A Generalized Scalar Pwm Approach for Three-phase Voltage-source
Inverter Fed Ac Drive
V.THAMIZHARASAN1, M.KARTHIKKUMAR2
1,2 ECE, Erode Sengunthar Engineering College, Perundurai, INDIA
Abstract: The generalized scalar pulse width modulation (PWM) approach, which unites the conventional PWM methods and most
recently developed reduced common mode voltage PWM methods under one umbrella, is established. Through a detailed example,
the procedure to generate the pulse patterns of these PWM methods via the generalized scalar PWM approach is illustrated. With
this approach, it becomes an easy task to program the pulse patterns of various high performance PWM methods and benefit from
their performance in modern three-phase, three wire voltage-source inverters for applications such as motor drives, PWM
rectifiers, and active filters. The theory is verified by laboratory experiments. Easy and successful implementation of various high-
performance PWM methods is illustrated for a motor drive.
Keywords: Carrier, common-mode voltage, discontinuous PWM (DPWM), generalized scalar PWM, inverter, modulation, near
state PWM (NSPWM), pulse width modulation (PWM), PWM implementation, scalar, space vector, space vector PWM
(SVPWM), zero sequence.
Received: November 9, 2022. Revised: May 13, 2023. Accepted: June 19, 2023. Published: July 18, 2023.
1. Introduction
THREE-PHASE, three-wire voltage-source inverters (VSI)
are widely utilized in ac motor drive and utility interface
applications requiring high performance and high efficiency.
In Fig. 1, the standard three-phase two-level VSI circuit
diagram is illustrated. The classical VSI generates ac output
voltage from dc input voltage with required magnitude and
frequency by programming high-frequency rectangular
voltage pulses. The carrier-based pulse width modulation
(PWM) technique is the preferred approach in most
applications due to the low-harmonic distortion waveform
characteristics with well-defined harmonic spectrum, fixed
switching frequency, and implementation Simplicity. Carrier-
based PWM methods employ the “per-carrier cycle volt-
second balance” principle to program a desirable inverter
output voltage waveform .In every PWM cycle, the reference
voltage, which is fixed over the PWM cycle, corresponds to a
fixed volt-second value. According to the volt-second balance
principle, the inverter output voltages made of rectangular
voltage pulses must result in the same volt-seconds as this
reference volt-second value. There are two main
implementation techniques for carrier based PWM methods:
1) scalar implementation and 2) space vector implementation.
Fig. 1. Circuit diagram of a PWM-VSI drive connected to an
R-L-E type load.
Fig. 2. Triangle intersection PWM phase “a” modulation
wave v*a , switching signal and va o voltage.
In the scalar approach, as shown in Fig. 2, for each inverter
phase, a modulation wave is compared with a triangular
carrier wave [using analog circuits or digital hardware units
(PWM units) as conventionally found in motor control
microcontroller or DSP chips] and the intersections define the
switching instants for the associated inverter leg switches.
The mathematics involved in the scalar method modulation
waves and the space vector calculations is related. With
proper modulation waves, the scalar and space vector PWM
pulse patterns can be made identical. Thus, both techniques
may have equivalent performance results. But the scalar
approach is simpler than the space vector approach from the
implementation perspective as the involved computations are
generally fewer and less complex .Based on the scalar or
vector approach, there are various PWM methods proposed in
the literature which differ in terms of their voltage linearity
range, ripple voltage/current, switching losses, and high-
frequency common mode voltage/current properties.
However, the implementation of these methods is not easy
and not reported in the literature to sufficient depth.
International Journal of Electrical Engineering and Computer Science
DOI: 10.37394/232027.2023.5.13
V. Thamizharasan, M. Karthikkumar
E-ISSN: 2769-2507
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Volume 5, 2023
Furthermore, the relation between the conventional and
recently developed reduced CMV (RCMV) PWM methods is
not well understood both in terms of implementation
characteristics and performance characteristics. Therefore,
difficulties arise in implementing and understanding the
performance attributes of these PWM methods. This paper
first reviews the PWM principles and establishes a general
scalar approach that treats the conventional and RCMV–
PWM methods, and unites most methods under one umbrella.
