de-watering the sheet as only mechanical action is required. Press felts historically were made from wool. However, today they are nearly 100% synthetic. They are made up of a polyamide woven fabric with thick batt applied in a specific design to maximise water absorption.

Presses can be single or double felted. A single felted press has a felt on one side and a smooth roll on the other. A double felted press has both sides of the sheet in contact with a press felt. Single felted nips are useful when mated against a smooth roll (usually in the top position), which adds a two-sidedness—making the top side appear smoother than the bot-tom. Double felted nips impart roughness on both sides of the sheet. Double felted presses are desirable for the first press section of heavy paperboard.

Conventional roll presses are configured with one of the press rolls is in a fixed position, with a mating roll being loaded against this fixed roll. The felts run through the nips of the press rolls and continues around a felt run, normally consisting of several felt rolls. During the dwell time in the nip, the moisture from the sheet is transferred to the press felt. When the press felt exits the nip and continues around, a vacuum box known as an Uhle Box ap-plies vacuum (normally -60 kPa) to the press felt to remove the moisture so that when the felt returns to the nip on the next cycle, it does not add moisture to the sheet.

Some grades of paper use suction pick up rolls that use vacuum to transfer the sheet from the couch to a lead in felt on the first press or between press sections. Pickup roll presses nor-mally have a vacuum box that has two vacuum zones (low vacuum and high vacuum). These rolls have a large number of drilled holes in the cover to allow the vacuum to pass from the stationary vacuum box through the rotating roll covering. The low vacuum zone

picks up the sheet and transfers, while the high vacuum zone attempts to remove moisture.

Unfortunately, at high enough speed centrifugal force flings out vacuumed water, making this less effective for dewatering. Pickup presses also have standard felt runs with Uhle box-

17

es. However, pickup press design is quite different, as air movement is important for the pickup and dewatering facets of its role.

Crown Controlled Rolls (also known as CC Rolls) are usually the mating roll in a press ar-rangement. They have hydraulic cylinders in the press rolls that ensure that the roll does not bow. The cylinders connect to a shoe or multiple shoes to keep the crown on the roll flat, to counteract the natural "bend" in the roll shape due to applying load to the edges.

Extended Nip Presses (or ENP) are a relatively modern alternative to conventional roll presses. The top roll is usually a standard roll, while the bottom roll is actually a large CC roll with an extended shoe curved to the shape of the top roll, surrounded by a rotating rub-ber belt rather than a standard roll cover. The goal of the ENP is to extend the dwell time of the sheet between the two rolls thereby maximising the de-watering. Compared to a standard roll press that achieves up to 35% solids after pressing, an ENP brings this up to 45% and higher—delivering significant steam savings or speed increases. ENPs densify the sheet, thus increasing tensile strength and some other physical properties.

6.2Dryer section

Fig. 6.2(a) Dryer section

Dryer section of an older Fourdrinier-style paper-making machine. These narrow, small diameter dryers are not enclosed by a hood, dating the photo to before the 1970s.

The dryer section of the paper machine, as its name suggests, dries the paper by way of a se-ries of internally steam-heated cylinders that evaporate the moisture. Steam pressures may range up to 160 psig. Steam enters the end of the dryer head (cylinder cap) through a steam joint and condensate exits through a siphon that goes from the internal shell to a centre pipe. From the centre pipe the condensate exits through a

joint on the dryer head. Wide machines require multiple siphons. In fast machines centrifu-gal force holds the condensate layer still against the shell and turbulence gene-rating bars are typically used to agitate the condensate layer and improve heat transfer.

The sheet is usually held against the dryers by long felt loops on the top and bottom of each dryer section. The felts greatly improve heat transfer. Dryer felts are made of coarse thread and have a very open weave that is almost see through, It is common to have the first bottom

18

dryer section unfelted to dump broke on the basement floor during sheet breaks or when threading the sheet.

Paper dryers are typically arranged in groups called sections so that they can be run at a pro-gressively slightly slower speed to compensate for sheet shrinkage as the paper dries. The gaps between sections are called draws.

The drying sections are usually enclosed to conserve heat. Heated air is usually supplied to the pockets where the sheet breaks contact with the driers. This increases the rate of drying. The pocket ventilating tubes have slots along their entire length that face into the pocket. The dryer hoods are usually exhausted with a series of roof mounted hood exhausts fans down the dryer section.

