LAPPEENRANTA UNIVERSITY OF TECHNOLOGY
Department of Electrical Engineering
MASTER’S THESIS
ENERGY SAVING ADJUSTABLE AC DRIVES FOR THE CEMENT
INDUSTRY EQUIPMENT
The topic of the Master’s thesis has been approved by the department council of the
Department of Electrical Engineering on 12 April 2006.
The supervisor from the ABB company is B.Sc Pekka Pulkki
The examiners is Professor, D.Sc. Juha Pyrhönen
The second examiners is D.Sc. Pia Salminen
Lappeenranta 6.06.2006
Anna Platonova
Karankokatu 4 A 7
53810 Lappeenranta
Finland
a.a.platonova@gmail.com
ABSTRACT
Author:
Platonova Anna
Title:
Energy saving adjustable AC drives for the cement industry equipment
Department: Electrical Engineering
Year:
2006
Place:
Lappeenranta
Master’s Thesis. Lappeenranta University of Technology. 81 pages, 42 figures and 6
tables.
Supervisor:
B.Sc Pekka Pulkki
Examiners:
Professor, D.Sc. Juha Pyrhönen and D.Sc. Pia Salminen.
Keywords:
variable speed drives, cement industry, energy, efficiency
The European power and automation technology companies have shown rapidly
increasing interest in the Russian market. Despite the fact that Russia brings big, new
perspectives to the European market, the Russian market is still relatively unstudied.
One area of particular interest is that of variable speed drives – which is a general term
applied to equipment that is used for control speed of a motor.
This Master’s thesis focuses on the cement industry market and more specifically on the
basic phases of cement production and the equipment used in the production process.
As a result, some estimates are given with regard to the energy conservation efficiency
of using VSD as opposed to standard methods for control.
FOREWORD
This Master’s thesis has been done for ABB Company. It is a research of the Russian
Cement Industry. It is shown that the upcoming cement market offers ABB Company
key perspectives, because for the development of the cement industry equipment a lot of
energy saving variable AC drives are needed.
I want to thank Professor Juha Pyrhönen, Julia Vauterin and Professor Erkki Lähderanta
and all professors from the Control Systems Department of Saint-Petersburg
Electrotechnical University for giving me the opportunity to study here.
Also I want to thank the Regional Director of ABB, Jussi Vanhanen for giving me the
opportunity to take part in this project and my supervisors and examiners: Pekka Pulkki,
Professor Juha Pyrhönen and Pia Salminen for their support and practical advices.
I would also like to express my thanks to all persons who with a simple smile and gentle
greeting helped me to feel myself at home in a pleasant working environment have.
I am also grateful to all my friends, especially to Yuri for love, patience, support and
also good technical advices. Also thanks to Julian, Nikulka and Dean for helping me
improve my English skills.
Special thanks go to my mother and my family, who have helped and supported me
throughout the years and helped me to achieve my personal goals.
Lappeenranta 06.06.2006
Anna Platonova
4
TABLE OF CONTENTS
Symbols and abbreviations .............................................................................................. 5
1. Introduction. ................................................................................................................. 7
1.1 Cement industry of Russia. .................................................................................... 8
1.2 Cement production process. ................................................................................. 20
1.3 Cement processing equipment ............................................................................. 22
1.3.1 Crushers .........................................................................................................22
1.3.2 Conveyors ......................................................................................................23
1.3.3 Mills ...............................................................................................................25
1.3.4 Separators.......................................................................................................28
1.3.5 Feeders ...........................................................................................................30
1.3.6 Kiln.................................................................................................................32
1.3.7 Fans ................................................................................................................33
1.4 Consumption and saving of electric energy. ........................................................ 34
2. AC-drive technology in cement producing. ............................................................... 38
2.1 AC-drive notion and properties............................................................................ 40
2.2 AC-drives control methods. ................................................................................. 54
2.2.1 Scalar control methods...................................................................................55
2.2.2 Vector control methods ..................................................................................58
2.3 Automation of AC-drives..................................................................................... 62
3 Comparison of traditional vane control and modern AC-drives control in Cesla
Cement factory. .............................................................................................................. 65
3.1 Efficiency ............................................................................................................. 68
3.2 Power savings and reactive power consumption. ................................................ 68
3.3 Power factor ......................................................................................................... 69
3.4 Location and Installation...................................................................................... 70
3.5 Specification......................................................................................................... 71
3.6 Operation.............................................................................................................. 71
3.7 Ability to control .................................................................................................. 73
3.8 Maintenance and spare parts ................................................................................ 73
3.9 Operating and installation costs ........................................................................... 74
4. Conclusion ................................................................................................................. 77
References ...................................................................................................................... 79
5
Symbols and abbreviations
AC
Alternating Current
CAN
Controller Area Network, protocol
CF
Controlled flow
CIPEC
Canadian Industry Program for Energy Conservation
CSI
Current Source Inverter
DC
Direct Current
DPF
Displacement Power Factor
DTC
Direct Torque Control
DOL
Direct on Line
ID
Induced-Draft
IM
Induction Motor
IP
Internet Protocol address
MS
Microsoft
PWM
Pulse Width Modulation
RMC
International public company, producing building materials,
Great Britain
ROSSTATA Russian federal service of state statistics
RUB
Russian ruble
SM
Synchronous Motor
SOED
Shorter Oxford English Dictionary
SPRS
Slip Power Recovery System
TCP
Transmission Control Protocol
THD
Total Harmonic Distortion
US
United States (of America)
USD
US dollar
USSR
Union of Soviet Socialist Republics
VDC
DC Voltage
VSD
Variable Speed Drive
VSI
Voltage Source Inverter
VVVF
Variable Voltage Variable Frequency
6
ZKG
Zement Kalk Gips, magazine
C
capacitance
f
frequency
I
current
i
current, instantaneous value
k
coefficient
L
inductance
M
mutual inductance
p
number of pole pairs
t
time
T
torque
u
voltage, instantaneous value
U
voltage
ω
electric angular speed
Ψ
flux linkage
η
efficiency
Subscripts
a, b, c
phases
d
direct, d-axis
N
phase
r
rotor
q
q-axis
s
stator
tri
triangular
x, y
axis
7
1. Introduction.
The world cement production reached 2,1 Billion tonnes in 2004 and the average annual
growth rate has remained constant at around 4%. The world cement production has
doubled in less than 20 years. Europe as a whole accounted for 14% of the world
production in 2004, i.e. 294 million tonnes. There are approximately 400 cement plants
in Europe that employ thousands of people and, in turn, greatly contribute to the
development of their local economies.
Cement production is an energy intensive process and energy costs may represent 30 40% of production costs. The cement industry is one of the largest industrial end users
of energy consuming about 2% of global electricity production. The industry has been
accused of wasting energy, basically due to the low efficiency processes that are
employed particularly in comminution. Energy prices in Europe are among the highest
in the world and therefore the European cement industry has over the years constantly
invested in measures to reduce energy consumption. However, the total consumption of
electrical energy is about 100 kWh/t in a modern cement plant.[15]
Increasing energy costs as part of the total production costs in the cement production
industry is highly significant. This warrants greater attention for higher energy
efficiency to improve the bottom line. Historically, energy intensity has declined,
although more recently energy intensity seems to have stabilized with the latest
improvements. Coal and coke are currently the primary fuels used in this sector,
supplanting the dominance of natural gas in the 1970s. Most recently, there is a slight
increase in the use of waste fuels, including recycled tires. Between 1970 and 1999,
primary physical energy intensity for cement production dropped by 1%/year from 8.5
GJ/tonne to 6.2 GJ/tonne.[16]
Despite of the historic progress, there is ample room and great demand for technologies
driving higher energy efficiency in the cement industry. Especially, the effectiveness of
the electrical drive systems in modern cement industry is in the focus of the work
reported here.
8
1.1 Cement industry of Russia.
The cement industry of Russia is a key branch of the building trade on which the
condition and development of the national economy as a whole largely depends.
Measured by its cement production capacity Russia is on the 5th place worldwide, and
measured by the volume of manufacture Russia is on the 6th place worldwide. During
1990-1998 the manufacture of cement fell to 31 % of the 1989 level but during the
latest seven years the manufacture of cement has constantly grown not less than by 7 %
a year. In 2002 its release has made 37,7 million tonnes.
The deterioration of the basic assets of the cement enterprises, according to Goskomstat
of the Russian Federation, by the end of 2004 was 70 %. This means that 70 % of the
expected lifetime of the equipment in average has been used. At present there are 18
unprofitable cement enterprises functioning in this branch of industry. The prime cost
and the selling prices of cement grow. Profitability averages 10.1% but it is obviously
not enough for the accumulation of the means necessary for updating of technology or
introduction of modern equipment.
In 2006 45 cement enterprises function, with a production capacity of 71,2 million
tonnes and actual capacity – 65 million tonnes per year. If the present situation in the
cement industry will be maintained by 2007 in the market of cement there will be a
steady deficiency of capacities that makes it impossible to increase the volume of
building trade in Russia.
The loading of the Russian production capacity has reached 88 %, but at a worldwide
practice the loading is 82 % in average (Figure 1-1). For the branch it is typical that
during winter time the production is lower than during the warm period of the year.
Soon, the capacity will not be large enough if new plants are not built.
%
9
100
87,9
79,9
80
69,3
60
40
20
0
2003
2004
2005
год
Figure 1-1: Utilization of the production capacity of the cement industry of Russia during the latest years,
according to ROSSTATA
For comparison:
•
in the world the maximum possible comprehensible level of the
loading of the production capacity is considered to be 82 % ;
•
The deterioration of the basic assets in the building trade of
Russia is 37,2 % [1]
The deterioration of the basic process equipment – rotating kilns, raw and cement mills
– has reached a critical level of 85 – 90 % (on the average on the branch the basic assets
%
are worn out by 70 %) (Figure 1-2).
100
80
73
76
79
2003
2004
2005
60
40
20
0
год
Figure 1-2: Degree of deterioration of the basic assets of the enterprises of the cement industry of Russia
during the latest years, according to ROSSTATA
10
Examples of the degree of deterioration in the primary equipment of the holding
company called «Eurocement Group» are present in Fig. 1-3. The «Eurocement Group»
- is the largest domestic holding company, specializing in cement production and
realization. Its production capacity reaches 33 million tonnes per year. «Eurocement
Group» consolidates 15 cement works in different cities in Russia.
Share of dredges older than the normative
lifetime of 20 years
(77 of a 122)
Share of rotating kilns older than the
normative lifetime of 30 years (60 of a 68)
Share of the cement mills working on old
technology without use of separators
(106 of a 113)
Figure 1-3: Example of deterioration in the primary equipment of the holding company “Eurocement
Group”.
During the latest 15 years, Russian cement enterprises have reduced their output
capacity by 20 million tonnes. These losses are irrevocable. During 2003-2005 the
reduction has further progressed and has totalled additional 4 million tonnes (Figure 14).
mln.tonne
11
production capacities of cement enterprises
of Russia
retirement of productive capacities
100
90
80
70
60
50
40
30
20
10
0
89
81.1
1995
74.2
1996
73.9
2001
year
71.2
2003
69.2
.8
19
.8
17
.1
15
.8
14
.8
11
9
7.
0
1990
77.2
2004
2005
Figure 1-4: Reduction in the production capacity in the Russian cement industry (according to
ROSSTATA)
This occurs because the maintenance expenses within the production capacity of the
cement enterprises are insufficient. Fig 1-5 shows the share of amortized deductions in
the expenditure pattern in manufacturing one tonne of cement in Western Europe
compared to Russian manufacturing. It can be seen that the market transformation has
sharply decreased in Russia from 28% in 1980 (USSR) to 2% in 2005 (Eurocement
Group, Russia).
28
30
25
%
20
14
15
10
5
2
0
«Eurocement Group»
2005 year
USSR 80-th
Western Europe
Figure 1-5: Amortized deductions in manufacturing of cement.
12
Fig 1-6 shows that the maintenance and operational costs of the equipment in the
manufacturing of cement have sharply decreased from 23,8 to 12,9 % in Russia [2].
23,8
25
21
20
15
%
12,9
10
5
0
«Eurocement Group»
2005 year
USSR 80-th
Western Europe
Figure 1-6: Share of the maintenance and operational costs of the cement manufacturing equipment
In Russia the power-intensive wet process of cement manufacturing prevails. Of 53
factories, 46 factories work on a full cycle, but of them only 4 factories making 15-20 %
of the total amount of cement work on a "dry" process (Figure 1-7). Globally 90% of
cement is produced by the dry process allowing savings of around 40 % of fuel. In fact
50-55 % of the price of cement consists of the cost of the electric and gas power used in
its production. In Germany, Spain, Italy, Japan and other countries with a developed
cement industry the companies have focused on the energy saving dry process where
the specific charge of fuel makes 110-115 kg of conditional fuel per one tonne of
clinker. Using the wet process, in Russia consumes 218, 7 kg of conditional fuel per one
tonne of clinker.
