1. Subject Name: POWER ELECTRONICS Subject Code: 10EC73
Prepared By: D.KAVITHA,M.PRIYADHARSINI
Department: ECE
Date: 8/12/2014
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2. INTRODUCTION TO POWER ELECTRONICS
UNIT 1
INTRODUCTION TO POWER ELECTRONICS
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3. Applications of power electronics Power Semiconductor devices
Topic details
Introduction
Applications of power electronics
Power Semiconductor devices
Control characteristics
Types of power electronics circuits
Peripheral effects
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4. Relationship of Power Electronics
INTRODUCTION
Relationship of Power Electronics
The electrical engineering field may be divided into three areas of specialization : Electronics , Power and Control
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5. Stability and response characteristics of closed loop system
Electronics
Processing of information at low power levels
Power
Generation,transmission,distribution & utilization of vast quantities of electrical power
Control
Stability and response characteristics of closed loop system
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6. Electrical energy in one form Power electronic system
Example: Input is AC and output is DC. This is conversion is performed by a rectifier ,so it is a power electronic system
Electrical energy in one form
Power electronic system
Electrical energy in the other form
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7. Applications of power electronics 1
Applications of power electronics 1. UPS and stand by power supplies for critical loads – Computers 2. Power control in resistance welding induction heating, electrolysis and process control 3. Power conversion for HVDC and HVAC transmission systems 4. Speed control of motors in traction drives, textile mills, cranes ,lifts etc 5. Solid state power compensators ,static contactors, transformer tap changers 6. High voltage supplies for electrostatic precipitators and x-ray generators 7. Power supplies for communication system,stelephone exchanges ,satellite system
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8. Power Controller or Converter Load
Power Source
Power Controller or Converter
Load
Control Unit
Sensing and feedback circuit
Reference settings and commands
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9. Power source : It can be a AC mains, generators or batteries Power Controller : Converts input power and it may be of SCR,GTO,MOSFET or IGBT Sensing & Feedback Circuit : Monitor the load condition & the feedback is given to control unit Control Unit : Drive switches and it is adjusted whenever there is a difference between actual speed and reference speed Reference Settings & Commands: Sends command to the control unit for proper functioning of power electronic system and load
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10. TYPES OF SEMICONDUCTOR DEVICES
Power devices
Diodes
Thyristors
Transistors
BJT
MOSFET
IGBT
SIT
General purpose
High speed
Schottky
SCR
GTO
RCT
SITH
GATT
LASCR
MCT
TRIAC
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11. DIODES General Purpose Diodes: High reverse recovery time of 25μs
Used in low speed applications such as rectifiers & converters
Operate up to 1khz
Ratings from 1A/50v to 1000A/500V
Fast Recovery Diodes:
RRT is less than 5μs
Used for high speed applications such as chopper & inverters
Ratings from 1A/50V to 100A/3KV
Schottky Diodes:
Pn junction is eliminated
Metal is placed directly on semiconductor
Storage time is absent
Turn-off time is very small
High Switching frequencys
Large reverse leakage currents
Breakdown voltage is less than 100v
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12. SCR SCR or thyristor, is a multi-junction device with latching switch behavior It is similar to a diode, but with one important difference. Two conditions have to be met for forward current to flow: 1. Forward-biased voltage 2. A pulse of current on the gate terminal 3. This allows the thyristor to control very large currents very easily A thyristor (or silicon-controlled rectifier) is a pnpn device with three terminals: 1. Anode (A) 2. Cathode (K) 3. Gate (G)
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13. Anode is positive with respect to cathode ,so is said to be forward bias Small positive voltage is applied between gate and cathode to turn on SCR When SCR is on, internal drop is negligible and current flows from anode to cathode Once SCR turns on , gate has no control over conduction Even if gate drive is removed , SCR does not turn off SCR turns off when current falls to zero.
