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Orissa Power Generation Corporation is a joint
venture of Govt.of Orissa & AES Corp. USA
It is having 2 x 210 MW Thermal Power Plant,
namely Ib Thermal Power Plant.
It is situated at Banharpalli in the district of
Jharsu uda,Orissa
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VARIABLE FREQUENCY DRIVE
ANENERGY SAVING
SOLUTION
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Motor driven equipment accounts for 65
percent of the electricity consumed in
industry. Industrial systems can account for a
large part of the energy used in manufacturing
processes and there is definitely an
opportunity for increased efficiency and
significant savings through system
improvements. Based on analysis of the motor
systems it is estimated that industrial motor
energy use could be reduced by up to 18
ercent.
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Saving energy by using adjustablespeed drives
An adjustable speed drive often uses lessenergy than an alternative fixed speed mode ofoperation. Fans and pumps are the most
common energy saving applications. When afan is driven by a fixed speed motor, theairflow may sometimes be higher/lower than it
needs to be. Airflow can be regulated by usinga damper which restricts the flow, but it ismore efficient to regulate the airflow byregulating the speed of the motor.
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Saving energy with adjustable speed pumpoperation is similar to saving energy with
adjustable speed fan operation. It is moreefficient to regulate the flow of fluid byregulating the speed of the motor rather thanby restricting the flow using a control valve.
Process control and energy conservation arethe two primary reasons for using anadjustable speed drive. Historically, adjustablespeed drives were developed for processcontrol, but energy conservation has emerged
as an equally important objective.
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Now a days solid state AC & DC Drives
maintain a good efficiency even at reducedspeed. The development in electronics &
simplicity of squirrel cage Motors are likely
to ensure that the invertor& squirrel cageMotors combination is mostly preferable
ones.
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The following are process control benefits thatmight be provided by an adjustable speed
drive: Smoother operation
Acceleration control
Different operating speed for each processrecipe
Compensate for changing process variables
Allow slow operation for setup purposes Adjust the rate of production
Allow accurate positioning
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VARIABLE SPEED DRIVE
Many electric motor-driven devices operate at
full speed even when the loads they are servingare less than their capacity. To match the output
of the device to the load, some sort of part load
control is in use for the majority of their life.Examples include pumps, fans, conveyors,
injection molding machines, air compressors
.Many part load control strategies wasteenergy. The most efficient method of part load
control, resulting in minimal wasted energy, is
the variable speed drive (VSD).
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What is a variable speed drive?
The basic function of a variable speed drive(VSD) is to control the flow of energy from the
mains to the process. Energy is supplied to the
process through the motor shaft.Two physical quantities describe the state of the
shaft: torque and speed. To control the flow of
energy we must therefore,ultimately, control thesequantities.
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Initially, DC motors were used as VSDs
because they could easily achieve the required
speed and torque without the need for
sophisticated electronics.
However, the evolution ofAC variable speeddrive technology has been driven partly by the
desire to emulate the excellent performance of
the DC motor, such as fast torque response andspeed accuracy, while using rugged,
inexpensive and maintenance free AC motors.
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The four milestones of variable speed drives:
DC MotorDrives
ACDrives, frequency control, PWMACDrives, flux vector control, PWM
ACDrives, Direct Torque Control
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DC Motor Drives
Field orientation via mechanical commutator
Controlling variables are Armature Current and
Field Current, measured directly from the motor
Torque control is direct
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In a DC motor, the magnetic field is created by the
current through the field winding in the stator. Thisfield is always at right angles to the field created by
the armature winding. This condition, known as
field orientation, is needed to generate maximumtorque. The commutator-brush assembly ensures
this condition is maintained regardless of the rotor
position.Once field orientation is achieved, the DC
motors torque is easily controlled by varying thearmature current and by keeping the magnetising
current constant.
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ADVANTAGE
Accurate and fast torque control
High dynamic speed response
Simple to controlDRAWBACK
Reduced motor reliability
Regular maintenance
Motor costly to purchase
Needs encoder for feedback
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AC Drive with frequency control
Controlling variables are Voltage and Frequency
Simulation of variable AC sine wave using
modulator Flux provided with constant V/f ratio
Open-loop drive
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Both voltage and frequency reference are fed into a
modulator which simulates an AC sine wave and
feeds this to the motors stator windings. This
technique is called Pulse Width Modulation (PWM)
and utilises the fact that there is a diode rectifier
towards the mains and the intermediate DC voltageis kept constant. The inverter controls the motor in
the form of a PWM pulse train dictating both the
voltage and frequency.Significantly, this method does not use a feedback
device which takes speed or position measurements
from the motors shaft and feeds these back into the
control loop.
