Low-Cost Low-Power Test-Bed for Experimental Investigation ofCommon-Mode Chokes for High-Power Applications

Annette Muetze, Bee See HengDept. of Electrical and Computer Engineering

University of Wisconsin-Madison, Madison, WI 53706 USAmuetze@engr.wisc.edu, bheng@wisc.edu

Abstract- A low-cost method for experimental investigationof common-mode chokes for reducing high-frequency motor Inveter otoground-currents of inverter-based drive systems of several hun-dred kW is presented. It provides a powerful tool during the 3 feed-throughdesign stage of such chokes to verify their predicted performance. common- MOdeThe method draws from the mainly capacitive behavior of - chokes in series -machines at very high frequencies. Results of experimental testsfo drve wit pea grudcurn amltue of mor tha 60 Fig. 2. Simpalified sketch to illustrate use of several singyle-turn feed-throughfor~~~~~~~drvewit pea grudcretapiue fmrhn6

common-mode choke in series instead of wound chokes.Amperes, carried out on a 4kW test-bed, are presented. Theyconfirm the feasibility of such tests as well as the capability ofsmall, inexpensive, single-turn chokes to effectively reduce theground-current. damage assessment [10]. More complicated structures, such

as filters that provide a connection to the inverter-dc link are

I. INTRODUCTION beyond the scope of this contribution. The paper focuses onthe realization of such a method and the rather straightforward,Theh-freq enys(HF) gondcure that canioc simple analysis supporting the development of similar test-

ihenominverer-asDepedndriveontheovesystemsccueirnotparasitic beds. The reported quantitative results shall provide orders ofphenomena. Depending on thoverall syste,n t he magnitudes of reference value for development of further test-drive size and presence of additional mitigation techniques,bdoftikndnohecnexsthese parasitic phenomena can lead to early drive failure due First, the pape iefly revesbto HF circulating bearing currents andl bearing currents dlue to

Frt h ae rel eiw akrudadcneto

rtorHFgcirculatingd b currentsand7These beffsarig ncuren nd the work (Section II). Then, the experimental test-setup is

rgly described (Section III), results are presented and discussedimportant with increasing machine size. Other phenomenainclude wide-band EMI and interference with ground faultprotection systems in industrial facilities, to name some.Common-mode chokes (CM) that are placed in the inverter-output (Fig. 1) can be a cost-effective method of reducing such The machine-inverter configuration is modeled by a per-ground currents [4], [6]. phase linear RLC network, similar to what is done in [4], [5].

Each switching transition is analyzed independently, what isjustified as the ring period is long compared to the transition

Inve er Motor time, particularly with large drives and with effectively usedinLverter = Mt.M.-ootor common-mode chokes [9].Common- The analysis also depends on the resistance value and-code - the degree of damping. As the inductance is increased, the

damping is decreased, assuming the losses in the inductor itselfFig. 1. Simplified sketch to illustrate placement of common-mode choke in are small. Thus, we are typically interested in low-dampingthe inverter-output. cases. Exemplarily, Fig. 3 shows the dependency of the peak

current on the inductance value for different capacitance valuesBecause of the large magnitude of both the HF ground- dv/dt = 2kV/,us, and R = 1Q (simulation results). With

current and the phase currents, where the latter leads to C r lOnF and C - 24nF for machines with 315mm andcomparatively "bulky" motor leads with large diameters, the 400mm shaft height respectively, the HF ground-current ofrequirements for such chokes differ from standard choke fan-cooled machine is 15 to 25% of the rated current when nodesign. Notably, use of several single-turn feed-through chokes mitigation technique is applied, what also has been observedin series is likely to be the more practical choice than wound experimentally [7].chokes [8], [9] (Fig. 2). With respect to the choke design, use of the simplified RLC-

This paper reports on a lost-cost technique to evaluate such circuit is limited, as saturation, non-linearity, and parameterCM-chokes for higher-power applications on low-cost low- dependency on frequency and magnetic utilization are not con-power test-beds. The method extends an approach that was sidered. Furthermore, core material characteristics at multiple,applied in the context of a series of test-runs for bearing high-frequency excitation are often only known approximately,

1 -4244-0743-5/07/$20.OO ©2007 IEEE 1582

Simulated peak HF ground current [A] B. Base-configuration, with CM-chokes80 undler-diampedL..80.nder.damped Five commercially available feed-through common-mode70 chokes, MI through M5, of three different types are used60 for the investigations (Table I); both in the form of selected

C 25 nF / individual chokes and of several combinations of the individual50 7a

C = 20 nF chokes.

