Efficiency of Motor Side Common Mode (CM) Filtering Techniques for PWM Inverters
Dennis Kampen, BLOCK Transformatoren-Elektronik GmbH & Co. KG, Germany
Nejila Parspour, ILEA- University of Stuttgart, Germany
Stefan Beyer & Stefan Laudahn, University of Applied Science Hannover, Germany
Abstract
In this paper several inverter output CM reduction filters are compared through experimental results. Filters with or without connection to ground and to the DC link are introduced. Quality criteria like leakage
current and voltage drop on motor side, conducted emissions and leakage current on line side, power
loss and inverter impact are used to compare all filters comprehensively.
1. Introduction
When using a PWM inverter to drive a motor there
are several problems along with the gain of flexibility.
First the inverter is a source of differential mode
(DM) disturbances. DM voltage is the voltage between two lines and has a rectangle shape at the
output of the inverter. The steeper the slew rate
of inverter output DM voltage and the longer the
motor leads the higher is the potential for overvoltages at the motor because of reflections [1].
These overvoltages may destroy the motor insulation and are avoided easily with a motor line inductor or better a sineformer, [2] which will be used by
default in this paper.
Common Mode (CM) disturbances appear because the sum of the inverter output voltages principally is not equal to zero, so that a CM current is
driven over the parasitic capacitances of the motor and the leads back through the power supply
to the DC link, which is the reference potential of
the CM voltage [3], [1], [4], [5]. The analytical expression for the CM voltage is
uCM =
u1M + u2M + u3M
3
(1)
where u1M , u2M and u3M are the voltages between
inverter output phases and inverter DC midpoint.
The CM voltage can have severe consequences
for the System. A voltage between motor shaft
and ground potential occurs, which leads to bearing currents and may destroy the motor bearings
[6]. If the motor is grounded local with the building ground, the CM leakage current can create a
large loop and is then a source of intensive radiated emissions [7]. Expensive shielded conduits
normally must be used to avoid the radiation. Fur-
thermore the CM leakage current uses the way
back through the supply line to the DC link, which
leads to conducted emissions and increased leakage currents on line side.
Conducted emissions are limited by the EN
61800-3 and leakage currents are limited by the
residual current detector (RCD) which is used in
the building.
2. Compared filters
In this investigation six filter topologies on the inverter output are compared with each other and
in relation to a measurement without filter. An
overview of the system and the placement of the
filters is given in figure 2.
Fig. 1: system overview
2.1. DM low pass LC filter
Standard DM filter, which consist of a three phase
DM inductor Lsin and three DM capacitor Csin connected between the phases to create a sinusoidal
voltage waveform between the phases (sine former). With three phase 3UI cores the filter only
has little influence on CM disturbances except the
damping effect of the leakage inductance of the
DM inductor.
Fig. 4: additional CM inductor
Fig. 2: DM LC low pass filter (sineformer)
2.4. additional CM LC low pass with
connection to ground
2.2. galvanic isolating transformers
With an isolating transformer the CM path is interrupted, so that CM currents cannot flow, except
over parasitic capacitances between the two windings [8].
Fig. 3: additional
former
galvanic
isolating
CM low pass LC filters consist of a CM inductor and a further capacitance CPE connected
between DM LC filter capacitors star point and
ground. The filter shortcuts the CM disturbances
before they reach the motor line and the motor
[10], [11].
transFig. 5: additional CM LC low pass with connection to ground
2.3. CM inductor
2.5. CM LC low pass with connection
to inverter DC link
CM inductors Lcom have one magnetic core where
all three phases are wound on. They offer inductive damping for CM current while affecting the DM
currents only by their leakage inductance [9].
CM LC low pass with connection to inverter DC
link have a DC link connection over a capacitance
CZK from the DM LC filter capacitors star point.
They do not use the ground to shortcut the CM
disturbances. They directly offer CM currents a
path back to the DC link [12], [13], [14], [15].
frequency. 50m 4G4 motor leads where used.