The paper then reviews the pulse patterns and performance of
the popular PWM methods, and provides guidance for simple
practical implementation which is favorable over the
conventional methods (space vector or scalar). The
experimental results verify the feasibility of the proposed
approach.
2. Generalized Scalar PWM Pproach
The generalized scalar PWM approach provides
degrees of freedom in the choice of both the zero-sequence
signal and the carrier waves. First, the zero-sequence signals
can be arbitrarily generated, but generating them based on the
phase reference voltages (original modulation signals)
provides significant advantages. Until present, most useful
PWM pulse patterns could be obtained by this approach.
Therefore, in most cases, the zero-sequence signals are
obtained by evaluating the reference voltages. Second, the
carrier waves of different phases can be selected arbitrarily.
However, recent studies have illustrated that employing
triangular carrier waves at the same frequency, and selecting
the phase relation between the carrier waves of different
phases based on the voltage references yields favorable
results. As a result, the generalized scalar PWM approach will
favor such constraints for the purpose of keeping the scope of
the paper within practical boundaries further relaxing the
constraints and investigating alternative pulse pattern switch
favorable characteristics is a subject matter of future research.
Given the described set of constraints, the generalized block
diagram of scalar PWM approach with zero-sequence signal
injection principle is illustrated in Fig. 3.
Fig. 3. Block diagram of the generalized carrier-based scalar PWM
employing the zero-sequence signal injection principle and multicarrier
signals.
. In the generalized scalar approach; according to the original
three-phase sinusoidal reference signals (voltage references
with single star superscripts) and zero-sequence signals, the
final reference (modulation) signals (voltage references with
double star superscripts) are generated.
Fig. 4. Block diagram of the conventional scalar PWM
method employing zero-sequence signal injection and a
common triangular carrier wave.
Then, the individual modulation and carrier waves are
compared to determine the associated inverter leg switch
states and output voltages. In the conventional approach (see
Fig. 6), which is the special case of generalized approach;
only one triangular carrier wave is utilized for all phases. In
the scalar representation, the modulation waves are defined as
follows:
Va∗∗ = Va + V0 = V1m cos(ωe t) + V0 (1)
V b ∗∗ = V b + V0 = V1m cos(ωe t 2π/3) + v0 (2)
V c ∗∗ = V c + V0 = V1m cos(ωe t +2π/3) + v0 (3)
Where Va,, V b and V c are the original sinusoidal reference
signals and V0 is the zero-sequence signal. Using the zero-
sequence signal injected modulation waves, the duty cycle of
each switch can be easily calculated in the following for both
single and multicarrier methods:
dx+ =(1/2)(1 + v x** /(Vdc/2) , for x * {a, b, c} (4)
dx = 1 dx+, for x * {a, b, c}. (5)
It is helpful to define a modulation index (Mi, voltage
utilization level) term at this stage. voltage (V1m-6-step =
2Vdc) is termed the modulation index Mi [1]
For a given dc-link voltage (Vdc), the ratio of the fundamental
component magnitude of the line to neutral inverter output
voltage (V1m) to the fundamental component magnitude of
the six-step mode
Mi = V1m/V1m-6-step (6)
There are two commonly used zero-sequence signals in
practical applications. The zero-sequence signal of the widely
utilized continuous PWM (CPWM) methods is generated by
employing the minimum magnitude test which compares the
magnitudes of the three phase original reference signals and
selects the signal which has minimum magnitude. Scaling this
signal by 0.5, the zero sequence signal of these CPWM
methods is found. Assume |v*a| |v*b |, |v*c |, then v0 = 5
v*a . This modulation wave is recognized as SVPWM
International Journal of Electrical Engineering and Computer Science
DOI: 10.37394/232027.2023.5.13
V. Thamizharasan, M. Karthikkumar
E-ISSN: 2769-2507
124
Volume 5, 2023
modulation wave in [1]. Inside the voltage linearity region, in
the CPWM methods, the modulation waves are always within
the carrier triangle peak boundaries; within every carrier
cycle, the triangle and modulation waves intersect, and ON
and OFF switching always occur. In discontinuous PWM
(DPWM) methods, the zero-sequence signal is injected such
that reference signal of one phase is always clamped to the
positive or negative dc bus. The clamped phase is alternated
throughout the fundamental cycle. In the most common
DPWM modulation wave (in the literature recognized as the
DPWM1 modulation wave), the phase signal which is the
largest in magnitude is clamped to the dc bus with the same
polarity [1]. Assume |v*a| ≥ |v*b |, |v*c |, then v0 = (sign(v*a
))
(Vdc/2) v*a . The inverter leg whose modulation wave
is clamped to the dc bus is not switched; therefore, switching
losses are reduced in DPWM methods. Using the discussed
CPWM modulation wave along with a common triangular
carrier wave for all phases, SVPWM method yields.