C 15 nF TABLE I30 :7 B : :PARAMETERS OF COMMON-MODE CHOKES USEDC = 10 nF j

20 5 t bt Choke Tabulated Window Outer LengthC = 5 nF_; ~~~~~~N\ ~region to obtain

10 r \ current reduction label inductance* diameter diameter---------------- ......... ......

practically irrelevant [/uH] mm mm mm

0 _ MI, M2 24.1 . 48.2 80 63 30M3 23.3...46.6 .63 50 30

1E-12 lE-10 LE-08 I.E-06 1E-04 1E-02 lE+00 M4 23.2..7 63 50 20Common mode inductance [H] M4, M5 13.2 29.7 63 50 20

Fig. 3. Dependency of peak current on inductance values, for different ) AL-value at 10kHz.capacitance values dv/dt = 2kV/,us, R = 1Q.

For evaluation, the chokes are placed between the inverterand the motor. If connected, the additional capacitors are on

and therefore often difficult to include with confidence in more the motor side of the choke (Fig. 5). The tests are carriedcomplex simulations. Thus, while the initial design step can be out with the different chokes and selected combinations, forbased on modeling techniques, experimental evaluation at the different values of Ca.power-level of the corresponding field of application greatlyincreases the degree of confidence on the performance of aCM-choke, leading to the test-setup described in this paper.

Squirrel-cage iinduction motor,only oine phase connected to oine inverter leg.

III. TEST SETUP_ ,- ~~Switch S

A. Base configuration, without CM-chokes, : . . : ~~~~~~~~~~~~~~~~~capacitance Ca_The base configuration consists of a 4kW 230V IGBT-

inverter, a 1 kW squirrel-cage induction motor, and one very Voltage source inveiter,' ~~~~~~onlyon1e phase comnected.

short #6 stranded wire motor-inverter interconnect of only o s25cm length. Only one motor terminal and one inverter leg Fig. 5. Base configuration with additional common-mode chokes. Compareare connected. The cable is kept at minimum length to limit with Fig. 4.its influence on the ground-current generation. Without anyadditional devices, the measured peak HF ground current isIp = 11.9A, which is a typical value for a drive of this C. Some nomenclaturerating. This amplitude is increased artificially by more than500% as follows: Drawing from the understanding that off-the

toThe measured currents and voltages are labelled according

shelf machines show mainly capacitive behavior at the high to the capactance of the additional capacitors Ca and label(s)frequencies of the CM-current, additional high AC voltage ofrtenta lled choke(s) aetpolypropylene foil capacitors are connected between the phase crrentan vog wterminal connection and the protective earth of the drive(Fig. 4).

D. Other configurationsIn a subsequent step, the test-setup is modified to further

analyze the influence of the motor winding on the HF groundSquirrel-cage current. To this aim, the inverter leg is either not connected toinduction motor, the motor at all, but only to the capacitors (Fig. 6) or to twoonly one phaseColnlnected to one of the three motor terminals (Fig. 7). Because of the results

Capacitor wit verterleg. obtained with these configurations (see below, Section IV), the,:~~~a ci nc Ca , Kone with all three motor terminals connected to one inverter

_______________ , F 2_ S ~~~~legwas not investigated.Voltage source invel er, Fo al he'ofgrtosueo Mcoe a nOnly one phase connected. - o o l he ofgrhn sfC-coewain

vestigated (e.g. Fig. 8). The results obtained with the baseFig. 4. Base configuration: one motor terminal connected to one inverter leg configuration (see below, Section IV-A) were used to selectand additional capacitors with capacitance Ca connected at the motor terminal, only two values of the additional capacitors, Ca =2.2nF and

Ca =3OnF.

