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Published on February 15, 2014

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Radio Frequency Based Detection of High Frequency Circulating Bearing Current Flow

This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TIA.2013.2296626, IEEE Transactions on Industry Applications On Radio-Frequency Based Detection of High-Frequency Circulating Bearing Current Flow Annette Muetze Graz University of Technology 8010 Graz, Austria muetze@tugraz.at Ville Niskanen Lappeenranta University 53850 Lappeenranta, Finland ville.niskanen@lut.fi Abstract—The possibility of bearing damage caused by inverter-induced bearing currents in modern variable-speed drive systems has been well recognised today. Further research is needed to develop appropriate non-intrusive methods for detection and monitoring of such currents. A radio-frequency based non-destructive method has been applied to detect discharge bearing currents. The method is understood to work on the energy that is radiated in the electric field during the bearing discharge event. We show that the method is also applicable to high-frequency circulating bearing currents that have so far been associated with ohmic bearing characteristics and no discharges occurring. The analysis and understanding of the applicability of the method to detect such currents also contributes to further understanding of the electric characteristics of the bearing, notably the moment the current conduction begins. CM DE HF HV LV MV NDE PD PE RF N OMENCLATURE Common mode. Drive-end. High frequency. High voltage. Low voltage. Medium voltage. Nondrive-end. Partial discharge. Protective earth. Radio frequency. I. M OTIVATION Jero Ahola Lappeenranta University 53850 Lappeenranta, Finland jero.ahola@lut.fi additional cost, and a trade-off has to be made between the per-default application of a mitigation technique and further analysis of the bearing currents occurring within a system. The choice will depend on the overall system, its configuration and cost, and the application. It is desirable to further reduce the additional cost and risk associated with such parasitic currents. Development and research on bearing current monitoring and diagnosis as well as on the current-conduction and damage mechanisms within the bearing will all contribute to this aim. Today, mostly intrusive techniques are applied to measure such bearing currents: Commonly, an electrically insulating layer is introduced into the current path. This electrical insulation is then shortened with a small wire, and the current flow through the wire measured. Such a method is not suitable for wide-spread cost-effective application in the field. Furthermore, the measurement circuit affects the measured currents. While models to conclude on the current flow in the respective system before modification are available (e.g. [18]), they only reflect the existing understanding and thus have limited applicability to enhance the understanding of the current flow mechanism. Thus, non-destructive methods for the detection and monitoring of inverter-induced bearing currents may be considered a great asset towards an even better understanding of such current flow and possible damage. A. Bearing Current Research, Monitoring, and Diagnosis B. Non-intrusive Detection of Inverter-Induced Bearing Currents Under all Operating Conditions The possibility of bearing damage caused by inverterinduced bearing currents in modern variable-speed drive systems has been well recognised today. Different authors have described the cause-and-effect chains, allowing the selection of appropriate mitigation techniques (e.g. [1]–[11]). Notably, distinction between (a) discharge bearing currents, that are directly related to the high-frequency (HF) common-mode (CM) voltage, and (b) HF circulating current that are caused through inductive coupling by the HF stator CM current and that are thus more prevalent with machines with larger frame sizes, is important. The mitigation techniques are frequently applied as preventative measures to avoid bearing failure. Common approaches include different types of filters and chokes, inverter modulation schemes that minimize the CM voltage, electrostatic shielding, slip and shaft grounding rings, and insulated or hybrid bearings (e.g. [6]–[17]). These techniques come with A radio-frequency (RF) based non-intrusive method to detect discharge bearing currents has been presented and used to evaluate and further understand the occurrence of discharge currents [19]–[21]. The method is based on the understanding of an electric machine as a spark gap transmitter with some of the energy stored within the bearing and machine (notably air gap) before the discharge being emitted as an RF signal. In contrast to these discharges occurring with discharge bearing currents, the bearings have so far been understood to have ohmic properties in the case of HF circulating bearing