Sound power adiated from an inverter-driven induction motor: experimental investigation

C. Wang, A. Astfalck and J.C.S. Lai

Abstract: The effects of various inverter strategies, switching frequency, random modulation levels and motor speeds on the radiated noise of a 2.2 kW induction motor with and without load were examined by measuring the sound power spectrum, rather than the sound pressure spectrum at a single point in space. Results show that the sound power level due to the aerodynamic and mechanical noise increases at a rate of 12dB per doubling of the motor speed. For an ideal, almost purely sinusoidal supply, electromagnetic noise dominates at low motor speeds. For a PWM inverter drive, the electromagnetic noise due to the switchmg harmonics could dominate the total sound power level at low motor speeds; however, the effects of switchmg harmonics appear to be insignificant at high motor speeds where the noise due to the fundamental frequency is dominant. At high speeds, the variation of sound power level with load is small, whereas at low speeds the total sound power level normally increases due to the low-frequency component.

1 Introduction

Because of the low cost and reliability, induction motors have been used extensively in industry wherever an AC supply is available. With the advent of electronic inverters the induction motor’s speed can be easily controlled to suit variable-speed applications. However, inverters may in- crease the acoustic noise emitted by the motor, as they introduce harmonics in the voltage/current supply due to switching.

Various strategies have been proposed to minimise and/ or eliminate the effects of these harmonics [l-lo], including modified PWM [l, 21, harmonic elimination [3, 41, altering the switching frequency [5] and random modulations [&SI. Most of these studies, however, are concentrated on the electromagnetic aspekts, with less attention paid to the acoustics. Since the systems used and the operating conditions differ from one study to another, the conclusions drawn may be incomparable and less representative. As pointed out by Timar and Lai [ l l , 121, the electromagnetic noise due to the change in speed can be very significant even for an ideal drive with no harmonics in the supply. Also, the electromagnetic noise could increase rapidly with speed and at some speeds, known as characteristic speeds, there could exist some local resonances producing hgh levels of the electromagnetic noise. So the study of the electromagnetic noise should be extended to the full speed range, with a thorough, systematic assessment of the acoustic effects of control schemes when the speed and load of the motor change.

0 IEE, 2002 IEE Proceedings online no. 20020067 DOL 10.1049/ip-epa:20020067 Paper first received 10th April 2001 and in revised form 3rd October 2001 C. Wang is with the Proving Ground of General Motors Corp., Mail code: 4b3- 394201, Milford MI48380, USA A. Astfalck is with the FASCO, Australia J.C.S. Lai is with the Acoustics and Vibration Unit, University College, The University of New South Wales, Australian Defence Force Academy, CanbelTa ACT2600, Australia

Most previous studies were involved with measurements of the sound pressure at a single point in space [S-lo]. Strictly spealung, the sound pressure at a single point in space is not an adequate or even appropriate indicator used to assess the acoustic behaviour of an induction motor. This is not only because the sound pressure measurement may be affected by the environment, but also because the acoustic radiation from motor structures is usually directional. Generally, such directivity may change with operating conditions and become more pronounced for a narrow band analysis at high frequencies. Thus the conclusions drawn from single-point measurements may be misleading. Sound power, being the total sound energy radiated per unit time from a specific device, has been widely accepted as the appropriate indicator of the noise-producing capacity of the device. For induction motors the sound power is normally given at the rated speed, which may not be useful to consumers when the motor is to be run over a wide speed range with the supply containing plenty of harmonics. There are, however, no common procedures or even an appropriate international standard available [ 131 to measure the sound power of a variable-speed induction motor. Timar and Hallenius [ 141 suggested that measurements should be made at the characteristic speeds; however, the characteristic speeds need to be confirmed and determined by experiments.

The objective of ths paper is to investigate the acoustic response of an inverter-driven induction motor under electromagnetic force excitation by measuring the radiated acoustic power for an almost sinusoidal supply and two different commercially available inverters. The effects of the switching frequency and the random modulation levels of the inverter, the motor speed and the load on the radiated acoustic noise are discussed.

