My report vfd

AC drives in cement industries
Department of Electrical engineering,AGI(Jaipur) Page 1
CHAPTER 1
INTRODUCTION
In the past, various control methods were employed to obtain flexibility and consistency during
manufacturing of cement. These included one or more methods of limited control of the speed of the
equipment, such as changing gear ratios or pulleys, or using hydraulic drives. Most often however, gas
and liquid flow rates were controlled with dampers or throttled valves while the fan or the pump kept
operating with a fixed speed motor.
The mechanical speed changing drives and some of the hydraulic drives are known to be hard wearing
on the components, are often inefficient and have high operating costs. Very few of these types of drives
remain in use today.
Due to technology advancements in semiconductor devices such as insulated gate bipolar transistors
(IGBTs), modern medium voltage (MV) drives are increasingly used in petrochemical, mining, steel and
metal, transportation industries among others to conserve electric energy, increase productivity and
improveproduct quality. The development of MV drives was also motivated by the proved improvement
in the efficiency, weight and volume of the motor and in the reduce installation costs in cabling, cable
trays etc.
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 on separators
The basic development directions of the cement industry of INDIA are:
1. Factory modernization and reconstruction with the purpose of using the dry cement
manufacturing process in 80-85 % of all factories;
2 Development and introduction of highly effective energy saving technologies.

Dept. of electrical engineering,AGI(Jaipur) Page 2
3 Providing the building trade with an assortment of cements with excellent construction
properties;
4 Preparing the cement industry for a transition after which the processes use coal and fuel-bearing
waste of the industry as the energy source;
5 Re-tooling of equipment and facilities as well as the organization of modernization of
equipment;
6 Reduction of harmful emissions in an atmosphere and improvement of working conditions.
High power consumption, defined not only by the process of manufacturing ,but also technically
obsolete equipment. Low profitability in manufacturing 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 conservation of energy is an essential step we can all take
towards overcoming the mounting problems of the worldwide energy crisis and environmental
degradation. In particular, developing countries are interested to increase their awareness on the
inefficient power generation and energy usage in their countries. However, usually only limited
information sources on the rational use of energy are available.
The know-how on modern energy saving and conservation technologies should, therefore, be
disseminated to government and industrial managers, as well as to engineers and operators at the plant
level in developing countries. It is particularly important that they acquire practical knowledge of
the currently available energy conservation technologies and techniques.

CHAPTER 2
CEMENT PRODUCTION PROCESS.
Portland cement is a calcined material comprising lime and silicates,which are mixed with sand
and stone.The manufacture of cement involves two steps.
The first step is the production of clinker. The production of clinker begins with the extraction
and ground, 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 found at depths of up to 10 m below the surface. 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,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 1600ºC 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 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.
2.1 Process In Cement Industry
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.

CHAPTER 3
CEMENT PROCESSING EQUIPMENT
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.
3.1 Crushers
Primarily crushers should be capable of accepting shot rock with the minimum of wastage or of
preliminary size reduction .Typically, the shot rock size should be less than 120 cm in diameter.
Commonly there are primary, secondary and, occasionally, tertiary crushers in 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.
a b
Figure 3.1 - crushers
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

3.2 Conveyors
Today continuous conveyors are acknowledged world-wide as a cost-efficient alternative.
Conveyors are used not only for transportation of limestone but they are also needed for transportation
of stones, raw meals.
Fig-a fig-b
Fig 3.2 -Conveyor belts
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.
3.3.1 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.
Of the four mill types, tube mills are known to be the most energy intensive mills.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 3.3 Tube mill
3.3.2 Vertical Roller Mill
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 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/tone depending heavily on the coarseness of the raw materials and the grinding circuit
configuration
3.3.3 Horizontal Roller Mill
Horizontal Roller Mill 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.Horizontal roller mills
consume only 65-70% of the energy used in ball mills.

Figure 3.4- VERTICAL ROLLER MILL

Figure 3.5- HORIZONTAL ROLLER MILL
3.4 Separators
Separators or classifiers are an important piece of equipment used in the grinding stages. 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.
Figure 3.6 - Separator

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 “high efficiency” 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.
3.5 Kiln
The primary element of the clinker production stage is the rotary kiln. 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.
Figure 3.7 Rotating 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.
3.6 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 3.8 - Fan in cement manufacturing plant
The air fan drives are relatively large drives. Typical motor powers vary between 20kW and 2000kW.
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.In this way large energy savings were
achieved compared to the conventional method of regulating the flow rate through dampers.