Additional sizing agents, including resins, glue, or starch, can be added to the web to alter its characteristics. Sizing improves the paper's water resistance, decreases its ability to fuzz, re-duces abrasiveness, and improves its printing properties and surface bond strength. These may be applied at the wet (internal sizing) or on the dry end (surface sizing), or both. At the dry end sizing is usually applied with a size press. The size press may be a roll applicator (flooded nip) or a blade type. It is usually placed before the last dryer section. Some paper machines also make use of a 'coater' to apply a coating of fillers such as calcium carbonate or china clay usually suspended in a binder of cooked starch and styrene-butadiene latex. Coating produces a very smooth, bright surface with the highest printing qualities.

Fig.6.2(b)Paper leaving the machine is rolled onto a reel for further processing.

6.3Calender section

Acalender consists of two or more rolls, where pressure is applied to the passing paper. Cal-enders are used to make the paper surface extra smooth and glossy. It also gives it a more uniform thickness. The pressure applied to the web by the rollers determines the finish of the paper.

After calendering, the web has a moisture content of about 6% (depending on the furnish). It is wound onto a roll called a tambour or reel, and stored for final cutting and shipping. The roll hardness should be checked, obtained and adjusted accordingly to insure that the roll hardness is within the acceptable range for the product.

19

7. Power plants in BILT

There are 4 power plants in BILT. All power plants are run by coal except PP4 which is run by both coal and liquor. Power plant details are given below.

Plant name capacity Output Voltage Fuel PP1 7.5MW 11KV coal PP2 12.5MW 11KV coal PP3 7.5MW 11KV coal PP4 40MW 11KV Coal &liqour

Table1.1 power plants in BILT 7.140 MW Power station(PP4) PP4 is the largest power plant in BILT. It is run by Liqour a by-product of wood while pulp-ing process. There are two boiler for PP4. One is CFBC boiler and other is Recovery boiler. Generally both produces steam in ratio 9:1 but it can be varied according to load and fuel avaibility. This power plant is synchronized MSEB supply. 7.1.1Circulating Fluidized Bed Boiler

Capacities: Up to 500 TPH

Design pressure & temperature: Upto 150 kg/cm2 (g) & 545ºC

Fuels: Handle high sulphur, high ash and high moisture fuels, pet-coke, sludge, washery rejects, lig-nite, biomass

Circulating Fluidized Bed combustion has given boiler and power plant operators a greater flexibility in burning a wide range of coal and other fuels. All this without compromising efficiency and with reduced pollution. How does the boiler work with this technology?

In the olden days blacksmiths used to heat the iron by placing it on a bed of coal. Bellows provide air to the coal from the bottom of the bed. Fluidized Bed combustion is something similar to this.

Fluidized Bed

At the bottom of the boiler furnace there is a bed of inert material. Bed is where the coal or fuel spreads. Air supply is from under the bed at high pressure. This lifts the bed material and the coal particles and keeps it in suspension. The coal combustion takes place in this suspended condition. This is the Fluidized bed. Special design of the air nozzles at the bottom of the bed allows air flow without clogging. Primary air fans provide the preheated Fluidizing air. Secondary air fans provide pre-heated Combustion air. Nozzles in the furnace walls at various levels distribute the Combustion air in the furnace.

20

Circulation

Fine particles of partly burned coal, ash and bed material are carried along with the flue gas-es to the upper areas of the furnace and then into a cyclone. In the cyclone the heavier parti-cles separate from the gas and falls to the hopper of the cyclone. This returns to the furnace for recirculation. Hence the name Circulating Fluidized Bed combustion. The hot gases from the cyclone pass to the heat transfer surfaces and go out of the boiler.

Bed Material

To start with the bed material is sand. Some portion is lost in the ash during the operation and this has to be made-up. In coal fired boilers the ash from the coal itself will be the makeup material. When firing bio fuels with very low ash content sand will be the makeup bed material. For high Sulphur coals Limestone addition to the bed material reduces SO2 emissions.

CFBC uses crushed coal of 3 to 6 mm size. This requires only a crusher not a pulverizer. From storage hoppers Conveyer and feeders transport the coal to feed chutes in the furnace. Start-up is by oil burners in the furnace. Ash spouts in the furnace remove the ash from the bottom of the furnace.

The diagram below shows the schematic of a CFB boiler.

Fig.7.1.1 Block diagram of CFBC boiler

Different boiler manufacturers adopt different methods of cyclone separation, the fluidiz-ing nozzles etc. But the basic principles remain the same.

