Acoustic Effects of Motor Supplied by Various VSI Inverters
R. Strzelecki" **, J. Kukluk', B. Hawrylak*
M. Klytta
'Technical University of.Zielona Gora
Industrial Electrical Engineering Institute
Podgorna 50, 65-246 Zielona Gora, Poland
strzelec@iep.pz.zgora.pl.
**ResearchInstitute ,,METALCHEM"
Sklodowskiej-Curie 55, 87-100 Torun, Poland
Sekretariataobr-metalchem.torun.pl.
Fachhochschule Gieflen-Friedberg,
Fachbereich Elektrotechnik 1,
Wiesenstral3e 14, 35390 GieRen, Germany
Marius.Klytta@el .fh-giessen.de
Abstract - The paper deals with acoustic effects of modern
drives with VSI-fed induction motors. The chosen testing
method, extensive experiments using typical industrial frequency converters and results of wide noise harmonic analysis are presented. The paper shows connections between the
inverter type (PWM-type, modulation frequency) and the
noise effects (spectrum, sound pressure) at various drive
frequencies. It can help in choice of the right inverter solutions in view of acoustic compatibility of the drive. The noise
spectrum analysis can be also, as shown, used for diverse diagnostic purposes.
wide analysis of inverter-stimulated acoustic effects with
comparative investigations should enable conclusions concerning advantageous technical solutions of inverters in
order to provide an optimal or satisfying solution to the
described problem. For their experiments and investigations, the authors chose typical industrial converters with
different modulation strategies and various modulation
frequencies.
I. INTRODUCTION
The realized testing stand (Fig. 1) was the common basis for the measurements.
The tests were carried out with several typical industrial
frequency converters, at different adjustments of their
modulation parameters and at various output (motor) frequencies, but with the same standard squirrel-cage motor
as sound source (Fig. 1). The main measured data were:
- acoustic spectrum of motor noise and motor current
spectrum;
- A-weighted sound pressure level LpA [&(A)] in idle
running of motor (conform to IEC 179 and DIN 45634).
During the measurements, the motor ventilator was cut
off. The measured noise signals were therefore very clean
electromagnetic stimulated vibrations, determinated by
harmonics generated by supplying inverter. The mechanically stimulated bearing noises were thereby band limited
and secondary [3]. Measured data evaluation and results
interpretation were thus easier.
The compatibility problems grew in the last years into
very important aspect of power electronics systems. especially in converter-fed industrial drives. The mainstream
of various investigations has concentrated up to now on
the electromagnetic compatibility (EMC).
Another compatibility field, the acoustic one [2, 61, is
mostly disregarded or discussed rather superficially [ 11.
The pulse-width modulated output voltages of frequency
converters cause vibrations of motor windings and annoying acoustic effects. The favoured method for reducing
such motor noise is to increase switching frequency in the
inverter part. On the other hand, this leads to higher
switching losses and additional EMC problems.
The paper is an attempt to start a more basic discussion
of this complex ( not only ) acoustic problem. The first
aim was to point out attributes of modern inverters with
PWM, which affect motor noise behaviour. After that,
II I1
Motor 1 ,I kW
111
11. INVESTIGATIONS AND THEIR RESULTS
-
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with
Sound Card
metre-
micr.
Fig. 1. Main components and configuration of the realized testing stand.
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2000
6000
10000
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!.-..-I1
2000
18000 Hz
6000
10000
-.L-_iIJ
14000
18000 Hz
Fig.2. Typical currents and voltages (a) and motor noise spectra (b) at extreme modulation frequencies of
2 kHz and 16 kHz ( drive frequency J,, = 50 Hz ; FC L.8600, see Tab.1).
61
59
57
55
53
5 1
49
47
45
Fig.3. Sound (noise) pressure characteristics for several motor fiequencies as functions of the modulation frequency ( inverter with voltage space vector
modulation L.8600). .
modulation frequencies in today’s IGBT - inverters. The
price, one has to pay, is extremely high. Among other
things it causes:
- intensified EMC problems, danger for the motor insulation;
- increased losses in inverter and reduction of drive
power. This reduction can reach up to 40% of the power
which the drive offers atfs = 2 - 4 kHz.
The typical influence of modulation frequency on the
motor noise (total A-weighted sound pressure level) is illustrated in Fig.3.
The noise decreases with higher frequencies fs, espe-
A . Influence of modulation frequency on the motor noise
spectrum
The Fig.2 shows typical results at extreme different
modulation frequenciesfs of an inverter. The first fundamental harmonic in the noise spectrum corresponds exactly to the switching frequency fs. Choice of this frequency above approx. 8-10 kHz sh& all upper noise
harmonics into the superacoustic (non audible) range. In
the borderline case, whenfs > 16 kHz, one can eliminate
all motor noise harmonics connected with switchings in
the inverter. This explains the trend towards such high
32
Flat-top PWM
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2000
6000
10000
14000
18000
Hz
Fig.4. Typical inverter output voltages and currents (a), motor current spectra (b) and motor noise spectra (c)
for sine and top flop modulation (fm= 15 Hz, fs =8 kHz)
cially in the range of lower motor frequencies and speeds.