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I30nF :,. -v3oflF ..................... 300Squirrel-cageinduction motor, 40----Not at allconnected.

capacitance Ca T0I~~ ~ ~ ~ ~ ~ a ~~~0

Voltage source inverter, -, 03,oonly one phase coinnected. -2 ------

Fig. 6. Further configurations: None of the motor terminals, but additional -40 -4------- .... V... 100capacitors with capacttance Ca connected to one inverter leg. Compare with

Fig. 4 ~~~~~~~~~~~~~~~~~~~~~~-60-200--------------- ~~~~~~~~~~~~~~~~~Time[us]

Squirrel-cage Fig. 9. Base configuration: measured currents and voltages with Ca OnFinduction motor, (Is, VO) and Ca -3OnF (I3OnF, V3OnIF).two phasesconnected to oneinverter leg.di cpctne TAs the peak amplitude of the HF ground current is approx-

Iimately proportional to C, we write Ip ox C. Then,Voltage source invem ter,only one phase connected. -1 Ca

Fig. 7. Further configurations: Two of the motor terminals connected to one '[p,nFcm'inverter leg and additional capacitors with capacitance Ca connected at the where Cm is the contribution from the motor winding and ismotor terminal. Compare with Fig. 4. a function of the frequency.

--------------- ~~~~~~~~~Weplot 'P,Ca /'p,nF as a function of Ca (Fig. 10; the figurewill be discussed below in Section IV-B3). We approximate

Toroidal common- Squirrel-cage/Ip,nF~with a second order polynomial and obtainmodechokes induction motor, TpC'pn -0 032C2 + 0.234Ca + 1.1045, (2)

Not at all ' ,al ,F .

conneted, from which we can deriveCapacitor with1

aacitacea Cm ~ nF 4.27nF, (3)~~~~~~~~~~~~~~~~~0.234

Voltage source inveiter,7 which is a typical value for machines of this frame size. Noteonly one phase conne ted - - also that the effective value of Cm at Ca= OnF will be

Fig. 8. Configuration with additional common-mode chokes and none of the somewhat smaller, as the dv/dt is higher.motor terminals connected. Compare with Fig. 6. Without additional capacitors, the dv/dt is approximately

2kV/,us, and with Ca =3OnF it is approximately 1kV/gus. ForCa =3OnF the ring frequency is approximately IMHz, giving

IV. RESULTS 10

A. Base configuration, without CM-chokesL-1

3+4272 062H =0.739puH, (4)

The peak current amplitude with no additional capacitors which is also a realistic value for the self inductance of the

nor common-mode chokes, 'p,OnFO,O is used as the reference short motor leads. Drawing from the measurements withoutvale t qantfytheinfuece f he ddiioal apcitrs ddtioalcapacitance, we obtain L =0.37,uH from Cm=Ca. With increasing value of Ca, the peak current amplitude 4. 27nF, which is still in line with the previous figure, as Cmincreases (Table II), reaching 'p,3OnF,O =63.1A =5.3 - pOF will have been taken somewhat too small and L also decreasesat Ca =3OnF (Fig. 9). with frequency.

TABLE' II B. Bas-cofgrtin ih Mcoe

TABLE IIIThe comparatively large value of the choke inductance sig-

MEASURED AMPLITUDES OF HF GROUND CURRENT, BASEMEASURED APTD OFnificantly reduces the ringing of the ground current, as can beCONFIGURATION, WITH COMMON-MODE CHOKES; 'p,0: PEAK CURRENT

clearly seen in the measurements. Exemplarily, Fig. 11 showsAMPLITUDE WITHOUT CHOKES

the currents and voltages measured in the base configurationCa Choke(s) Ip 'p/'p,0 with Ca = 3OnF and choke M3 and with chokes M3, M4,A

no M3 6.2 0.53 and M5, using the same time-scale as Fig. 9. As a matter ofMl 5.6 0.47 fact, for Ca = 3OnF, an increase of the period time by a factorMI, M2 3.1 0.26 of ten gives an inductance value of 73.9,uH. Fig. 12 showsM3, M4, M5 4.4 0.37

