Variable frequency Drives.pdf
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Transcript of Variable frequency Drives.pdf
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VFD
Manoj Barsaiyan
-
Presentation Outline
What is VFD
How it is Useful
Types of VFD
Components of VFD
Advantages & Disadvantages
Case studies
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What is an Electric Drive?
Motor + Control = Drive
Speed-Torque Characteristics are adjusted
according to the requirements of the load
Feedback is necessary to take corrective
action so that the output matches the exact
requirement.
-
4
STARTING OF SYNCHRONOUS MOTOR
A synchronous motor has no self-starting torque i.e., a synchronous
motor cannot start by itself.
-
Variable Frequency Drives (VFD)
VFD substitutes the
requirement of energy
efficient motor and
gear box assembly/
scoop coupling.
The VFD schemes are
based on V/F constant.
-
WHAT IS VARIABLE SPEED
DRIVE
variable voltage and variable frequency drives
Supplied from the fixed voltage and frequency mains supply source
Converts the power twice, first into DC through source converter and then into a variable voltage and frequency output.
-
Operating principle
RPM = Revolutions per minute
f = AC power frequency (hertz)
p = Number of poles (an even number)
-
Rectifier is that special type of converter that converts AC to DC
Inverter is that special type of converter that converts DC to AC.
-
RECTIFIER I/P - CONSTANT a.c
O/P- VARIABLE d.c THYRISTOR
FIRING
1 -- 2
2 -- 3
3 -- 4
4 -- 5
5 -- 6
6 -- 1
1 -- 2
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Rectifier
O/p
-
VD = 1.35 VRMS
Voltage and current
waveforms during
commutation
-
VFD controller Variable frequency drive controllers are solid state electronic
power conversion devices.
The usual design first converts AC input power to DC
intermediate power using a rectifier bridge.
The DC power is then converted to quasi-sinusoidal AC power
using an inverter switching circuit. The rectifier is usually a three-
phase diode bridge, but controlled rectifier circuits are also used.
-
VFD controller Insulated-gate bipolar transistor (IGBT) became the device used in
most VFD inverter circuits
AC motor characteristics require the applied voltage to be
proportionally adjusted whenever the frequency is changed in order to
deliver the rated torque. constant volts per hertz ratio is maintained.
In addition to this simple volts per hertz control more advanced
control methods such as vector control and direct torque control
(DTC) exist. These methods adjust the motor voltage in such a way
that the magnetic flux and mechanical torque of the motor can be
precisely controlled.
-
VFD controller The usual method used to achieve variable motor voltage is pulse-
width modulation (PWM). With PWM voltage control, the inverter
switches are used to construct a quasi-sinusoidal output waveform by
a series of narrow voltage pulses with sinusoidally varying pulse
durations.
-
PWM Sine Wave Synthesis
-
Drives & Motors Fundamentals
RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER
-
Drives & Motors Fundamentals
RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER
-
Drives & Motors Fundamentals
RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER
-
Drives & Motors Fundamentals
RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER
-
Drives & Motors Fundamentals
RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER
-
Drives & Motors Fundamentals
RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER
-
Drives & Motors Fundamentals
RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER
-
Drives & Motors Fundamentals
RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER
-
Drives & Motors Fundamentals
RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER
-
Drives & Motors Fundamentals
RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER
-
Drives & Motors Fundamentals
RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER
-
Drives & Motors Fundamentals
RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER
-
Drives & Motors Fundamentals
RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER
-
Drives & Motors Fundamentals
RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER
-
Drives & Motors Fundamentals
RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER
-
Drives & Motors Fundamentals
RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER
-
Drives & Motors Fundamentals
RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER
-
Drives & Motors Fundamentals
RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER
-
Drives & Motors Fundamentals
RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER
-
Drives & Motors Fundamentals
RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER
-
Drives & Motors Fundamentals
RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER
-
Drives & Motors Fundamentals
RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER
-
Drives & Motors Fundamentals
RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER
-
Drives & Motors Fundamentals
RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER
-
Drives & Motors Fundamentals
RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER
-
Drives & Motors Fundamentals
RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER
-
Drives & Motors Fundamentals
RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER
-
Drives & Motors Fundamentals
RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER
-
Drives & Motors Fundamentals
RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER
-
Drives & Motors Fundamentals
RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER
Area Under The Square-Wave Pulses
Approximates The Area Under A Sine Wave
Frequency
Volt
age
-
Drives & Motors Fundamentals
-
Drives & Motors Fundamentals
RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER
How Often You Switch From Positive
Pulses To Negative Pulses Determines
The Frequency Of The Waveform
Frequency
Volt
age
-
Drives & Motors Fundamentals
Frequency = 30Hz
Frequency = 60Hz
-
RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER
Motor
PULSE WIDTH MODULATION (PWM)
-
INVERTER
RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER
Motor
-
RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER
Motor
INVERTER
-
RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER
Motor
INVERTER
-
RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER
Motor
INVERTER
-
RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER
Motor
INVERTER
-
RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER
Motor
INVERTER
-
RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER
Motor
INVERTER
-
RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER
Motor
INVERTER
-
RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER
Motor
INVERTER
-
RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER
Motor
INVERTER
-
RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER
Motor
INVERTER
-
i
2
1
6
5
4
3
-
R Ph
Y Ph
B Ph
1 2 3 4 5 6 1
FIRING SEQUENCE & PHASE CURRENTS
-
In power plant we require:
FLOW CONTROL-condensate, Feed
water ,Air Flow etc.
PRESSURE CONTROL-Draft, PA Header
Pressure,Seal Air Pressure Control etc.
-
Mechanical Capacity Control
Inlet Guide Vane
Outlet Damper Control
Blade Pitch Control
Hydraulic coupling
-
VFD motor
3 phase induction motor
3 phase synchronous motor
-
Four Quadrant Operation :
-
Inverter Duty Motors Inverter duty motors are specially designed to
withstand the new challenges presented by the use of
inverters. There are a number of ways to designate
motors "inverter duty,"
Class F insulation - to withstand the higher heat
generated by non-sinusoidal current from the drive.
-
Inverter Duty Motors
A proper inverter duty motor will have special rotor bar
construction designed to withstand variations in air gap
flux densities and rotor harmonics.
Cooling of VFD motors.
-
Type of Loads Constant torque loads. These loads represent 90% of all general
industrial machines (other than pumps and fans). Examples of
these load types include general machinery, hoists, conveyors,
printing presses, positive displacement pumps, some mixers and
extruders, reciprocating compressors, as well as rotary
compressors.
Constant horsepower loads. These loads are most often found
in the machine-tool industry and center driven winder
applications. Examples of constant horsepower loads include
winders, core-driven reels, wheel grinders, large driller machines,
lathes, planers, boring machines
-
Type of Loads Variable torque loads. Variable torque loads are most often
found in variable flow applications, such as fans and pumps.
Examples of applications include fans, centrifugal blowers,
centrifugal pumps, propeller pumps, turbine pumps, agitators, and
axial compressors.
VFDs offer the greatest opportunity for energy savings when
driving these loads because horsepower varies as the cube of
speed and torque varies as square of speed for these loads.
-
Type of Loads Variable torque loads.
For example, if the motor speed is reduced 20%, motor
horsepower is reduced by a cubic relationship (.8 X .8 X .8), or
51%.
As such, utilities often offer subsidies to customers investing in
VFD technology for their applications. Many VFD manufactures
have free software programs available for customers to calculate
and document potential energy savings by using VFDs.
Flow Speed
Pressure (Speed)2
Energy (Speed)3
-
Variable speed drives for power
generation applications
Pumps
Boiler feed-water pump
Condensate extraction pump
Cooling water pump
Ash Water pumps
Fans
Primary air fan
Secondary air fan
ID fan
Other
Raw coal feeder
Coal mill
air compressor
Gas turbine starter
-
Major Application Areas
Pumps
Fans
Compression type load
-
Efficiency v/s Different Control is as follow
-
Energy Saving with VFD VFD gives the highest efficiency at part load applications. So we
can say that whenever our application requires part load application
we must go for VFD.