7.1.2 Recovery boiler

Recovery boiler is the part of Kraft process of pulping where chemicals for white liquor are recovered and reformed from black liquor, which contains lignin from previously processed

21

wood. The black liquor is burned, generating heat, which is usually used in the process or in making electricity, much as in a conventional steam power plant. The invention of the re-covery boiler by G.H. Tomlinson in the early 1930s was a milestone in the advancement of the kraft process. Recovery boilers are also used in the (less common) sulfite process of wood pulping.

Concentrated black liquor contains organic dissolved wood residue in addition to sodium sulfate from the cooking chemicals added at the digester. Combustion of the organic portion of chemicals produces heat. In the recovery boiler heat is used to produce high pressure steam, which is used to generate electricity in a turbine. The turbine exhaust, low pressure steam is used for process heating.

Combustion of black liquor in the recovery boiler furnace needs to be controlled carefully. High concentration of sulfur requires optimum process conditions to avoid production of sulfur dioxide and reduced sulfur gas emissions. In addition to environmentally clean com-bustion, reduction of inorganic sulfur must be achieved in the char bed.

Several processes occur in the recovery boiler:

Combustion of organic material in black liquor to generate steam Reduction of inorganic sulfur compounds to sodium sulfide, which exits at the bottom as

smelt Production of molten inorganic flow of mainly sodium carbonate and sodium sulfide, which

is later recycled to the digester after being re-dissolved Recovery of inorganic dust from flue gas to save chemicals Production of sodium fume to capture combustion residue of released sulfur compounds

Fig.7.1.2 Block diagram of recovery boiler

22

A modern recovery boiler consists of heat transfer surfaces made of steel tube; furnace-1, superheaters-2, boiler generating bank-3 and economizers-4. The steam drum-5 design is of single-drum type. The air and black liquor are introduced through primary and secondary air ports-6, liquor guns-7 and tertiary air ports-8. The combustion residue, smelt exits through smelt spouts-9 to the dissolving tank-10.

The nominal furnace loading has increased during the last ten years and will continue to in-crease. Changes in air design have increased furnace temperatures. This has enabled a signif-icant increase in hearth solids loading (HSL) with only a modest design increase in hearth heat release rate (HHRR). The average flue gas flow decreases as less water vapor is present. So the vertical flue gas velocities can be reduced even with increasing temperatures in lower furnace.

The most marked change has been the adoption of single drum construction. This change has been partly affected by the more reliable water quality control. The advantages of a sin-gle drum boiler compared to a bi drum are the improved safety and availability. Single drum boilers can be built to higher pressures and bigger capacities. Savings can be achieved with decreased erection time. There is less tube joints in the single drum construction so drums with improved startup curves can be built.

The construction of the vertical steam generating bank is similar to the vertical economizer, which based on experience is very easy to keep clean. Vertical flue gas flow path improves the cleanability with high dust loading. To minimize the risk for plugging and maximize the efficiency of cleaning both the generating bank and the

economizers are arranged on generous side spacing. Plugging of a two drum boiler bank is often caused by the tight spacing between the tubes.

The spacing between superheater panels has increased. All superheaters are now wide spaced to minimize fouling. This arrangement, in combination with sweetwaterattemperators, ensures maximum protection against corrosion. With wide spacing plugging of the superheaters becomes less likely, the deposit cleaning is easier and the sootblowing steam consumption is lower. Increased number of superheaters facilitates the control of superheater outlet steam temperature especially during start ups.

The lower loops of hottest superheaters can be made of austenitic material, with better corro-sion resistance. The steam velocity in the hottest superheater tubes is high, decreasing the tube surface temperature. Low tube surface temperatures are essential to prevent superheater corrosion. A high steam side pressure loss over the hot superheaters ensures uniform steam flow in tube elements.

Safety

One of the main hazards in operation of recovery boilers is the smelt-water explosion. This can happen if even a small amount of water is mixed with the solids in high temperature. Smelt-water explosion is purely a physical phenomenon. The smelt water explosion phe-nomena have been studied by Grace.By 1980 there were about 700 recovery boilers in the world. The liquid - liquid type explosion mechanism has been established as one of the main causes of recovery boiler explosions.