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Advantages
Low cost
No feedback device required simpleDrawbacks
Field orientation not used
Motor status ignored Torque is not controlled
Delaying modulator used
Torque cannot be controlled with any degree ofaccuracy. Furthermore, the technique uses amodulator which basically slows downcommunication between the incoming voltage and
frequency signals and the need for the motor to
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AC Drives -flux vector control using PWM
Features
Field-oriented control - simulates DC drive
Motor electrical characteristics are simulated
- Motor Model
Closed-loop drive
Torque controlled indirectly
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With flux vectorPWM drives, field orientation is
achieved by electronic means rather than the
mechanical commutator/ brush assembly of the DCmotor. Firstly, information about the rotor status is
obtained by feeding back rotor speed and angular
position relative to the stator field by means of a
pulse encoder. A drive that uses speed encoders isreferred to as a closed-loop drive. The electronic
controller of a flux-vector drive creates electrical
quantities such as voltage, current and frequency,which are the controlling variables, and feeds these
through a modulator to the AC induction motor.
Torque, therefore, is controlled indirectly.
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Advantages
Good torque response
Accurate speed control
Full torque at zero speed
Drawbacks
Feedback is needed
Costly Modulator needed
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AC Drives -Direct Torque Control
In DTC technology field orientation is achieved
without feedback using advanced motor theory tocalculate the motor torque directly and without using
modulation. The controlling variables are motor
magnetising flux and motor torque.
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Advantage
Torque & Flux are being directly controlled no need
for a modulator.Improve speed accuracy & faster Torque control.
.
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Variable frequency drives are making their
way into systems and equipment for energy
savings and as a means of part load control. Inthe future, expect more centrifugal chillers to
be installed with variable frequency drives as
opposed to inlet vane control, as well as anincreased use of variable frequency drives for
screw air compressors.
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Asprices continue to fall, variable frequency
drives will be used more frequently for soft
start capabilities and even for balancing fluid
systems as opposed to using balancing
(throttling) valves.
The convenience of adding a drive to precisely
and easily control equipment speed rather than
complex, expensive, and maintenance intensive
mechanical drives, or using guesswork to
control processes, will continue to increase
demand for this technology.
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Fluid flow control
The majority of variable frequency drive applications
are for centrifugal pumps and fans.The savings potential for these devices is the largest
since the theoretical input power varies with the cube
of fan/pump speed and volume.
For example, a fan operating at half speed will requireonly about 13 percent of full speed power. Losses in
the variable frequency drive will reduce savings
somewhat, but the savings are still very impressive.Air and water flow control is accomplished by either
of several methods, including recirculating a portion
of the flow, throttling, variable inlet vanes, and
variable frequency drives.
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Recirculating part of the flow results in the fan or
pump operating at full volume all the time. Only a
portion of the flow is used for the system or processand the rest is recirculated back to the inlet of the fan
or pump. This is the least efficient means of
controlling flow.
Throttling essentially chokes the outlet of the pump orfan to decrease flow much like holding your thumb
over the end of a garden hose. The pressure increases
and the flow decreases. This results in some energysavings over a constant volume recirculating system
but is still wasteful. So VFD provides most efficient
method to save energy.
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WORKING PRINCIPLE OFVFD
In a Motor the stator and rotor contain pole pairs
wound with copper wire. When a current isapplied, a magnetic field is generated and the
north/south field rotates through the stationary
stator as the rotor spins to catch up to therotating field. The spinning of the rotor provides
the torque necessary to drive a load.
An electric motor turns at a given speeddepending on the number of poles in the motor
and the frequency of the alternating current
applied.
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Motor speed can be changed by changing
the alternating current frequency.
Nearly all variable frequency drivesmanufactured today are referred to as
pulse width modulation drives. These
drives contain electronic circuitry that
converts the 50 Hertz line power to direct
current, then pulses the output voltage forvarying lengths of time to mimic an
alternating current at the frequency
desired.
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Variable frequency drives can be installed
with manual or automatic bypasses. In the
early days of the variable frequency drives,bypasses were more common since variable
frequency drives were not as reliable as they
are at the present time. The bypasses wereinstalled in the event of a drive failure to
ensure the system or process would remain on
line.
