2. Contents
• Importance and purpose
• Cement processing equipments
• Functional working of an electric drive system
• Applications
• Slip power recovery
• AC drives control method
• Motor control through VFD
• Reasons of using drives and case example
3. 1. Energy Conservation and its Management has become a prime
factor for the nation, society and individual due to high cost and
non availability of energy.
Why Important?
2. Non awareness of latest technologies and methods, non-
conventional energy sources and renewable energy sources and
how to run the plant and equipment in an energy efficient manner.
4. Our Concern
1. Electrical motors when not selected properly can cause energy
losses as high as 25% to 30%.
2. The motors designed in lower frame might not be properly selected
and have lower margins when operated at abnormal operating
conditions run with very low power factor and efficiency and result
in to high losses and consumption
5. The Purpose
1. Energy savings on most pump and fan applications.
2. Better process control and regulation.
3. Speeding up or slowing down a machine or process.
4. Inherent power-factor correction
5. Emergency bypass capability
6. Protection from overload currents
7. Safe Acceleration
6. Introduction to cement industry
1. Cement production is an energy intensive process
2. The industry has been accused of wasting energy
3. Demand for technologies driving higher energy efficiency in the
cement industry.
4. The deterioration of the basic process equipment – rotating kilns,
raw and cement mills has reached a critical level of 85 – 90 %
5. The power-intensive wet process of cement manufacturing prevails
6. High power consumption, defined not only by the process of
manufacturing but also technically obsolete equipment
7. Cement Processing Equipment
• Crushers
– Powers of the drives typically vary from 500 kW to 3000 kW.
– a very high start-up torque.
– The drive is typically supported by a flywheel in the drive train.
• Separators
−2.8 – 3.8 kWh electric energy per tonne of raw material can be saved.
– Motors are often used in braking mode to guarantee fast and accurate
speed control and also during stopping of the rotating separator.
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8. • Conveyors
– a cost-efficient alternative for transportation
– not only for transportation of limestone but they are
also needed for transportation of stones, raw meals.
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9. • Mills
1. Tube Mill (or Ball Mill)
– 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.
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10. • Fans
– used practically in all stages of the producing
cement and consume 30% of all electric energy
– 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
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11. Electric power consumption in a
cement plant
Quarrying & preblending 6 kWh/tonne 5 %
Raw milling 28 kWh/tonne 24 %
Blending 7 kWh/tonne 6 %
Burning & cooling 25 kWh/tonne 22 %
Finish milling 44 kWh/tonne 38 %
Conveying,
packing & loading
6 kWh/tonne 5 %
total 116 kWh/tonne
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14. Induced Draft Fan for Cement Kiln
• The induced draft fan induces kiln air flow, which must be
continuously controlled to match the process requirements. Because
cement making is a thermal and a chemical process, both air volume
and mass flow must be controlled.
• Advantage of using VFD for induced draft fan of Cement Kiln:
• Maintenance of uniform working temperature, reduced fan noise
and saving of valuable floor space in the plant.
• The ID fan power can be several thousand hp and using a drive to
control air flow can result in considerable energy savings
• VFD optimises the fan RPM in such a manner that it precisely
matches the system operating conditions.
15. The input power decreases significantly as the speed is decreased to
reduce flow rate.
The power of a fan is proportional to the cube of the mechanical
angular frequency
Pfan ≈k Ω3
16. • To reduce the flow rate, the vane is partially
closed .
• Speed can be controlled by varying the
frequency, which controls the synchronous
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.
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18. Cement Kiln Speed and Torque Control
Requirement of kiln drive are for
1.Motor current limit protection during starting stage.
2.High starting torque. A special design requirement is the
starting torque.It is typically 250 percent of full load torque
for the first three to five seconds before dropping to around
200 percent and gradually reducing further during the next 15
to 20 seconds at the end of which period the full speed is
attained.
20. Mills
# In cement production, mills are process critical. Reliable and precise
control has a high impact on production throughput and operating
costs. Controlling them with VSD’s results in
#Energy savings: Grinding mills can consume more than 60 percent of
the plant’s total electrical energy. Controlling them with VFD’s result in
significant energy savings .
21. AC drives control methods
Scalar Control Method
• The main task of scalar control is to form a phase voltage
based on preset values of amplitude and frequency
• Scalar control methods of an AC-motor can be generally
realized with voltage-fed inverters, current-fed inverters
and slip power recovery control.
• 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.
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22. Generalised motor control through VFD
• 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
• Rectifier voltage command is generated directly from the
• frequency signal through a U/f = constant gain constant
24. In pumps and fans using constant speed motor with conventional control, variation in flow is
achieved by means of throttling valve or damper. The Power consumed=P Q H/ (m xp)
Where Q = Delivery of pump in m3/sec
H = pressure head (m)
m = motor efficiency
p = pump efficiency
Variable Speed Drives
Q-H characteristics of pump
0
100
200
300
400
500
600
0 1 2 3 4 5 6
Discharge(Q) in m3/sec
Pressure(H)inmmofwater
H
Hr1
Hr2
25. Q-H Curve with variable speed motor
0
100
200
300
400
500
600
0 1 2 3 4 5 6
Discharge(Q) in m3/sec
Pressure(H)inmmofwater
H1
H2
Hr
26. Reasons for variable AC DRIVES in
cement industry
1. Soft starting and reversing of motor
2. Accurate speed Regulation
3. Dynamic torque Regulation
4. High power factor
5. Regenerative braking
6. Flying start
7. Considerable maintenance savings and
increased productivity.
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