currents. Based on this understanding, such currents can thus not be detected with an RF based method that detects the electric field in the frequency range radiated from any discharge, because of the lack of occurrence of a discharge and subsequent release of energy that can be radiated outside of the bearing. Note that this possibility of detection is not related to the maximum amplitude of the bearing current. (HF 0093-9994 (c) 2013 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TIA.2013.2296626, IEEE Transactions on Industry Applications circulating bearing currents generally have larger amplitudes than discharge currents.) To the best of the authors’ knowledge, systematic detection of flow of HF circulating currents with an RF technique and occurrence of a discharge along with such currents to be detected have yet to be shown. Such work would both show that bearing currents under all operating conditions–i.e. all types of bearing currents–can be detected using an RF technique and provide further insight into the moment the current conduction begins. vCM iCM 3 3 -0.6 -0.6 vb 3 -0.6 ib 3 -0.6 II. C ONTRIBUTION AND O RGANIZATION OF Discharge of voltage across the bearing We show that HF circulating bearing currents, too, can be detected using an RF based non-intrusive method. This is in contrast to the common understanding of purely ohmic bearing characteristics when such currents flow. It closes the gap that only some types of bearing currents have been shown to be detectable through this technique, and allows RF based detection of inverter-induced bearing currents under all operating conditions. Analyzing HF circulating bearing currents further, we present results from investigations of the switching instant during which the HF voltage between the two bearings increases, the bearing lubrication film cannot maintain electrically insulating properties, and HF circulating currents start to flow. The understanding of the applicability of the RF based method is tightly coupled with further findings on the electric characteristics of the bearing, notably the moment the current conduction begins: We have observed instantaneous capacitive behavior of the bearings already at low rotational speed and discharges that can be associated with the subsequent flow of HF circulating currents. Experimental results with supporting theoretical considerations are given (Sections V–VIII) following short reviews of the two HF bearing current mechanisms referred to above, the RF based method, the test setup and an overview of the types of tests carried out (Sections III and IV). ib mirrors iCM Discharge Bearing Currents PAPER HF circ. Bearing Currents Fig. 1. Comparison of discharge and HF circulating bearing current mechanisms: discharge of capacitively coupled CM voltage across the bearing versus inductive coupling; vCM : CM voltage, iCM : CM current, vb : voltage across bearing, ib : bearing current. 2) HF circulating bearing currents are caused by inductive coupling through the HF stator CM current (Figs. 2 and 3). In contrast to the discharge currents, the circulating currents occur the moment a switching event takes place. The frequencies of these currents are typically in the range of a few hundred kilohertz, with the first halfperiod of the oscillation sometimes reaching one to two megahertz. In general, it is understood that the voltage in the loop driving the HF circulating current leads to the bearing lubricating film being “punctured”, the bearing shows ohmic behavior, and the bearing resistance is so small that it is usually neglected in the proposed equivalent circuits. stator core stator frame F0 stator winding end shield ib III. R EVIEW OF HF B EARING C URRENTS AND RF BASED B EARING C URRENT D ETECTION A. Review of HF Bearing Currents The nonzero HF CM voltage at the output of modern fast-switching inverters typically changes with every inverter switching instant and arrives at the motor terminals with a high dv/dt, where it interacts with the HF machine impedance. Discharge bearing currents are directly caused by the HF CM voltage, HF circulating bearing currents–which are in the focus of this paper–by the HF CM current that flows as a result of this interaction (Fig. 1) (e.g. [1]–[11]): 1) Discharge bearing currents result from the stator winding HF CM voltage charging the bearings via a capacitive voltage divider, and occur–statistically distributed–as discharge current pulses (of up to a few amp` res) when e the threshold voltage of the bearings (that depends on the operating conditions and typically is in the range of a few up to some tens of volts) is exceeded. bearing shaft rotor core Fig. 2. Path of HF circulating bearing current [22]. Common mode current 20 A/Div Bearing currents (NDE) (DE) 5 A/Div 1 ms/Div Fig. 3. Measured HF circulating bearing currents, induction motor, frame size 400 mm, 500 kW rated power, motor speed n = 3000 rpm, bearing temperature θb ≈ 70°C [22]. 