In the smelt water explosion even a few liters of water, when mixed with molten smelt can violently turn to steam in few tenths of a second. Char bed and water can coexist as steam blanketing reduces heat transfer. Some trigger event destroys the balance and water is evap-orated quickly through direct contact with smelt. This sudden evaporation causes increase of

23

volume and a pressure wave of some 10 000 – 100 000 Pa. The force is usually sufficient to cause all furnace walls to bend out of shape. Safety of equipment and personnel requires an immediate shutdown of the recovery boiler if there is a possibility that water has entered the furnace. All recovery boilers have to be equipped with special automatic shutdown se-quence.

The other type of explosions is the combustible gases explosion. For this to happen the fuel and the air have to be mixed before the ignition. Typical conditions are either a blackout (loss of flame) without purge of furnace or continuous operation in a substoichiometric state. To detect blackout flame monitoring devices are installed, with subsequent interlocked purge and startup. Combustible gas explosions are connected with oil/gas firing in the boiler. As also continuous O2 monitoring is practiced in virtually every boiler the noncombustible gas explosions have become very rare.

7.2Generator

40MW generator is used in PP4 whose specification is given below.

Output 50000 KVA No. of phases 3 No. of Poles 4 Volts(AC) 11000V Current(AC) 2624A Speed 1500 Rpm. Power Factor 0.8 Frequency 50Hz Limiting speed 1800Rpm Type of stator Star Excitation Volts 425V Excitation Currents 610A Altitude <1000 Type TC220 Weight 86000Kg Enclosure system IP54 Cooling system IC9A1W7 Coolant temperature 33*C Max. temp. rise of stator by RTD 58*C Class of insulation F Brgs DE STEEV NDE STEEV GRS/oil ISOV446

Table 1.1 Generator specification.

7.2.1 Brushless Excitation System of generator

In all the excitation systems discussed so far, the D.C. power generated or derived from dif-ferent means is fed to the generator fielded throw brushes to slip ring. The brush gear and slip ring have become such a vital parts that required high maintenance and are a source of failures, thus forming week links in the system. With the advent of mechanically robust sili-cone diode capable of converting A.C. to D.C. at a high power levels, brushless excitation system has become popular and being employed. The basic arrangement of a typical brush-

24

less excitation system presently used in BHEL machines. This system consists of main components as listed below:- (I). Three phase pilot exciter. (II). Three phase main exciter. (III). Rotating rectifier wheels. (IV). Cooler. (V). Metering and supervisory system. Three Phase Pilot Exciter :- Three phase pilot exciter has a revolving field with permanent magnet poles. The controlled rectified d.c. is fed to the main exciter field. The induced Three Phase a.c. voltage is rectified in the rotating rectifier bridge and is fed to the generator rotor winding through the d.c. leads in the shaft. The pilot exciter has 16 poles. The output is 220V + - 10%, 400 Hz. Ten mag-nets are housed together in a non magnetic enclosure and this make one pole. These magnets are braced between the hub and external pole shoe with bolts. Three Phase Main Exciter :- The three phase main exciter is a six pole rotating armature unit. The field poles with the damper windings are arranged in the stator frame. Laminated magnetic poles are arranged to form the field winding. To measure the exciter current a quadrature axis coil is fitted between two poles. The winding conductors are transposed within the core length, and the end turns of the rotor windings are secured with steel bands. The connections are made at rectifier wheel end. A ring bus formed at the winding end and leads to rotating rectifier wheel are also connected to the same. The complete rotor is shrunk fit on the shaft. The rotor is supported on a journal bearing positioned between the main and the pilot exciters. Lubrication of the bearing is formed from the turbine oil system. Rotating Rectifier Wheels :- The silicon diodes are arranged on the rectifier wheels in three configurations. The diodes are so made that the contact pressure increases during rotation due to the centrifugal force. There are two diodes. Coolers :- Because of these properties, hydrogen will extract more heat per unit volume/min. Thus for a given rise of temperature, machine capacity can be increased. It has been estimated that by use of Hydrogen 20% reduction in active construction materials can be affected. At 0.035 kg/cm² of hydrogen, machine rating is increased by 22-25% and at 2.109 kg/cm² the rating increase is 35%.

