ISSN 2319-8885
Vol.04,Issue.08,
April-2015,
Pages:1570-1576
www.ijsetr.com
Low Modulation-Index Operation of a Multilevel Diode Clamped Inverter
for A PMSM Drive
MAGGIDI LATHA1, D.LAVANYA2
1
PG Scholar, Dept of EEE, CVSR College of Engineering, Ghatkesar, RR (Dt), Telangana, India.
Associate Professor, Dept of EEE, CVSR College of Engineering, Ghatkesar, RR (Dt), Telangana, India.
2
Abstract: In this project author discussed comparison analysis of induction motor drive and PMSM drive. Among AC motors,
Induction motors are the most preferred one but now-a-days much attention is given to PMSM machines. Permanent Magnet
motors are generally AC synchronous machines which operate at unity power factor and are more efficient then induction motor.
Simulation of Five-level diode-clamped pulse width-modulated (PWM) inverter intended for a medium-voltage motor drive with
a constant-torque load but no regenerative braking. It is applicable to drilling rigs, extruders, and rubber mixers. The power
conversion system consists of a three-phase six-pulse diode rectifier, a five-level diode-clamped PWM inverter, and a dc voltagebalancing circuit including a single coupled inductor. The five-level inverter is characterized by injecting a common ninth
harmonic zero-sequence voltage on each of the three-phase reference voltages in a low-modulation-index region. Simulation and
analysis is also carried out in Matlab-Simulink software.
Keywords: Motor Drives, Diode-Clamped Inverters, Voltage Balancing.
I. INTRODUCTION
The diode clamped inverter, particularly the three-level
one, has drawn much interest in motor drive applications
because it needs only one common voltage source. Also,
simple and efficient PWM algorithms have been developed
for it, even if it has inherent unbalanced dc-link capacitor
voltage problem [1]. However, it would be a limitation to
applications beyond four-level diode clamped inverters for
the reason of reliability and complexity considering dc-link
balancing and the prohibitively high number of clamping
diodes [2]. Multilevel PWM has lower dv/dt than that
experienced in some two-level PWM drives because
switching is between several smaller voltage levels [3]. The
authors of [6] have discussed a 6.6-kV transformerless
medium-voltage motor drive using a five-level diodeclamped PWM inverter and a three-phase diode rectifier used
as the front end for energy savings of fans, blowers, and
pumps without regenerative braking. The authors of this
paper have proposed a new dc-voltage-balancing circuit
including a single compact coupled inductor, which is
suitable for the five-level inverter [7].
The motor drive combining the five-level inverter with the
frontend diode rectifier is applicable not only to fan/blower
loads but also to constant-torque loads without regenerative
braking such as drilling rigs for oil and geothermal energy
resources [9], extruders [10], and rubber mixers [11], [12]. In
particular, a drilling rig has often used long cables for
delivering electric power from an inverter to a motor, and the
cable length has ranged over several hundreds of meters.
Such a long cable acts as a distributed parameter circuit in a
frequency range of 100 kHz or higher. Impedance mismatch
is accompanied by reflection at both inverter and motor
terminals. This reflection causes an overvoltage at the motor
terminals. When a conventional two-level inverter is used, it
reaches double the inverter dc-link voltage [13]. In contrast,
the five-level inverter would cause a much smaller
overvoltage because the voltage steps are one-fourth of those
of the two-level inverter. Hence, the five-level inverter is also
suitable for the drilling rigs in terms of mitigated overvoltage.
However, such a constant-torque load requires the rated
torque and the rated current even in a low-speed region,
whereas the torque of fans and blowers is proportional to a
square of motor speed except for starting. The five-level
inverter has to supply the rated current with much lower
frequencies than the base frequency to a constant-torque load.
This paper addresses low-modulation-index operation of a
five-level diode-clamped PWM inverter equipped with a dc
voltage-balancing circuit for a constant-torque motor drive.
Attention is paid to solving the following concerns:
 Magnetic-flux fluctuation of the coupled inductor in the
balancing circuit;
 Imbalanced power losses of IGBT modules in the fivelevel inverter.