2 Experimental configurations and procedures

2. I Motor The test motor was a Brook Crompton Betts three-phase, four-pole 415V, 50Hz, 2.2kW induction motor with 44 rotor slots and 36 stator slots.

46 IEE ProcElectr. Power Appl., Vol. 149, No. I , January 2002

2.2 inverters Three configurations for the speed controllers were tested: benchmark, a PWM inverter and a random modulation inverter, as described below.

2.2. I Benchmark: The purpose of using an almost sinusoidal drive is to provide an ideal state in which the effects of the harmonics can be neglected, thus setting the benchmark for other inverters to compare with. The drive, hereafter referred to as the benchmark, consists of a General Electric 6.6 kW DC motor connected in shunt ,and mechanically coupled to drive a three-phase 400 V 5 kW synchronous generator, as shown in Fig. 1. This setup provides a Variable-frequency, variable-voltage three-phase output. The harmonics in the output current and voltage from this configuration are more than 20 dB lower than the fundamental frequency in magnitude, as shown in Fig. 2a.

bridge rectifier& variac

bridae

I l l LT'

Fig. 1 Schematic diagram of benchmark conjguration

2.2.2 PWM inverter (inverter- I): The sinusoidal PWM inverter used in the tests is type MSC manufactured by Zener Electric, based on symmetric PWM, controlled by a microcontroller and updated every cycle. The switching frequency of the inverter is proportional to the test speed of the motor at a rate of 21 Hz/Hz. At the rated speed of the motor, 1500 rev/min, the switching frequency was 1050 Hz. Fig. 2b shows that when the fundamental frequency was 35Hz, the 19th and 23rd harmonics in the current and voltage output of this inverter were even hgher in magnitude than the fundamental.

2.2.3 Random modulation inverter (inverter-;?): The random modulation inverter used is type VSC2000, also manufactured by Zener Electric, based on symmetric PWM, controlled by a microcontroller and updated at 1 kHz independent of the switchng frequency. T h s inverter allows eight different fixed switchmg frequencies of 1, 2, 3, 4, 5, 6, 7 and 8 kHz. Also, three random modulation levels in which level 0 represents no random modulation, levels 1 and 2 represent a pseudorandom modulation with a spread of 10 and 20%, respectively, are available. With the switching frequency at 8 kHz, which is much higher than the fundamental frequency of 50 Hz for the rated speed, the harmonic content of this inverter below 8 kHz is of the same order as the benchmark, as shown in Fig. 2c. In the following discussion, unless otherwise stated, the switchmg frequency and the random modulation level of this inverter was set to 8 kHz and level 0, respectively.

To maintain a constant flux in the air gap between the rotor and the stator, and hence a constant torque over the speed range, the ratio between the voltage and the fundamental frequency for the three inverter configurations was kept the same in the measurements.

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2.3 Procedure Experiments were conducted with the motor under no load and load conditions. For no-load tests the motor was driven by the three inverters described above separately for a range of motor speeds, as given in Table 1. In addition, for inverter-2, the effects of different switching frequencies and random modulation levels on the acoustic noise were also examined.

For the load tests the motor was mechanically coupled to a 1.2 kW DC motor via a torque transducer, as shown in Fig. 3. The load motor was operated in its generating mode with its armature connected to a variable resistor bank. The

IEE Proc.-Electr. Power Appl.. Vol. 149, No. I , January 2002 47

Table 1: Speeds tested for sound power measuremen* ~~

Speed Aerodyn. No load (rev/min) noise

Full load

benchmark inverter-I inverter-2 benchmark inverter-I . inverter-2

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900

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1250

1260

1280

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1360

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1450

1460

1500

v v v

v v v

v v

v v v v

v v v

v

v v v v

v v

v v

wire frame

motor

load mot01

torque transducer

Fig. 3 Experiinentalset-up for load test in anechoic room

field current for the load motor was supplied by a variable DC power supply. By varying the voltage of the supply the load provided can be adjusted. The full load of the motor tested was 15". Note that the load of the motor would change the rotor slip and therefore the motor speed [15]. The effects of different loads were examined by keeping either the fundamental frequency of the supply constant or the motor speed constant.