Likewise, using the discussed DPWM modulation wave along
with a common triangular carrier wave for all phases,
DPWM1 yields. Using the same modulation waves, but
varying the triangle polarities (resulting in multicarrier waves,
vtri-a , vtri-b , vtri-c ) additional methods yield... Thus, the
proposed approach is broad and covers most methods
reported in the literature. Considering that the methods
reported in this paper (both conventional and the recently
developed RCMV–PWM methods) and the other
conventional methods are the most popular methods (due to
their superior performance when compared to other methods),
the approach yields sufficient results from the practical
utilization perspective. Using the proposed approach and
creating a variety of triangular carrier wave patterns and
modulation wave shapes, further PWM pulse patterns, and
thus new methods can be invented. This paper will be
confined to the methods discussed in this paragraph.
However, it can be shown that they can be included under the
generalized scalar PWM umbrella. In the alternating polarity
triangular carrier wave methods, the alternating triangle
polarities, for example those in Table I, can be obtained by
determining the regions of Fig. 4. This goal can be easily
achieved by comparing the modulation waves. For example,
the v*a v*b, v*c condition corresponds to the A1 region.
Such operations can be very easily performed with the PWM
units of the modern digital signal processors used in ac motor
drives and PWM rectifiers. Before getting involved in
implementation details and experimental results, the
aforementioned discussed PWM methods will be reviewed in
terms of pulse patterns and performance.
3. Pulse Patterns and Performance
Characteristics of Popular PWM
Methods
Since a large number of PWM methods exist and each
method with unique pulse pattern yields unique performance
characteristics, full investigation of all PWM methods in
regard to their pulse pattern and performance is a laborious
task. However, the theoretically infinite choice of methods, in
practice reduces to a relatively small count when a systematic
evaluation and performance comparison (among the many
methods) is made. This section aims to review the popular
PWM methods with the focus on their pulse patterns such that
in the following section the practical implementation can be
discussed to sufficient depth. Observing the pulse patterns,
the performance characteristics can be studied, and thus major
advantages of these methods can be recognized. Therefore,
while reviewing the pulse patterns of the popular methods, it
will also be possible to demonstrate why these methods are so
well accepted. The pulse pattern of SVPWM is illustrated in
Fig. 8 for the region A1. With equally partitioned two zero
states [(0 0 0) and (1 1 1)], SVPWM provides very low PWM
ripple and provides voltage linearity for the modulation index
range of 0<Mi <0.907. As it has symmetric zero stages, from
the PWM ripple reduction perspective SVPWM is the best
method. But as Mi increases the ripple also increases and it
loses its advantage around Mi 0.6 where discontinuous
PWM methods can be used .For example increasing the
switching frequency by 50% and employing the DPWM1
method for region A1B2 the switching count and, therefore,
the switching losses remain the same (as one of the phases
ceases switching during the PWM period) while the ripple of
DPWM1 becomes less compared to SVPWM. Due to
reduction of the total zero state duty cycles at high Mi and
50% increase of the carrier frequency (decrease of the carrier
cycle), the effect of the zero states on ripple decreases and
they can be lumped.Thus, low ripple or low loss results.