100
90
90
Russia
85
World
80
70
%
60
50
40
30
20
10
15
10
0
Wet way
Dry way
Figure 1-7: Wet and dry manufacturing processes
13
The production of cement per employee working in Russia is only one sixth of the
levels found in foreign countries (Figure 1-8).
7,5
8
thousand tonnes
7
5,5
6
5
4
3
2
1,05
1
0
Russia
Foreign countries (old Foreign countries (new
factories)
factories)
Figure 1-8: Production of cement per employee.
The basic development directions of the cement industry of Russia are:
•
Factory modernization and reconstruction with the purpose of using
the dry cement manufacturing process in 80-85 % of all factories;
•
Development and introduction of highly effective energy saving
technologies.
•
Providing the building trade with an assortment of cements with
excellent construction properties;
•
Preparing the cement industry for a transition after which the
processes use coal and fuel-bearing waste of the industry as the
energy source;
•
Re-tooling of equipment and facilities as well as the organization of
modernization of equipment;
•
Reduction of harmful emissions in an atmosphere and improvement
of working conditions.
Cement manufacturing is a high capital-intensive industry. So, for the construction of a
new modern factory with a capacity of one million tonnes of cement per year it is
necessary to invest 150 million euros. Reconstruction demands about 50 euros for each
tonne of cement production, etc.
14
The current status of industry is characterized by a number of the parameters negatively
influencing an operating efficiency of the enterprises. High power consumption, defined
not only by the process of manufacturing (the "wet" process prevails), but also
technically obsolete equipment. Low profitability in manufacturing (on the average
about 12%) and insignificant capital allowances do not allow the industry to carry out
re-investment in its infrastructure and in due time to invest in modernization or
qualitative repair of the existing equipment. Deterioration of the basic assets is very
high in cement factories (about 70%).
The cement enterprises take the second place after the power enterprises on the volume
of gas consumption. Since 2001 the annual limits of the natural gas, allocated to cement
works covers about 50% of the necessary volume of gas consumption. Purchase of the
other 50% is conducted at prices between 130-150% of the official price established by
the federal energy committee-FEC.
At present, despite of the steady growth of cement consumption (on the average about
9% a year) the development of the Russian cement industry is complicated by a high
competition between the cement producers. The low prices of the products do not allow
the enterprises to update their basic assets, such as suitable railroad cars, which breaks
the timeliness of deliveries, especially, during summers.
Contrary to the forecasts of the cement market experts in 2004 the cement sales
increased and provided in turn, growth in the manufacturing of cement in all regions of
the country, except in the North and in the Far East. Domestically during 2004 the
country has produced 45,6 million tonnes of cement which is 4,6 million tonnes more
(11,3 %), than in 2003.
The greatest volume of the manufacture of cement (8,1 million tonnes or 17,7% from
the general release in the country) is received at the enterprises of the Central economic
region. The second place on the manufacture of cement belongs to the Ural economic
region. In the enterprises of this area in 2004 7,3 million tonnes of cement has been
manufactured (16,0% of the production in the country). The third place in the
manufacture of cement takes place in the Volga region. In 2004 the enterprises of this
area manufactured 6,9 million tonnes which is 15 % of the total production. By the
15
enterprises of the Central Black Earth economic region during the latest year 6,7 million
tonnes has been produced, and by the North-Caucasian area 5,1 million tonnes (14,8 %
and 11,2 % of the production in the country respectively). At the enterprises of VolgoVjatskogo and of the West-Siberian economic regions it has been produced 2,8 million
tonnes of cement. In total it made 12 % of the production in the cement industry. In
Northwest, East-Siberian, Far East and Northern economic regions it has been
manufactured – accordingly - 2,1; 1,7; 1,2 and 0,7 million tonnes of cement which have
made 4,7; 3,8; 2,6 and 1,6% from the total production of cement in the country. The
data are illustrated in Fig 1-9.
Far East
economic region
2.6%
East-Siberian
economic region
3.8%
Northen economic
region 1.6%
Others 0.6%
Northwest
economic region
4.7%
Volgo-Vjatskiy
and WestSeberian
economic region
12%
Central economic
region 17.7%
Ural economic
region16%
North-Caucasian
economic region
11.2%
Volga economic
region 15%
Central Black
Earth economic
region 14.8%
The total amount was 45,6 million tonnes of cement
Figure 1-9: Manufacturing of cement in different regions of Russia in 2004.
The growth of investments and the revival of housing and industrial building practically
in all economic regions of the country have caused an increase in the demand of cement.
Cement consumption in 2004 in comparison with 2003 has grown by 4,6 million tonnes
or by 11,4 %, including import and the domestic production of Russia – which has
grown by 4,1 million tonnes or by 10,6 %.
16
The gain of production of cement in 2004 is achieved at 32 cement enterprises of
Russia. About 60 % of the general gain of the release of cement - 2,7 million tonnes - is
received
at
13
cement
enterprises,
-
«
Oskolcement».
«Novoroscement»,
«Sebryakovcementт», «Mordovcement», « Kavkazcement » and others, - leading
enterprises of the cement industry entering into the number of 15 in which more than 70
% from the general release of cement in the country is made.
The greatest volume of release and the maximal development of the operating capacities
on manufacturing of cement in 2004 as well as the latest years belongs to «Maltsovskiy
Portland cement» (Tab. 1-1). Except for the mentioned enterprises, the significant gain
of release of cement for this year has been received on « Katavskiy Cement » (692
thousand tonnes), "Urals Mountains-cement" (356 thousand tonnes), « Nev'yanskiy
Cementnik » (161 thousand tonnes), « Krasnoyarskiy Cementnik » (251 thousand
tonnes), « Pikalevskiy Cement » (110 thousand tonnes), « Schyurovskiy cement » (83
thousand tonnes), « Mikhailovcement » (100 thousand) and others.
The cement industry of Russia has entered 2005 without any substantial improvement
of equipment and technology while the reduction of mid-annual capacity has been 2,0
million tonnes.
17
Table1-1: Rating of the cement enterprises of Russia on production output for 2001-2004 years[3].
Cement output, thousand tonnes
Cement enterprises
2001
2002
2003
2004
Russia
35373,9
37705,8
41001,7
45622,5
«Maltsovskiy portlandcement»
3423,4(1)
3591,4(1)
3555,8(1)
3637,5(1)
«Oskolcement» (Belgorod Region)
1633,3(7)
2218,8(5)
2764,3(2)
3105,3(2)
«Novoroscement» (Krasnodar Region)
1935,8(4)
2233,0(3)
2648,6(3)
3011,7(3)
«Mordovcement» (Mordovia)
2058,3(3)
2337,5(2)
2513,2(4)
2773,9(4)
«Sebryakovcement» (Volgograd Region) 2122,0(2)
2230,0(4)
2270,0(5)
2538,0(5)
«Suholozhskcement» (Ural Region)
1814,8(6)
1835,0(6)
1935,0(6)
2003,0(6)
«Vol’skcement» (Saratov Region)
1250,7(13) 1425,5(12) 1726,0(7)
1895,1(8)
«Mikhailovcement» (Ryazan Region)
1451,0(10) 1430,0(11) 1702,0(8)
1802,0(9)
«Pikalevskiy Cement» (Leningrad
1350,0(12) 1418,0(13) 1560,0(9)
1670,5(11)
(Bryansk Region)
Region)
«Lipetskcement» (Lipetsk Region)
1600,0(8)
1487,0(10) 1535,0(10) 1647,2(12)
«Kavkazcement» (Karachaevo-Circassia) 1038,4(14) 1347,4(14) 1525,9(11) 1939,9(7)
1530,0(9)
1614,0(8)
1520,3(12) 1470,3(14)
1821,7(5)
1616,7(7)
1517,4(13) 1706,8(10)
«Voskresenskcement» (Moskow Region) 1358,3(11) 1500,3(9)
1511,0(14) 1491,1(13)
«Topkinskiy Cement» (Kemerovo
Region)
«Belgorodskiy Cement» (city of
Belgorod)
«Ulyanovskcement» (Ulyanovsk Region) 959,0(15)
1095,0(15) 1133,7(15) 1369,5(15)
The capacity of the operating enterprises on the manufacturing of cement in the
beginning of 2005 was 68 million tonnes. The capacity is 19 million tonnes smaller than
in 1990. In addition to the reduction in the production capacity, cement manufacturers
in Russia continue to face other problems. One of them typical for the most of the
cement enterprises is high power intensity, raw material intensity and labour
18
intensiveness of the manufacturing of cement. The share of the material expenses during
the latest 15 years in the cement industry are at the highest level and has reached 74,7%.
At the same time the expenses of fuel and the electric power on the average make about
42,0%, and at some enterprises even 50-52%.
In the cement industry of Russia for the latest 15 years the structure of production
output for the mode of production has not varied. Practically no technology
improvements were applied.
The leading players of the cement market of Russia are the companies «Eurocement
Group» and «Inteko». By results of consolidation of 15 cement works holding the
«Eurocement Group» has entered into the group of the ten largest cement companies of
the world (Tab. 1-2). [3]. In addition to the above mentioned there are the French
«Lafarge» German “Heidelberg Cement” and the Swiss «Holcim» on the Russian
market. Despite of these there still exists a significant part of factories, owned by
regional industrial groups.
19
Table1-2: The largest cement companies of the world [4].
N Company
Country
Production
capacity of
cement. Still
May, 2005.
(million
tonnes)
1 Holcim
Switzerland
154,1
102
16,3
1,11
2 Lafarge
France
150,7
119
19,6
1,35
3 Cemex
Mexico,
97*
Great Britain
58
8,1
1,33
Cement
output in
2004
(million
tonnes)
Proceeds
in 2004
(billion
USD)
Net profit
in 2004
(billion
USD)
4
Heidelberg
Cement
Germany
77,9
65
8,6
-0,41
5
Italcementi
Group
Italy
70,0
48
5,6
0,36
6
Anhui Conch
China
Cement
41,5
32
1,0
0,12
Buzzi
7 Unicem+
Dyckerhoff
Italy,
Germany
40,5
32
3,4
0,35
8 Taiheiyo
Japan
38,0
25
8,3
0,16
9
Votorantim
Cimentos
Brazil
34,5
18**
-
-
10
"Eurocement
Russia
Group"
33,0
9,6***
-
-
*Include RMC Group;** Estimation;*** Without taking acquisitions within 2004-2005.
20
1.2 Cement production process.
Cement is a substance applied to the surface of solid bodies to make them cohere
firmly. Specifically, cement is a powder to which water is added to make a soft paste.
The paste (which hardens upon drying) is applied to bind together bricks, stones, etc in
building (SOED). Portland cement is a calcined material comprising lime and silicates,
which are mixed with sand and stone. Upon hydration, this mixture forms a plastic
material, which sets and hardens into the rock-like material known as concrete.
Confusion between cement and concrete is common among the uninitiated.
Even though the Romans invented cement as they studied volcanic phenomena it may
be seen that cement is not a natural material and the manufacturing process is expensive
and power-intensive. However, the result is worth the effort, and cement is one of the
most popular building materials in the world. It can be used alone, or as a component of
other building materials (for example, concrete and ferro-concrete). Cement works are
generally situated very near the deposits of raw materials required for the manufacture
of cement.
The manufacture of cement involves two steps. The first step is the production of
clinker. The production of clinker begins with the extraction and processing of raw
lime. Excavation of limestone deposits is carried out usually by pulling down, i.e. a part
of mountain is «pulled down», uncovering the layer of yellowish-green limestone. This
layer is, as a rule, found at depths of up to 10 m below the surface. Up to this dept often
four layers of limestone, typically about 0.7 m thick, can be found. The extracted
limestone is sent on an electrically driven conveyor for crushing in a ball crusher into
small pieces, approximately 2,5 cm in diameter. The limestone pieces are then dried,
ground, and mixed with other components. In the final stage in this production of
clinker, the lime mix is roasted with very high temperatures ranging from 1400 to 1600
ºC where the raw materials are combined. The raw mixture is heated in an electrically
driven kiln, a gigantic slowly rotating and sloped cylinder, with temperatures increasing
over the length of the cylinder up to ~1480°C. The temperature is regulated so that the
product contains sintered but not fused lumps. Too low a temperature causes
21
insufficient sintering, but too high a temperature results in a molten mass or glass. In the
lower-temperature part of the kiln, calcium carbonate (limestone) turns into calcium
oxide (lime) and carbon dioxide. In the high-temperature part, calcium oxides and
silicates react to form dicalcium and tricalcium silicates (Ca2Si, Ca3Si). Small amounts
of tricalcium aluminate (Ca3Al) and tetracalcium aluminoferrite (Ca4AlFe) are also
formed. The resulting material is clinker, and can be stored for a number of years before
use. A prolonged exposure to water decreases the reactivity of cement produced from
weathered clinker.
