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Power Electronics
Lecture 9
9 October 2003
MMME2104
Design & Selection of Mining Equipment
Electrical Component
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Lecture Outline
What are power electronics
Power electronics building blocks
Rectifiers
Converters
Inverters
Applications in motor drives
Efficiency in power electronics
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Power Electronics
The task of power electronics is to process and control theflow of electric energy by supplying voltages andcurrents in a form that is optimally suited to the load.
Power electronics allows you to:
convert from AC DC (rectifiers)
convert from DCAC (inverters)
convert from DC DC or ACAC (converters)
control the frequency of AC supplies
control the voltage and current output of AC and DCsupplies
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Power Electronics Building Blocks
Inductors Capacitors
Semiconductors
Diodes Switches
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Inductors and Capacitors
Inductor: V = L di/dt
The current in an inductor cannot change instantaneously!
Capacitor: i = C dV/dt
The voltage across a capacitor cannot change instantaneously!
Thesepassive components are fundamental to the
operation of all power electronics.
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Rectifiers (AC-DC)
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The Most Basic Diode Rectifier
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Full Bridge Diode Rectifier
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Single-Phase Full Bridge Rectifier
Converts single-phase AC to DC: VDC = 2 VAC
e.g.
240VAC
e.g.
340VDC
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Three-Phase Full Bridge Rectifier
Converts 3-phase AC to DC: VDC = 2 VAC
Example application: Brushless excitation in
synchronous machines
e.g.
415VAC
e.g.
590VDC
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Three-Phase Full Bridge Rectifier
Input
Output
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DC-DC Converters
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The Most Basic DC-DC Converter
This switching technique is known aspulse-width
modulation (PWM)
The duty ratio D = ton/Ts
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Step-Down (Buck) Converter
Converts DC to a lower DC voltage: Vo = DVd
Example application: field control in DC motors
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Step-Down (Buck) Converter
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Step-Up (Boost) Converter
Converts DC to higher voltage DC: Vo = Vd/(1-D)
There are also buck-boostDC-DC converters, whichconvert to higher and lower voltages
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Full-Bridge DC-DC Converter
Provides bi-directional DC-DC power flow
Application: 4-quadrant control of DC motors
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Inverters (DC-AC)
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Single-Phase Full-Bridge Inverter
Converts DC to single-phase AC
Same topology as DC-DC full bridge
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Single-Phase Full-Bridge Inverter
Control
signals
Output
= filtered output
Switching frequency
Carrier frequency
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Three-Phase Full-Bridge Inverter
Converts DC to 3-phase AC
Example application: AC motor drives
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Variable Speed AC Drive
E.g. VVVF control of induction motor
3-phase
rectifier
3-phase
inverter
AC
motor
415V
3-phase
supply
600V DC link
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AC-AC Conversion via Power Electronics
Instead of using a transformer:
AC-AC conversion can be performed indirectly
via a DC link (as per previous slide). AC-AC conversion can also be achieved directly
with a matrix converter topology (beyond the
scope of this course)
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Semiconductor Devices
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Losses in Power Electronics
In semiconductor components:
Switching losses
Conduction losses
In passive components (C & L):
Effective series resistance (ESR)
Power electronics are normally designed as efficient as practical for two
reasons:
Cost of wasted energy
Difficulty in heat removal
Typical efficiencies are in the range of 90-99% for each conversion
stage, depending on the exact converter topology.
f
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Reference
N. Mohan, T. Undeland & W. Robbins (1995)
Power Electronics: Converters, Applications and
Design, 2nd Edition, Wiley, New York.
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