Consequently, SVPWM at lowMi , and DPWM1 at high Mi
have been widely used. It has been illustrated that transition
between methods is seamless (the load current is not
disturbed) and a combination of the two methods has been
successfully used in commercial drives.
However, for both SVPWM and DPWM1, as shown at the
bottom of the pulse pattern diagrams, the inverter CMV is
high. In motor drive applications, higher CMV is associated
with increased common mode current (motor leakage
current), higher risk of nuisance trips, and reduced bearing
life.The CMV of the three-phase VSI is defined as the
potential of load star point with respect to the midpoint of dc
bus of the inverter and the CMV at the inverter output can be
expressed as follows:
vcm = (vao + vbo + vco)/3 . (7)
the CMV of SPWM, SVPWM, and DPWM1 methods (and all
other common carrier wave PWM methods), may take the
values of Vdc/6 or Vdc/2; depending on the inverter switch
states. As these methods utilize zero vectors V0 and V7, a
CMV of Vdc/2 and Vdc/2 can be created. This high CMV is
illustrated in Figs. 8 and 9(a) at the bottom traces. Since they
do not utilize zero states, NSPWM, AZSPWM1, and
AZSPWM3 (and all other RCMV PWM methods) avoid a
CMV with a magnitude of Vdc/2 and their CMV magnitude is
confined to Vdc/6. Comparing the pulse patterns of Fig. 9(a)
and (b), it can be observed that NSPWM is a discontinuous
International Journal of Electrical Engineering and Computer Science
DOI: 10.37394/232027.2023.5.13
V. Thamizharasan, M. Karthikkumar
E-ISSN: 2769-2507
125
Volume 5, 2023
PWM method and it yields equivalent advantages to DPWM1
in terms of switching loss reduction. This is obtained
differently from the DPWM1 method.
While in DPWM1 one of the zero states is avoided, in
NSPWM both zero states are avoided and an active vector is
introduced instead. The switching count reduction is obtained
by proper sequencing the voltage vectors. Similar to
DPWM1, its ripple is low in the high Mi range as the active
vectors close to the reference voltage vector dominate.The
main disadvantage of NSPWM involves voltage linearity.
NSPWM is only linear in the range of 0.61
Mi
0.907
(DPWM1 linearity range: 0 < Mi < 0.907). While in PWM
rectifier applications this range is sufficient, in motor drives
operation at low Mi is required and NSPWM must be
combined with another method. In motor drive applications,
operating at lowMi and obtaining low CMV is possible by
employing AZSPWM1 or AZSPWM3 in this range. The two
AZSPWM methods yield low CMV with Vdc/6 magnitude and
AZSPWM3 has a lower CMV frequency which is favorable.
However, it has simultaneous switching actions which may
yield undesirable results of motor terminal overvoltage or
increased leakage current during switching instants (similar to
RSPWM and AZSPWM2). As a result, while AZSPWM1 has
found application AZSPWM3 has been cautiously
approached. It should also be noted that AZSPWM methods
have very high ripple at low Mi (due to the fact that the zero
states are replaced with active vectors with opposite direction
to the reference voltage vector) and at high Mi they are
inferior to NSPWM as the switching losses are higher [16].
Thus, although they are linear in the range of 0 < Mi < 0.907,
AZSPWM methods should only be used in regions where all
other methods do not meet the performance requirements of
an inverter drive. It should be noted that in all PWM methods,
the injected zero sequence signal is a low-frequency signal
(periodic at 3ωe and/or its multiples, lower than the carrier
frequency by at least an order of magnitude) which causes
low-frequency CMV. At such frequencies the parasitic circuit
components (capacitances) are negligible (open-circuit) and,
therefore, the zero-sequence voltage has no detrimental effect
on the drive and yields no common mode current (CMC),
unless low common mode impedance path is provided by
filter configurations included in the drive for the purpose of
ripple reduction . High-frequency CMV, on the other hand,
can be harmful and can be reduced by PWM pulse pattern
modification. Thus, the high-frequency CMV effects (at the
carrier frequency and higher) have been considered here.
