the CM currents to flow on a motor side of the system and reduces the conducted emissions measured on a supply side of the inverter (e.g. by means of LISN).
Common Mode Current Paths and Their Modeling in PWM Inverter-Fed Drives Adam Kempski, Robert Smolenski, Ryszard Strzelecki University of Zielona Gora Institute of Electrical Engineering ul. Podgoma 50 65-246 Zielona Gora
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components, especially in the motor cable and between the stator windings and a frame. It causes spreading of the CM currents over the drive system including a supply and grounding arrangement. The CM currents on their way hack to the source of interference can flow through the supply system of the inverter, as it is suggested in some papers [1-2] or through DC-link to heat sink capacitance [3-51. The aim of our work is to establish real paths of the CM currents and construct a model, as simple as possible, which would be capable of mapping of the measurement results.
Absfrad This paper investigates the paths of common mode currents in a PWM drive system. On the ground of the experimental results a high frequency model of this system has been established. Separate analysis of the drive components and their contributions to common mode current flows have been done. On this basis we have ascertained that heat sinkto-DC link capacitance is essential for creating the grounding currents paths. The large value of this capacitance constrains the CM currents to flow on a motor side of the system and reduces the conducted emissionsmeasured on a supply side of the inverter (e.g. by means of LISN). In the case of a small value of heat sink-to-DC Link capacitance a larger part of the CM currents can flow via mains, and the inductance of this path contributes to a creation of a low frequency part of the CM currents. In spite of lumped simplitications the model allows to predict the mechanisms of creations of oscillations of the CM currents in a real system with a good accuracy.
RESULTS 11. EXPERIMENTAL
I. INTRODUCTION The origin of the common mode conducted EM1 (ElectroMagnetic Interference) is the source of the common mode (CM) voltage inevitably existing at the output of the inverter. The s u m of phase voltages in a neutral point of system with respect to ground is not equal to zero as the result of tempomy electrical asymmetry. The CM voltage is a staircase function with the step i1/3 Ud (U, - dc link voltage), Fig.1.
In order to trace the real CM currents paths the measurements have been taken in a typical PWM drive system, which has been supplied and grounded in a manner commonly used in practice. The experimental arrangement and measuring points are depicted in the Fig.3. We have tested 2-pole 1.5 kW induction motor fed by a commercial available industrial inverter. All measurements have heen done using the current probes with linear frequency range up to 50MHz. Fig.2. shows the experimental results of a CM currents passage through the system, and the CM currents paths are marked in Fig.3. by means of the arrows of different thickness.
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The steep pulses of the CM voltage excite the parasitic capacitive couplings in the drive system This work has been supported by rhe Polish Committee for Scientific Research under Grant 8TIOA 034 21.
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The common mode currents, which are generated by switching states of the inverter, close partly inside the cable. Next, the CM currents split according to the
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it 4 proportion of the HF impedance of a PE cable wire (or shield) and the HF impedance of the grounding arrangement between the grounding points of the inverter and the motor. The main retum path for the CM currents leads via heat sink-to-DC link capacitance. The CM current causes a CM voltage drop on this capacitance. In a blocking state of diodes of the rectifier only a small part of this current flows through the inverter supply arrangement. In the conduction state this voltage drop causes oscillation of small amplitude and relatively low frequency in a closed loop consisting of DC-link-to-heat sink capacitance and resultant inductance of the mains (or LISN), the cable and the input filter. 111. MODELING OF CM CURRENT PATH
A. Introduction The modeling of the common mode paths in the multiphase circuits is a specific process. Firstly, in a threephase PWM inverter-fed motor drive (a balanced circuit for differential signals) three legs of the invexter can be reduced to its single-phase CM voltage source, and three legs of a load of the inverter can be effectively connected in parallel [4]. Secondly, very short switching times of output voltages of the inverter and the capacitive couplings in the CM current paths make the waveforms and amplitudes of the CM currents very sensitive to the shape of CM voltage. Therefore, if we want to get a proper representation of the real CM currents waveforms, we have to apply the real waveform of output voltage (not ideal switching) in the model. Finally, because of large dimensions of the circuits and HF spectrum of interferences the phenomena in the PWM inverter-fed motor drive have essentially a wave character.
large value of DC link capacitor makes the buses of DC l i effectively equipotential for the high frequency CM currents [4], in our model inverter is represented by its DC link-to-heat sink capacitance and a CM voltage source based on a real switching instant of the inverter used in the measurements. Some types of the investigated inverters can create EM1 noise themselves. However, this current does not affect the CM currents flows on the load side of the inverter. C. Modeling of cable On account of high dddt at the output of the inverter, the CM currents in the motor cable are, in fact, traveling waves phenomena, especially in case of a very long cable [6]. Amplitudes of the CM currents in the three-core cables strongly depend on the location of the cable due to different values of transverse to-ground capacitances. To fmd this factor, the measurements on the cable with a PE wire have been taken. The results in that arrangement in a qualitative manner correspond to a shielded three-core cable. Fig.4. shows the CM currents at the output of the inverter in the cables of a different length. The cables have been openended. In this case, the theory of a long line is applicable. For this purpose, we have applied the model of a long transmission lossy line. Fig.5. depicts the results of simulation of an open-ended line fed by the source of interferencebased on the real switching instant As we can see in the figures, for the shorter lengths of the cable the CM current due to high dddt is influenced by currents resulting from the multiple reflections from the both ends of the cable. For a long length of the cable the damped oscillatoly waveform of the CM current is formed according to the lossy line theory. As we can see in the Fig.4. in this case, amplitudes are higher.
