IEEE Power Electronics Magazine Compendium - March 2018 - 99

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Isolation Transformer

Induction
Motor

LC Filter

Inverter

FIG 6 The use of the isolation transformer.

inductor in the LC filter must be used in most highpower drive systems, but that causes a higher voltage
drop across the inductor.
The increase in the capacitor value of the filter
reduces LC resonant frequency, which is affected by the
parallel connection of the filter capacitor and motor magnetizing inductance. This leads to instability in the drive
system. To overcome this issue, active damping could be
proposed while at the same time suppressing LC resonance to achieve high efficiency [18]. Furthermore, the
use of an LC filter introduces a phase shift between the
voltages at the output of the feeding converter and the
voltage at the motor terminal [19]. This phase shift may
pose a control problem if not taken into account. Hence,
the control algorithm should be modified accordingly.

Furthermore, a dual inverter-fed open-end winding (neutral
if the winding is removed) induction motor drive with
two isolated dc power supplies for CM voltage elimination and to maintain dc-link capacitor voltage balancing
is reported in [12]. The neutral point of the dc link and
motor and/or star point of the output filter capacitors
have been grounded through a grounding network using
isolation transformers to reduce CM voltage. The use of
the isolation transformer is shown in Figure 6.
At a low modulation index, the three-phase system looks
continuous, which leads to very low dwell time, resulting in
an increase in CM voltage. There will also be high spikes in
the dc side that affect the reliability and performance of the
motor drive. To overcome this, the dwell time is modified to
decrease CM voltage. The adjusted dwell time is compensated
for in the subsequent cycles of the switching periods [23].

CM Voltage
Use of Inverter Output Filters
Passive and active filter-based solutions are employed to mitigate the problems that arise due to PWM actions [19], [24],
[25]. Presently, passive filtering is commonly used for such
problems. Passive filters are hardware circuits that are
installed at the output terminals of the converter structure
[26]. The most common approach is using filters based on
low-pass LC filters, CM chokes, and CM transformers [26].
For reducing the overvoltages at the motor terminals (especially in the case of long cable connections), differential-mode
LC filters are used [16]. On the other hand, differential-mode
LC filters make it extremely difficult to apply precise control

Speed

ion
era
t
Ac
cel

Cruise

rati

Power

Cruise

ele

Psource
Pstorage
Pmotor
Speed

Dec

The CM voltage on the motor side is produced because of
the switching actions of the power converters. This phenomenon has to be taken into consideration while
designing the motor drive [20]. CM voltage is mostly
responsible for the ground leakage current through stray
capacitances that ultimately may damage the motor bearing. Replacement of the bearing is an expensive and timeconsuming process; hence, unplanned maintenance must
be avoided. Normally, the bearings should be replaced or
maintained during the scheduled or planned maintenance. Great effort has been invested to minimize the CM
voltage in MV drives to save the drive system from catastrophic failure. The most widely used approach is to
modify the pulse-width modulation (PWM) strategy
toward minimizing the CM voltages. Another approach is
to employ passive filters at the output of the PWM
inverter. However, the weight and cost of the drive system will increase [20].
For the MIs, the CM voltage is similar to the traditional
two-level inverter, but with a lower level. Therefore, this
topic is still a subject of research, and several contributions
have been reported in recent years [21]. An effective solution to the issues mentioned above might be found mainly
by offering a lower-switching-frequency drive system with
very low harmonic content. Other possible solutions for
such a problem could be grounding the brushes on the motor
shaft [16] and using dv/dt-resistant winding insulation [22].

Time

0
Energy
Energy Due
Supplied
to Charging
During
from Source Acceleration

Energy
Recovered
During
Deceleration

FIG 7 The power (source, storage, and motor) and speed trajectories.

June 2016

z IEEE PowEr ElEctronIcs MagazInE

Table of Contents for the Digital Edition of IEEE Power Electronics Magazine Compendium - March 2018