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15. GTO (Gate Turn-off Thyristor) Similar to SCR But can be turned off by gate terminals Gate has full control over the conduction of thyristor Do not require any commutation circuit Used for low power applications Gate drive required to turn off is very large
VI-CHARACTERISTICS
SYMBOL
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16. SITH (Static Induction Thyristor)
It can be turned off by negative gate pulse
Gate has full control over the conduction
Do not require any commutation circuit
Characteristics is similar to GTO
Used for medium power converter circuit
SYMBOL
VI CHARACTERISTICS
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17. MCT (MOS-CONTROLLED THYRISTOR) Turn on by giving negative voltage to gate terminal Turn off by giving positive voltage to gate terminal Has low power ratings SYMBOL VI-CHARACTERISTICS
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18. LASCR (LIGHT ACTIVATED SCR) Turned on by light signal Gate is photo sensitive and SCR turn on when light falls on it Isolation between gate and drive circuit Used in HVDC in transmission
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19. RCT (Reverse Conducting SCR) It has an SCR antiparallel with the diode High current during forward direction Less current during reverse direction Suitable for inductive loads GATT (Gate Assisted Turn-off Thyristor) Suitable for high speed switching & Mediun power applications Require force to turn off the circuits TRIAC (Bidirectional Triode Thyristor) Considered as antiparallel SCR Conducts in both the direction It has a single gate Used for heat control, light control and motor control Turned off by line commutation of AC supply Not suitable for inductive loads
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20. TRIAC SYMBOL AND CHARACTERISTICS
VI-CHARACTERISTICS
SYMBOL
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21. CONTROL CHARACTERISTICS SCR
Vs is applied through load across anode-cathode
Vg is applied across gate-cathode
Positive Vg turns on SCR, hence complete Vs appears across the load
Gate has no control over the conduction of SCR
Even if we apply negative gate pulse does not turn-off SCR
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22. CONTROL CHARACTERISTICS OF GTO &SITH
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23. GTO & SITH has the similar control characteristics
Vs applied through load across anode-cathode
Whenever device turns - on , voltage applied across the load
Whenever device turns - off, voltage is zero
Positive gate turns-on GTO & SITH
Negative gate turns-off GTO & SITH
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24. CONTROL CHARACTERISTICS OF MCT
Negative voltage between gate – cathode turn –on the device
Positive voltage between gate – cathode turn –off the device
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25. CONTROL CHARACTERISTICS OF BJT
Positive voltage is applied between collector and emitter
Control voltage is applied between base and emitter
When base-emitter voltage is positive , BJT turns on and Vs appears across the load
When base-emitter voltage is negative , BJT turns off and Vo is zero
Circuit diagram of BJT
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26. CONTROL CHARACTERISTICS OF MOSFET
Positive gate-source voltage turn-on the device,Vs appears across the load
Negative gate-source voltage turn-off the device, load voltage is zero
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27. CONTROL CHARACTERISTICS OF IGBT
Positive gate-emitter voltage turn-on the device,Vs appears across the load
Negative gate-emitter voltage turn-off the device, load voltage is zero
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28. TYPES OF POWER ELECTRONICS CONVERTERS:
It is classified into 5 categories depends upon input /output
AC to DC converters
DC to AC converters
DC to DC converters
AC to AC converters
AC Regulators
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29. AC to DC converters
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30. 1φ or 3φ AC supply available from mains
It converts AC into controlled DC voltage & current
It includes diode rectifiers as well as controlled rectifiers
Controlled rectifiers use SCR, since input is AC supply
SCR is turned off by natural commutation
External commutation are not required
AC to DC converters are also called line commutated converters
Used for DC drives , UPS & HVDC systems
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31. Converters are commonly called as inverters
DC to AC Converters
Converters are commonly called as inverters
Input to inverters is fixed DC voltage
The DC voltage is from batteries
Output of the inverter is fixed or variable frequency
AC voltage magnitude is also variable
Inverters are mainly used whenever mains are not available
Applications:
Used for speed control of induction motors
Used in standby & emergency power supplies
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32. Input waveform
Vdc t
Output waveform Vo t
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33. It takes the input from the fixed voltage DC supply
DC to DC Converters
It takes the input from the fixed voltage DC supply
Output is fixed or variable DC voltage
It is used in DC drives
Speed of motor can be controlled in forward & reverse directions
It is also used in SMPS
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34. Input waveform
Vdc t
Output waveform Vo t
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35. AC to AC Converters (Cycloconverters)
Input to the cycloconverters is 1Ф or 3ф AC
Input is fixed voltage & fixed frequency
Output has variable voltage & variable frequency
Output frequency is lower than input frequency
Cycloconverters are used mainly for AC traction drives
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36. Input is fixed voltage AC mains
AC Regulators
Input is fixed voltage AC mains
Output frequency is same as input frequency
It does not change the frequency , whereas cycloconverters
change the frequency
Used for speed control of large fans & pumps
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37. It takes input power from power source & delivers to the load.
Peripheral Effects
It takes input power from power source & delivers to the load.
Normally uses switches to convert input power in required form.
Because of switching action following things takes place:
Switching voltage/current pulses are induced in power supply
Harmonics are induced in power supply due to improper waveforms
Load contains voltage/current spikes & harmonics
Interference is radiated due to switching of drives
These effects certainly affect other loads which are connected to same power supply
Similarly output power converter is also distorted
Hence, it affects the performance of the load connected to it.
These problems can be reduced by putting filters before & after the power converters.These filters attenuate the harmonics & noise spikes.
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