0093-9994 (c) 2013 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TIA.2013.2296626, IEEE Transactions on Industry Applications B. Review of RF Based Bearing Current Detection The RF based bearing current detection works similarly to partial discharge (PD) detection in the insulation of electric machines. This technique has mainly been developed to determine the quality and state of the insulation and prevent its premature ageing and failure (for a comprehensive reference, see [23]). While such techniques have long been established for form-wound high-voltage (HV) and medium-voltage (MV) machines, with many devices commercially available (e.g. [24]–[27]), the increased use of power-electronics converters has also led to such techniques being developed for detection of PD in random-wound, low-voltage (LV) windings (e.g. [28]–[32]). Earlier techniques, notably for the HV and MV machines were designed for offline application, with the rotor removed to increase the confidence in the signals, but methods for online detection have found recent interest, notably in the context of PD occurring with converter-fed, LV machines. As for discharge bearing currents, it has been shown that a fraction of the energy released during the discharge is radiated outside from the electric motor and can be detected as RF electric field measurement by appropriate equipment [19], [20]. In this case, the machine itself operates as a spark gap transmitter. The characteristics of the RF radiation (e.g. power, frequency range, radiation pattern) are determined by the characteristics of the electrical discharge and those of the electric machine as the transmitting antenna. The frequency band of the radiation has been determined to be 90−400 MHz; the machine shaft end has been shown to play a key role in the transmitting characteristics [33], and the radiated power, even though small, is sufficiently large to be detected. Any antenna tuned to detect electric fields in the identified frequency range may be used for this technique, although an antenna with directivity is highly recommended since it reduces possible effects of external interferences. A device based on a similar principle can be obtained on the market, too, [34]. Note that this device detects RF emissions above a certain threshold. (Determined at 10 mV within our laboratory tests with a 50 Ω load.) In contrast to direct detection with an RF antenna, it thus does not provide any further insight into the strength of the emitted RF signal. Note also that many of the commercially available devices to detect PD in HV and MV windings are not suitable for the purpose of detecting bearing currents, since they are operating in a much higher frequency range of a few gigahertz. IV. T EST S ETUP A. Drive Systems Two drives with two different power levels and thus frame sizes are used for the experiments. Both are 230/400 V, 50 Hz, ∆-connected, 4-pole induction motors operated by three-phase 400V, 50 Hz inverters. The two motors and inverters are referred to as motors MA-15 and MB-75 as well as inverters IA-15 and IB-75 respectively. MA-15 is a 160 mm frame size 15 kW, MB-75 a 280 mm frame size 75 kW machine. The two 400 V inverters are rated at 14.8 A and 82 A respectively. The smaller inverter is operated at 4 kHz (scalar control, constant switching frequency), the larger one at 3 kHz (direct torque control, average switching frequency). Both machines are grounded through the PE conductor of the motor cable only. Bearing temperatures during operating were in the range of ≈ (25 . . . 65)°C. In order for the bearing currents to be measured, the bearings of the example case machines are insulated towards the housing using an electrically insulating layer of 5 mm thickness applied around the outer bearing race that was shortened with a short wire. As discussed above, this technique is intrusive and will slightly alter the HF current flow when compared to the unmodified case. Using the techniques presented in [18], this influence is estimated to reduce the amplitudes of the HF circulating bearing currents by (-10 . . . 15)% for MB-75, and by up to 25% for MA-15. B. Measurement Equipment The measurement equipment included an EMCO 93148 antenna that has a bandwidth of 200 MHz to 2 GHz and that was placed at approximately one meter distance from the motor, pointing towards the motor shaft, a Textronix TDS7140 oscilloscope with a bandwidth of 1 GHz and a maximum sampling rate of 10 GS/s, and an RF bandpass filter Mini-Circuits BHP100+ with a bandwidth of 90−400 MHz (input impedance set to 50 Ω). The HF bearing currents on the nondrive-end (NDE) and drive-end (DE) sides were measured either with Tektronix TCP202 50 MHz passive (measurements shown in Figs. 6(a), 6(b), 7(a), 7(b), 9–12) or R&S ZC20 50 MHz active current probes (measurements shown in Figs. 6(c), 6(d), 7(c), 7(d), 8(a)–8(c)); the voltage across the bearings with a Tektronix P5210 50 MHz high voltage differential probe (Figs. 4 and 5). The HF