II. 6.6-KVMEDIUM-VOLTAGE MOTOR DRIVE
A. Power Conversion System
Fig. 1 shows the 6.6-kV transformerless medium-voltage
motor drive using 4.5-kV IGBTs and diodes, which consists
of a three-phase six-pulse diode rectifier, a five-level diodeclamped PWM inverter, and a dc-voltage-balancing circuit.
Copyright @ 2015 IJSETR. All rights reserved.
MAGGIDI LATHA, D.LAVANYA
As for a concern about line-side harmonic currents, a
would remain among the four split dc capacitors because
transformerless hybrid active filter has been proposed to
saturation voltage drops occur in the IGBTs and forward
devote itself to the six-pulse diode rectifier used as the front
voltage drops appear in the diodes. The authors have
end [15]. Let the voltage between two nodes X and Y be vX−Y
introduced two balancing control methods to the balancing
, where X and Y correspond to P2, P1, M, N1, or N2,
circuit as follows [7]:
respectively. For example, vP 2−M is the voltage between
 Duty-factor control.
nodes P2 and M. In addition, let the dc mean voltage of one
 Phase-shift control.
capacitor be Vdc.
Fig. 3. Relation between the node currents and the
magnetic flux.
The former is used for voltage balancing between the two
positive-side dc capacitors, and between the two negativeside dc capacitors. The latter achieves voltage balancing
between a set of the two dc capacitors at the positive side and
the other set at the negative side.
Fig.1. 6.6-kV motor drive system based on the five-level
diode-clamped PWM inverter equipped with the dcvoltage-balancing circuit.
Fig. 2. Two representative circuit states of the dc-voltage
balancing circuit. (a) Q1 and Q2 remain turned ON. (b)
Q3 and Q4 remain turned ON.
B. DC-Voltage-Balancing Circuit
Fig2 shows the basic principle of the dc-voltage-balancing
circuit, and two representative circuit states where the
positive side IGBTs (Q1 and Q2) and the negative-side IGBTs
(Q3 and Q4 ) are complementarily turned ON or OFF. 1 The
energy stored in Cdc1 is transferring to Cdc3 in Fig. 2(a),
whereas the energy stored inCdc4 is transferring toCdc2 in Fig.
2(b). The two circuit states repeat alternately so as to
discharge Cdc1 and Cdc4, and to charge Cdc2 and Cdc3. Finally,
the dc mean voltages of the four split dc capacitors can be
balanced. In practice, however, small voltage differences
III. CONCERNS OF LOW-MODULATION-INDEX
OPERATION
A. Magnetic-Flux Fluctuation within the Coupled Inductor
Let the currents flowing out of nodes P1 and M be iP1 and iM ,
the current flowing into node N1 be iN1 , and the output
current of the inverter be iO , as shown in Fig. 1. Each of
node currents iP 1, iM , and iN1 contains an amount of ac
component, the frequency of which is three times as high as
the inverter output frequency fO [16]. This brings voltage
fluctuation with a frequency of 3fO to the four dc capacitors.
Moreover, the magnetic flux within the coupled inductor also
fluctuates at the same frequency. Note that the node currents
flowing into, or out of, nodes P2 and N2 do not affect the
capacitor voltages because they mainly flow through the
diode rectifier. Fig. 3 shows a simple block diagram that
indicates the relation among the node currents, the capacitor
voltages, and the magnetic flux Φ. If the amplitudes of the
node currents are constant, the magnetic flux is in inverse
proportion to a square of frequency of the node currents.
Hence, the magnetic-flux fluctuation tends to be larger and
larger as the inverter output frequency gets lower and lower.
Fig. 4 illustrates relations between the line-to-neutral voltages
reference e∗ and the duty factors DP2, DP1, DM, DN1 , and DP1
, where Vdc is the dc mean voltage of one capacitor. Each
duty factor represents a ratio of a time interval, during which
the output current iO flows into, or out of, each node. If the
voltage reference e∗ does not exceed |Vdc|, that is, the
modulation index MI is less than 0.5, DP2 and DN2 are always
zero. Therefore, the output current iO flows into, or out of,
only node P1, M, or N1. This makes the ac components of iP
1, iM , and iN1 large.