To separate the sound power due to the electromagnetic noise (whch is the primary concern of t h s paper) from the total sound power, the sound power of the aerodynamic and mechanical noise at seven different speeds (Table 1) was measured with the motor uncoupled from the supply and driven by the 1.2 kW DC motor.

The acoustic measurements were conducted in an anechoic room with dimensions 3.5 x 3.5 x 3.5m and a lower cut-off frequency of 150 Hz. The motor was mounted in the centre of the room and rested on a test frame. All the equipment for driving the motor was located outside the anechoic room. The acoustic power was measured by scanning a sound intensity probe over five surfaces (the bottom surface, being a lOmm thick aluminium plate, is acoustically rigid) enclosing the motor according to IS09614Part 2 [16]. In load tests a 10" thck aluminium plate, softly connected on the frame, was inserted between the motor and the load to minimise the noise contribution

from the load, as shown in Fig. 3. In this case, only four surfaces were scanned. The use of a sound intensity measurement system also helps to suppress the noise of the load from contributing to the overall measured acoustic power. The sound intensity system comprises a B&K type 2032 dual-channel FFT analyser and a B&K type 3519 sound intensity probe. Since the switching frequency of inverter-2 could be up to 8 kHz, two spacers of 12 and 3 mm were used respectively, for a pair of phase-matched half- inch microphones, B&K type 4181, and a pair of phase- matched quarter-inch microphones, B&K type 41 35. Thus the valid frequency range for these two spacers covers from 125Hz to about 18kHz. The Hanning window with 50% overlap and 400 spectra averages were used to obtain the sound intensity spectra. The quarter-inch microphones were only used for high-frequency measurements. The sound intensity measurement system was calibrated using a sound intensity calibrator, B&K type 3541. In addition, a multi- function acoustic calibrator, B&K type 4226, was used for the calibration at 8 kHz. The frequency resolution of the analyser was set to 8 Hz for the 12 mm spacer, and 16 Hz for the 3mm spacer. The sound intensity data were processed by a HP series 300 microcomputer to give the sound power of the motor.

3 Results and discussions

Typical narrowband (8Hz) sound power spectra for no load tests are shown in Figs. 4 and 5 for two operating speeds, 900 and 1500rev/min. It is clear from the results that irrespective of the motor speed, the motor driven by inverter-I is the noisiest while the benchmark drive is the quietest. Also, for frequencies above lOOOHz, the harmo- nics for inverter-1 are densely packed, while inverter-2 has a prominent peak centred around 8kHz. The modal behaviour of the motor structure was discussed in [17].

3.1 Effects of motor speed The total sound power produced by the motor driven by the two inverters and the benchmark at each motor speed with no load is compared in Fig. 6a. Clearly, for all three drives the total sound power increases with speed. For compar- ison, the aerodynamic and mechanical noise measured by driving the motor at different speeds is also plotted in Fig. 6a. It can be seen that for the motor tested the sound

IEE Proc-Elrctr. Power Appl . Vol. 149, No. I . Junuriry 2002 48

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Soundpower spectru of motor driven bv three inverters ut

power of the aerodynamic and mechanical noise increases with speed linearly at a rate of about 12dB, rather than 15dB [18, 191, per doubling of the speed. Compared with this, the total sound power produced by the benchmark drive is about 5dB higher on the average, and increases with the speed at about the same rate (above 900 revjmin). By subtracting the aerodynamic and mechanical noise from the total sound-power, the sound power due to electro- magnetic noise of the three drives is compared with that of the aerodynamic and mechanical noise in Fig. 6b. It can be seen that the electromagnetic noise with only the funda- mental frequency in the supply is on average about 3 dB higher than the aerodynamic and mechanical noise even at high speeds. By examining the power spectra of the benchmark inverter and aerodynamic and mechanical noise shown in Figs. 4 and 5, it can be inferred that as the motor speed increases, it is the low-frequency noise around 1 kHz that dominates the total sound power level.