This first step is the most labour-intensive step, and therefore the most expensive, and it
accounts for up to 70 % of the cost of cement. In the second step, the clinker is
powdered. The second step consists of several stages, crushing the clinker, drying of
mineral additives, crushing of plaster stone, and then grinding all of these ingredients
together.
However, it is necessary to take into account that the properties of raw materials are not
always identical, and the physical characteristics (such as durability, humidity, etc.) of
raw material vary. Therefore it has been necessary to develop certain methods to control
the quality of each raw material. These methods ensure a good homogeneous grinding
and full mixture of the ingredients.
There are three main processes used by the Cement industry; Wet, Dry and Combined.
The Wet method manufactures cement from swept (carbonate component), clay (a
silicate component) and ferriferous additives (converter sludge, ferriferous product,
pyrite dross). The humidity of the clay should not exceed 20 %, and the humidity of the
swept should not exceed 29 %. The wet process gets its name because the crushing of
the raw meal is performed in a liquid environment. The result is sludge with a humidity
of 30 - 50 % of weight. This sludge is then heated in a large electrically driven kiln,
typically 7m in diameter and 200m long. This is done for two reasons. Firstly, to
dissociate calcium carbonate to calcium oxide with the creation of carbon dioxide, and
then to react calcium oxide with the other components to form calcium silicates and
aluminates which partially fuse at temperatures up to 1450ºC. The ingredients leave the
kiln as a black nodular material, clinker. The clinker is finally interground with a small
22
proportion of gypsum (to control a rate of hydration) yielding the finished cement
product.
The Dry method simply means that raw materials are dried before or during grinding.
The result is a dry cement powder. The Combined method, as the name suggests, is a
combination of both the Wet and Dry methods. The Combined method has two
varieties. The first uses the Wet Method to produce sludge. The sludge is then
dehydrated on filters to a humidity of 16 - 18 % , and sent to the kiln for heating as a
moist mass. The second variant of preparation is opposite to the first: the Dry method is
used for manufacturing a raw meal, and then 10-14 % of water is added. The ingredients
are then turned into granules approximately 10 - 15mm in diameter, and in turn, these
granules are sent to the kiln.
1.3 Cement processing equipment
For each process of manufacturing cement, well-defined equipment and manufacturing
methods are used. Additionally, cement producing is an energy-intensive industry. An
average of 110 kWh of electricity per tonne of cement produced is consumed. Energy
typically accounts for at least 30% of the cost of cement. The most energy-intensive
stage of the process is the clinker production, which accounts for up to 90 % of the total
electric energy use, and also virtually all of the fuel use.
1.3.1 Crushers
Primarily crushers should be capable of accepting shot rock with the minimum of
wastage or of preliminary size reduction (Figure 1-10). Typically, the shot rock size
should be less than 120 cm in diameter and, either the feed hopper should be protected
by an appropriate grizzly, or a hydraulic breaker may be installed to reduce oversized
rock. Commonly there are primary, secondary and, occasionally, tertiary crushers in
23
series. Most crushers are operated in open circuit though, frequently, they are also
preceded by a screen or grizzly to bypass fine material direct to product.
Figure 1-10: Crushers. [6]
Electric drives for stone crushers account for 5% of all drives used in a typical cement
plant. The powers of the drives typically vary from 500 kW to 3000 kW. These drives
should provide enough energy to produce the highest torque values in order to crush the
pulped limestone fragments extracted from the rock quarry. These drives should have
by default, a very high start-up torque, as it is possible for the crusher to be blocked by
stones. The drive is typically supported by a flywheel in the drive train. Reductions in
speed will help to discharge the stored energy in the flywheel. The drive must be able to
recover from these speed dips within a short time. For stone crushers slip ring motors
are typically used as they can be started forward and backward to break away and also
because they reduced starting current but still high starting torque. The most advanced
version of such a drive is the Scherbius cascade where the rotor slip energy is supplied
to the network via cyclo-converter Figure 2-12.
1.3.2 Conveyors
Today continuous conveyors are acknowledged world-wide as a cost-efficient
alternative. For example in the past, limestone was often transported by trucks, which
meant both high cost and environmental pollution. Conveyors (Figure 1-11) are used
24
not only for transportation of limestone but they are also needed for transportation of
stones, raw meals and so on.
Figure 1-11: Conveyor Belts. [6]
That is why they find numerous applications in a cement plant. For down-hill conveyors
(Figure 1-12) typically 50 kW to 500 kW motors are used. It should be ensured that
braking capacity required for the down hill will be sufficient, when massive stones are
transported down. For these conveyors usually direct on line motors and DC or AC
drives are used.
Figure 1-12: Downhill conveyor.[17]
Other conveyors use motors with power range 20 kW to 400 kW, working on DOL
motors and AC drives.
25
1.3.3 Mills
Roller mills: The grinding of raw materials and of the cement mixture are both
electricity-intensive steps and account for about 60-65% of total electrical energy use in
cement production. Roller mills for grinding raw materials and coal and separators or
classifiers for separating ground particles are the two key energy-consuming pieces of
equipment at the raw materials preparation process stage. For the dry-process cement
making, the raw materials need to be ground into a flowable powder before entering the
kiln. There are four main types of grinding systems in use:
- Tube Mill (or Ball Mill) Extracted materials are crushed inside a rotating cylinder
which is typically 6 m in diameter and roughly 20 m long. The cylinder contains
spherical metal balls which - upon rotation - work to crush the extracted materials.
(Figure 1-13). Of the four mill types, tube mills are known to be the most energy
intensive mills (The energy comparisons are based on grinding material of the same
hardness to the same level of fineness and are given by Rosemann and Ellerbrock [5]).
Over 95% of the energy input to the machines ends up as heat. Only 1 to 2 % of the
input power is used to create new material surface.
Figure 1-13: Tube Mill (or Ball Mill).[18], [19]
- Vertical Roller Mill – For decades, vertical roller mills (Figure 1-14) have been used
for grinding of cement raw materials, and it is very well known that these machines are
much more energy efficient than the ball mills. In such mills materials are crushed
between a rotating grinding table and 2 to 4 grinding rollers positioned slightly less than
26
90 degrees from the table surface and pressed hydraulically against it. This particular
application also tends to lead to steep particle size distribution of the material. Upon
completion of the milling process, separators are used to filter fine particles from the
mill output, as the coarse elements must be recycled to the mill for further
disintegration. Installing highly efficient separators has added enormously the efficiency
to the grinding process. The crushing of the cement’s raw materials, using currently
available technologies consumes approximately 16-22 kWh/tonne. The energy
consumption of the final stage of cement grinding is around 28-55 kWh/tonne
depending heavily on the coarseness of the raw materials and the grinding circuit
configuration. The grinding of additives is also an energy intensive task. The process
may require up to 55kWh/tonne. Vertical roller mills use approximately 70-75% of the
energy used in ball mills.
Figure 1-14: Vertical Roller Mill [20]
- Horizontal Roller Mill – Horizontal Roller Mill (Figure 1-15) was first introduced in
1994 at the industrial scale with 80 tonne/h clinker grinding capacity. It is a material
bed-grinding machine operated with multiple compression. Materials are crushed inside
of a rotating mill cylinder. The cylinder also contains grinding rollers which are
compressed against the inside surface of the tube using hydraulic actuators. This
technology has not been widely applied in the cement industry due to the inherent
27
mechanical problems encountered with it. Horizontal roller mills typically consume
only 65-70% of the energy used in ball mills.
Figure 1-15: Horizontal Roller Mill. [21]
- Roller Press (or High-pressure Grinding Rolls) – In the mid eighties the roller press
was introduced to the cement industry. It has been used in different process applications
that enable a large reduction in specific energy consumption. As far as energy
consumption is concerned, the roller press grinding system appears to be attractive. Two
counter-rotating rollers are used to crush the materials (Figure 1-16). The counterrotating rollers may be up to 2 m in diameter and 1.4 m long. However, low temperature
in the grinding circuit and the narrow particle size distribution means that the quality of
the cement is different from the normal requirements of the cement consumers.
Generally, the maximum capacity of these roller presses will be no more than around
150t/h. Roller presses, typically, use 50-65% of the energy used in ball mills.
Figure 1-16: High-pressure Grinding Rolls. [22]
28
Choices of grinding mills vary due to a number of factors. While the power
consumption (and hence energy costs) at ball mills are more expensive, they do have
reduced operational and maintenance costs over other types of mills. Also, capital costs
are difficult to compare, because site-specific constraints must be considered. Relative
factors which affect investment decisions may include; the moisture content of the raw
materials; vertical roller mills which grind both dry and wet materials, thus are most
suitable for the processing of raw materials with higher moisture content, while roller
presses and horizontal roller mills may require a separate dryer. Consequently, another
investment factor is the desired coarseness of the resulting product. Two types of mills
can be operated in circuit to take advantage of the different advantages of each. But
more than 80% of new raw mills are vertical roller mills, though many tube mills are
still in use. Roll presses are also used, particularly in upgrading existing tube mill
circuits either to increase production or to reduce specific power consumption.
Raw mill grinders are typically driven by electrical motors with power from 1000 kW to
3000 kW. They should provide high starting torque, about 250% of nominal (except
modern vertical roller mills which may be started with lifted rolls, so that the starting
torque is less than the nominal torque of the mill). Often slip ring AC or separately
excited or series wound DC motors are used. In coal mill grinders DOL motors are
used, usually slip ring AC or DC motors with power 100 kW to 400 kW.
1.3.4 Separators
Separators or classifiers are an important piece of equipment used in the grinding stages
(Figure 1-17). It is used to filter out larger particles so that they may be re-ingested into
the system for further grinding. The efficient separation of substance which is of
sufficient fineness reduces the need for re-grinding of materials and ensures lower
energy consumption.
29
Figure 1-17: Separator. [6]
Equipment called ‘high-efficiency classifiers’ or ‘high-efficiency separators’, accurately
filter out larger particles (which need to be re-ingested into the mill) from materials
which are acceptable and can be passed through the system, so that the energy use in the
grinding mill is decreased. Some studies suggest that in the preparation stage, 2.8 – 3.8
kWh electric energy per tonne of raw material can be saved and at the grinding stage
and another 1.7 – 2.3 kWh/tonne cement can be saved by the use of such “highefficiency” classifiers.
Typically motors used in separators have output power from 150 kW to 500 kW.
Motors are often used in braking mode to guarantee fast and accurate speed control and
also during stopping of the rotating separator. Motors should be able run through a
possible voltage break down. Also flying start should be provided. The speed setting of
the separator drive must be exact and quick. The high inertia of the separator requires a
four-quadrant drive (Figure 1-18). In voltage source converters this may be realised by
using an active network bridge or a braking resistor (Figure 1-19) in the DC circuit.
30
Inverter
(rectifier)
Control Logic
Inverter
L1
L2
L3
Motor
Figure 1-18: A fully four-quadrant drive allowing regenerative braking.
DC Link
Braking
resistor
L1
L2
L3
Inverter
Control Logic
Converter
Motor
Figure 1-19: A four quadrant motor drive with a braking resistor.
1.3.5 Feeders
Feeders are used e.g. in the blending silos (Figure 1-20). Both the chemical composition
and the rate of feed of raw meal to the kiln must be consistent to avoid kiln instability
and to minimise fuel consumption.
31
Figure 1-20: Feeders[6]
There are various blending silo designs. The two major types involve turbulence (in
which the material is tumbled about by the injection of high volume air through air pads
on the silo floor) and controlled flow (where sequenced light aeration of segments of air
pads caused layers of material in the silo to blend by differential rates of descent within
the silo). Controlled flow silos may have multiple discharge chutes, or an inverted cone
over a centre discharge within which the meal is fluidised. Modern blending silos are
generally of continuous flow type. A given silo will show a lower blending efficiency if
the feed is itself consistent.
Short term feed fluctuation (e.g. hunting of the feeder control) as well as average feed
rate should be monitored. Feeders need typically speed controlled motor drives the
power of which varies between 5 kW to 25 kW. The dives connected to the factory
automation system in order to get accurate proportions of the mixed materials.