Fig.4. Reference line-to-neutral voltage e and duty factor
D.
International Journal of Scientific Engineering and Technology Research
Volume.04, IssueNo.08, April-2015, Pages: 1570-1576
Low Modulation-Index Operation of a Multilevel Diode Clamped Inverter for A PMSM Drive
B. Imbalanced Power Losses in IGBT Modules
B. Reduction of the Magnetic-Flux Amplitude
In case of a modulation index of MI < 0.5, the output
Fig. 6 shows the circuit state of periods 3 and 7. SinceQ2
voltage gets a three-level PWM waveform, and some specific
and Q3 remain turned on during the periods, the winding
IGBTs do not switch [4]. On the other hand, the other IGBTs,
currents iLP and iLN circulate through DB1 and DB4, respectively,
keeping on switching, continue producing power losses.
so that the winding voltages are zero. Note that the balancing
Thus, power losses of the IGBT modules in the five-level
circuit neither charges nor discharges the four dc capacitors
during the periods 3 and 7. Fig. 6 illustrates the theoretical
inverter get imbalanced.
waveforms of either winding voltage vwind, or the magnetic
IV. CONTROL OF THE DC-VOLTAGE-BALANCING
flux Φ, where leakage inductances are disregarded. Fig. 6(a)
CIRCUIT
assumes that the balancing circuit is effective only during
It is impossible to completely eliminate the magnetic-flux
periods 1 and 5 that correspond to the two representative’s
fluctuation although the ninth-harmonic zero-sequence
circuit states shown in Fig. 2.3 In this case, vwind is a square
voltage injection contributes to mitigating it. The peak value
waveform and Φ is a triangle waveform. On the other hand,
of the magnetic flux tends to get larger than its designed
Fig. 6(b) includes periods 3 and 7 so that vwind is a three-level
value if the fluctuation remains. As a result, the volume of the
waveform and Φ is a trapezoid waveform. The amplitude of
coupled inductor tends to increase under the same maximum
the trapezoid waveform is smaller than that of the triangle
magnetic flux density of the core.
waveform. Hence, including the periods 3 and 7 contributes
to reducing the magnetic-flux amplitude.
A. Switching Sequence and Control Block
Fig. 5 shows the switching sequence of the dc-voltage
V. LABORATORY MOTOR DRIVE SYSTEM
balancing circuit, which consists of eight periods. The
Permanent magnet synchronous motors (PMSM) are
negative side switches (Q3 and Q4) are phase-shifted by 180◦
typically used for high-performance and high-efficiency
from the positive-side ones (Q1 and Q2). Fig. 4 shows the
motor drives as shown in Fig,7. High-performance motor
diagram of the duty-factor control, where the negative-side
control is characterized by smooth rotation over the entire
carrier signals vtriN is phase-shifted by 180◦ from the
speed range of the motor, full torque control at zero speed,
positive-side one vtriP[7]. The duty-factor difference between
and fast acceleration and deceleration. To achieve such
Q1 and Q2, or between Q3 and Q4. where TR is the period
control, vector control techniques are used for PM
during which one switch (Q2 or Q3 ) turns on whereas the
synchronous motors. The vector control techniques are
other switch (Q1 or Q4 ) turns off, and TS is the switching
usually also referred to as field-oriented control (FOC). The
period. Note that periods 2, 4, 6, and 8 are significantly short,
basic idea of the vector control algorithm is to decompose a
and produce little effect on the magnetic flux although they
stator current into a magnetic field-generating part and a
play an important role in the duty-factor control. Therefore,
torque generating part. Both components can be controlled
let Tk (k = 2, 4, 6, 8) be zero in the following discussion. On
separately after decomposition. Then, the structure of the
the other hand, the phase-shift control is carried out by means
motor controller (vector control controller) is almost the same
of adjusting the phase difference between the positive-side
as a separately excited DC motor, which simplifies the
switches and the negative-side ones [7].
control of a permanent magnet synchronous motor.