25

Fig.Fig. 7.2.1 Brushless Excitation System

Increase in life of machine :- This is effected because of: Enclosed construction, which keeps the dirt and moisture out from winding and ventilation passages. No deterioration of armature insulation due to corona. During corona discharge, ozone, nitric acid and otter chemical compounds are formed due to oxidation, which attack organic bounding materials of insulation. Leading increased output from the same machine, with the increase in pressure, the heat transfer co-efficient increases appreciably and also in same space more H2 by weight can be employed. Thus, the denser H2 will have improved capacity to absorb and remove the heat with the result that from the same machine, output may be increased. It has been estimated that for every 0.07kg/cm² increase in pressure up to 1 kg/cm², an increase of 1% in out can be achieved, while theCorresponding figure will be 1\2 % for to 2kg/cm². Incidentally this will resultseither in allowing lower quantity of cool-ing water or higher inlet temperature of cooling water without impairing output of efficiency of the machines. The permitted increase in the temperature 0.56°c up to 1 kg/cm² for 0.035 kg/cm² rise of pressure and 0.280°c between 1 kg/cm² to 2 kg/cm². Hydrogen /air mixture between 5/95% and 75/25% are explosive and hence normally a 95/5% - 98/2% content is employed. In modern systems it is more general to restrict hydro-gen/air mixture to 98/2%.

26

8. Direct online Starter All motors in BILT are under 5 HP. So it is efficient to use direct on line starter.

Different starting methods are employed for starting induction motors because Induction Motor draws more starting current during starting. To prevent damage to the windings due to the high starting current flow, we employ different types of starters.

The simplest form of motor starter for the induction motor is the Direct On Line starter. The Direct On Line Motor Starter (DOL) consist a MCCB or Circuit Breaker, Contactor and an overload relay for protection. Electromagnetic contactor which can be opened by the thermal overload relay under fault conditions.

Typically, the contactor will be controlled by separate start and stop buttons, and an auxilia-ry contact on the contactor is used, across the start button, as a hold in contact. I.e. the con-tactor is electrically latched closed while the motor is operating.

Fig. 8.1 Direct Online Motor Starter - Square D

8.1 Principle of Direct On Line Starter (DOL)

To start, the contactor is closed, applying full line voltage to the motor windings. The motor will draw a very high inrush current for a very short time, the magnetic field in the iron, and then the current will be limited to the Locked Rotor Current of the motor. The motor will develop Locked Rotor Torque and begin to accelerate towards full speed.

As the motor accelerates, the current will begin to drop, but will not drop significantly until the motor is at a high speed, typically about 85% of synchronous speed. The actual starting current curve is a function of the motor design, and the terminal voltage, and is totally inde-pendent of the motor load.

The motor load will affect the time taken for the motor to accelerate to full speed and there-fore the duration of the high starting current, but not the magnitude of the starting current.

Provided the torque developed by the motor exceeds the load torque at all speeds during the start cycle, the motor will reach full speed. If the torque delivered by the motor is lessthan

27

the torque of the load at any speed during the start cycle, the motor will stops accelerating. If the starting torque with a DOL starter is insufficient for the load, the motor must be replaced with a motor which can develop a higher starting torque.

The acceleration torque is the torque developed by the motor minus the load torque, and will change as the motor accelerates due to the motor speed torque curve and the load speed torque curve. The start time is dependent on the acceleration torque and the load inertia.

DOL starting have a maximum start current and maximum start torque.

This may cause an electrical problem with the supply, or it may cause a mechanical problem with the driven load. So this will be inconvenient for the users of the supply line, always ex-perience a voltage drop when starting a motor. But if this motor is not a high power one it does not affect much.

8.2 Parts of DOL Starters

8.2.1 Contactors & Coil

Fig. 8.2.1Contactor

Magnetic contactors are electromagnetically operated switches that provide a safe and con-venient means for connecting and interrupting branch circuits.

Magnetic motor controllers use electromagnetic energy for closing switches. The electro-magnet consists of a coil of wire placed on an iron core. When a current flow through the coil, the iron of the magnet becomes magnetized, attracting an iron bar called the armature. An interruption of the current flow through the coil of wire causes the armature to drop out due to the presence of an air gap in the magnetic circuit.

Line-voltage magnetic motor starters are electromechanical devices that provide a safe, con-venient, and economical means of starting and stopping motors, and have the advantage of being controlled remotely. The great bulk of motor controllers sold are of this type.

28

Contactors are mainly used to control machinery which uses electric motors. It consists of a coil which connects to a voltage source. Very often for Single phase Motors, 230V coils are used and for three phase motors, 415V coils are used. The contactor

has three main NO contacts and lesser power rated contacts named as Auxiliary Contacts [NO and NC] used for the control circuit. A contact is conducting metal parts which com-pletes or interrupt an electrical circuit.

NO-normally open NC-normally closed

8.2.2 Over Load Relay (Overload protection)

Overload protection for an electric motor is necessary to prevent burnout and to ensure maximum operating life.