CM currents were measured using an R&D EZ-17 100 MHz passive current probe. Bearing temperatures were measured using an AZ8868 infrared thermometer between the shaft and the motor end shield. C. Types of Tests Four types of tests are carried out: I. HF circulating bearing current flow “as is”: The electric machine is operated at low rotational speed and the HF circulating bearing currents–if flowing–are detected. (Section V.) II. Generation of increased HF circulating bearing current flow: The NDE bearing and its insulation are shortened to decrease the impedance of the path of the circulating current. This additional measure decreases the HF impedance in the path of the HF circulating currents and the thus the likeliness for these to occur at increased rotational speed and/or low bearing temperatures. (Section VI.) III. Bearing currents due to rotor ground currents: The NDE bearing insulation was again left open. The machine was grounded via the rotor, i.e. the stator grounding connection eliminated and the rotor connected to ground with the help of a sleeve made from the tinned copper braid of the coaxial shield of the supply cable. In this configuration, any HF CM current would return to 0093-9994 (c) 2013 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TIA.2013.2296626, IEEE Transactions on Industry Applications bearing was externally supplied with a HF voltage within the frequency range typical for HF circulating bearing currents using the HF signal generator. (Section VIII.) For all types of tests, the HF bearing currents, voltage, and impedance, and RF emission/detection properties are analyzed. V. R ESULTS I: HF C IRCULATING B EARING C URRENT F LOW “A S I S ” A. Experimental Results with Motor MA-15 Because of its small frame size, in line with the scaling laws for inverter-induced bearing currents, only discharge bearing currents occurred in this first type of tests and thus measurement results obtained with this machine are not discussed in this section. B. Experimental Results with Motor MB-75 Fig. 4. Measurement setup of motor MA-15 (15 kW). Fig. 5. Measurement setup of motor MB-75 (75 kW). ground via the rotor grounding connection, thereby passing the DE bearing current. Since HF circulating bearing currents are generated through inductive coupling of the HF CM current, these bearing currents have the same waveforms as HF circulating bearing currents, but larger amplitudes. (Section VII.) IV. External supply of HF bearing currents using a Hameg HM8131-2 15 MHz signal generator: The NDE bearing insulation was left open: In this configuration, any current flow across the NDE bearing is negligible. The DE With this machine, HF circulating bearing currents occurred–along with discharge bearing currents–up to slightly above 200 rpm rotational speed for lower bearing temperatures in the range of some (20 . . . 30)°C. Above, only discharge bearing currents were observed. Maximum amplitudes of these HF circulating bearing currents reached up to 1.2 A for 100 rpm. For elevated bearing temperatures above 60°C, very small HF circulating bearing currents of a few hundred milliamp` res could be observed up to even 3000 rpm. The e HF CM current was measured to approximately 10 A. The frequency of the HF CM and the bearing currents was in the range of 1 MHz. As the bearing voltages and currents were measured, too, the bearing current type could be verified for any bearing currents detected through the non-intrusive RF based method. Conventionally, the HF circulating bearing current has been understood to flow through the bearing that has mainly ohmic behavior. During such purely ohmic behavior, there would be an energy conversion due to ohmic loss within the bearing, but no energy release as a result of a discharge. However, as shown in Figs. 6(a)–6(d), also such HF circulating bearing currents can be detected through the RF based method: The measured HF bearing currents through the NDE and DE bearings have the same waveforms and amplitudes and opposite signs, which is a clear indicator of HF circulating bearing currents. The moment the currents start to flow, an RF current pulse is detected. This important finding will be further analyzed below. C. Analysis The detected RF pulse indicates that some energy has been released and radiated outside the machine the moment the individual current has started to flow. A detailed consideration of the voltages measured across the NDE and the DE bearings shows a relatively steep voltage rise notably in one of the two bearings (Figs. 6(a) and 6(b): NDE bearing, Figs. 6(c) and 6(d): DE bearing) the moment the current starts to flow. The voltage and current waveforms are not fully proportional as one would expect for purely ohmic behavior. 0093-9994 (c) 2013 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