With the availability of real-time on-line analysis of mill feed or product, it is possible
to maintain chemistry within narrow limits and modern plant designs frequently
dispense with kiln feed blending.
32
1.3.6 Kiln
The primary element of the clinker production stage is the rotary kiln (Figure 1-21).
These kilns are often 6-8 m in diameter and 60 m -100 m long. They are set at a slight
incline (3-4 degree angle) and rotate 1 to 3 revolutions per minute. These kilns are fired
at the bottom end while cement materials move toward the flame as the kiln rotates.
These materials reach operating temperatures of 1400-1500 degrees C in the kiln.
Several important processes occur with the unprocessed material mixture, during pyroprocessing. Firstly, all moisture is removed from the materials. Next, calcium carbonate
in limestone is divided into carbon dioxide and calcium oxide (free lime); this process is
called calcination. Finally, the lime along with other raw minerals reacts to form
calcium silicates and calcium aluminates, which are the main components of clinker.
This process is known as clinkerization.
Figure 1-21: Kiln.
Kiln drives have typically powers from 200 kW to 1000 kW. Motor designs should
provide for short-term loading of up to about 250% the motor rated current and torque
to overcome inertia and static friction. Also smoothing the high torque peaks, speed
regulation, soft starting and reversing should be provided for normal operation. The kiln
drive can also be designed as a twin drive to assure a safe operation. If the hot kiln stops
the tube will bend and be damaged. The electric energy used in the kiln (excluding
grinding) is roughly estimated at 40-50 kWh/tonne clinker. Variable speed drives,
improved control strategies and high-efficiency motors can help to reduce the power use
in cement kilns.
33
1.3.7 Fans
Fans (coolers) have an important role on a cement plant. They are used practically in all
stages of the producing cement and consume 30% of all electric energy in cement
production. There are four main types of fans used in the production line: induced-draft
(ID) fans, exhaust fans, filter fans, cooling fans (Figure 1-22). The air fan drives are
relatively large drives. Typical motor powers vary between 20kW and 2000kW. The
best availability and the best drive efficiency are extremely important.
Figure 1-22: Centrifugal fans with and without AC-drive control.
Fans can be propelled by a constant speed motor with damper or vane or by a variable
speed drive. The use of motors together with air volume control through dampers is a
low cost solution. Variable speed drives will reduce the energy consumption during
operation. As an example a Greek cement plant controls their ID fans with AC
converters 630 kW [6]. In this way large energy savings were achieved compared to the
conventional method of regulating the flow rate through dampers. Power consumption
was reduced by 163 kW with the following benefits:
Energy saving about 1'250 MWh/year
Reduction in CO2 emissions 625 t/year (Calculated from the electric power
generation.)
Other benefits:
Reduced reactive power
Payback period 1,8 years
Reduced need of maintenance
34
1.4 Consumption and saving of electric energy.
Cement production is an energy-intensive industry. For example 60 to 130 kilograms of
fuel oil or an equivalent fuelling amount per tonne of cement produced is needed. The
power consumption rose significantly with the introduction of dry process kilns and has
continued to rise with conversion to coal, increased fineness of cement, and with
demands of environmental protection. Typically, the electric power consumption is
presently 110-120kWh/tonne cement which may be broken down as shown in table 1-3
[7]:
Table1-3: Electric power consumption in a cement plant. [7]
Quarrying & pre-
6 kWh/tonne
5%
Raw milling
28
24
Blending
7
6
Burning & cooling
25
22
Finish milling
44
38
Conveying,
6
5
blending
Conveying, packing
& loading
5%
Blending
6%
Finish milling
38 %
Burning & cooling
22 %
Raw milling
24 %
packing & loading
Total
Quarrying &
preblend
5%
116kWh/tonne
The most energy-intensive stage of the process is the clinker production, which
accounts for up to 90 % of the total energy use, where milling requires less than half of
total energy consumption. The greatest part of all energy consumption falls at the
motors and drives which are used throughout the cement plant to drive fans (pre-heater,
cooler, alkali bypass), to rotate the kiln, to transport materials and, most importantly, to
grind the materials. In a typical cement plant, 500-700 electric motors may be utilized,
varying from a few kW to MW size.
35
Methods of Energy efficiency are an important point for every manufacturer, because
the cost of energy increases every day. The conservation of electrical power should first
address such areas as:
Blending (if turbulent)
- convert to Controlled Flow
Pneumatic conveying
- convert to mechanical
Milling
- install pre-grinding
ID fans
- eliminate air in-leakage, use high efficiency
impellers
Cooler fans(with outlet dampers)
- convert to inlet vane or variable speed
Plant air compressors(if central)
- minimize system loss and convert to
distributed system
Plant lighting
- basic lighting can be augmented by
additional lighting as required with timed
shut-off
Electrical drives consume the largest amount of electric power and energy in cement
manufacturing. The most drives have fixed speed AC motors. However, the most motor
systems are often operated at partial or variable loads, especially in cement plants where
there are large variations in load. Various technologies may be utilized to control these
motors.
Reducing energy losses in the system and increasing efficiency can be achieved through
the installation of variable speed drives (VSD), frequency converter drive systems
(Variable Voltage Variable Frequency (VVVF)), cascade converters (also called Slip
Power Recovery Systems (SPRS)) and so on.
Using VSDs provides energy savings in a wide array of applications. The amount of
savings depends on the flow pattern and loads. It is estimated that the savings range
from 7 to 60%. VSD equipment is used more and more in cement plants in various
applications depending on electricity costs. Some of the different applications in cement
plants are fans in the kiln, coolers, pre-heaters, separators, mills, and other equipment.
For example, Blue Circle’s Bowmanville plant (Canada) installed a variable air inlet
fan, reducing electricity and kiln fuel usage (because of reduced inlet air volume),
36
creating a yearly energy cost saving of approximately $47,000(USD) (CIPEC, 2001).
VSDs for clinker cooler fans have a low payback, even when energy savings are the
only reason for installing VSDs. Energy savings greatly depend on the application and
flow pattern of the system on which the VSD is installed.[16]
Although savings are significant, not many studies are available for the cement industry.
One presumed case study estimates the savings at 70%, compared to a system with a
vane (or 37% compared with a regulated system) for the raw mill fan. In practice,
savings of 70% are unrealistic without exceptionally high efficiency. Lafarge Canada’s
Woodstock plant replaced their kiln ID fan drives with VSDs and reduced electricity
consumption by 5kWh/tonne. The potential savings are at 15% for 44% of the installed
power, or roughly equivalent to 7kWh/tonne cement [8].
VSD systems are used for standard solutions and large drives in cement manufacturing.
For most large fan applications, frequency converter drive systems offer significant
power savings over damper or inlet vane controlled fixed speed AC drives albeit at
higher capital cost. However, there are still some disadvantages of using variable speed
single drives, like braking possibility, harmonic distortion, and higher cabling request
and space requirements As the market demands a solution that includes all of the
advantages of variable speed single drives and at the same time reduces many of the
disadvantages mentioned, a solution was created especially for such multi-motor
applications. The solution is called “Multi-drive”.
Multi-drives are characterized by:
– Reduced cabling, due to the single power entry for multiple drives,
– Energy-saving motor-to-motor braking,
– Reduced space requirement,
– Easy to build 12-pulse line supply section, thereby lower harmonics,
– All the benefits of a single variable speed drive are retained,
– Possibility to apply a regenerative supply unit being able to reduce current
harmonics.
– Possibility of using regenerative braking without brake resistor or active bridge
because the braking energy may typically be used in the other inverters of the
multi-drive
37
It is a relatively simple matter to decide which investment to choose when only the
direct investment costs are considered. It has been shown, however, that the operating
costs should play a far more important role in such decisions.
Despite of the advances in energy efficiency technologies in the cement manufacturing
sector, there is still much that can be done to improve efficiency on a plant by plant
basis. Unfortunately, many cement plants and their investors continue to look at the
expenses involved with energy efficiency as unnecessary capitol expenditures rather
than a cost saving/profit creating opportunity over time.
Finally, when a company’s environmental policies come under the scrutiny of global
cement markets, cement producers and their major share holder are more easily willing
to look at alternative means of energy savings as a means of boasting their own
environmental image to the customers and share holders. Keeping this in mind, it is
obvious that there are many possibilities for modernizing the cement mills with the
above-mentioned technologies, if not for energy saving purposes, then for political
concerns of the cement industry itself.
38
2. AC-drive technology in cement producing.
Motor Drives are used in a widely varying power range, from fractions of watts to
several thousand kilowatts, with applications ranging from extremely precise, highperformance position-controlled drives used in robotics and automation, to variablespeed drives used for the adjustment of flow rates in pumps and fans. In all AC drives
where speed and position are controlled, an electronic power converter is required as an
interface between the input power and the motor.
Electrical drives have an important role as electromechanical energy converters in
transportation and production processes. The simplicity of controlling electrical drives
is an important point, required for meeting the increasing demands of the end user with
respect to flexibility and precision, as a result of the technological progress in this
industry.
On the contrary, the control of electrical drives previously provided strong incentives to
control engineering, forcing the development of new control structures and their
introduction in other areas of control. This is true because of the stringent operating
conditions and widely varying specifications, additionally a drive may require control
for torque, acceleration, speed or position and the fact that most electric drives have in
contrast to chemical or thermal processes well defined structures and consistent
dynamic characteristics.
At power higher than several hundred watts, there are the following three basic types of
motor drives: DC motor drives, AC induction motor drives and AC synchronous motor
drives. In some special cases synchronous reluctance motor drives and switched
reluctance motor drives are used. Controlled electrical drives are made up of several
parts: the electrical machine, the power converter, the control equipment and the
mechanical load, all of which are dealt with in varying depths.
Figure 2-1 shows a block diagram of an electric motor drive. Responding to an input
command, the electric drives efficiently control the speed and/or the position of the
39
mechanical load, thereby eliminating the need for mechanical control device like vane.
The controller, by comparing the input command for speed and/or position with the
actual values measured through sensors, will provide an appropriate control signal to the
converter which consists of a power semiconductor device. [9]
Electric Drive
Converter
Motor
Load
Fixed form
Electric Source
(utility)
speed /
position
Adjustable
Form
Sensor
Power
Signal
Controller
Input command
(speed / position)
Figure 2-1: Block diagram of an electric drive system.[9]
Figure. 2.1 shows, the converter getting its power from a utility source with singlephase or three-phase sinusoidal voltages which are of fixed frequency and constant
amplitude by nature. The converter, which responds to the control inputs, effectively
converts these fixed-form input voltages into an output in the appropriate form, which is
best suited for the operating of the motor.
Input commands to the electric drive in Fig. 2-1 come from a process computer, which
calculates the objectives of the overall process and issues adequate commands to control
the mechanical load. However, in general-purpose applications, electric drives operate
in an open-loop manner without any real feedback. [9] As it is common place to find
AC drives in the cement industry, we will consider only them in the following.
40
2.1 AC-drive notion and properties.
The main power components of an AC drive, have to be able to supply the required
level of current and voltage in a form the motor can use. The controls have to be able to
provide the user with necessary adjustments such as minimum and maximum speed
settings and the torque control, so that the drive can be adapted to the user's process.
Variable frequency drive technology employs solid-state electronic techniques to vary
motor speed, thereby varying the operating speed of a piece of equipment. Except the
cooling fans these drives have no moving parts and hence require minimal maintenance
when compared to other non-electronic alternate final control elements. The only
usually service-needing part is the DC-link electrolytic capacitor that typically must be
replaced every ten years.
Induction motors and synchronous motors are the workhorses of industry because of
their low cost and rugged construction. When operated directly from the line voltage,
such motors operate at a nearly constant speed. However, it is possible to vary the speed
of the motors by using modern power electronic converters.
There are many ways to classify the AC drives: The classification may be based on the
type of the motor, on the type of rectifier and inverter, on the power range, on the
applications of AC-drives or on the control methods. Let us consider some of them.
By the type of the motor AC drives can be classified on induction motor drives and on
synchronous motor (SM) drives. As the name implies the induction motor drive is based
on the induction motor and the synchronous motor drive on the synchronous motor
accordingly. Both of them have wide applications in the cement industry. The
synchronous motor drives may also be divided in separately excited SM drives,
Permanent magnet SM drives or synchronous reluctance motor drives.
Also the drives can be classified on high-power, medium-power and low-power drives.
Using of one or another power range depends on requirements in each case. In cement
41
plant high and medium power drives are mostly used. High-power power electronic
drives can be e.g. divided on the amount of intermediate link DC voltage levels and
schematic representation (three or four-level and so on).
The AC drives can be classified into two broad categories based on their applications:
•
Variable speed drives. One important application of these drives in
process control by controlling the speed of fans, compressors,
pumps, capacity modulated heat pumps, blowers and the like.