Under any condition of overload, a motor draws excessive current that causes overheat-ing. Since motor winding insulation deteriorates due to overheating, there are established limits on motor oper-ating temperatures to protect a motor from overheating. Overload relays are employed on a motor control to limit the amount of current drawn.

The overload relay does not provide short circuit protection. This is the function of over current pro-tective equipment like fuses and circuit breakers, generally located in the disconnecting switch en-closure.

The ideal and easiest way for overload protection for a motor is an element with current-sensing properties very similar to the heating curve of the motor which would act to open the motor circuit when full-load current is exceeded. The operation of the protective de-vice should be such that the motor is allowed to carry harmless over-loads but is quickly removed from the line when an overload has persisted too long.

Fig.8.2.2Thermal Overload Relay

Normally fuses are not designed to provide overload protection. Fuse is protecting against short circuits (over current protection). Motors draw a high inrush current when starting and conventional fuses have no way of distinguishing between this temporary and harmless in-rush current and a damaging overload. Selection of Fuse is depend on motor full-load cur-rent, would “blow” every time the motor is started. On the other hand, if a fuse were chosen large enough to pass the starting or inrush current, it would not protect the motor against small, harmful overloads that might occur later.

29

The overload relay is the heart of motor protection. It has inverse-trip-time characteristics, permitting it to hold in during the accelerating period (when inrush current is drawn), yet providing protection on small overloads above the full-load current when the motor is run-ning. Overload relays are renewable and can withstand repeated trip and reset cycles

without need of replacement. Overload relays cannot, however, take the place of over cur-rent protection equipment.

The overload relay consists of a current-sensing unit connected in the line to the motor, plus a mechanism, actuated by the sensing unit, which serves, directly or indirectly, to break the circuit.

Overload relays can be classified as being thermal, magnetic, or electronic:

1. Thermal Relay: As the name implies, thermal overload relays rely on the rising tem-peratures caused by the overload current to trip the overload mechanism. Thermal overload relays can be further subdivided into two types: melting alloy and bimetallic.

2. Magnetic Relay: Magnetic overload relays react only to current excesses and are not affected by temperature.

3. Electronic Relay: Electronic or solid-state overload relays, provide the combination of high-speed trip, adjustability, and ease of installation. They can be ideal in many precise ap-plications.

30

9. Variable-frequency drive (VFD) A variable-frequency drive (VFD) (also termed adjustable-frequency drive, variable-speed drive, AC drive, micro drive or inverter drive) is a type of adjustable-speed drive used in electro-mechanical drive systems to control AC motor speed and torque by varying motor input frequency and voltage.

Fig.9 variable-frequency drive for small motor

VFDs are used in applications ranging from small appliances to the largest of mine mill drives and compressors. However, about a third of the world's electrical energy is consumed by electric motors in fixed-speed centrifugal pump, fan and compressor applications and VFDs' global market penetration for all applications is still relatively small. This highlights especially significant energy efficiency improvement opportunities for retrofitted and new VFD installations.

Over the last four decades, power electronics technology has reduced VFD cost and size and improved performance through advances in semiconductor switching devices, drive topolo-gies, simulation and control techniques, and control hardware and software.

VFDs are available in a number of different low and medium voltage AC-AC and DC-AC topologies.

The AC electric motor used in a VFD system is usually a three-phase induction motor. Some types of single-phase motors can be used, but three-phase motors are usually preferred. Var-ious types of synchronous motors offer advantages in some situations, but three phase induc-tion motors are suitable for most purposes and are generally the most economical motor choice. Motors that are designed for fixed-speed operation are often used. Elevated voltage stresses imposed on induction motors that are supplied by VFDs require that such motors be designed for definite-purpose inverter-fed duty in accordance to such requirements as Part 31 of NEMA Standard MG-1.

31

9.1Controller

The VFD controller is a solid state power electronics conversion system consisting of three distinct sub-systems: a rectifier bridge converter, a direct current (DC) link, and an inverter. Voltage-source inverter (VSI) drives are by far the most common type of drives. Most drives are AC-AC drives in that they convert AC line input to AC inverter output. However, in some applications such as common DC bus or solar applications, drives are configured as DC-AC drives. The most basic rectifier converter for the VSI drive is configured as a three-phase, six-pulse, full-wave diode bridge. In a VSI drive, the DC link consists of a capacitor which smooths out the converter's DC output ripple and provides a stiff input to the inverter. This filtered DC voltage is converted to quasi-sinusoidal AC voltage output using the invert-er's active switching elements. VSI drives provide higher power factor and lower harmonic distortion than phase-controlled current-source inverter (CSI) and load-commutated inverter (LCI) drives (see 'Generic topologies' sub-section below). The drive controller can also be configured as a phase converter having single-phase converter input and three-phase inverter output.