CHAPTER 4
CONSUMPTION AND SAVING OF ELECTRIC ENERGY.
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 .
Electric power consumption in a cement plant
Quarrying & preblending 6 kWh/tone 5 %
Raw milling 28 kWh/tone 24 %
Blending 7 kWh/tone 6 %
Burning & cooling 25 kWh/tone 22 %
Finish milling 44 kWh/tone 38 %
Conveying,
packing & loading
6 kWh/tone 5 %
Total 116 kWh/tone
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.In a typical
cement plant, 500-700 electric motors may be utilized, varying from a few kW to MW size.
4.1 Efficency methods
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:
1. Blending (if turbulent) - convert to Controlled Flow
2. Pneumatic conveying - convert to mechanical
3. Milling - install pre-grinding
4. ID fans - eliminate air in-leakage, use high efficiency impellers
5. Cooler fans(with outlet dampers) - convert to inlet vane or variable speed

6. Plant air compressors(if central) - minimize system loss and convert to distributed system.
7. Plant lighting - basic lighting can be augmented by additional lighting as required.
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.
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 ), 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.
One presumed case study estimates the savings at 70%, compared to a system with avane (or 37%
compared with a regulated system) for the raw mill fan.VSD systems are used for standard solutions and
large drives in cement manufacturing.However, there are still some disadvantages of using variable
speed single drives, like braking possibility, harmonic distortion, and higher cabling request and space
requirements.
The solution is called “Multi-drive”. Multi-drives are characterized by:
1. Reduced cabling, due to the single power entry for multiple drives,
2. Energy-saving motor-to-motor braking,
3. Reduced space requirement,
4. Easy to build 12-pulse line supply section, thereby lower harmonics,
5. All the benefits of a single variable speed drive are retained,
6. Possibility to apply a regenerative supply unit being able to reduce current harmonics.
7. 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.

CHAPTER 5
AC-DRIVE TECHNOLOGY IN CEMENT PRODUCTION.
5.1 BASIC ELECTRIC DRIVE IN CEMENT INDUSTRY
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.
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 5.1 - Block diagram of an electric drive system
Figure 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 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.
Figure shows, the converter getting its power from a utility source with single phase 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 Figure come from a process computer, which calculates the objectives of the overall process and
issues adequate commands to control the mechanical load.
5.2 AC-drive Motion 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 speed, thereby varying the operating speed of a piece of equipment. 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.
The AC drives can be classified into two broad categories based on their applications:
1. 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 .
2. 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.
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.
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.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.
Consider a simple example of an induction motor driving a fan as shown in figure below 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 thedesired flow rate, as in Fig. below
Fan: a) constant-speed drive: b) Variable-speed drive.
Figure 5.2- Fan speed control using vane and vsd
In the system in Fig.-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 motor is remarkable since the power of a fan
Pfan is proportional to the cube of the mechanical angular frequency Ω as
Pfan ≈k Ω3
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.
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 variable frequency
converters are used. They must satisfy the following basic requirements:
i. Ability to adjust the frequency according to the desired output speed
ii. Ability to adjust the output voltage so as to maintain a constant air gap flux in the constant-
torque region
iii. 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.
The utility input is converted into DC by means of 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
DC
AC 50-Hz
(1-Ф or 3-Ф)
Input
OUTPUT
(VARIABLE VOLTAGE AND
FREQUENCY)
Figure 5.3 - Variable-frequency converter
These converters can be classified based on type of rectifier and inverter used in Figure. The
rectifier can be controllable (thyristor or transistor rectifier) and uncontrollable (diode rectifier). The
diode rectifier allows only rectification of voltage, but cannot 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:
RECTIFIER
FILTER
INVERTER MOTOR

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.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.
The most commonly used drive for these applications is PWM-VSI.
An AC-drive should often provide also electromagnetic braking. During it the power is flowing
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. In order to dissipate the
braking energy, when the capacitor voltage exceeds a preset level.
Figure 5.4 - Electromagnetic braking in PWM-VSI (regenerative braking.)
As shown in Fig. 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.