•
Servo drives. By means of sophisticated control, induction and
permanent magnet synchronous motors can be used as servo drives
in computer peripherals, machine tools, automatic motion control
and robotics.
Variable speed and servo drives can be divided on accuracy of maintenance of speed
and on accuracy of positioning accordingly.
The servo drives have not found a wide manufacturing application in cement industry,
because cement plants have big fans, pump applications and practically do not have
robotics we will consider only variable speed drives in the following.
One of the primary advantages of variable frequency drive technology is that it can be
applied to induction and synchronous motors that are commonly used in hazardous and
non-hazardous locations throughout industry. Virtually any induction and synchronous
motors can be driven with a voltage source variable frequency drive; however, there are
some special considerations that must be taken into account when applying one.
It should be recognized that despite of the high efficiencies motors still generate
substantial amounts of heat during operation. This heat must be dissipated to maintain
cool motor operation and motor longevity. Motor operation at reduced speeds results in
a reduction of fan cooling effectiveness, which can cause the motor to heat excessively
and can bring about premature motor failure. Harmonics, due to the short rise times of
the synthesized waveforms, add to motor heat dissipation requirements. The most
operationally flexible solution to the heat dissipation problem is to utilize a motor with a
42
1.15 service factor or equip the motor with a higher insulation class that is designed to
operate under the above conditions.
When the motor does not have the required service factor, the motor should not be
operated at full rated torque regardless of speed, because higher harmonics, produced by
frequency modulation will lead to additional heating and can damage the insulation.
Depending on the nature of the load, this is accomplished e.g. in fan and pump
applications by limiting the maximum speed at which the motor will operate. In other
applications, appropriate adjustment of the load or control algorithm may alleviate the
problem. The net result is a light power derating of the drive system.
As fan-cooled motors can get excessively hot when operated at less than 10 to 30
percent of line speed, depending on the characteristics of the load, it may be appropriate
to limit the minimum speed of the motor. In most applications, the motor should not be
allowed to operate fully loaded at less than 10 percent of motor speed. However, this is
not a concern for motors with active ventilation.
Consider a simple example of an induction motor driving a fan as shown in figure 2-2-a,
where the motor and the fan operate at a nearly constant speed. To reduce the flow rate,
the vane is partially closed. This causes loss of energy across the vane. This energy loss
can be avoided by eliminating the vane and driving the fan at a speed that results in the
desired flow rate, as in Fig. 2-2-b.
Figure 2-2: Fan: a) constant-speed drive: b) Variable-speed drive.
In the system in Fig. 2-2-b, the input power decreases significantly as the speed is
decreased to reduce flow rate. This decrease in power required by the fan from the
43
motor is remarkable since the power of a fan Pfan is proportional to the cube of the
mechanical angular frequency Ω as
Pfan ≈ k Ω 3
(2.1)
where k is a constant of proportionality.
If the motor and the fan energy efficiencies can be assumed to be constant as their speed
and loading change, then the input power required by the induction motor would also
vary proportional to the cube of the speed. Therefore, in comparison with a vane to
control the flow rate, a variable-speed fan can result in significant energy saving in
cases where reduced flow rates are required for long periods of time. Moreover, fan
systems are usually designed to provide a flow margin of 20-30% over the maximum
values of their actual flow.
Speed can be controlled by varying the frequency, which controls the synchronous
speed (and, hence, the motor speed, if the slip is kept small), keeping the air gap flux
constant by varying the stator voltage in a linear proportion to the frequency. Varying
the stator frequency and voltage is the preferred technique in most variable-speed
induction motor drive applications.
For connecting the utility power system and the inductor motor in AC drives variablefrequency converters are used. They must satisfy the following basic requirements:
•
Ability to adjust the frequency according to the desired output
speed
•
Ability to adjust the output voltage so as to maintain a
constant air gap flux in the constant- torque region
•
Ability to supply a rated current on a continuous basis at any
frequency
Except for a few special cases of very high power applications were cyclo-converters
are used, variable-frequency drives employ inverters with a DC input. Figure 2-3
illustrates the basic concept where the utility input is converted into DC by means of
44
either a controlled or an uncontrolled rectifier and then inverted to provide three phase
voltages and currents to the motor, variable in magnitude and frequency.
Variable frequency converter
DC
AC
Rectifier
Filter
Inverter
50-Hz
(1-Ф or 3-Ф)
AC
Motor
Output
(variable voltage &
frequency)
Figure 2-3: Variable-frequency converter.
These converters can be classified based on type of rectifier and inverter used in Fig. 23. The rectifier can be controllable (thyristor or transistor rectifier) and uncontrollable
(diode rectifier). The diode rectifier allows only rectification of voltage, but can not
regulate the magnitude of the rectified voltage. A thyristor or transistor rectifier allows
rectification of a voltage with controllable average magnitude and provides recuperation
of electrical energy to the supply. Inverters can be based on applying transistors as
power switches (Voltage Source Inverters) or thyristors (Current Source Inverters). The
VSI contains DC-link capacitors and the CSI has a DC choke. As the name implies, the
basic difference between the VSI and the CSI is the following: In VSI, the DC input
appears as a DC voltage source (ideally with no internal impedance) to the inverter. On
the other hand, in the CSI, the DC input appears as a DC current source (ideally with the
internal impedance approaching infinity) to the inverter. In practice, the DC voltage
source inverters totally dominate the frequency converter markets at present. Voltage
source inverters are available as two-level, three-level and multi-level versions. The
two-level converter is the most common in industrial applications since it covers all the
standard industrial voltages lower than 1000 V. The power range reaches up to 5000
kW at 690 V level. Three level converters are available at 3 kV and 6 kV levels and
powers up to more than 20 MW. Multi-level converters cover the same power range but
voltages up to 10 kV and even more may be covered.
For the Cement industry there is - in most cases - no need for using AC-drives with
recuperative mode. The most commonly used drive for these applications is PWM-VSI
(Pulse Width Modulated Voltage Source Inverter). Nevertheless, let us consider these
45
drives with one and four quadrant mode. Figure 2-4-a shows the schematic of a PWMVSI drive with a diode rectifier, assuming a three-phase utility input. A PWM inverter
controls both the frequency and the amplitude of the voltage output. Therefore, at the
input, an uncontrolled diode bridge rectifier is generally used. The traditional analogous
method of generating the inverter switch control signals is by comparing three
sinusoidal control voltages (at the desired output frequency and proportional to the
output voltage magnitude) with a triangular waveform at selected switching frequency,
as shown in Fig. 2-4-b.
+
is
Ud
a
Motor
b
c
50-HZ
AC input
N
(a)
U tri ucontrol, a
ucontrol, b
ucontrol, c
uS
0
iS
t
iS1
ωt
0
uaN
t
ubN
uS
t
iS
iS1
0
uab
ωt
uab1
Ud
t
(b)
Figure 2-4:Two-level PWM-VSI: (a) schematic; (b) waveforms, input voltage and current in
1Φ− and 3Φ−systems, PWM output on the right[10].
In a PWM inverter, the harmonics in the output voltage appear as sidebands of the
switching frequency and its multiples. Therefore, a high switching frequency results in a
sinusoidal current (plus a superimposed small ripple at a high frequency) in the motor.
46
Because the ripple current through the DC bus capacitor is at the switching frequency,
at higher switching frequencies the input DC source impedance seen by the inverter
would be smaller, consequently, a small value of capacitance suffices in PMW
inverters. However this capacitor must be capable to carry the ripple current. Results in
a better input current waveform drawn from the utility source can shown by using a
small capacitance across the diode rectifier. Nevertheless, care should be taken in not
letting the voltage ripple in the DC bus voltage become too large, which would cause
additional harmonics in the voltage applied to the motor.
The input AC current draw by the rectifier or PWM-VSI drive contains a large amount
of harmonic. Its waveform is shown in Fig. 2-4-b for a single-phase and three-phase
input. A small DC-link capacitance will result in a better waveform.
Drive operates from the utility system. The power factor of this drive is practically
independent of the drive speed and the motor power factor. It is just a slight function of
the load power, which is improved slightly at a higher power. As can be observed from
the input current waveforms of Fig. 2-4-b, the displacement power factor (DPF) is
approximately 100%.[8]
An AC-drive should often provide also electromagnetic braking. During it the power is
flow from the motor to the variable-frequency controller. During braking, the voltage
polarity across the DC-bus capacitor remains the same as in the motoring mode.
Therefore, the direction of the DC bus current to the inverter gets reversed. PWM-VSI
drives cannot reverse, because current direction through the diode rectifier bridge
normally used. Some equipment must be providing to handle this energy during
braking; on the other hand the DC-bus voltage can reach destructive levels. One of the
methods to realize this goal is to switch on a resistor in parallel with the DC-bus
capacitor. You can see it in Fig. 2-5-a. In order to dissipate the braking energy, when the
capacitor voltage exceeds a preset level.
47
AC
P
Diode
Rectifier
+
Ud
-
Inverter
Motor
Inverter
Motor
R brake
(a)
4-quadrant
AC Switch
Mode
P
Converter
+
Ud
-
(b)
Figure 2-5: Electromagnetic braking in PWM-VSI: (a) dissipative braking; (b) regenerative
braking.
An energy-efficient technique is to use a four-quadrant converter (switch-mode on
Fig.1-18 or a back-to-back connected thyristor converter on Fig. 2-6 [23]) at the front
end in place of the diode bridge rectifier.
Transformer
12-pulse rectifier
3 level GTO bridge
50-HZ
AC input
Motor
Figure 2-6: Back-to-back connected thyristor converter for four-quadrant operation
As shown in Fig. 2-5-b the energy recovers from the motor-load inertia for being fed
back to the utility supply, because the current through the four-quadrant converter used
for interfacing with the utility source can reverse in direction. As the recovered energy
is not wasted this is called regenerative braking. The decision to use regenerative
braking instead dissipative braking depends on the additional equipment cost in
comparison with the savings on energy recovered, unity power factor operation from the
utility source and the desirability of sinusoidal currents.
For high-power drives with large output voltage (3.3; 4.16; 6 kV) multilevel converters
are used. Multilevel technology allows getting the output voltage curve close to sine
48
(with minimal THD). As a consequence we have less high harmonics, therefore less
motor losses and less insulation stress. An example of a multilevel inverter can be seen
in Fig. 2-7.
18-pulse Rectifier
Multilevel inverter
Transformer
+20
Optional
du/dtFilter
0
AC Motor
-20
2-7: Multilevel drive.
The power section of the multilevel drive consists of the following:
•
Transformer – provides e.g. three phase shifted voltages for the 18-pulse
rectifier and reduces common mode voltages at the motor.
•
18-pulse rectifier – generates the DC link voltage. The thyristors of the
rectifier are used as diodes under normal operating conditions, i.e. no
phase control. In the event of a fault, the thyristors are switched off,
quickly interrupting the short circuit current. The thyristors also provide
a convenient method for pre-charging the DC link. The DC link voltage
is approximately 6000 V DC under normal operating conditions. The DC
link floating capacitors are part of the multi-level inverter. The
transistors switches have a complex control algorithm.
•
Multi-level inverter – generates the sinusoidal output voltage of up to
4160 V AC.
•
Optional du/dt filter – reduces switching transients to allow operation
with long motor cables or old/unknown motor winding insulation.
•
Motor – any high voltage (up to 4 kV) standard motor can be connected.
49
Multi-Level Switching Topology.
Different inverter topologies can be categorized according to the PWM voltage
waveform they generate. The following diagrams compare PWM output waveforms of
different inverter topologies, without any output filter.
Classical solution for low-voltage drives is 2 level inverters. The output voltage of a two
level inverter can either be +UDC or 0 (or – UDC ) (Fig.2-8). The sine waveform is
approximated by PWM switching between these voltages. As the waveform is within
the du/dt insulation limits for low voltage motors, the two-level inverter is the standard
simple solution for low voltage drives.
2-8: 2 level inverter waveform.
For medium voltage drives, the 2 level inverter waveform exceeds the du/dt motor
insulation limits and requires significant filtering. The high filter losses may
significantly reduce the overall drive efficiency.
The 3 level inverter is an improvement, since it can output an intermediate voltage level.
Its output voltage can be UDC or ½ UDC or 0 (and negative). Since the voltage steps are
smaller, less filtering is needed to achieve the same output du/dt sustainable for a
medium voltage motor. The filter losses are still significant.
50
2-9: 3 level inverter waveform.
The next progression is a multi-level (4 level) inverter. The even smaller steps make the
output waveform more sinusoidal and within the insulation requirements for most
modern motors. Specific installation requirements, such as long motor cables, can be
met with only minimal additional filtering, which results in the highest overall drive
efficiency. The scheme provides this waveform shown on Fig.2-10.