For the nominal frequency, one observes no clear influence of this kind. All harmonics of the noise spectrum
then reach a higher level anyway (see Fig.2b) - this is the
result of ,,sideband effect" (compare Fig.5) and of the increase in ,,mechanical" noises in this speed range.
These distinct differences between the two PWM strategies disappear, however, in operating range fm 2 50 Hz
(Fig.5).
B. Signijkance of inverter modulation type
Extended measurements confirmed very strong and interesting influences of the drive working frequency. The
reason can be found in the sidebands appearing around the
modulation harmonics (Fig. 5).
The groups around oddfs - harmonics have frequencies:
C.Influence of drive frequency and modulation type on
motor noise level
The type of chosen inverter modulation is also of importance. Sine modulation and flat top modulation are
modulation types which are often offered and used. The
advantage of the flat top modulation is to be found in reduced inverter switchings and as a consequence lower inverter losses.
But from the acoustic point of view, this modulation is
disadvantageous: it brings additional and higher motor
current harmonics and therefore more stronger acoustic
harmonics in the noise spectrum. Fig.4 illustrates this fact
at medium values of the modulation and drive frequency.
(2i-I).fs
and
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and the groups for even fs - harmonics have frequencies:
--
and
2i. f s k ( Z j - I ) . f 7 , , ;
i,jeN
this means that for even values there are only sideband
components [5].
33
Flat-top PWM
Sine PWM
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Hz
Fig.5. "Sideband effect" : influence of motor frequency on motor noise spectnim (fs= 8 kHz; FC L.9300).
For higher drive frequencies these movable harmonics
cause typical widening and flattening of the corresponding
spectrum parts and the noise level rises, reaching for
nominal frequency almost the same values for both PWM
types (Fig.5, Fig.6).
In the Figure 6 most of the obtained results are presented in the complex way. The characteristics show advantages of sine modulation in practically whole frequency and speed range as well as the influence of the inveter modulation frequency and of the operating point
(drive frequency).
The noise spectrum carries a.0. informations about possible fault operation states of drive. For instance the motor phase failure (moment t, in the Fig.8) could be clear
identified.
111. CONCLUSIONS
The paper shows relations between modulation type and
modulation frequency of typical industrial inverters and
on the other hand noise behaviour of drives with converter-fed induction motors (Fig.2 - Fig.6).
34
Z k H z sin
4 k H 2 f l a t top
60
CI
3
55
B
3
50
45
40
0
I
I
I
I
20
40
60
80
f,
100
WHzl
Fig. 6 Inverter output frequency dependence of motor noise level LpAfor different PWM algorithms and modulation fiequencies ( FC L.9300 )
Very high modulation frequency (fs 2 16 kHz ), as a typical method for improving acoustic behaviour, increases
losses in inverter part and leads to sharp EMC problems
in the modern converters. The investigations show that
sine PWM with approx. 8 - 10 kHz is the right solution in
most drive applications. Higher modulation frequencies
bring no clear change in the noise level (Fig.3, Fig.6). In
the case of flat top modulation, modulation frequency ,of
approx. 4 kHz seems to be optimal for acoustic behaviour
(Fig.6).
The described ”sideband effect” clears the specific
sound changes and noise level increase with higher drive
frequencies ( Figs. 5, 6 ). Because of this effect, the noise
difference between both analysed modulation types grows
less in the upper speed range.
The sine modulation is clearly better at all drive frequencies ( Fig. 6 ). In the range of the drive fiequencies
fm 2 fN noise levels and appearance of the whole spectra
become similar, no matter what modulation it is
(Fig.5 and Fig.6). Better results can be achieved with
modern control algorithms (Fig.7 ).
The spectrum analysis can be effectively used to identify
fault operation states of converter-fed drives (Fig.8). The
obtained data are also suitable for other analysis methods,
for instance using Higher-Order Statistics [4]. It opens
further chances for effective diagnostic.
60
56
F:
52
f
48
s
B
44
40
0
8
4
12
f,
16
W z l
Fig. 7 Inverter modulation frequency dependence of motor noise level bA
for different motor flux control algorithms ( U/f = const, Yvslor
=const ) and different drive frequencies ( FC L.Motec ).
TABLE 1. List of the investigated frequency converters [7]
Country
Firm
Germany
Denmark
LENZE
DANFOSS
35
36