CHAPTER 6
Multi-Level Switching Topology
MULTILEVEL DRIVE
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.
Figure 6.1 - Multilevel drive.
The power section of the multilevel drive consists of the following:
1. Transformer – provides e.g. three phase shifted voltages for the 18-pulse rectifier and reduces
common mode voltages at the motor.
2. 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.

3. 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.
4. Multi-level inverter – generates the sinusoidal output voltage of up to 4160 V AC.
5. Optional du/dt filter – reduces switching transients to allow operation with long motor cables or
old/unknown motor winding insulation.
6. Motor – any high voltage (up to 4 kV) standard motor can be connected.
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 ) .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.
Figure 6.2 – Voltage waveform of 2 level inverter
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.
Figure 6.3- Voltage waveform of 3 level inverter
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.
below.
Figure 6.4- Voltage waveform of 4 level inverter
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
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.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.
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.

CHAPTER 7
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:
1. Should the control be open loop or closed loop?
2. Is it a position-, speed-, or torque-controlled system?
3. What types of power converters should be used?
4. Should it be a one-quadrant, two-quadrant, or four-quadrant drive system?
5. Is it a single-machine or a multi-machine drive?
6. What is the range of speed? Does it include zero speed , field weakening region?
7. What are the accuracy and response-time requirements?
8. Is the drive required to give robust or parameter-insensitive response?
9. 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 skin effect, adding further nonlinearity to the system.
7.1 SCALAR CONTROL METHODS:
Figure 7.1 – Block diagram of V/F control method

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.
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
= constant
method is often used in centrifugal pump and fan drives. A simple and popular open-loop U/f = constant
speed control method for an inductor motor shown in Fig. above.
The power circuit consists of a phase-controlled rectifier with a single- or three-phase AC supply. LC
filter, and inverter. The frequency WC 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.
Wound rotor IM slip power recovery control methods are the static Kramer and the static Scherbius
methods. The static Kramer and static Scherbius 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.
Figure 7.2 -Static Scherbius drive

Figure 7.3 - 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 d I 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, d
I approaches zero and the machine slows down with load torque. Then as the speed error tends to zero in
the steady state, d I 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 .
A drive controller incorporates one or more of the currently available solid state power electronic
devices, such as diodes, silicon control rectifiers (SCRs), insulate gate bipolar transistors (IGBTs), gate
turn-off thyristors (GTOs) and integrated gate controlled transistors (IGCTs). All of these devices are
designed to electronically convert the input frequency of the power supply (50/60 Hz), to a variable
frequency at the output.

7.2 DRIVE SECTIONS IN A VFD
An electrical circuit topology for a VFD is shown .The drive has three major identifiable component
sections:
The first section is called the converter section. Its main purpose is to rectify naturally (diode rectifier)
or in a controlled fashion (SCR or IGBT), the input supply voltage and frequency into a fixed DC
voltage.
The second section is called the DC link bus, which may include a reactor or a capacitor bank. The
primary function of this section is to maintain a fixed reference voltage to be switched by the inverter
section.
The third section is called the inverter section and is designed to switch in a controlled fashion the fixed
DC link bus voltage into a variable frequency output. Depending on the current ratings this section may
contain GTOs, IGBTs, or more recently IGCTs.
The type of converter switching device and DC link voltage defines the operating principle of the
inverter: voltage source or current source.The latest available drive technology employs a voltage source
inverter for switching IGBT or IGCT devices.
The switching of these devices is controlled by state of the art digital controllers, which contain the
algorithms for firing and protecting the solid state devices and the motor.
The digital controller also maintains and displays an accurate log of drive faults, which are essential in
troubleshooting drive problems.
Most of the modern digital controllers will include algorithms referred to as constant volts per Hertz,
(V/Hz) and Flux Vector control.
The Flux Vector is a sophisticated algorithm providing 100:1 speed control range with 0.01 percent
accuracy and without the requirement of a speed encoder.
Fan, kiln drive, which normally requires a 250% torque at zero speed, for up to 60 seconds, a speed
encoder is always recommended to prevent motor overheating at standstill.
More importantly, however, is the selection of the drive, which must be designed to deliver sufficient
current to the motor to satisfy the torque requirements of the specific application.