2-10: 4 level inverter waveform.
As a minimum, variable frequency drives should be powered via a disconnect switch,
which allows maintenance personnel to completely and safely disconnect the drive from
the power line for repair without de-energizing other circuits as well. Most variable
frequency drives contain start/stop circuitry that eliminates the need for a combination
51
starter. Most manufacturers will incorporate disconnects and/or starters into the design
of the drive when specified.
Variable frequency drives inherently produce a soft motor start. While a motor
connected across the line will accelerate rapidly to full speed and draw up to 6…8 times
its full load current, variable frequency drives accelerate the motor at a controlled rate
that is adjustable and limited by the current capability of the drive. Full motor torque is
available during motor acceleration.
Variable frequency drives decelerate the motor at a controlled rate by putting an
electrical load on the motor, thereby dynamically braking it. To understanding braking
in the inductor motors, it should be said that it is possible to operate an induction
machine as a generator by mechanically driving it above the synchronous speed (which
is related to the supply voltage frequency). The electromagnetic torque developed in this
mode is negative and acts in an opposite direction to the direction of rotating magnetic
field. Some drives are designed to recuperate the rotational energy of the motor, using
regenerative braking circuits that generate electricity during braking and put power back
onto the line. These drives are more complex and may be economical when the motor is
constantly accelerated and decelerated.
Alteration of the firing sequence of the power semiconductors allows many variable
speed drives to operate a motor in the forward and reverse directions. Reversing logic
circuits are usually incorporated into the drive, which ramps the motor to a halt before
accelerating the motor to the desired speed, in the opposite direction.
Power factor correction is a desirable feature that can also in some cases affect the
electric bill. Usually, filters for the 5th, 7th, 11th and 13th are used in the factory level to
reduce the adverse effects of the low order harmonic in the supply network. Variable
frequency drives may also contain power factor correction and operate at a power factor
of approximately 94 to 96 percent at full load (with diode rectifier, active network
bridge rectifiers this value is more close to 100%), but may be significantly lower at
lower loads. The large drives are designed to operate in this region under all load
conditions. Depending on size and manufacture, some drives may contain no power
52
factor correction equipment at all, which, when applied to constant torque and many
centrifugal loads, can increase the electric bill.
Power factor correction equipment can be economically advantageous by reducing the
current required to operate the drive at reduced speeds, thereby reducing transformer
loading. This reduces the transformer and cable heat losses, as well as kVA demand.
The economic benefits of power factor correction equipment should be examined to
determine the viability of additional expenditures for this purpose. It should be noted
that when traditional power factor equipment is used (capacitors and inductors), the
power factor will vary somewhat as a function of speed. The best way of using the
power factor correction in VSDs is to use one active rectifier in conjunction with a
multi-drive system.
The efficiency of a variable frequency drive is the ratio of the electric power delivered
to the motor to the input power consumed by the drive. The measurement of these
parameters is relatively difficult due to the non-sinusoidal nature of the input and output
current waveforms. However, efficiency can be thought of in terms of the heat losses
associated with the power semiconductors and the relatively constant heat loss
associated with the electronic circuitry. These losses can represent approximately 2 to 6
percent or more of full load power in typical variable frequency drives. The majority of
the heat losses associated with medium and large drives are due to power semiconductor
heat effects and vary as a function of motor current.
More important to the user is the efficiency with which the variable frequency drive
utilizes billed electrical consumption to produce brake power and, ultimately, useful
work. Firstly, the power factor correction feature will maintain a near unity power factor
regardless of the actual motor power factor, effectively reducing kVA demand.
Secondly, in many applications proper instrumentation and control can keep the motor
fully loaded, thereby maintaining optimum motor efficiency at reduced speeds.
Detracting from overall efficiency are the effects of synthesized non-sinusoidal
waveforms on motor operation. The relatively short rise time of these waveforms
53
creates harmonics, which result in some heating effects that adversely affect motor
efficiency.
The ultimate measure of efficiency to the end user is the system efficiency, that is, the
ratio of useful work done by the drive to the electrical energy taken from the network.
This focuses on the utilization of electricity in a cost-effective manner. For example, at
full speed and full load, there is virtually no difference in the efficiency of the various
types of variable frequency drives; however, the current source inverter tends to
maintain its efficiency as the speed is reduced, the economic effects of which may be
partially offset by the increased power factor offered by a pulse width modulating drive
that maintains a near unity power factor regardless of speed. In short, it should be noted
that no one type of drive is universally applicable. The full-load efficiency of present
day voltage source inverters is typically in the range of 98.5 %. If an active network
bridge is used the full load efficiency will be in the range of 97 %. Using e.g. moderate
3 kHz switching frequency reduces a typical induction motor efficiency by about 0.5 %
units. Often the reduced losses in the whole drive system far exceed in amount the
losses of the converter and the extra losses in the motor. The efficiency of a diode
rectifier equipped frequency converter might be estimated as
η = 1 - (0.005…0.01 + 0.01 × Load/Full load)
(2.2)
Variable frequency drives are electronic devices that supply current to the motor. The
current is limited by the current rating of the output semiconductors. Under overload
conditions, the variable frequency drive will trip itself to protect the drive and the
motor. Typically very shortly a 200 % current may be drawn and 150 % may be taken
for 10 s. Depending on manufacturer, variable frequency drives may also trip as a result
of other abnormal conditions, such as ground fault, over-voltage, and the like.
Dedicated lights for each fault are available from some manufacturers, while others
utilize diagnostic cards to display a code that identifies the fault. In any case, proper
diagnostic indication can greatly enhance the ability of maintenance personnel to
identify and correct a problem.
When properly sized, some variable frequency drives are capable of operating more
than one motor connected in parallel, while multiple drives may be capable of
54
synchronizing the motor speeds of various motors. Motor speeds can also be controlled
to be a ratio of one another within certain accuracy limits. It is well advised to consult
with a manufacturer when considering multi-motor variable frequency drive
applications.
2.2 AC-drives control methods.
The control of AC machines is very complex and the complexity increases if exact
performance specifications are demanded. The complications take place as AC
machines have complex dynamics, variable-frequency power supplies, AC signals
processing etc. The AC machines can have various methods of control, and what
method will be adopted depends on its intended application. The decision about the
control method is usually based on the following questions:
•
Should the control be open loop or closed loop?
•
Is it a position-, speed-, or torque-controlled system?
•
What types of power converters should be used?
•
Should it be a one-quadrant, two-quadrant, or four-quadrant drive
system?
•
Is it a single-machine or a multi-machine drive?
•
What is the range of speed? Does it include zero speed and field
weakening region?
•
What are the accuracy and response-time requirements?
•
Is the drive required to give robust or parameter-insensitive response?
•
Do pulsating torque, harmonics, and input power factor need control?
The AC- motor drive system is basically a multi-variable control system, and so the
state variable control theory should be applicable. Usually the voltage and frequency are
the control inputs and the outputs may be speed, position, torque, air gap flux, stator
current, or any combination of these. The AC-machine model is nonlinear and
represents a complicated object which describes a set of complex differential equations.
Additionally, the parameters of this machine may vary with saturation, temperature, and
55
skin effect, adding further nonlinearity to the system. There are two main types of
drives control methods: scalar and vector control.
2.2.1 Scalar control methods
The main task of scalar control is to form a phase voltage based on preset values of
amplitude and frequency derivable by pulse-width modulation (PWM) of the inverter,
preset values envelopes a three-phase voltage for supply of the AC-motor. This
principle is the simplest way of frequency control and due to due to its low cost, it is
widely used for a drive of the mechanisms which do not need high requirements to
insure the quality of speed regulation, first of all, concerning electric drive of the
pumps, fans, and compressors. The given class of mechanisms creates many
opportunities for energy- and resource-saving which are successfully realized with the
application of these converters. Scalar control methods of an AC-motor can be generally
realized with voltage-fed inverters, current-fed inverters and slip power recovery
control. Scalar controls realize only magnitude and frequency control. The most usual
speed control methods are U f = constant and U f 2 = constant. The U f 2 = constantmethod is often used in centrifugal pump and fan drives. Some of the control principles
of voltage-fed inverters are also valid for current-source inverters, so we will consider
control methods of an AC-motor just with voltage-fed inverters. A simple and popular
open-loop U f = constant speed control method for an inductor motor shown in Fig.211.
AC line
Inverter
Rectifier
+
+
Uo
Gain
constant
U/f
Motor
speed
command
2.-11: Open-loop U/f speed control, the rectifier produces a variable DC-output.
56
The power circuit consists of a phase-controlled rectifier with a single- or three-phase
AC supply. LC filter, and inverter. The frequency ωc is the speed command variable
and it is close to the motor speed, neglecting the small slip frequency. The scheme is
defined as U f = constant control because the rectifier voltage command is generated
directly from the frequency signal through a U f = constant gain constant. In this
scheme, the speed will tend to drift with variation in load torque and fluctuation of
supply voltage. If open-loop speed fluctuation is not applicable, closed-loop speed
control schemes may be used.
Wound rotor IM slip power recovery control methods are the static Kramer and the
static Scherbius methods. The static Kramer and static Scherbius (Fig. 2-12) systems are
popular in large power pump and compressor-type drives, where the variation of speed
is usually limited. Kramer cascade consists of AC motor with a wound-rotor, converters
and transformer (Fig 2-13).
3-phace
AC
supply
AC-motor
-
+
+
+
-
Cycloconverter
Bidirectional slip
power flow
Figure 2-12: Static Scherbius drive.[11]
-
57
3-phace AC supply
Power
input
Power
feedback
Wound-rotor
AC-motor
Transformer
Diode rectifier
Inverter
Id
Slip power
2-13: Static Kramer drive system.
The stator windings of the AC motor are connected directly to the grid, but the rotor
windings are connected to the grid via slip rings and a frequency converter. The Kramer
cascade provides sub-synchronous region speed control principles where the slip power
is recovered back to the line through a converter cascade. Control of this system is
rather simple, but the disadvantage is the drive system can be controlled only in one
quadrant. Control is realized by current and speed control loops. If the command speed
is increased by a step, the motor accelerates at constant developed torque corresponding
to the I d where the limit is set by the speed control loop. As the actual speed
approaches the command speed, the DC link current is reduced to balance it with the
load torque. If the speed command is decreased by a step, I d approaches zero and the
machine slows down with load torque. Then as the speed error tends to zero in the
steady state, I d it re-establishes the balance with the load torque. The air gap flux
maintains a near constant state during the entire operation, as dictated by the stator
voltage and frequency [11]. The static Kramer drives speed control system of on Fig.214.
58
3-phace
AC
supply
Transformer
Wound-rotor
AC-motor
Diode
rectifier
rotor
speed
Inverter
-
Gain
constant
1
Id'
+
+
rotor
speed'
Id
-
Gain
constant
2
Id
2-14: Speed control of static Kramer drive system.
2.2.2 Vector control methods
As AC motors are very complicated devices, scalar control methods can often be
insufficient. Vector control methods allow the regulation of output values (speed,
position, torque, air gap flux etc) using not only magnitude and frequency of input
values. If we know the mathematical model of the motor, the magnitude of phase
voltage and current, and its phases, AC drives can form a vector control of the motor.
For real-time AC motor control good software and good hardware are needed. The ACmachine model is represented as a complicated object which describes a complex
differential equations set which AC drive have to solve it during the control process. For
this reason, AC-drive usually encloses the mathematical model of the motor.
Vector control methods provide similar characteristics with AC-motors as are typically
found in high-quality DC-drives. These system properties are achieved by channel
separation of the flux linkage regulation and the torque regulation of the motor. This is
not possible when using scalar control methods. With these systems, a vector
representation of physical quantities is used. The converters using these control
59
principles have rather high cost and are used in equipment where high quality
specifications for rate control: machines lifts, cranes. Although in combination with
good feedback sensors (position or speed sensor) in most cases scalar control converters
can manage these tasks. It is also necessary to note, the industry has a tendency of
rejecting the speed sensors, and the development of motor control algorithms are based
on the measured phase currents and voltages.
We understand that the total control of the electric drive is provided by the
motors’ electromagnetic torque control. During the construction of an AC-drive control
system, it is possible to use the following equations in regard to the shaft
electromagnetic torque The torque is the vector product of the stator flux linkage Ψs and
the stator current vector is or the vector product of the rotor flux linkage Ψr and the rotor
current vector ir with opposite sign.