For inverter-1, the aerodynamic and mechanical noise is 30 and lOdB lower than the total sound power level (Fig. 6a) and the sound power level of electromagnetic origin (Fig. 6b) at 450 and 1500rev/min, respectively. Therefore over the speed range tested, the electromagnetic noise js dominant so that the influence of the aerodynamic and mechanical noise can be neglected. Ths is because the switching frequency of this inverter is around 900Hz and plenty of harmonics are populated in the supply. In fact

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Fig. 5 1.500 revlmin a Benchmark and Inverter-1 b Benchmark and Inverter-2

Soundpower spectra of inotor driven by three iniierters at

Fig. 6a shows that below 1000rev/min the total sound power level is almost independent of the operating speed, but from 1000 to 1500rev/min the total sound power level increases only by about 5 or 6 dB. Similar to the benchmark drive, ths increase is mainly attributed to the increase of the low-frequency noise at around 1 kHz (Figs. 4 and 5).

For inverter-2, there is a distinguished peak around the 8 kHz switchmg frequency in the sound power spectrum. The total sound power contributed by only the components below 6 kHz is very close to that of the benchmark drive, as shown in Fig. 6. Ths can also be observed from the corresponding sound power spectra in Figs. 4 and 5. Thus the switching frequency of an inverter contributes signifi- cantly to the total sound power level. Fig. 6 shows that the effect of the switching frequency on the total noise is substantial at low speeds. As the operating speed increases, the difference between the sound power level for 0-6 kHz and that for (rl8kHz decreases. This indicates that the increase of the total sound power level due to the switching harmonics is not as much as that due to the fundamental frequency in the supply. This is because, from the sound spectra in Fig. 5 at high motor speeds the peak around 1 kHz (whch is a mode of the stator [ 171) is hgher than that around the switching frequency of 8 kHz.

Although the benchmark drive at 450 rev/min .was not tested the sound power results of inverter-2 for M k H z may be used instead because Fig. 6 shows that the results of inverter-2 at low speeds are almost the same as those of the benchmark. Fig. 6 clearly shows that at low motor speeds

IEE Proc-Electr. Power Appl., Vol. 149, No. I , Jrmunry 2002 49

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a

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400 500 600 700 800 900 10001100 120013001400 1500

b rotor speed, revlmin

Fig. 6 Soundpower aguinst motor speed a Total sound power b Sound power of electromagnetic noise: * aerodynamic and mechanical noise, 0 benchmark, A inverter-1, 0 inverter-2, + inverter-2 below 6 kHz

the electromagnetic noise dominates the total sound power even though there are virtually no harmonics in the supply.

Timar and Lai [ l l , 121 argued that for variable-speed induction motors there should exist some characteristic: motor speeds at which the total sound power exhbits peaks due to the coincidences between the motor speed and the resonances of the motor structure. From Fig. 6 it can be seen that this speed does exist for all three inverters, for example at 1280 revimin.

3.2 Effects of switching frequencies and random modulation levels Figs. 4 and 5 show that the sound power spectra for all three inverters are similar for frequencies below 1 kHz. The difference between inverter-1 and the benchmark drive becomes more pronounced above 1 kHz, while that between inverter-2 and the benchmark spectra is more significant around 8 kHz. This is because the switching frequency of inverter- 1 is around 900 Hz and that of inverter-2 is 8 kHz. To further examine the effects of the switching frequencies the sound power at a fixed motor speed of 1500 rev/min with no load but for various switchng frequencies of inverter-2 was measured. The random modulation level wzs still set at level 0. Fig. 7 shows that the peaks of the sound power spectrum are shifted to higher frequencies in accordance with the switchng frequency. Fig. 8 shows that generally the variation of the total sound power level with the switching frequency is small. This is because the low- frequency noise, say around 1 kHz, is more dominant at 1500 rev/min. However, when the switchng frequency is 1 kHz the switchng harmonics enhance the noise in this region, and increase the total sound power level by about 2 dB compared with a switching frequency of 2 kHz. It has been argued that, as far as noise reduction is concerned, the