Fig.5. Circuit state of periods 3 and 7.
Fig.7. 200-V 5.5-kW laboratory motor drive system.
The operation beyond the machine base speed requires
the PWM inverter to provide output voltages higher than its
output capability limited by its DC link voltage. To overcome
the base speed limitation, a field-weakening algorithm can be
implemented. A negative d-axis required current will increase
the speed range, but the applicable torque is reduced because
of a stator current limit. Manipulating the d-axis current into
Fig.6. Winding voltage v wind and magnetic flux: (a)
without period 3 or 7 and (b) with periods 3 and 7.
International Journal of Scientific Engineering and Technology Research
Volume.04, IssueNo.08, April-2015, Pages: 1570-1576
MAGGIDI LATHA, D.LAVANYA
the machine has the desired effect of weakening the rotor
field, which decreases the BEMF voltage, allowing the higher
stator current to flow into the motor with the same voltage
limit given by the DC link voltage. The characteristics of a
permanent magnet machine are highly dependent on the rotor
structure. The rotor can be implemented in various ways.
When employing the modern permanent magnet materials,
the rotor can be constructed even completely without iron. In
that case, the rotor frame is constructed for instance of
aluminum, onto which the shaped permanent magnets are
glued so that the sinusoidal flux density distribution is
achieved in the air gap of the machine. An ironless rotor
structure wastes permanent magnet material, since the
magnetic circuit closes through air in the rotor side.
Therefore, a thin steel rim, to which the magnets are attached,
is employed. The rim can be either a laminated structure, in
which case the eddy current losses of the rotor remain very
low, or a thin steel tube; however, in this case, there is a
danger that the rotor warms up excessively due to the effect
of the time harmonics of the stator.
VI. SIMULATION RESULTS
Simulation results of this system is shown in bellow Figs.8 to
18.
Fig.10. Shows the capacitor voltage, primary current,
secondary current and Magnetizing Current under
Steady state without any voltage injection.
Fig.8. shows the Matlab/simulink model of proposed
converter with PMSM drive.
Fig.9. shows the waveforms of phase voltage, line voltage,
and current under steady state without any voltage
injection.
Fig.11. Shows the 3-Ph Five Level DCI Steady-State
Waveforms with Ninth- Harmonic Voltage Injection.
International Journal of Scientific Engineering and Technology Research
Volume.04, IssueNo.08, April-2015, Pages: 1570-1576
Low Modulation-Index Operation of a Multilevel Diode Clamped Inverter for A PMSM Drive
Fig.12. Shows the capacitor voltage, primary current,
secondary current and Magnetizing Current with NinthHarmonic voltage injection.
Fig.14. Shows the capacitor voltage, primary current,
secondary current and Magnetizing Current without any
voltage injection.
Fig.13. Shows the 3-Ph Five Level DCI Fed PMSM Drive
Transient Waveforms without Any Voltage Injection.
Fig.15. Shows The 3-Ph Five Level DCI Fed PMSM Drive
Transient Waveforms With Ninth-Harmonic Voltage
Injection.
International Journal of Scientific Engineering and Technology Research
Volume.04, IssueNo.08, April-2015, Pages: 1570-1576
MAGGIDI LATHA, D.LAVANYA
Fig.16. Shows the capacitor voltage, primary current,
secondary current and Magnetizing Current with NinthHarmonic voltage injection.
Fig.17. Shows the performance characteristics
Induction motor drive under no-load condition.
of
VII. CONCLUSION
This paper has described low-modulation-index operation
of a five-level diode -clamped PWM inverter equipped with a
dc-voltage-balancing circuit for a constant-torque motor drive
without regenerative braking. The ninth-harmonic zerosequence voltage injection contributes not only to mitigating
the magnetic-flux fluctuation of the coupled inductor but also
to make uniform power losses produced by the eight IGBT
modules per leg. The proposed concept is implemented using
Matlab/simulink software and the output waveforms are
obtained.