Fig 5: Pulse pattern
Fig 6: voltage and current waveforms
Fig 7: Rotor Current and Stator Current
Fig 8: Rotor Speed
Fig 9: Electromagnetic Torque
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DOI: 10.37394/232027.2023.5.13
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E-ISSN: 2769-2507
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Volume 5, 2023
4. Implementation
In this section, the practical implementation of the PWM
methods of Section IV is discussed. Only several comparisons
among the three sinusoidal references (original modulation
waves) and several algebraic operations are necessary to
obtain the zero-sequence signal, as shown in Section III via
examples. Having obtained the zero sequence signal, it is
added to the original modulation waves and the final
modulation signals result. The duty cycles are checked and
bounded to the range of 0 to 1 (in case over modulation
condition occurs). For SVPWM and DPWM1, a common
triangular carrier wave and for AZSPWM1, AZSPWM3, and
NSPWM using a common triangular carrier wave but
alternating its polarity according to Table I, are generated.
Determining the triangle polarity also involves only a
comparison of the sinusoidal references. These operations can
be lumped together with the zero-sequence signal
determination stage to further minimize the computational
burden. This task is left to the implementation engineer as a
straightforward procedure. Once the final modulation signals
and triangles are obtained, the remaining task involves the
sine-triangle comparison stage. This comparison is performed
to determine the switching instants via a comparator for each
phase. The scalar PWM implementation of all the discussed
methods is an easy task compared to the space vector
implementation (discussed at the end of Section II). The
implementation can be best realized on digital PWM units
such as the modern motion control or power management
microcontrollers and DSP chips (which involve a well
developed software programmable digital PWM unit). Most
modern commercial drives and power converters utilize such
control chips which are offered at economical price. Thus, the
approach can be readily employed in most power converter
control platforms. Employing such digital platforms, and
considering the simplicity of the scalar PWM algorithms, it is
easy to program (combine) two or more PWM methods and
online select a modulator in each operating region in order to
obtain the highest performance [1]. Combination of SVPWM
at low Mi and DPWM1 at high Mi is common in industrial
drives. Combination of AZSPWM methods at low Mi and
NSPWM at high Mi is an alternative approach for low CMV
requiring applications While operating at high Mi (such as full
speed operation of a drive or PWM rectifier operating under
the normal operating range), over modulation condition may
occur due to dynamics or other line or load variation
conditions.
5. Experimental Results
In the laboratory, the discussed PWM methods are
implemented using the Texas Instruments TMS320F2808
fixed-point DSP chip. In this chip, the PWM signals are
generated by the enhanced PWM (EPWM) module of the
DSP [30] which is a hardware digital circuit dedicated for the
purpose of generating the PWM signals. The PWM period
(thus the switching frequency), the dead time, etc. are all
parameters that are controlled by the user via software. The
EPWM module includes an internal counter clock (termed as
“PERIOD”) which corresponds to the triangular carrier wave.
The value of this counter defines the half of the PWM period.
One carrier cycle is completed as the counter counts up from
“0” to “PERIOD” and then decrements down to “0” again.
