IEEE Power Electronics Magazine Compendium - March 2018 - 100

Speed Range 18X

12X

400
3

Natural Frequency (Hz)

Excitation Torque

Dynamic Magnifier =

Induced Shaft Torque

converter that provides the regeneration at reduced harin these motors. Control system design under low-speed conmonics and operation at a high power factor and active/
ditions is complicated because of phase shift between voltreactive power combination [29]. Therefore, it is important
ages at the input and output of the LC filter installed at the
in some applications to use an AFE multilevel converter prooutput of an inverter. Also, there is a voltage drop across the
viding harmonic mitigation in the power grid and an almost
LC [26]. Usually, it is assumed in the drive control that the outunity power factor for all operating
put voltage and current of an inverter
points. It is highly advantageous for
are equal to the motor input voltage and
MV applications that require regencurrent. The region of proper motor
Complex multiwinding
eration capability. Figure 7 shows
operation remains limited if there are
transformers can
a cycle of power and speed trajectoany discrepancies. Hence, it is necesries [29]. The use of multilevel AFE
sary that the measurement circuits or
mitigate harmonics
solutions makes it possible to improve
the control algorithms of the electric
through phase
the input power factor, power qualdrive be modified [27].
ity, total cost, and volume by possiUsing MIs in drive systems results
shifting in modular
bly eliminating line side transformer.
in a reduction of motor winding insuconverters.
Back-to-back converters (converters
lation stress. However, the reliability
with AFE) are almost double the
and efficiency of the inverter may
cost of a single converter. Still, they
decrease. The practice is to make it
are a worthy investment to achieve the solution with
possible to use very small LC filters or even exclude them
nearly perfect sinusoidal input currents at nearly unity
by ensuring low harmonic voltage waveforms while mainpower factor and with regenerative capability.
taining a low switching frequency. Furthermore, the control
strategy that is chosen must actively damp filter oscillations
and allow fast dynamic operation with very low switching
torsional Vibration
frequencies [28].
In the MV drive, torsional vibration can occur because of
the large inertia of the motor and its mechanical load. This
occurs when the natural frequency of the mechanical sysregeneration capability and Power Factor correction
tem matches the frequency of torque pulsations caused by
In general, for all electric devices, a high power factor is
the motor current harmonics [30]. Excessive torsional vibradesired. Therefore, rectifiers with low current harmonics
tions at resonance can result in high twisting torque and,
and those capable of operating at almost unity power factor
hence, breakage of shafts and couplings. The undamped
are required as utility interfaces for many ASDs. The
response is shown in Figure 8(a) [31]. The torsional vibrarequirement of a high power factor is especially important
tions can also damage other mechanical components in the
for MV drives due to their high power rating. The rectifier
system. PWM control with harmonic elimination can be
design is a decisive factor for the optimum size, cost, and
helpful to overcome such problems. The operating points
efficiency of the MV drive.
that excite the shaft in PWM inverter-fed motors can be preThe most popular solution for high-power regeneradicted without torque sensors as shown in Figure 8(b) [31].
tive loads is the use of a multilevel active front-end (AFE)

0
0

1
2
3
Frequency Ratio (ω /ω n )

(a)
FIG 8 (a) An undamped response. (b) A Campbell diagram.

100 IEEE PowEr ElEctronIcs MagazInE

z June 2016

300

200
Critical or
Resonance
Points

100
0

500

1,000

1,500

2,000

Speed (r/min)
(b)

2,500

3,000

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