2.2nF M3 6.9 0.33 the same results as Fig. 11 but using a larger time-scale. DueMl 6.9 0.33 to the large damping of the system, and only approximately,Ml, M2 4.4 0.21M3, M4 5.0 0.24 oscillation occurs only for half of the period time.

l0nF M3 10.6 0.29Ml 9.4 0.25MI, M2 6.3 0.17M3, M4, M5 7.5 0.20 V30nF,M3,4,5 300

2OnF M3 14.4 0.27M l11.90.22 40 -------------- ...

MI, M2 7.5 0.14 I3OnFM3 200M3, M4, M5 8.1 0.15 .

3OnF M3 16.9 0.27ml 13.8 0.2210M1, M2 6.9 0.112 0M3, M4, M5 9.4 0.15 0V30nF,M3 2 3 4 5 >

-20~~~~~~~~!.............. .................

=...=W-40considered (Fig. 10). For the given orders of magnitudes,the approximately linear increase of the peak ground current -60 -200amplitude with increasing values of Ca is also found with the Time [us]use of chokes.

6 Fig. 11. Base configuration: measured currents and voltages with Ca = 3OnFand choke M3 (/3OnFM3, V3onFM3) and with Ca = 3OnF and chokes M3, M4,and M5 (/30nF,M3,4,5, V30nF,M3,4,5)- Compare with Fig. 9.

5

4no chokes=

cL 3 - % M3 60V3onF,M3,4,5 300

2~~~~~~~~~~~~~~~~~~~~~~~~~~~..... 40 '13OnF,M --/---~=200

M3,4,5 ¢2020~ ~ ~ ....... i

u t ~~~~~~~~~~~~~~/__IF34ioo0 0 ct~~~~~~~~~~~~~~~~~~~~~~~~~~

0 5 10 15 20 25 30 V2 3 4 5 6 7 8 9 10O >Ca [nF] -20 -

-40 -100Fig. 10. Base configuration: current increase with additional capacitance \30nFM3Ca without choke and with different chokes, Ip,onF: peak amplitude with no -60 - -200additional capacitors, Ip: peak current amplitude with additional capacitors Time [us]with capacitance Ca.

With respect to the effectiveness of the chokes to reduce the Fig. 12. Base configuration: measured currents and voltages with Ca = 3OnFgroundcurrent,chokes MI and M3 with approximately the and choke M3 (I3OnFM3, V3onFM3) and with Ca = 3OnF and chokes M3, M4,ground current,chkes also t lte approximately and M5 (I30nFM3,4,5, V30nFM3,4,5). Larger time-scale than Fig. 11.

same tabulated inductances also translate into approximatelythe same reduction of the HF ground current amplitude. Animportant finding is that this current reduction is approxi-mately the same for all values of Ca, except for Ca =OnF C. Other configurations(down to 30%). In a subsequent step, the test-setup is modified with respectThe approximately 2 or 2.5 times higher inductances of to the number of motor terminals that are connected (Sec-

the combinations of chokes do lead to an additional current tion JJJ-D). The results of these investigations are summarizedreduction of 10 to 15%. in Table IV and also illustrated in Fig. 13.

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TABLE IVTABLEREAMPLIUDESOHF GROUNDCURRENT,DIwork. However, the effect needs to be paid attention to whenMEASURED AMPLITUDES OF HF GROUND CURRENT, DIFFER investigating drive systems with relatively small rated power.