Controller advances have exploited dramatic increases in the voltage and current ratings and switching frequency of solid state power devices over the past six decades. Introduced in 1983, the insulated-gate bipolar transistor (IGBT) has in the past two decades come to dom-inate VFDs as an inverter switching device.

In variable-torque applications suited for Volts per Hertz (V/Hz) drive control, AC motor characteristics require that the voltage magnitude of the inverter's output to the motor be ad-justed to match the required load torque in a linear V/Hz relationship. For example, for 460 volt, 60 Hz motors this linear V/Hz relationship is 460/60 = 7.67 V/Hz. While suitable in wide ranging applications, V/Hz control is sub-optimal in high performance applications in-volving low speed or demanding, dynamic speed regulation, positioning and reversing load requirements. Some V/Hz control drives can also operate in quadratic V/Hz mode or can even be programmed to suit special multi-point V/Hz paths.

The two other drive control platforms, vector control and direct torque control (DTC), adjust the motor voltage magnitude, angle from reference and frequency so as to precisely control the motor's magnetic flux and mechanical torque.

Although space vector pulse-width modulation (SVPWM) is becoming increasingly popular, sinusoidal PWM (SPWM) is the most straightforward method used to vary drives' motor voltage (or current) and frequency. With SPWM control (see Fig. 1), quasi-sinusoidal, vari-able-pulse-width output is constructed from intersections of a saw-toothed carrier frequency signal with a modulating sinusoidal signal which is variable in operating frequency as well as in voltage (or current).

Operation of the motors above rated nameplate speed (base speed) is possible, but is limited to conditions that do not require more power than the nameplate rating of the motor. This is sometimes called "field weakening" and, for AC motors, means operating at less than rated V/Hz and above rated nameplate speed. Permanent magnet synchronous motors have quite limited field weakening speed range due to the constant magnet flux linkage. Wound rotor synchronous motors and induction motors have much wider speed range. For example, a 100 hp, 460 V, 60 Hz, 1775 RPM (4 pole) induction motor supplied with 460 V, 75 Hz (6.134 V/Hz), would be limited to 60/75 = 80% torque at 125% speed (2218.75 RPM) = 100% power. At higher speeds the induction motor torque has to be limited further due to the lowering of the breakaway torque of the motor. Thus rated power can be typically pro-

32

duced only up to 130-150% of the rated nameplate speed. Wound rotor synchronous motors can be run at even higher speeds. In rolling mill drives often 200-300% of the base speed is used. The mechanical strength of the rotor limits the maximum speed of the motor.

Fig. 9.1: SPWM carrier-sine input & 2-level PWM output

An embedded microprocessor governs the overall operation of the VFD controller. Basic programming of the microprocessor is provided as user inaccessible firmware. User pro-gramming of display, variable and function block parameters is provided to control, protect and monitor the VFD, motor and driven equipment.

The basic drive controller can be configured to selectively include such optional power components and accessories as follows:

Connected upstream of converter - circuit breaker or fuses, isolation contactor, EMC filter, line reactor, passive filter

Connected to DC link - braking chopper, braking resistor Connected downstream of inverter - output reactor, sine wave filter, dV/dt filter.

9.2Operator interface

The operator interface provides a means for an operator to start and stop the motor and ad-just the operating speed. Additional operator control functions might include reversing, and switching between manual speed adjustment and automatic control from an external process control signal. The operator interface often includes an alphanumeric display and/or indica-tion lights and meters to provide information about the operation of the drive. An operator interface keypad and display unit is often provided on the front of the VFD controller as shown in the photograph above. The keypad display can often be cable-connected and mounted a short distance from the VFD controller. Most are also provided with input and output (I/O) terminals for connecting pushbuttons, switches and other operator interface de-vices or control signals. A serial communications port is also often available to allow the VFD to be configured, adjusted, monitored and controlled using a computer.