T=
T=
3
3
p(Шs × is ) = − p(Шr × ir )
2
2
3
3M
3M
M (ir × is ) = −
p(Шs × ir ) =
p(Шr × is )
2
2 Ls
2 Lr
(2.3)
(2.4)
where is p - number of pole pairs
M - mutual inductance
Ls, Lr – are stator and rotor inductances accordingly.
along with other derivatives from these equations. Vector controls require the
independent control of vector components which are included in the electromagnetic
torque equation. The selection of the equations for the control systems plays an
important role, as many parameters, particularly in the squirrel-cage motor, cannot be
measured. Moreover this choice essentially influences to complexity of system transfer
functions, occasionally increasing the order of equations into few times. However, in
any cases the electromagnetic torque equation structure will be similar to
T=
3
3
3
p(Шrx + jШry ) × (irx + jiry ) = − p(Шrx iry − Шry irx ) = p(Шry irx − Шrx iry ) (2.5)
2
2
2
where subscripts x and y means decomposition to the x and y axes accordingly.
60
The general principle of the simulation and of the AC-drive control systems
construction consists of a coordinate system which is constantly focused in the vectors
direction. This vector determines the electromagnetic torque. Then with a vector
projection on another coordinate axis, corresponding to its variable in the
electromagnetic torque equation (2.5) will be zero. This equation formally becomes
identical to the electromagnetic torque equation for DC-motors which is proportional to
the armature’s magnetic flux. The vector is chosen where the direction of the coordinate
system is oriented. Choosing the vector is a random process. The vector has been
defined in simple terms, and as an opportunity of AC-drive model realization. For
example, in case of orientation on rotor flux linkage
Шr = Шrd ;Шrq = 0 torque can be represent as
T=
3
3
p (Шrq ird − Шrd irq ) = − pШrd irq
2
2
Шrq = 0
T=
3M
3M
p(Шrd irq − Шrq ird ) =
pШrd irq
2 Lr
2 Lr
Ш
(2.6)
(2.7)
rq = 0
where d and q are axis of the rotating coordinate system. Rotor flux linkage is
oriented on the d axis.
Obviously, the first equation for the control of a squirrel-cage motor is not interesting
because it is practically incapable of measurements and control rotor current. The
second one, allows under certain conditions that rotor flux linkage to be constant,
controls the electromagnetic torque by variation of a stator current projection on a crosssection axis isq . So, when creating vector control system for AC-drive we need to
choose a vector where the coordinate system will be oriented, then choose a suitable
equation for the electromagnetic moment. Finally, defining the values included in it
from the equations for a stator and/or rotor circuit.
Using vector control, both stator current and rotor speed are measured and these signals
are fed into the mathematical model of the motor, which is stored in the inverter
microprocessor memory. The motor model calculates the two components which
represent the motor torque and its magnetic flux, thus improving the dynamics of the
drive. The vector control makes it possible to obtain full torque albeit at zero speed.
61
Speed control by means of frequency (and voltage) variation also allows the capability
to operate the motor not only at speeds below the rated speed but also above the rated
speed. This provides a controlled alternative to the following ways of speed reduction,
which are not as attractive, for example mechanical brakes, which waste energy
associated with load-motor inertia and whose brake pads wear out with repeated use;
letting the motor coast to a halt, which could take a long time; and “plugging” where the
phase sequence of the utility supply to the motor is suddenly reversed, causing large
currents to flow into the utility source and bringing the motor to a halt in an
uncontrolled manner.
New directions in the field of high-quality control systems development are systems
with a direct torque control (DTC). The basic idea of DTC-control is during each
calculation step to define the optimal condition of the voltage converter, using values of
the stator torque and stator flux linkage. The pulse-width modulator is excluded from
system as a separate part. The system realizes vector speed regulation and the
mathematical apparatus of this vector speed regulation are based on the differential
equations of the dynamics changes in an AC-motor and also vector correlations. The
method is equally correct both for transient and steady-state processes. This method
essentially raises the dynamic range of the system’s work, as a result, for example, there
is absence of speed failures at load surge. The aim of a speed loop – is to set
instantaneous current vector position, necessary for the maintenance of the preset speed.
The aim of a current loop – is to provide actual position and current vector amplitude,
equal to preset values.
The inverter switching time, does not depend on PWM, but depends on an actual error
of the current vector. The selection criterion of the inverter status is very important and
allows:
•
To minimize switching frequency of the inverter at small error
amplitude;
•
To reduce large short-term current error over a short period of time with
a minimum of inverter switching.
62
This current control method has essential advantages in comparison with PWM-control.
It allows the construction of higher-speed systems, instantly responding to external
influences, and saving energy in power switches in comparison with PWM.
Let us consider the example of the controlling torque results of scalar and vector control
(Tab. 2.1). With torque control, the motor torque is always controlled by the torque
reference. The motor speed operating point is defined by the intersection of motor
torque and load torque curves. Vector control is used when a very fast or accurate
torque control is required.
Table 2.1: Typical performance of torque controls in scalar- and vector controlled AC
motors[12]:
Scalar control
Vector control
Resolution
1:1000
1:1000
Nonlinearity
+/-12%
+/- 4%
Reproducibility
+/-4%
+/-1%
Response time
150 ms
10-20 ms
As we can see in Table 2.1 the vector control method has essential advantages in
comparison to the scalar control method. It allows the creation of higher-speed systems
which can instantly react to external influences with less nonlinearity.
2.3 Automation of AC-drives
At the moment, AC drives cannot decide which rotation speed the motor should be
rotated. Therefore AC drives have remote controls in addition to local controls
belonging to the AC drive systems which we discussed in the previous chapter.
Generally, automatic control systems in the production process include the majority of
modern AC drives in the plant. Data communication between this system and single
drives are realized by means of field buses. The most commonly used are ProfiBus,
63
Ethernet and CAN. Industrial control systems trace current operation factors of
production processes and compute the capacity requirements (with reference to the AC
drive and its required speed of rotation). The aim of the AC drive is master control
execution with the prescribed accuracy and high efficiency.
Every industrial control system is developed with specific requirements, but each has a
common structure. The system is intended for gathering and processing information in
the separate technological object and carries out functions of the registration, the
control, and the protection from failure. Some possible applications are power supply,
water supply, heat-and-power engineering, technological processes of some
manufactures, etc. Industrial control receives parameters from technological process
objects, transferring them to an operator's console, and controls the technological
processes objects in an automatic mode which have been prescribed in algorithms or in
a manual mode on commands from an operator's console, and logging the report of
work from the process equipment. The system allows the transference of parameters as
well as the operation of objects in real time.
operator's
console 2
operator's
console 1
server
router
Network TCP/IP
(Internet)
router
controller
controller
controller
equipment
equipment
equipment
2-15: Structure of industrial control system.
The system represents the network of data transmission having arbitrary architecture.
Units of this network are Controllers, Routers, a Server, and Operator's consoles:
64
•
The controller. It is connected to sensors and actuating units of the
process equipment. The controller measures and fixes values of sensors,
transfers information to higher level system equipment and it operates
the process equipment, connected to it.
•
The router is telecom standard communication equipment used in
intranet or internet networks. The router contains channel-forming
equipment for communication with network objects (controllers, the
server). The router is used for the data receive and storage of it from the
controllers The router stores the dynamic models files from the
controllers control programs and providing network services for
controllers such as loading, transferring of date and time and so on.
•
A server - the centre of system, contains the software for queries,
transformation and archiving information from controllers and provides
web-based applications accessible from objects within the network
including system and system work archive.
•
An operator's console. Uses standard computer and software technology
which is commonly used in offices and for internet connectivity. The
operator's console is used as the control-centre and for displaying global
system status and information for each sub-system in real-time. In a
network of systems there may be multiple operators’ consoles.
The minimal configuration required by a system is a controller and an operator's
console. These are simultaneously fulfilling the functions of the server and the router.
Besides this, independent work of the controller is possible, and removal of the saved
data occurs during its connection to the Server, or as a manual process at the operator's
console.
Data transfer between the systems network objects are facilitated using the standard
TCP/IP Protocol. Information about the controller settings and status and about status of
controllable objects is displayed on operator's console. This means that for the systems
network components, standard software solutions are used to display status visualization
of the system control in common web browsers (e.g. MS Internet Explorer, Netscape
Navigator) is usually used.
65
3 Comparison of traditional vane control and modern ACdrives control in Cesla Cement factory.
Let us consider a comparison of traditional and modern AC-drive control
methods by a given example in Saint-Petersburg Cement Plant “Cesla” shown in Fig 31 and Fig 3-2.
3-1: Cement factory “Cesla” in Slanci.
3-2: Picture of Cement factory “Cesla”.
The factory is located in the town Slanci 200km away from Saint Petersburg. The
German “Heidelberg Cement Group” is a basic shareholder of "Cesla". At the end of
2002, the Heidelberg Cement Group has acquired the “Cesla” factory and has invested
nearly 40 million euros in its reconstruction and development. Because of the
66
realization of the investment program cement manufacturing will increase
approximately 3 times during the next 3 years- and the factory will produce more than
one million tonnes of cement per year. The increase of cement output in this plant is
shown in table 3-1.
Table 3-1: Cement output on the cement plant “Cesla” [13],[14]:
Year
1990
1995
1999
2000
2001
2002
2003
2004
2005
2006(planned)
Cement output
(thousands of tonnes)
1000
100
170
239.5
263.4
340
400
500
680
> 1000
The “Cesla” factory has the entire production line starting with limestone extraction and
finishing with packing of cement. In 2005, the “Cesla” cement factory started
manufacturing modernization. This revised output estimates were determined by
installing new equipment and AC-drive control in the manufacturing line. The total cost
of this project reached more than one million euros. The installing of the new equipment
is planned for completion in 2006. In addition, the dust-controlling equipment has also
been installed. An expansion of the manufacturing line is set for the future as well as the
start-up of a second production line in the “Cesla” factory. This expansion will allow
the production of more than one million tonnes of cement per year, and - creates a base
for high-quality building materials manufacturer in the developing construction market
of the Northwest region.
By this moment, almost all equipment (mills, dryers, fans, conveyors, separators,
feeders) are working without any means of accurate control. All motors in this
equipment are directly connected to the supplying network. The feeders (Fig.1-20) have
DC-drives with voltage-regulators in the range up to 220V and allow fuel feeding
control. In the dryer systems, slip ring motors with power up to 120 kW are used. The
rotation speeds of these motors are regulated by switching of the windings, and the
dryer can be operated with power of 60, 90 and 120kW. In this plant, 6 ball mills can be
67
used, but only 3 are currently working. Materials are crushed inside a rotating tube – up
to 6m in diameter and 20m long – containing metal balls that tumble against the
materials (Figure 1-13). Tube mills are the most energy intensive systems. They have
synchronous motors with power of 1000kW; and they are directly connected to supply.
For soft start providing they have thyristor exciters. The drives for separators (Fig.1-17)
and fans (Fig. 1-22) have manual regulation by changing the angle of the vanes and
dampers. Without any regulation only the conveyors (Fig. 1-11) remain, and they are
operating with AC motors. We can easily see the large amount of energy which is
consumed with such types of regulation.
During 2005-2006 the Kiln and ID fan were modernized. After the installation of the
AC-drives into the kiln, the rotating speed increased from 1.15rpm to 2rpm and
productive capacities increased from 28 tonnes/h to 50 tonnes/h. It not only provides an
increase of production capacities, but also secures soft starts and energy savings.
Fans and pumps are used in practically all stages of the produced cement and consume
30% of all electric energy in the cement production. Fans can be propelled by a constant
speed motor with dampers or vanes or by a variable speed drive. A comparison between
them should prove helpful in determining the advantages and disadvantages of each
technology in a given application. Let us compare some of the characteristics (energy
efficiency, reactive power consumption, effects of the mechanical construction and so
on) of fans with AC-drive and DC-motor with vane.
Control vanes are the most commonly applied control elements of fans in this factory.
Variable speed drive technology begins to be used in the same applications. Variable
speed drive systems represent technical opportunities and the possibilities of significant
economic rewards. It was initially thought that this work should focus on the advantages
and disadvantages of these technologies. The following comparison focuses on the
differences between the technologies that may be applicable in making the decision for
the installation of variable speed drive technology or to use a control vane technology in
a given application.
The most fundamental difference between the control vane and variable speed drive
technologies is the difference in the type of control used. We should attempt to
68
understand that a control vane is a device that dissipates energy, and a variable speed
drive is a device that can regulate the amount of energy consumption.
As an analogy, the application of a control vane can be thought of, as the case of a car
where the engine is operated continuously at full torque while the brake is manipulated
to control the speed of the car. Alternatively, variable speed drive technology can be
thought of as the operation of the car when the brake is totally released and engine
torque is used to control the car speed.