Figure 7.4 – Sections in a VSD
7.3 Operating Power Factor
The power factor of a typical modern VFD is 0.95 lagging and is maintained constant over the speed
range. This is due to the diode inverter front-end and capacitor DC link. These drives use the IGBT and
IGCT devices.
For the current source inverters and other regenerative drivers, the power factor varies with speed. To
maintain good power factor, at or above 0.95 lagging, the VFD suppliers will correct the power factor
and install additional capacitors and filter reactors in the drive enclosure. 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.Active power factor
correction may be reached only by active transistor rectifier. If 12 or 18 even 24 pulse diode rectifiers
are used the power factor of the network side approaches unity but not with a six pulse diode bridge.

7.4 Voltage and Power Ratings
The selection of the operating voltage level is a matter of economics and that of the power distribution
system.Modern variable frequency drives are available to operate at voltages up to 7200 V three phase,
50/60 Hz and can handle large loads.
In general for drives rated 750 kW and lower, 380V to 720V provide the most economical solution.
Larger drives and motors are best operated at medium voltage of 2.3kV to 7.2 kV.
Earlier generations of drives could operate only at low voltages 380V to 720 V.
Modern drives are designed to operate with power cells rated for 690 V or 3300V per cell with fewer
components and without the need of a second step up transformer.

CHAPTER 8
Comparison of traditional vane control and modern AC drives control
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. A comparison between them should prove helpful in
determining the advantages and disadvantages of each technology in a given application.
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. 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 are the
difference in the type of control used. We should attempt to 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.
Fan 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.
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.The net result in this application is: VSD technology is more preferable and allows very much cost
savings.

8.1 Summary of performance
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.
Item Control vane AC VSD
Equipment efficiency Low High
Motor efficiency High Optimal
Power savings None High
Flexibility of location IM rated Inverter installation is free
Exposure to environment Fully exposed Better
Specification - Better
Shut off capability Better
Ability to control - Better
Potential of leaks - Better
Maintenance: valve/drive - Better
Equipment - Better
Expertise Better
Spare parts - Better
Installation costs Better -
Operational costs High Low
Figure8.1- Power consumption of a VSD fan Figure 8.2- Power factor improvement

8.2 Reasons to choose variable AC DRIVES for cement industry
Feature Advantage Benefit
Soft starting and
reversing of motor
Lower starting currents and reduced mechanical
stress during starting.
Minimized wear and tear of mechanics, means
improved reliability and prolonged crusher
lifetime.
Smooth reversing in case of blocked crusher.
Savings through smaller sized
cables and supply switchgear.
Considerable maintenance savings
and increased productivity.
High uptime of crusher and
consistent production.
Accurate speed
Regulation
Optimization of crusher speed and smooth
change of direction of rotation.
Reduced operational and
maintenance costs compared to
slip-ring motor solutions.
Dynamic torque
Regulation
Reduced mechanical stress caused by high torque
peaks.
High torque during start and operation if
required.
Less maintenance and lower costs.
High uptime and increased
throughput.
High power factor Lower reactive power consumption and reduced
need for compensation equipment compared to
other control methods.
Lower installation costs and
substantial energy savings.
Regenerative braking Braking energy is fed back into the plant
electrical network.
Reduced utility bills.
Flying start Separator can be started when spinning. Time savings through immediate
starting and no need for braking.

CHAPTER 9
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.
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.
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.
1. The application of variable frequency drive technology can reduce the number of pieces of
equipment that are exposed to the dusty process.
2. Whereas control vanes typically provide shutoff capability, variable frequency drive
applications may require piping changes to prevent backflow and provide tight shut-off
capability.
3. AC variable frequency drives provide better control characteristics because of good
resolution and negligible dead time.

CHAPTER 10
REFERENCES
[1] http://cleantechindia.wordpress.com/2008/07/16/indiancement industry losses.
[2] Study report on traditional vane drive in Tesla Cement Plant,2009
[3] “Cement Production Rate In India” survey report by government of India in 2010
[4] A Text book on Industrial Drives by S.K.Dubey
.

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