At 80% flow rate:
Damper control absorbs 93% motor power
IGV control absorbs 70% of motor power
Eddy current coupling control absorbs 67% of motor power
VFD control absorbs 51% of motor power
-
Energy Saving with VFD
-
Energy losses Vs
Investment Cost
-
TYPES OF VARIABLE SPEED DRIVE
VOLTAGE SOURCE INVERTER(VSI)
A capacitor is used at the diode bridge
output to maintain voltage at input of the
Inverter
CURRENT SOURCE INVERTER(CSI)
A series high voltage reactor is used between
diode bridge and the Inverter to maintain
current during variations at supply end.
-
VFD controller
Voltage Source inverter control
-
CSI CURRENT SOURCE INVERTER
ONE SYSTEM CONTROLS ONLY ONE MOTOR
SUITABLE FOR LARGE DRIVES
PRECISE CONTROL OF MOTOR IS POSSIBLE WITH
FEEDBACK SIGNAL
-
Load commutated inverter fed
synchronous motor
Current Source Inverter
-
Comparison of CSI/VSI
CSI is more reliable than VSI
Due to presence of large inductor CSI drive
has higher cost, weight, volume and slower
dynamic response.
CSI is not suitable for multimotor drives
whereas a single VSI can feed a number of
motors connected in parallel.
-
Characteristic of VFD drive
V/f ratio is kept constant below the base speed and above the
base speed only V is maintained constant.
Tmax is proportional to square of V/f ratio.
-
WHAT IS LCI ?
LCI means Load Commutated Inverter.
Commutation is the process whereby changing
voltage cause one cell to stop conducting and
another to begin. For a very short period of time
two phases shall be short-circuited.
In Other words Control can turn ON a thyristor,
but we need the changing voltage relationship to
turn it OFF Commutation takes place.
-
LCI: main features
Not requires commutation circuits
size and cost of the inverter gets reduced.
Frequency of operation can be much higher.
can operate at higher power levels.
-
Load Commutated Inverter
(LCI) Fed Drive
Used with synchronous motor operating at
leading power factor.
At leading power factor, the inverter
thyristors can be commutated by the
armature induced voltages.
Can use either rotor position sensors or
motor terminal voltage sensors.
-
RECTIFIER INVERTER
REACTOR
BRUSH
LESS
Exciter
SYNC.
Motor
AC Line
DC Link
1 3 5
4 6 2
2 6 4
1 3 5
-
THYRISTOR FIRING
1 53
2
1
4 66 2
1 2 3 4 5 6 1 2 3
120 deg.
-
1 3
4 6
5
2
I
DC
Voltage
Source
Synchronous Motor Stator winding
R Ph
Y Ph
B Ph
-
1 3
4 6
5
2
I
DC
Voltage
Source
R Ph
Y Ph
B Ph
Synchronous Motor Stator winding
-
1 3
4 6
5
2
I
DC
Voltage
Source
R Ph
Y Ph
B Ph
Synchronous Motor Stator winding
-
1 3
4 6
5
2
I
DC
Voltage
Source
R Ph
Y Ph
B Ph
Synchronous Motor Stator winding
-
1 3
4 6
5
2
I
DC
Voltage
Source
R Ph
Y Ph
B Ph
Synchronous Motor Stator winding
-
1 3
4 6
5
2
I
DC
Voltage
Source
R Ph
Y Ph
B Ph
Synchronous Motor Stator winding
-
1 3
4 6
5
2
I
DC
Voltage
Source
R Ph
Y Ph
B Ph
Synchronous Motor Stator winding
-
Mitigating Harmonics
Sometimes filters are used with VFD to reduce
harmonics.
VFDs Using Some VFDs are manufactured with
IGBT rectifiers. The unique attributes of IGBTs
allow the VFD to actively control the power input,
thereby lowering harmonics, increasing power
factor and making the VFD far more tolerant of
supply side disturbances.
