It contains overview of thermal power plant with single line diagram. Next motors used in power plants and starters of various types. After, All the necessary information about Variable Frequency Drive with Diagrams which will assist in understanding easily and more convenient way.

1. 1
OVERVIEW OF THERMAL
POWER PLANT
&
VARIABLE FREQUENCY DRIVE
Sharad Akbari ( B.E. Electrical )

2. 2
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220 KV Switchyard
220 KV IPBD or Cable
Generator X’mer
220 / 10.5 KV
Unit X’mer
10.5 / 6.9 KV
Unit Service X’mer
6.6 / .433 KV
Station X’mer
220 / 6.6 KV
Station Service X’mer
6.6 / .433 KV
6.6 KV6.6 KV
• BFP Motor
• ID Fan
• FD Fan
• PA Fan
• Pulverizer
Electrostatic
Preceptor
• HFO Pump
• Cooling Water Pump
• HVAC
• Cooling Tower
^
^
^
^
^
^
^
^^
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^
^
STG
10.5 KV, 50 Hz
3000 rpm
BoilerTurbine

3. Motors Used in Power Plant
3
• Boiler Feed Pump
• P.A Fan
• I.D Fan
• F.D Fan
• Coal Processing
• Pulverizer
• Various Pumps

4. Starters
4
1.DOL Starter
2.Star-Delta Starter
3.Auto Transformer Starter
4.Soft Starter
5.Forward & Reverse Starter

5. 5
 Variable Frequency Drive
 A Variable Frequency Drive is a type of motor controller that drives an electric motor by varying the
frequency and voltage supplied to the electric motor. Other names for a VFD are variable speed
drive, adjustable speed drive, adjustable frequency drive, AC drive, microdrive, and inverter.
Motor Speed N=120*f / P
Example
At 50 Hz N=120*50/2 = 3000 rpm
60 Hz N=120*60/2 = 3600 rpm
What is VFD?

6. 6
Block Diagram of VFD
AC to DC
Converter
DC to AC
Inverter
DC Bus Motor
AC Input
Detail Diagram of VFD

7. 1. AC to DC Converter
7
• Comprised of six diodes
• Allow current to flow in only one direction
• Whenever A-phase voltage is more positive than B
or C phase voltages, then that diode will open and
allow current to flow.
• When B-phase becomes more positive than A-
phase, then the B-phase diode will open and the A-
phase diode will close.
• The same is true for the 3 diodes on the negative
side of the bus.
• We get six current “pulses” as each diode opens
and closes. This is called a “six-pulse VFD”

8. 2. DC BUS
8
• We can get rid of the AC ripple on the DC
bus by adding a capacitor
• Capacitor absorbs the ac ripple and delivers
a smooth dc voltage.

9. 3. DC to AC Inverter
9
Switching Sequence for converting DC to AC
When we close one of the top switches in the inverter, that phase of the motor is connected to the
positive dc bus and the voltage on that phase becomes positive.
1. 2.

10. 10
Switching Sequence
When we close one of the bottom switches in the converter, that phase is connected to the negative dc bus and
becomes negative. Thus, we can make any phase on the motor become positive or negative at will and can thus
generate any frequency that we want. So, we can make any phase be positive, negative, or zero.
3. 4. 5.

11. 11
Pulse Width Modulation (PWM)
• Waveforms obtained earlier are nowhere sinusoidal and therefore are not acceptable. They contains harmonics.
• The Solution is to use Pulse Width Modulation Technique to eliminates Harmonics.
• Pulse Width Modulation (PWM) VFDs provide a more sinusoidal current output to control frequency and voltage
supplied to an AC motor. PWM VFDs are more efficient and typically provide higher levels of performance.