For every PWM cycle, the per phase duty cycles are
calculated first. Then, the duty cycles are converted to the
count number as they are scaled with the PERIOD. Since the
EPWM module has two comparator registers (COMPA and
COMPB) per phase, the count number, its complementary,
zero (0), or PERIOD value are loaded to these counters
depending on the pulse pattern to be generated. In this
application, the switching rule is defined as the PWM signal
of the upper switch of an inverter phase (for instance, Sa+ ) is
at logic level “1” when the counter value is between the
loaded values of comparator register A (COMPA) and
comparator register B (COMPB) comparator registers and at
logic “0” otherwise (Sa
logic is complementary of Sa+ ).The
conventional PWM pulse patterns are generated such that the
PWM cycle starts and ends with the upper switches in the
high “1” state, (often termed as “active high” except for the
PWM periods where the switching ceases in DPWM
methods.. Employing the scalar implementation approach and
utilizing the TMS320F2808 DSP platform, PWM signals are
programmed and applied to a three-phase VSI. This
demonstrates that all the intended pulse patterns can be easily
generated. Note that SVPWM utilizes zero vectors (000) and
(111) which cause a CMV of
Vdc/2 and Vdc/2, respectively.
In AZSPWM1, only active switch states are utilized; thus, the
CMV is limited to Vdc/6. While in SVPWM the line-toline
voltages are unipolar, AZSPWM1 line-to-line voltages are
bipolar. Bipolar voltage pulses result in higher output current
ripple compared to unipolar voltage pulses, and they may
cause overvoltages at the motor terminals in long-cable
applications. A 4 kW, 1440 min
1 , 380 Vll
rms induction
motor is driven from the VSI in the constant V/f mode (176.7
Vrms/50 Hz) and the PWM frequency is 6.6 kHz for CPWM
methods and 10 kHz for DPWM methods to provide equal
average switching frequency. Operation at Mi = 0.8 (180.3
Vrms/51 Hz, 1510 min
1 ) is discussed. As can be seen from
the diagrams all the discussed methods provide satisfactory
performance at the high Mi . NSPWM provides low motor
current ripple and CMV/CMC. SVPWM and DPWM1 have
low current ripple but high CMV/ CMC. AZSPWM3 has high
CMV and CMC magnitude compared to NSPWM, but its
CMV/CMC frequency is less. AZSPWM1 has higher PWM
current ripple and comparable CMV/CMC to NSPWM. The
aforementioned waveforms demonstrate that the various
PWM methods can be easily implemented with the
generalized scalar PWM approach.
6. Conclusion
PWM principles are reviewed for three-phase, three-wire
inverter drives. The characteristics, pulse patterns, and
implementation of the popular PWM methods are discussed.
International Journal of Electrical Engineering and Computer Science
DOI: 10.37394/232027.2023.5.13
V. Thamizharasan, M. Karthikkumar
E-ISSN: 2769-2507
127
Volume 5, 2023
The generalized scalar PWM approach is established and it is
shown that it unites the conventional PWM methods and most
recently developed reduced common mode voltage PWM
methods under one umbrella. Through a detailed example, the
method to generate the pulse patterns of these PWM methods
via the generalized scalar PWM approach is illustrated. It is
shown that the generalized scalar approach yields a simple
and powerful implementation with modern control chips
which have digital PWM units. With this approach, it
becomes an easy task to program the pulse patterns of various
high performance PWM methods and benefit from their
performance in modern VSIs for applications such as motor
drives, PWM rectifiers, and active filters. The theory is
verified by laboratory experiments. It is demonstrated that
with the proposed approach both the conventional PWM
pulse patterns (such as those of SVPWM and DPWM1) and
the recently developed improved high-frequency common
mode voltage/current performance method pulse patterns
(NSPWM, AZSPWM1, and AZSPWM3) can be easily
generated. Applying such pulse patterns to motor drives, it
has been demonstrated that NSPWM, AZSPWM1, and
AZSPWM3 methods provide good performance. Thus, the
inverter design engineers are encouraged to include such
pulse patterns in their designs.
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Scientific Article or Scientific Article Itself
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Conflict of Interest
The authors have no conflicts of interest to declare
that are relevant to the content of this article.
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International Journal of Electrical Engineering and Computer Science
DOI: 10.37394/232027.2023.5.13
V. Thamizharasan, M. Karthikkumar
E-ISSN: 2769-2507
128
Volume 5, 2023