CONFIGURATIONS; Ipo: PEAK CURRENT WITHOUT CHOKES

# of connected motor terminals2 1 0 V. CONCLUSIONS

Ca, Ip 1p/1p,O Ip 1p/1p,O Ip 1p/1p,O When designing common-mode chokes for use in theChoke(s) A A A2.2nF inverter-output of higher-power inverter-based drive systems,M3 7.8 0.33 6.9 0.33 6.9 1.83* experimental evaluation of such chokes during the designMl 9.1 0.39 6.9 0.33 5.9 1.58* stage is a valuable tool. For this purpose, a low-cost techniqueMI, M2 6.3 0.27 4.4 0.21 4.1 1.08* .M3, M4, M5 6.3 0.27 5.0 0.24 5.62 1.50* using a low- power test-bed can be applied to replace a more

3OnF costly, higher-power test-bed. This was illustrated exemplarilyM3 21.9 0.31 16.9 0.27 20.3 0.30 by using a 4kW drive to experimentally investigate theMl 17.2 0.24 13.8 0.22 15.6 0.23MI, M2 14.1 0.20 6.9 0.11 9.4 0.14 performance of several common-mode chokes to reduce HFM3, M4, M5 17.1 0.24 9.4 0.15 12.5 0.19 ground currents with amplitudes typical for drives up to

*): System is underdamped. A higher value of Ca is required. 5OOkW rated power.

1.0 REFERENCES

[1] 5. Chen, T.A. Lipo, and D. Novotny, "Circulating type motor bearing0.8 current in inverter drives," Proceedings 31th IEEE Industry Society

2 motor terminals, Ca = 2.2nF Annual Meeting, vol. 1, pp. 162-167, 1996.OG X X?2motor terminals Ca 3OnF [2] J. Ollila, T. Hammar, J. Lisakkala, and H. Tuusa, "A new reason forX 0.6 1 t bearing current damanges in variable speed drives," Proceedings 7th}I R X / / 1rInotor terminal, Ca= 2.2nF$; : X //-1motor terminanl Ca 30nF European Conference on Power Electronics and Applications (EPE),

04 0 motoroterminals pp. 2539-2542, Trondheim, 1997..[3]pJ. Link, "Minimizing electric bearing currents in ASD systems," IEEE

[4] S. Ogasawara and H. Akagi, "Modeling and damping of high-frequency0.0 ~~~~~~~~~~~~~~~~~leakagecurrents in PWM inverter-fed AC motor drive systems," IEEE

Transactions in Industry Applications, vol. 32, pp. 1105-1114, 1996.no chokes M3 Ml M1,2 M3,4,5 [5] I. Boldea and S.A. Nasar, Electric Drives, CRC Press LLC, Florida,

Choke(s) 1999.[6] C. Mei, J.C. Balda, W.P. Waite, and K. Carr, "Minimization and

Fig. 13. Current reduction with use of different chokes (different config- cancellation of common-mode currents, shaft voltages, and bearingurations - # of connected motor terminals), Ca = 2.2nF/3OnF; Ip,0: peak currents for induction motor drives," Proceedings 34th Power Eectronicsamplitude without chokes, Ip: peak current amplitude with chokes. Specialist Conference (PESC), vol. 3, pp. 1127-1132, Cape Girardeau,

2003.[7] A. Muetze, Bearing currents in inverter-fed AC motors, PhD-Thesis,

Darmstadt University of Technology, Shaker Verlag, Aachen, 2004.The results show that the current reduction achieved with [8] A. Muetze, "Scaling issues for common mode chokes to mitigate

ground currents in inverter-based drive systems," Proceedings 40th IEEEthe chokes is almost independent of the number of connected Industry Society Annual Meeting, vol. 3, pp. 1860-1867, Hong Kong,motor terminals, except for the case of the motor not connected October 2-6, 2005.

[9] A. Muetze and C.R. Sullivan, "Simplified Design of Common-Modeand Ca 2. 2nF Fora2.nF,tesystmisnderdmped, Chokes for Reduction of Motor Ground Currents in Inverter Drives,"

and a larger value of Ca is required to properly investigate Proceedings 41th IEEE Industry Society Annual Meeting, Tampa, FL,the performances of the chokes under larger currents. These October 8-12, 2006.

lgvathe large [10] A. Muetze, A. Binder, H. Vogel, and J. Hering, "What can bearings bear?larger values of Ca are already required to generate the large - How much current is too much? How much current reduction enough?"common-mode current amplitudes that are typical drives with IEEE Magazine on Industry Applications, vol. 12, no. 6, pp. 57-64,comparatively large rated power, which are in the focus of this November/December 2006.

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