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Bypasses are still available but not always
installed. The criticality of the application must
be considered in each case to determinewhether the added cost and security of a bypass
is warranted.Harmonic filtering may be
necessary in some applications. Variablefrequency drives can produce harmonics that
can make their way back to the rest of the
building and interfere with sensitive electronicequipment and machines. Line reactors can be
used on smaller drives of 20 hp and less to
dampen and mitigate harmonics.
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For larger applications, an isolation
transformer may be warranted. The isolation
transformer can be installed either on the drive
or on the piece of equipment to be protected
from the harmonics.Because the equipment being protected may
be much lower in power, it may be most
economical to isolate the piece of equipment
from the building supply.
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Energy
Saving
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In Ib Thermal PowerStation Ball Tube Mill isused for pulverization of the Coal .Primary air
is used inside the Mill for the transportation of
the Coal to the Furnace.So the Mill ispressurised to primary air pressure of
600mmwc.As the inside of the Mill is
presssuried so there is chance of leakage ofCoal dust from Mil trunnions.Seal Air Fan is
used for the sealing of the trunnions so that the
coal dust will not escape outside.
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The Seal Air header is always maintained at
pressure 1200mmWC.But Seal Air Fan isdesigned to develop a pressure of
1660mmWC & flow of 13344Mcube
/hr.The Fan is inlet damper control with
damper is on suction side of the Fan.The
Fan sucks clean air from atomsphere
through auto viscous filter.The Fan is
driven by 120KW,2990RPM,415V ,205
amp induction motor.
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Single stage,Centrifugal typeOrientation: Horizontal suction Top Horizontal
delivery.
Medium: Air.Capacity: 3.9 m3 /sec
SP wet of medium: 1.0535 kg / m3
Total Head leveloped: 1662 mmwc
Speeed: 2880 RPM
Drive Motor
Rating: 120 kw
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The required flow & pressure was achieved
by throttling ofSuction damper with a 30 %
opening. Since the Motor was running at full
speed so there is a loss of energy & also
during starting the motor was taking 5 to 6
times full load current stressing the insulation
consequently failure of motor.
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A considerable energy saving is achieved by
running the motor at lower speed with 100%
damper opening. Conventional dampercontrol & inverter control with 100% damper
opening, at different air flow quantities is
shown in the graph.At 100% air flow quantity the amount of
power consumption is same for both damper
control & frequency control. The amount ofpower consumption is quite considerable
when the air flow quantity requirement is less.
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ENERGY CALCULATION BEFORE
COMMISSINING OFVFD
Average Energy advance for 1 day = 2489 KWH
Average KW during operation = 104 KW
Average Power Factor during operation = .945
Average Current during operation = 144 Amps.
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ENERGY CALCULATION AFTER
COMMISSINING OFVFD
Average Energy advance for 1 day = 1806 KWH
Average KW during operation = 74 KW
Average Power Factor during operation = .990
Average Current during operation = 97.2 Amps.
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CALCULATION OF SAVINGS
Avg. Energy Adv. before comm. ofVFD for 1 day = 2489 KWHAvg. Energy Adv. after comm. ofVFD for 1 day = 1806 KWH
Energy saving for 1 day = 2489 1806 = 683 KWH
% Energy Saved = 27.5 %
1. Energy saving of 27.5% achieved after commissioning ofVFD in1 Sear Air Fan.
2. Energy saved for 1 day = 683 KWH.
3. Payback period for 1 drive is 1.4 years.
4. Improvement of power factor from .945 to .990 is achieved due tocommissioning ofVFD.
5. Reduction of average KW from 104 to 74 is achieved due to
commissioning ofVFD.
6. Reduction of Motor current from 144 Amps to 97.2 Amps
is achieved due to commissioning of VFD.
C i
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Conclusion:
1.Variable speed drive saves energy by reducing
throttle loss & improving fan/pump efficiencyat low flow.
2.Variable speed drive provide elegant options
to vary flow so that advantage can be taken ofproduction variation ambient temp. variation.
3.Variable speed drive can be provided by belt,
pulley, gear hydraulic & compressor to saveenergy.
4.Increased life of rotating components due to
lower operating speed.
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5. Reduced noise & vibration level.6. Smooth starting without any jerk& without
any transient overloading of supply.
7. System harmonies with use of electronicsvariable speed drives can be mitigated by the
use of filter.
8. Improve system power factor.
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PLEASE.PLEASE.
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THANKS