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Variation of sound power spectra with switching frequency

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Fig. 8 switching frequency at 1500 revlmin

Totalsound power of motor driven by inverter-2 against

switching frequency should be as high as possible because hgher harmonics could be shfted out of the audible frequency range [5 ] . However, Fig. 8 shows that the sound power still could increase even when the switching frequency is relatively high, for example at 6 kHz. Malfait et al. argued that ths may be caused by the subharmonics for a specific switching frequency [5]. It can be seen from Fig. 7 that for the switching frequency of 6 kHz, significant peaks appear at 2 and 1 kHz. This is attributed to the complex interactions between the subharmonics of the

IEE Proc.-Electr. Power Appl., Vol. 149, No. 1, January 2002 50

switching frequency, the vibration modes and the frequency of updating (1 kHz) in the PWM algorithm. The magni- tudes of these peaks vary periodically as the switching frequency changes as shown in Fig. 7. As a result, the total sound power level at the switching frequency of 6 kHz is about 6 dB higher than that at 7 kHz. Thus, although it is generally true that shifting the harmonics out of the audible frequency range could reduce the noise emitted from a motor, the switching frequency should be set properly to avoid the enhancement of the low-frequency vibration modes by the switching harmonics. It would therefore be worthwhile to study in future the generation mechanism of subharmonics for a particular switchng frequency and the interaction between the switchng harmonics and the low- frequency vibration modes.

At low motor speeds where the switching harmonics could dominate the total sound power level, a significant reduction in radiated sound power may be achieved by selecting a high switchng frequency, which generally implies higher switching losses. By comparison with inverter-1 of which the switching frequency is around 900Hz, it can be seen from Fig. 6 that at all tested speeds, inverter-2 is much quieter although there is no random modulation.

The effect of the random modulation on acoustic noise was studied by implementing random modulation levels 1 and 2 on inverter-2 at two operating speeds, 1500 and 900 rev/min respectively. The switching frequency was set at 8 kHz. The total sound power spectrum for modulation level 0 at 1500rev/min is shown in Fig. 5. For random modulation levels I and 2, it was found that although the spikes around 8 kHz were greatly reduced, the total sound power level does not vary by more than 0.2dB, even at 900rev/min, supporting the argument of Covic et a1 [8]. Ths result suggests that the randomisation of the switching pattern only changes the spectrum of the electromagnetic force but not the magnitude of the total force. At high frequencies, because the vibroacoustic response of a motor structure is more uniform due to a hgh modal density, the power input to the structure would not change very much. It may therefore be argued that, if the magnitude of the total force could not be effectively reduced, the total sound power might only be reduced when the random modulation technique is implemented with a low switching frequency. This is because at low frequencies the vibration modes do not cluster together, and the spreading of the electromag- netic force spectrum may effectively avoid the coincidence between the harmonics and vibration modes and reduce the power input to the motor structure. However, this is a fallacy because if the switching frequency is set high there are few harmonics in the low-frequency range. Further- more, in the experiment it was subjectively perceived that the motor was significantly quieter when random modula- tion level 2 was implemented. T h s is because the tonal nature of the high-frequency noise is greatly reduced. Therefore to reduce noise radiation from a motor, the harmonics as well as the total energy input to the motor structure have to be reduced.

3.3 Load test results and analysis In load tests, three sets of measurements were made: the variation of the sound power with the motor speed at full load, the variation of the sound power level with load at a constant fundamental frequency and at a constant speed. By examining narrowband sound power spectra for the three drives [20], it has been found that similar to the no- load tests, inverter-1 drive is still the noisiest and the benchmark drive is still the quietest under load.