VIII. REFERENCES
[1] A. Nabae, I. Takahashi, and H. Akagi, “A new neutralpoint-clamped PWM inverter,” IEEE Trans. Ind. Appl., vol.
17, no. 5, pp. 518–523, Sep./Oct. 1981.
[2] F. Z. Peng, J.-S. Lai, J. McKeever, and J. VanCoevering,
“A multilevel voltage-source converter system with balanced
DC voltages,” in Proc. Conf. Rec. IEEE Power Electron.
Spec. Conf., Jun. 1995, pp. 1144–1150.
[3] L. M. Tolbert, F. Z. Peng, and T. G. Habetler, “Multilevel
converters for large electric drives,” IEEE Trans. Ind. Appl.,
vol. 35, no. 1, pp. 36–44, Jan./Feb. 1999.
[4] L. M. Tolbert, F. Z. Peng, and T. G. Habetler, “Multilevel
PWM methods at low modulation indices,” IEEE Trans.
Power Electron., vol. 15, no. 4, pp. 719–725, Jul. 2000.
[5] M. Saeedifard, R. Iravani, and J. Pou, “Analysis and
control of DCcapacitor- voltage-drift phenomenon of a
scad.com/ passive front-end five-level converter,” IEEE
Trans. Ind. Electron., vol. 54, no. 6, pp. 3255–3236, Dec.
2007.
[6] N. Hatti, K. Hasegawa, and H. Akagi, “A 6.6-kV
transformerless motor drive using a five-level diode-clamped
PWM inverter for energy savings of pumps and blowers,”
IEEE Trans. Power Electron., vol. 24, no. 3, pp. 796–803,
Mar. 2009.
[7] K.Hasegawa and H. Akagi, “A newdc-voltage-balancing
circuit including a single coupled inductor for a five-level
diode-clamped PWM inverter,” IEEE Trans. Ind. Appl., vol.
47, no. 2, pp. 841–852, Mar./Apr. 2011.
[8] J. Li, S. Bhattacharya, and A. Q. Huang, “A new ninelevel active NPC (ANPC) converter for grid connection of
large wind turbines for distributed generation,” IEEE Trans.
Power Electron., vol. 26, no. 3, pp. 961–972, Mar. 2011.
[9] T. Hoevenaars, I. C. Evans, and A. Lawson, “New marine
harmonic standards,” IEEE Ind. Appl. Mag., vol. 16, no. 1,
pp. 16–25, Jan./Feb. 2010.
[10] R. A. Hannna and S.W. Randall, “Medium-voltage
adjustable-speed-drive retrofit of an existing eddy-current
clutch extruder application,” IEEE Trans. Ind. Appl., vol. 36,
no. 6, pp. 1750–1755, Nov./Dec. 2000.
[11] H. Schreiber, “The application of a vector controlled AC
drive for internal batch mixing of rubber compounds,” in
Proc. Conf. Rec. IEEE 43rd Annu. Conf. Electr. Eng.
Problems Rubber Plast. Ind., Apr. 1991, pp. 6–9.
[12] K. Nagata, T. Okuyama, H. Nemoto, and T, Katayama,
“A simple robust voltage control of high power sensorless
induction motor drives with high start torque demand,” IEEE
Trans. Ind. Appl., vol. 44, no. 2, pp. 604–611, Mar./Apr.
2008.
Fig.18. Shows the performance characteristics of PMSM
drive under no-load condition.
International Journal of Scientific Engineering and Technology Research
Volume.04, IssueNo.08, April-2015, Pages: 1570-1576
Low Modulation-Index Operation of a Multilevel Diode Clamped Inverter for A PMSM Drive
[13] H. Akagi and I. Matsumura, “Overvoltage mitigation of
inverter-driven motors with long cables of different lengths,”
IEEE Trans. Ind. Appl., vol. 47, no. 4, pp. 1741–1748,
Jul./Aug. 2011.
[14] [Online]. Available: http://p.
International Journal of Scientific Engineering and Technology Research
Volume.04, IssueNo.08, April-2015, Pages: 1570-1576