B. Modeling of inverter output Our measurements have shown that in a inverter no-load condition, in spite of presence of the switching instants, a current in the PE wire of the inverter is nearly about zero. On the other hand, in the load conditions the capacitance between DC-link and heat sink can he a main part of a retum path of the CM currents from the cable and the motor, as it has been presented above in Fig.3. Because the
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Fig.5. Common mode c m t s in open-ended cable with PE or shield (simulationresults).
D.Modeling of motor Fig.6, shows the experimental results of the influence of the length of a PE motor wire on the shape and frequency of the CM current. The motor has been supplied via a threecore cable (as sholt as possible). The measurements taken in such a system have been the hasis for determining the structure and parameters of the motor model.
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The measurement and simulation results of the CM currents flows in the system are compared in Fig.10.
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The simulation and experimental results fit together very well. In the model we have applied the ladder representation of the long line, because the part of a CM current generated in the motor flows back along the whole PE wire of the cable (or shield). The simple algebraic representation of the lossy line and its implementation in the common use simulating packages is less u s e l l in this case. However, as we have proved, an application of the “algebraic” lossy line and the lumped representation of the PE cable wire do not affect considerable the CM currents shapes and frequencies. It is a subject of our further researches.
REFERENCES
[21.
~31. 141.
PI. V. CONCLUSIONS
In this paper the paths of CM currents have been analyzed. On the basis of the experimental results and the model it has been shown that the high frequency CM currents, which are generated on the motor side of the drive system, on their way back to the CM voltage source close mainly by the heat sink-to-DC link capacitance. Its value determines the conducted emissions measured on the supply side of the inverter (e.g. by means of LISN). In the case of a large value of heat sink-to-DC link capacitance the CM currents, which almost entirely close by this capacitance, cause a CM voltage drop on it. In the conduction state of diodes of the rectifier this voltage drop produces oscillations of small amplitude and relatively low frequency in a closed loop consisting of DC-link-to-heat sink capacitance and resultant inductance of the supply side of the converter. This part of the CM currents is not visible on the motor side of the system in discussed case. However, at the same time, on the motor side of the inverter the HF emissions, which are not measured on the supply side of the system, can disturb other equipment. In the case of a small value of heat sink-to-DC link capacitance the larger part of the CM currents can constrain to flow via mains, and inductance of this path contributes to a fanning of the low frequency part of CM currents on the motor side of the drive system. In spite of lumped simplifications the model allows to predict the mechanisms of creations of the oscillations of the CM currents in a real system with a good accuracy. In particular, the model could be the basis for comparative analysis and selection of proper methods of the improvement of electromagnetic compatibility of PWM drives.
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R. 1. Kerlwas D. Schlegel, “EM1 Emission of Modem PWM AC Dnves”, IEEE Indwny Applications Mngarine, NovemberiDeeember 1999 pp.47-79. D. Macdonald, W. Gray, “PWM drive related bearing failures’: IEEE Indurby Appl. Magorhe. JulyiAugust, 1999 pp. 41-41. P. Link “Minimidng clecmc bearing currents in ASD systems”, IEEE Indwby Appl. Magoline JulyIAugust, 1999 pp. 5 5 4 7 . Li Ran, S. Go% I. Clare, K. J. Bradley, C. Chistopoulos ‘Conducted Elecmmgnetic Emission in Induction Motor Drive System Part I: Time Domain Analysis and Identification of Dominant Modes” IEEE Tmns. on Power Elecnonics, vol. 13, No. 4, July 1998 pp.768-776. Li Ran, S. Gokani, 1. Clare, K. 1. Bradley, C. Christopaulos ‘Conducted Elfftromagnetic Emission in Induction Motor Drive System Parl 11: Frequency Domain Models” IEEE Trans. on Power Elecoonics. vol. 13, No. 4, July 1998 pp.768-776. A. de Lima, H. W.Dommel, R.M. Stephan, ”Modeling AdjustableSpeed Drives with Lang Feeders”BEE Trom. on Ind. Eleclmnim., vol. 47, No. 3, June ZOO0 pp.549-556. M.MelfZ A.M Jason Sung, S.Bell, G.L.Skibinski, Effect of surge Voltage Risetime on Insulation of Lowvoltage Machines Fed by PWM Converters, IEEE Trans. On Ind. Appl. Va1.34, No.4, 1998, pp. 766-774. A. Kwpski, R Stnelecki, R. Smoleaski Z. Fedycak, “Bearing current path and pulse rate in PWM-inverter-fed induction motor,” IEEE 32ndPESC’OI 2001 Val. 4, pp. 2025-2030 G. Skibinski,
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