Download 1 – Introduction

1-Introduction
What is power electronics?
-a branch of electronic and electrical engineering that deals with the conversion and control of
electric energy
-involves the study of electronic circuits intended to control the flow of electrical energy, these
circuits handle power flow at levels much higher than the individual device ratings.
-the word ‘power’ in power electronics implies high amplitudes of current and voltages
Why is power electronics needed?
-because AC power from power companies are of a fixed AC voltage, current, and fixed
frequency, this kind of power cannot be always used directly
Ex. Computers need stable and uninterrupted supply of power
Ex. Emergency power supplies for medical equipment and other critical loads
What are the basic functions of a power electronic system (PES)?
-the 2 basic functions are the control and conversion of electric energy
Electric energy
in one form
Power electronic
system
Electric energy in
another form
control and conversion
1. Conversion – converting one form of electrical energy to another form
-AC to DC (Rectification / Conversion)
(Rectifiers / Converters)
-DC to AC (Inversion)
(Inverters)
1- Introduction
Power Electronics Track 1 Lecture Notes
A. R. Sombilla
Page 1
-AC to a different form of AC
-DC to a different form of DC
2. Control – controlling the amount of electric energy to be given to the load or output.
Some applications of Power Electronics
1. UPS (Uninterruptible Power Supply)
-standby power supplies for critical loads such as computers and medical equipment
2. Power control in industrial situations
-resistance welding (a group of welding processes such as spot and seam welding that
produce coalescence of faying surfaces where heat to form the weld is generated by the
electrical resistance of the combined with the time and force used to hold the materials
together during welding)
-induction (a process of heating an electrically conducting object (metal) by
electromagnetic induction where eddy currents are generated within the metal and
resistance leads to joule heating of the metal.
-electrolysis (a method of using DC to drive an otherwise non-spontaneous chemical
reaction; is commercially highly important as a stage in the separation of elements from
naturally occurring sources such as ores using an electrolytic cell.
-and many others…
3. Power conversion for HVDC (High Voltage Direct Current) and HVAC (High Voltage
Alternating Current) transmission systems
4. Speed control of motors
Used in:
-traction drives
-textile mills
-rolling mills
-cranes
-lifts
-compressors
-pumps
-etc…
5. Solid-state power compensators
-static contactors
-transformer tap changers
6. HV (High Voltage) supplies for electrostatic precipitators and X-ray generators
7. Power supply for communication systems, telephone exchanges, satellite systems etc…
1- Introduction
Power Electronics Track 1 Lecture Notes
A. R. Sombilla
Page 2
Advantages of Power Electronic Systems
1. fast and dynamic response due to non-moving devices
2. highly efficient conversion due to low losses in electronic devices
3. compact size and light-weight due to solid state technology
4. increased operating life and less maintenance due to non-moving parts
5. highly flexible operation because of the use of digital and microprocessor-based control
6. EMI and acoustic noise are lessened because of solid-state technology
Disadvantages of Power Electronic Systems
1. generate harmonics which affect performance of other loads
2. power factor is very low
3. costly for simple conversion requirements
But over-all advantages outweigh disadvantages
Block Diagram of a Power Electronic System
PP
Power Source
Reference settings
and commands
Power Controller or
Converter
Control Unit
Load
Sensing and
Feedback
Circuits
Power Source
-can be an AC source or generator or it can also be batteries
Power Controller
-converts the input power to a form suitable for the load
1- Introduction
Power Electronics Track 1 Lecture Notes
A. R. Sombilla
Page 3
Sensing and Feedback Circuits
-monitor the load conditions
Control Unit
-consist of drive circuits
-the drives of the switches are adjusted according to feedback and reference settings
Types of Power Electronic Converters
Power Electronic Converters can be classified according to:
a. their input and output
b. the job they perform
1. AC to DC (Converters)
Single-phase
or three phase
supply
AC to DC
converter
Variable or fixed DC output
-examples: diode rectifiers and controlled (SCR) rectifiers
-also called as LINE COMMUTATED CONVERTERS
-used for DC drives, UPS, and HVDC systems
2. DC to AC (Inverters)
DC to AC
inverter
Fixed voltage DC
variable voltage, variable frequency AC
-used whenever AC mains supply is not available
-also used in speed control of induction motors and standby and emergency power
supplies
3. DC to DC (Choppers)
Fixed voltage DC supply
v
DC to DC
chopper
Fixed or variable DC voltage
-used in DC drives, Switched Mode Power Supplies (SMPS)
1- Introduction
Power Electronics Track 1 Lecture Notes
A. R. Sombilla
Page 4
4. AC to AC (Cycloconverters)
Fixed voltage,
fixed frequency AC
v
(single-phase or three-phase)
AC to AC
Cycloconverter
Variable frequency,
Variable voltage AC
-output frequency is lower than the input frequency
-used in AC traction drives
5. AC to AC (AC regulators)
Fixed voltage AC
v
AC regulator
Variable voltage AC
-output frequency is the same as the input frequency
-used for speed control of large fans and pumps
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1- Introduction
Power Electronics Track 1 Lecture Notes
A. R. Sombilla
Page 5