9.3Drive operation

Referring to the accompanying chart, drive applications can be categorized as single-quadrant, two-quadrant or four-quadrant; the chart's four quadrants are defined as follows:

Quadrant I - Driving or motoring, forward accelerating quadrant with positive speed and torque Quadrant II - Generating or braking, forward braking-decelerating quadrant with positive speed

and negative torque

33

Quadrant III - Driving or motoring, reverse accelerating quadrant with negative speed and torque

Quadrant IV - Generating or braking, reverse braking-decelerating quadrant with negative speed and positive torque.

Fig.9.3 Electric motor speed-torque chart

Most applications involve single-quadrant loads operating in quadrant I, such as in variable-torque (e.g. centrifugal pumps or fans) and certain constant-torque (e.g. extruders) loads.

Certain applications involve two-quadrant loads operating in quadrant I and II where the speed is positive but the torque changes polarity as in case of a fan decelerating faster than natural mechanical losses. Some sources define two-quadrant drives as loads operating in quadrants I and III where the speed and torque is same (positive or negative) polarity in both directions.

Certain high-performance applications involve four-quadrant loads (Quadrants I to IV) where the speed and torque can be in any direction such as in hoists, elevators and hilly con-veyors. Regeneration can only occur in the drive's DC link bus when inverter voltage is smaller in magnitude than the motor back-EMF and inverter voltage and back-EMF are the same polarity.

In starting a motor, a VFD initially applies a low frequency and voltage, thus avoiding high inrush current associated with direct on line starting. After the start of the VFD, the applied frequency and voltage are increased at a controlled rate or ramped up to accelerate the load. This starting method typically allows a motor to develop 150% of its rated torque while the VFD is drawing less than 50% of its rated current from the mains in the low speed range. A VFD can be adjusted to produce a steady 150% starting torque from standstill right up to full speed. However, motor cooling deteriorates and can result in overheating as speed decreases such that prolonged low speed motor operation with significant torque is not usually possi-ble without separately-motorized fan ventilation.

With a VFD, the stopping sequence is just the opposite as the starting sequence. The fre-quency and voltage applied to the motor are ramped down at a controlled rate. When the fre-quency approaches zero, the motor is shut off. A small amount of braking torque is available to help decelerate the load a little faster than it would stop if the motor were simply switched off and allowed to coast. Additional braking torque can be obtained by adding a braking cir-cuit (resistor controlled by a transistor) to dissipate the braking energy. With a four-quadrant rectifier (active-front-end), the VFD is able to brake the load by applying a reverse torque and injecting the energy back to the AC line.

34

9.4Energy savings

Many fixed-speed motor load applications that are supplied direct from AC line power can save energy when they are operated at variable-speed, by means of VFD. Such energy cost savings are especially pronounced in variable-torque centrifugal fan and pump applications, where the loads' torque and power vary with the square and cube, respectively, of the speed. This change gives a large power reduction compared to fixed-speed operation for a relatively small reduction in speed. For example, at 63% speed a motor load consumes only 25% of its full speed power. This is in accordance with affinity laws that define the relationship be-tween various centrifugal load variables.

9.5 Control platforms

Most drives use one or more of the following control platforms:

PWM V/Hz scalar control PWM field-oriented control (FOC) or vector control Direct torque control (DTC).

9.6 Load torque and power characteristics

Variable frequency drives are also categorized by the following load torque and power char-acteristics:

Variable torque, such as in centrifugal fan, pump and blower applications Constant torque, such as in conveyor and displacement pump applications Constant power, such as in machine tool and traction applications.

9.7 Available power ratings

VFDs are available with voltage and current ratings covering a wide range of single-phase and multi-phase AC motors. Low voltage (LV) drives are designed to operate at output volt-ages equal to or less than 690 V. While motor-application LV drives are available in ratings of up to the order of 5 or 6 MW, economic considerations typically favor medium voltage (MV) drives with much lower power ratings. Different MV drive topologies are configured in accordance with the voltage/current-combination ratings used in different drive control-lers' switching devices such that any given voltage rating is greater than or equal to one to the following standard nominal motor voltage ratings: generally either 2.3/4.16 kV (60 Hz) or 3.3/6.6 kV (50 Hz), with one thyristor manufacturer rated for up to 12 kV switching. In some applications a step up transformer is placed between a LV drive and a MV motor load. MV drives are typically rated for motor applications greater than between about 375 kW (500 hp) and 750 kW (1000 hp). MV drives have historically required considerably more application design effort than required for LV drive applications. The power rating of MV drives can reach 100 MW, a range of different drive topologies being involved for different rating, performance, power quality and reliability requirements.