IEE Proc.-Electr. Power Appl., Vol. 149, No. I , Janucrry 2002

Fig. 9 shows that at hgh motor speeds the variation of the acoustic power level for the three inverters with the load torque is small (within 3dB) both when the fundamental frequency is kept constant at 50Hz and when the motor speed is maintained at 1375 rev/min. It has been argued that an external fluctuating load of a motor would produce additional harmonics in the electromagnetic force and thus in the acoustic response [16]. However, as far as the total acoustic power level is concerned, the effects of a steady external load appear to be insignificant. Fig. 10a shows the variation of the sound power due to the electromagnetic noise (obtained by subtracting the aerodynamic and mechanical noise from the total sound power) with the motor speed at full load. For inverter-I the variation of the sound power level is small (within 2 dB), while for inverter-:! and benchmark drive (the sound power of inverter-2 below 6 kHz is considered to be the same as that of the benchmark inverter) the sound power level increases with the motor speed similar to the no load condition.

rotor speed, revlmin

1460 1426 1370

- % 60 - 0 10 15

torque, Nm a

fundamental frequency, Hz

45 46 48 50

I I I 0 5 10 15

torque, Nm b

Fig. 9 Vuriution of sound power level with loud torque a Fundamental frequency = 50 Hz b Motor speed= 1375rev/min: A inverter-1, 0 inverter-2, 0 benchmark

The variation of the sound power level of the electro- magnetic noise with speed for inverter-I and inverter-2 at full load and no load is compared in Fig. lob. At a low speed of 450 rev/min, the sound power for the two inverters increases by about 5dB from no load to full load. As the motor speed increases, the difference between the no-load and full-load results decreases. Consequently, for inverter- 1 the sound power level at full load is not very sensitive to the motor speed, while for inverter-2 the sound power level at full load does not increase with the motor speed as fast as that at no load. By comparing the narrowband sound power spectra at full load [20] with no load results, it was found that the difference in the total sound power level is due to the low-frequency components, which are attributed to the external loads. Therefore in future work, attention should be paid to the low-frequency noise, especially when the motor is loaded.

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400 500 600 700 800 900 1000 1 100 1200 1300 1400 1500 a

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55 F 250 500 750 1000 1250 1500 1750

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Fig. 10 Soundpower against motor speed ut full load U Sound power of electromagnetic noise: * aerodynamic and mechanical noise, A inverter-I, 0 inverter-2, + inverter-2 below 6 kHz b Comparisons of sound power level due to electromagnetic noise a t full load with no load: 0 inverter-1, no load, 0 inverter-2, no load

4 Conclusions

0 The sbund power level due to the aerodynamic and mechanical noise increases at a rate of 12dB per doubling of the motor speed. 0 The electromagnetic noise also increases with the motor speed and can be a significant noise source even at high motor speeds.

0 As the motor speed increases there exist specific speeds which could excite particular structural modes, causing peaks in the total sound power.

0 The switching harmonics in the supply affect the noise radiated from a motor very much. At low motor speeds the noise due to the switching harmonics could dominate the total sound power level. Therefore it would be effective tu reduce the radiated noise by setting the switching frequency as high as possible. At hgh motor speeds, because the noise due to the fundamental frequency is dominant, the effect of the switchng harmonics on the total sound power level is insignificant in general. However, a better effect could be obtained by setting the switching frequency to avoid the interaction between the switching harmonics and the low- frequency vibration modes.

0 The effects of the random modulation technique might depend on the switching frequency settings. For high switching frequencies the sound power would not change much although the tonal nature of the noise may be reduced. For low switching frequencies the sound power

could be reduced with hgher modulation level by avoiding the coincidence between the harmonics and vibration modes.

CB The effect of a steady external load on the total sound power level depends on the motor speed. At high speeds the variation of the sound power level with the load torque is small. At low speeds the total sound power level normally increases due to the low-frequency component.

5 Acknowledgments

The project has been supported by the Australian Research Council under the large grant scheme. The motor and the inverters were provided by Fasco, Australia, and Zener Electric, Australia, respectively. C.Wang acknowledges receipt of an overseas postgraduate research scholarship for the pursuit of ths study.

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References

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52 IEE Proc-Electr. Power Appl.. Vol. 149, No. 1. Januriry 2002