12. 12
• It is desirable to provide the maximum torque at any operating frequency, a VFD is designed so that the magnetizing flux density is the
same at every operating frequency.
• The flux density is determined by the equation
V = 4.44*f*N*A*B
V is the RMS voltage, f is the frequency, N is the number of turns, A is the core area and B is the peak flux density.
• From that equation it can be seen that if B is constant, V/f must be constant.
• Torque reduces at the square of the voltage reduction when the frequency remains the same.
• In a VFD, you are changing not only the voltage but the frequency at the same time; maintaining that same V/Hz ratio, so torque remains
the same throughout the speed range. That is the fundamental criteria for using a VFD.
• As the speed reduces and torque remains the same, the total shaft power is of course reduced, being that the power is a function of
torque and speed. So at 20% speed (assuming you have 50Hz primary power), the motor power will be 20% of what it would be at full
speed.
V/f Control

13. 13
Controlling of VFD
1. Scalar Mode
• A standard VFD (lets call it a Scalar Drive) puts out a PWM pattern designed to maintain a constant V/Hz
pattern to the motor under ideal conditions. How the motor reacts to that PWM pattern is very dependent
upon the load conditions. The Scalar drive knows nothing about that is happening in Motor, it only tells the
motor what to do.
• A 400V scalar drive is told to run a 400V, 50 Hz motor at 50% speed Following V/F pattern
Drive Motor
Speed Setting = 50% Motor Speed = 50%
No Load
Drive Motor
Speed Setting = 50% Motor Speed = 40%
Increased Load

14. 14
2. Vector Control (Field Oriented Control)
• More Accurate than V/f Control.
Two types
• Field Oriented Control with sensor
• Sensor less Field Oriented Control
• No feedback through speed sensor
• Feedback is derived through motor terminals
• Drive need to go through “Auto tuning”
• Feedback through encoder
• Better speed regulations up to 0.01%
• Faster response to load variations

15. 15
Acceleration and Deceleration Time
• The acceleration time is the time required of output
frequency from 0 Hertz to the maximum frequency
• The deceleration time is from maximum frequency
decrease to 0 Hz.
• Acceleration time setting requirement
Limit the acceleration current under the over-current
capacity of the variable frequency ac drive, to avoid VFD
drive tripped in speed loss of over current.
• Deceleration time set point
Avoid excessive voltage of the smoothing circuit, prevent
ac drive tripped from regeneration over voltage.

16. 16
Stop Methods
Frequency
Run Command Command removed
DC Break
DC Brake
Frequency
Command removed
Ramp to Stop Coast to Stop
1. Ramp to stop 1. Coast to stop
Stop According to deceleration time Free Running to stop

17. 17
Benefits
1. Efficient Energy Saving
• In the United States, an estimated 60-65% of electrical energy is used to supply motors, 75% of which are variable-
torque fan, pump, and compressor loads. Eighteen percent of the energy used in the 40 million motors in the U.S.
could be saved by efficient energy improvement technologies such as VFDs.
Horsepower α (Speed)³

18. 18
2. Low Inrush Motor Starting Current
• AC induction motors to draw 6 to 8 times their full load amps when they are started across the line.
• Starting Current is never exceeding Full Load Current of the Motor in VFD

19. 19
3. High Power Factor
• KVA = Volt * Amp * 1.732
• KW = Volt * Amp * P.F * 1.732
• Decreasing Reactive current will increase Power Factor
• The VFDs include capacitors in the DC Bus that perform the same function and maintain high power
factor on the line side of the VFD.
• Eliminates the need to add power factor correction equipment to the motor or use expensive capacitor
banks.
Power Factor = KW / KVA

20. 20
4. Reduced Mechanical and Thermal stress at starting
5. Easy Installation
6. Tighter Process Control with Variable Speed Drives
7.Extended Equipment Life and Reduced Maintenance
Other Benefits

21. 21
Applications
• Machinery
• Hoists
• Conveyors
• Printing presses
• Positive displacement pumps
• Mixers and extruders
• Compressors
• Pumps
• Fans
Constant Torque
Variable Torque

22. 22
Thank You