3.1 Efficiency
One fundamental distinction between control vane and variable speed drive (VSD)
technologies is that the vane dissipates the excessive energy not required by the load
and the efficiency of the vane decreases in direct proportion to the flow, while the VSD
generates only the required amount of hydraulic energy necessary to load and the
efficiency of an AC Drive is high throughout the control range. This difference is
important as the capacity of the fan is higher than the maximum required demand. In the
“Cesla” factory the average efficiency of fans is 60%. The net result in this application
is: VSD technology is more preferable and allows very much cost savings.
3.2 Power savings and reactive power consumption.
As we can see in Fig 3-3, the VSD technology allows reduced power consumption and
reactive power savings.
69
Reactive power savings with VSD
technology
Power consumption of fan
1000
800
500
Power, kW
Rective Power, kVA
600
400
300
200
600
400
VSD
200
100
0
Vane
0
50
60
70
80
90
100 110 120 130 140
Air flow, 1000m/h
45
50
55
60
65
Air flow, 1000m/h
3-3: Reactive power savings and power consumption of typical ID-Fan.
3.3 Power factor
The power factor of a motor operating at full speed in a control vane application can be
over 90% in larger motors operating under full load conditions, but it can drop off
significantly as the load is reduced. If we have a diode-bridge-supplied frequency
converter drive instead the displacement power factor is high but the overall power
factor not so high due to the harmonic input currents (see Fig. 2-4). Active power factor
correction may be reached only by active transistor rectifier Fig. 1-18. If 12 or 18 even
24 pulse diode rectifiers (on Fig. 2-7 is shown multilevel drive with 18-pulse rectifier)
are used the power factor of the network side approaches unity but not with a six pulse
diode bridge. The large drives are designed to operate in this region under all load
conditions, they include circuits for power factor correction, which help to maintain
near unity power factor independent of the load. Depending on size and manufacture,
some drives may contain no power factor correction equipment at all, which, when
applied to constant torque and many centrifugal loads, can increase the electric bill.
Fig 3-4.shows the differences in power factors found in different types of controls.
70
Power factor, %
120
100
80
60
40
VSD
20
Vane
0
50 60 70 80 90 100 110 120 130 140
Air flow, 1000m/h
3-4: Power factors of typical ID-Fan with VSD and vane control technologies.
3.4 Location and Installation
In a large majority of applications, control vanes must be located in the field and are
subjected to the local environment, which may be dangerous, corrosive, hot, cold, wet,
or snowy. Electronic variable speed drives may be located either in a motor control
centre, where the drive will be protected from the environment, or in the field, if
required, making the location of a variable speed drives much more flexible in this
regard.
The installation of a typical pneumatic control vane in a continuous operation, involves
considering the installation of bypass piping. Air supply piping and control wiring
compete for space in the process area. The installation of a variable speed drive needs
sufficient space and access in the motor control centre or in the local environment, if
located there. Some electronic variable speed drives may require that the cables to the
motor be routed separately from power cables. Often, variable speed drive installations
are much more straightforward.
Reconditioned electronic variable speed drive applications can be installed in a
straightforward way when enough space is available. In most cases, the existing cables
connecting the motor to the starter can be reconnected to the drive and reused. It should
be noted that there are many applications that may require extensive electrical and
71
mechanical work, such as where space for the drive is not available or major electrical
rework must be done because of the existing cable route.
While electronic variable speed drives can typically be located in a protected
environment, the dusty parts of a control vane are exposed to dust, which may be
abrasive and subject to physical wear and damage. While appropriate construction
techniques and materials can alleviate these problems, the vane must eventually be
rebuilt due to environmental damage (e.g. dust) as well as age.
3.5 Specification
The vane specification represents the product of many technical and economic
compromises and ultimately decisions. In a typical control vane application, substantial
amounts of process data and compatibility information must be accumulated in order to
size and select a control vane. As a minimum, the control vane selection entails
specification of vane type, vane trim and its inherent characteristic, materials of
construction for parts, accessories, and so on.
Specification of electronic variable speed drives is usually simpler than that of a control
vane because it is primarily dependent on the motor and type of load, information about
which is more available than is process data and compatibility information. The
specification of other variable speed drive technologies can be considerably more
complex than that of specifying a control vane. The stability of variable speed drive
technology to the problem of material compatibility represents a distinct technical
advantage over control vane technology.
3.6 Operation
While control vanes are capable of completely closing, stopping flow in both directions
(within the vane leakage specifications), variable speed drive equipment do not share
this characteristic. For example, when the pressure supplied by a fan is insufficient to
72
overcome the pressure in the piping system, the flow will be reversed through the fan.
Reverse flow (which may affect flow meter performance) can be minimized by
installing a check vane at the discharge; however, stopping the motor does not create
zero flow and positive shut off, as does a control vane.
In short, where positive shut off is required, control vane technology should be seriously
considered, but a variable speed drive with an appropriately controlled on-off vane may
also be applicable.
Control vane applications typically include bypass facilities which allow the removal of
the control vane for service while operating the bypass vane manually. It should be
noted that experience has shown that, in many cases, bypass vanes and the control vane
tend to fail to operate properly after approximately the same length of service. This
often leads to process shutdown for repairs when the problem is intolerable.
A similar functionality can be designed to remove a variable speed drive from service
where parallel equipment is operated at line speed with its discharge throttled with a
manual vane. Some electronic variable speed drives can be purchased with an across the
line starter that enables the motor to operate while the drive is being serviced.
In control vane applications, the rotating equipment operates at full speed with full
equipment capacity available at all times. On the other hand, a control system that
incorporates a variable speed drive will control motor speed corresponding to load. As a
result, under abnormal conditions the equipment may be momentarily incapable of
overcoming the demand of the piping system, resulting in reverse flow through the
equipment (with a possible motor reversal) and potential equipment damage, process
disturbance, and environmental emissions. Assuming that all equipment is functioning
properly, removing the cause of the disturbance condition or modifying the control
strategy can usually alleviate this problem.
73
3.7 Ability to control
Sluggishness can cause instability of the control loop, due to the time required for the
control vane to react to changes, and the dead band associated with vane positioning.
The input converter introduces a time delay between the controller output and the
pneumatic signal to the actuator. Friction caused by the actuator and packing causes the
control vane to ignore the small controller output changes encountered when the process
variable is near its set point, or when the process reacts slowly. The resulting hysteresis
causes the process variable to shift near the set point or become unstable. Control vane
positioners are typically used to minimize these problems, however some delays remain.
In the case of manual controlling of the vane it is especially difficult to set the specified
pressure requirement, which can be made only with a large approach.
Some alternative final control elements, notably electronic variable speed drives, control
motor speed with negligible converter dead time and an almost infinite resolution. This
eliminates most of the hysteresis problems found with control vanes, therefore
improving the control of the process variable.
3.8 Maintenance and spare parts
As a minimum, the maintenance costs associated with control vanes include fixing leaks
and periodically rebuilding the vane, especially in the dustiest environments. The vane
and its filter should be periodically replaced. The amount of maintenance required for a
given number of vanes is difficult to assess accurately. However, one rule of thumb
states that the maintenance expenditures associated with a vane over 5 years of
operation will equal its purchase price.
The electronic troubleshooting skills necessary to repair electronic variable speed drives
are very different from the mechanical skills necessary for control vane repair. On the
other hand, such a drive located in an area protected from the process environment
should exhibit a relatively good maintenance record, although this is difficult to
74
quantify. The conditions where maintenance and troubleshooting are performed on
electronic variable speed drives are generally better than those of a control vane.
The application of electronic variable speed drives will tend to reduce the amount of
equipment maintenance, as reducing the speed and maintaining a relatively constant
load will reduce the wear and tear on the equipment. This reduction in maintenance is
difficult to quantify, since it varies with each application.
Spare parts requirements for control vanes can entail many specialized items due to the
different vane types, sizes, and materials of construction. Even with the standardization
of manufacturer throughout a plant, this remains true. As electronic variable speed drive
technology needs not address to materials problems, the number of spare parts may be
fewer than those for control vanes and will consist of more universal components.
Initial spare parts cost for these drives may be more expensive than those for a typical
control vane; however, a faulty part can often be returned to the manufacturer for repair
or be repaired at the plant.
3.9 Operating and installation costs
In typical applications, variable speed drive technology offers the user increased
efficiency over vane control. The net result is that variable speed drives can reduce the
electric power expenditure and will often yield electrical energy savings sufficient to
justify most applications.
While the purchase price of a variable speed drive for a new application is significantly
higher than that for a control vane with accessories, its running costs are generally
remarkably less than that of the vane control after the equipment and accessories,
bypass vanes, air piping, conduit, cable, starter, breaker, labour, and so on are taken into
account. In old installations replacing a vane control system by a variable speed drive is
typically more costly than just replacing a worn out control vane, as the vane control
electrical and pneumatic utilities are already installed.
75
The investment cost of larger variable speed drives can greatly exceed that of a vane
control system; however, the potential operating cost savings are usually larger as well.
In some variable speed drive applications, the size of the equipment and also the motor
can be smaller than those of a vane control application due to the reduced hydraulic
energy requirements.
As an example an energy saving calculation for traditional flow control methods
compared to variable speed AC drive control with software “FanSave” v 4.0.B (ABB) is
represented in Fig.3-5. The centrifugal fan with radial blades and a control vane which
works with an average efficiency of 60% was compared with the similar fan running
with a variable speed drive. As we can see yearly energy saving is 228443kWh. With
energy price 1.20RUB/kWh (Price for Russian enterprises) the annual cost savings will
be 274131RUB. If the investments for the installation of the VSD instead of control
vane will 100 000 RUB, the payback period will be 5 months.
3-5: Calculation of Energy saving for traditional flow control methods compared to VSD control
The following is a summary of the performance comparison of a typical application
where a control vane or an electronic variable speed drive would be applicable (table 3-
76
2). This selection of variable speed drive technology represents a widely applicable
technology with a good price/performance ratio.
Table 3-2: Comparison of vane control and VSD technologies :
Item
Control vane
AC VSD
Equipment efficiency
low
high
Motor efficiency
high
optimal
Power savings
none
high
IM rated
high
no flexibility
inverter installation is
Power factor
Flexibility of location
free
Exposure to environment
fully exposed
Better
Ease of installation
-
-
Specification
-
Better
Shutoff capability
Better
-
Ability to control
-
Better
Potential for leaks
-
Better
valve/drive
-
Better
equipment
-
Better
expertise
Better
-
spare parts
-
-
Installation costs small units
-
Better
Better
-
high
low
Maintenance:
Large units
Operational costs
From the above table, we can see that neither technology is superior in all respects, but
some subtle observations can be made:
•
AC variable frequency drive operation is typically more efficient than
control vane operation; however, the economic impact may or may not
justify its installation.
•
The application of variable frequency drive technology can reduce the
number of pieces of equipment that are exposed to the dusty process.
77
•
More expertise is usually required to troubleshoot variable frequency
drives than control vanes.
•
Whereas control vanes typically provide shutoff capability, variable
frequency drive applications may require piping changes to prevent
backflow and provide tight shut-off capability.
•
AC variable frequency drives provide better control characteristics
because of good resolution and negligible dead time.
4. Conclusion
Variable speed drives – especially voltage source inverter drives – in modern industry,
globally, have became a standard energy saving and quality enhancing equipment. The
reliability of the present day voltage source inverters is high and only few maintenance
objects exist in the inverters. Typically the cooling fan must be replaced every now and
then and the DC capacitors must be replaced every 10 years.
The inverter technology also has some drawbacks: better cabling than in DOL drives
and sometimes some filtering in the inverter output must be used in order to ensure high
motor lifetime. If six-pulse diode rectifiers are used instead of active network bridges
low order harmonics are induced in the supplying network worsening the power factor.
There, however, are solutions for all the practical problems related to the variable speed
drives. Hence at present all the benefits of the AC VSDs are available. The cultural
change, however, is big when moving from traditional mechanical flow control systems
to modern power electronic systems.
Saving energy using AC-drives in such factories as the “Cesla” factory in Russia is
clearly not an easy task. Practically all factories were built in the time of the former
USSR and refurbishing and modernization are required. This demands large
investments in the Russian Cement Industry. Currently the Russian construction
industry suffers from a large deficit of quality cement. There are good opportunities for
78
the development of the Russian cement industry. Based on our example of the “Celsa”
factory, we can clearly see that the installation of new equipment such as AC variable
speed drives, will allow significant increases in production capacities and energy cost
saving. The drawback is that more skilled personnel is needed in the maintenance of
high-technology equipment and thus lots of education for the operating personnel is
needed when variable speed drives are adopted in the production.
79
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