An Active Front End Technology (AFE) drive
provides the best way to take advantage of VFD
benefits and minimize harmonics.
-
Multi-Pulse VFDs
There are a minimum of six rectifiers for a three
phase AC VFD. A standard six-pulse drive has six
rectifiers, a 12-pulse drive has two sets of six
rectifiers, an 18-pulse drive has three sets of six
rectifiers and so on.
If the power connected to each set of rectifiers is
phase shifted, then some of the harmonics
produced by one set of rectifiers will be opposite
in polarity from the harmonics produced by the
other set of rectifiers. The two wave forms
effectively cancel each other out.
-
Single Channel (6 Pulse)
Duel Channel (12 Pulse)
Duel Channel has inherent advantage of lower torque pulsation,as well as reduction in harmonics after converter.
VFD Driven ID Fans
-
Block diagram of 12 pulse VFD
-
LCI
DRIVE
-
Thyristor
comutation
-
2-channel VFD
-
Maintenance of VFD
VFD maintenance requirements fall into three
basic categories:
keep it clean;
keep it dry;
keep the connections tight.
-
Photo 1, Fan Injecting Dust into Drive Enclosure
-
Photo 2, Corrosion on Board Traces Caused by Moisture
-
Photo 3, Arcing Caused by Loose Input Contacts
Photo 4, Arcing Caused by Loose Output Contacts
-
ADVANTAGE OF VFD OVER OTHER SPEED/FLOW CONTORLLLERS
1. Very high efficiency at lower loads.
2. Very smooth starting of Drives. There is no inrush of huge starting current which in turn increases motor insulation life.
3. As the starting current is low, it eliminates the disturbance in Electric System.
4. No limitation on number of Start/Stop of Drives.
5. Reduced Vibration & wear and noise on mechanical system. Increased life of mechanical system.
6. Precise speed control possible.
7. Regeneration of stored energy in form of inertia is also possible.
-
Drawback of VFD System Increased number of Switchgear Equipments. This
multiplies its cost many fold.
Use of complex Electro-Electronic System makes it more complicated. There has been many starting problem in the past.
It requires Air Conditioning System which in addition to adding cost also consumes substantial Aux. Power
Load current has ample quantity of Harmonics which in addition to contributing to the loss also make torque pulsating. This is more pronounced during starting.
For supporting VFD and a.c. system, additional civil structure is also required which further add the cost.
-
EOT CRANE APPLICATION USED FOR MAIN HOIST,
CROSS TRAVEL & LONG TRAVEL
ADVANTAGES
IMPROVED RELIABILITY
PRECISE SPEED & TORQUE CONTROL
SAFELY LOWERS LOAD IN THE EVENT OF A BRAKE FAILURE (BRAKE SLIP DETECTION CAPABILITY)
ENERGY SAVING IN OPERATION AS WELL IN REGENERATIVE BREAKING.
-
Agenda
Agenda
Cooling Tower Application
Fan Motors Used to remove heat from the water
By evaporation of sprayed water
By Forced draft
Traditional approach of Forced Draft
ON/OFF of Fan motors
Dual speed Fan motors
Variable pitch axial fans
-
Agenda
Agenda
VFD Application for Cooling Tower Fan
TemperatureSensor
Cooling TowerFan Drive
CoolingTower Cells Chiller
CondenserWater Pump
Energy Savings on load profile but could be as high as 40% also with extreme climate variations
Optimum Return Water Temperature for optimum Chiller Efficiency
Reduced Water Loss
Other advantages of Drive usage like pf improvement, no starter, only three(3) motor cables etc.
-
CONCLUSION
Considerations while selection of VFD:
Proveness of the system
Requirement of Inverter Duty motor
Distance of the Equipment from the Inverter
Redundancies in Power and Control circuit hardware
Logics build in the system to avoid tripping of drive
unit in case of any one channel failure.
Harmonic generated in the system
Noise level limitations in operation