12/04/2013
Front & Backend end of Power
Electronic Systems
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Single phase
Introduction of phases
Utility power conversion
AC-DC
DC-AC
• A single phase consists of a generator with
two wires
• Circuits operating on same frequency but at
different phase angles are termed polyphase systems
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Two Phase System
• Two circuits (four wire) is used with voltage phases differing by
90 deg.
• First largest generators in the world @ Niagara Falls were 2
phase
• Advantage? – Simple self starting motoring – two phase mean
it provided torque from zero motor speed
• Easy to analyse as phases are completely separate
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Two Phase System
• Started out to power an
induction motor – two
coupled generators
• No interaction between V &
I from one phase to other –
Hence easy to analyse
• Three phase were hard to
understand
• Still used in stepper motors
– AMD hard drives
2 phase to three-phase transformers installed at
Niagara Falls in 1895 (photo courtesy Hall of
Electrical History at the Schenectady Museum,
Schenectady, New York).
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Why Obsolete?
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3-Phase Vs Single Phase
• The HP rating of 3 ph motors and KVA rating of 3ph
Transformers is about 150% greater than for single
phase for same frame size
• Power deliver pulsates for both– single phase falls to
zero three times – 3 phase never falls to zero
• Power delivery to the load is same at any instant in 3 ph
• Constant Torque – less vibrations
• Needs only 75% of size of conductors for balanced 3
phase for same KVA rating
Invention of ‘symmetrical components’ in
1918
Wires: 2 phase use 2 pairs. If 3 wires used,
the common conductor carries the vector
sum of the phase currents – large
conductor
– 3 phase use 3 wires for long transmission
– Saving of one wire
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Power transfer on 3 phase with balanced
loads is constant
– Twice the line frequency in 1 phase
– 4 times the line frequency in 2 phase
• Increased mechanical noise in motor drive
shafts and generator
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• If three separate coils are
placed 120 degrees apart,
three voltages 120 out of
phase with each other will
be produced when the
magnetic field cuts through
the coils.
• There are two basic three
phase connections: Y and 
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Power Conversion
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AC-DC Rectification
A conversion may be required to
change the form of electricity (lowhigh V, AC to DC etc)
The conversion involves
semiconductor switches, transfer
electrical energy for the required
voltage and flow of power
Filters are then needed to smooth
the pulse of energy into a
continuous flow
• Four possible directions of
conversion
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• Power diodes are traditional
employees
• Thyristor (SCR)phase
controlled rectifier is used for
obtaining variable DC
voltage from a constant AC
voltage and frequency
source
• SCR is a 4 layer pnpn
device that can be analysed
as a couple of back to back
transistors having an
additional control terminal Gate
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DC to AC
Types of Rectifier Front-Ends
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DC-AC inverters are used to convert:
– DC Battery voltage to fixed or variable frequency AC voltage
– AC input voltage rectified to DC, then inverted back to single or 3 phase
fixed or variable frequency output voltage
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Applications:
– HVDC / solar/wind/batteries power generation
– Speed control of induction and synchronous motors
– UPS
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(a )



( b)
(c)
Diode-bridge rectifiers: power flows only in one direction
Switch-mode converters: power flow can reverse
Thyristor converter: power flow can be made bi-directional
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AC-DC power supply
Half wave rectifier
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Full wave or Bridge Rectifier
3 phase bridge
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Which is better?
Filters
• Half wave – most ripple
• Full wave –
• 3 phase – least ripple
For a source with same voltage and
current output and same ripple, the 3
phase will:
• Half & full wave convert non average
sinusoidal waveform to one with average
value
• But we want constant voltage
• The rectified waveform have large amount
of ripple superimposed on top of average
value
– Small diode currents and
– Smaller filter capacity
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Capacitors
Regulator
• Add a filter capacitor
to smooth out the
ripple
• DC-DC Conversion
• Types:
– Linear – simple, less efficient,
lower output V only
– Switching – more flexible, any
output, opposite polarity
• For DC-DC regulator is used
to transform the voltage level
• For AC-DC, to transform the
voltage and smooth out the
ripple
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Single phase uncontrolled Full Wave
Rectifier
Single phase HW Rectifier
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Pulse Number
DC to AC Inverter
• An important rectifier characteristic is
Pulse Number – repetition rate in the
direct output voltage during one complete
cycle of input AC supply
• A useful way to judge the quality of the
required dc output is by contribution of its
superimposed ac harmonics. The
harmonic or ripple factor RF is defined by
• Converts DC to AC – switching DC
input voltage (or I) in a predetermined
sequence (eg. Sine wave) to generate
AC output.
• Types:
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– Voltage Source (VSI)
– Current Source (CSI)
• Common analysis using inductive loads
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FF is Form Factor
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Power Flows
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4 Quadrant Operation
• Negative power flow indicates that
the power is fed back from load to
source.
• Hence, inverter must have “4
quadrant” capability to cater for all
possible load types.
• Practically, this can be achieved
by placing an anti-parallel diode
across each switching device.
Assume Load is drawing lagging power factor:
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Operation? – Square wave inverters
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Sequence 1
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Sequence 2
Summary
• Three phase system
• AC – DC utility conversion
• DC- AC inductive load inversion
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