The 15kW motor did not have isolated bearings,
so that for estimating bearing currents the motor
leakage current and the motor star point to ground
voltage were measured which can be done for estimating the bearing currents [16]. All filter prototypes compared here were designed cost effective
and as small as possible by the professional filter
design department of BLOCK TransformatorenElektronik GmbH & Co. KG.
3.1. motor star point to ground
voltage
Fig. 6: additional CM LC low pass with connection to inverter DC link
2.6. CM transformer
CM transformers are CM inductors with a fourth
winding. The CM current is lead from the DM filter capacitors star point through the fourth winding to the DC link. The current in the fourth winding induces the CM voltage to the three phases
so that the CM voltage in the motor leads is canceled. This is called voltage cancellation principle
[13], [14], [15].
Fig. 7: additional CM transformer
The measurement of the motor star point to
ground voltage uY−PE shows that the isolating
transformer and the CM transformer offer best reduction, although it must be noticed that the isolating transformer only damps the low frequency
components while the high frequencies are not
damped due to the stray capacitances between
the windings. The sine former and especially the
CM inductor (CM_ind) boost the low frequency
parts of uY−PE which worsens the bearing problems. High frequency components are damped
very good with a simple CM inductor. For damping of the lower frequencies, Lcom would have had
to be uneconomically high to set the corner frequency low enough.
Fig. 8: motor star point to ground voltage comparison
3. Experimental results
3.2. motor leakage current
To test the filter comparatively, an invariant drive
configuration was built up. The 11kW inverter had
an internal class A line filter. The switching frequency fs was set to 8kHz at 50Hz fundamental
It can be stated that the motor leakage currents
are damped very good with all three CM low pass
LC filters, whereas the CM transformer offers the
best damping.
Fig. 9: motor leakage current comparison
Fig. 11: inverter output current comparison
3.5. line leakage current
3.3. filter power loss
The power loss in all CM filters in comparison to
the single sine former and especially the isolating transformer is significant lower. The isolating
transformer losses are high because it was built of
laminated iron with a high material insertion. The
sine former has higher losses because of the CM
currents, which are not damped in contrast to the
other filters.
CM currents emitted by the inverter output flow
partly through the ground to the power line and
back to the inverter. The line side leakage current is often monitored by residual current detectors (RCD), which could trigger because of the CM
currents. It can be stated that the best damping effect could be reached by the CM transformer and
the CM inductor with connection to the DC link.
The CM inductor with connection to the ground in
opposite increases the leakage current drastically
because the ground is used for short circuiting CM
disturbances.
Fig. 10: filter power loss comparison
Fig. 12: line leakage current comparison
3.4. inverter impact
3.6. conducted emissions on line side
The inverter output phase current is a degree for
the inverter stress. Without filter there are very
high inverter output peak currents due to recharging of motor and lead parasitic capacitances. With
all filters the inverter stress could be reduced significantly.
To comply with EN61800-3 limits, line side filters
are used. In this investigation it can be figured
out that filtering on the inverter output lead to significant reduction of conducted emissions on line
side. All CM filters provide a further reduction in
comparison to a simple sine former up to 10MHz.
[5] C HEN, S.; L IPO, T.; and F ITZGERALD, D.
Modeling of motor bearing currents in PWM
inverter drives. Industry Applications, IEEE
Transactions on, 1996.
[6] B USSE, D.; E RDMAN, J.; K ERKMAN, R.;
S CHLEGEL, D.; and S KIBINSKI, G. The effects of PWM voltage source inverters on
the mechanical performance of rolling bearings. Applied Power Electronics Conference
and Exposition, 1996. APEC ’96. Conference
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Fig. 13: conducted emissions on line side
comparison
4. Conclusion
This paper categorizes and compares motor filters
which reduce the CM emissions of PWM inverters. The CM transformer has shown to be the superior solution for a whole system improvement,
including bearing currents, line side leakage currents and conducted emissions, power loss and
inverter stress. It can also be stated that the CM
transformer filter has been the filter with the smallest size.
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