Voltage [V] Voltage [V] Current [A] This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TIA.2013.2296626, IEEE Transactions on Industry Applications 1 Inde Ide 0 -1 3 4 5 6 Time [us] 7 8 9 10 Unde Ude 5 0 -5 3 -3 x 10 4 5 6 Time [us] 7 8 9 10 2 RF 0 -2 3 4 5 6 7 8 9 10 Time [us] Voltage [V] Voltage [V] Current [A] (a) 100 rpm rotational speed and ≈ 30°C bearing temperature. 1 Inde Ide 0 -1 3 4 5 6 Time [us] 7 8 9 10 Unde Ude 5 0 -5 3 -3 x 10 2 4 5 6 Time [us] 7 8 9 10 RF 0 -2 3 4 5 6 7 8 9 10 Time [us] Current [A] (b) 100 rpm rotational speed and ≈ 30°C bearing temperature. 1 Inde Ide 0 -1 1 2 3 4 5 6 7 8 Voltage [V] Time (us) 2 0 -2 1 Unde Ude 2 3 4 5 6 7 8 Voltage [V] Time (us) x 10 2 0 -2 1 -3 RF 2 3 4 5 6 7 8 Time (us) Current [A] (c) 400 rpm rotational speed and ≈ 55°C bearing temperature. 1 Inde Ide 0 -1 0 1 2 3 4 5 6 7 Voltage [V] Time (us) 2 0 -2 0 Unde Ude 1 2 3 4 5 6 7 We interpret this as follows: The moment the voltage in the loop increases, the bearing lubricating film still has electrically insulating properties. A certain voltage is required for the film to lose these insulating properties. At a certain threshold, the bearing(s) start(s) to conduct. In the case of discharge bearing currents, the energy stored across the bearing(s) is released instantaneously. In the present case of HF circulating bearing currents some of this energy is absorbed by the change of the electrical properties of the bearing. This reduces the bearing impedance further whereby part of the energy is released and radiated through the electric machine antenna structure. With HF bearing currents–in contrast to the case with discharge bearing currents–both bearings are required to have electrically conducting properties. As observed in Figs. 6(a)– 6(d), the establishment of the current conducting path might not be uniformly distributed across the two bearings: One bearing might first suffer from a steeper voltage rise and the subsequent voltage breakdown (thereby also releasing energy that is eventually radiated through the machine antenna structure) than the other. Note also that, depending on the drive, some CM capacitive coupling might exist additionally. Such voltage would add to the differential voltage induced by the CM current generated HF flux, increasing the latter across the one and decreasing it across the other bearing before the HF circulating bearing current flow begins. The energy released has been computed from the energy stored in the total capacitance (two bearing capacitances and one rotor-to frame capacitances) (eqs. (2) and (3) in [20], see Appendix). The calculated values range between 10 and a few 100 nJ. This is within the lower part of the range of energies released in the case of discharge bearing currents (between a few and a few thousands of nanojoules). The latter explains that the RF signals detected in the case of discharge bearing currents were typically in the order of 10 mV, whereas those to detect HF circulating bearing currents were rather in the range of a few millivolts. Comparing Figs. 6(a) and 6(b), both show the relatively steep voltage rise the moment the current starts to flow. The detected RF pulse is larger for Fig. 6(b) where the voltage rises faster and higher. However, note that the maximum bearing current is larger in Fig. 6(a), indicating that the amplitudes of the bearing current and of the RF pulse might not be strongly related. The detected HF circulating bearing currents shown in Figs. 6(c) and 6(d) occur not only at much higher rotational speed, but also significantly higher bearing temperatures. Here, much less voltage builds up before the current conducting path is established resulting in much smaller emitted RF signals. Voltage [V] Time (us) x 10 2 0 -2 0 -3 VI. R ESULTS II: I NCREASED HF C IRCULATING B EARING C URRENT F LOW RF 1 2 3 4 5 6 7 Time (us) (d) 1500 rpm rotational speed and ≈ 65°C bearing temperature. Fig. 6. Motor MB-75 and inverter IB-75: HF circulating bearing current detected through non-intrusive RF based method; measured currents, voltages and RF signals. A. Experimental Results with Motor MA-15 Some flow of HF circulating bearing currents in the smaller machine could be generated through the decreased impedance of the bearing current path. With this machine, the preliminary findings were obtained that indicated the applicability of the method and that led to further research on the larger machine. For this reason they are also briefly mentioned in this paper. 0093-9994 (c) 2013 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

VII. R ESULTS III: ROTOR G ROUND C URRENTS A. Experimental Results with Motor MA-15 With the machine grounded via the rotor instead of the stator, and the NDE bearing fully insulated, the total HF CM current of about 8 A and close to 1.5 MHz flows through the DE bearing (Fig. 8). Here again, similar to the case of “true” HF circulating bearing currents, the currents flow through the bearing only after the current conducting path has been established what brings along an emission of an RF pulse. Voltage [V] Voltage [V] Current [A] Voltage [V] Voltage [V] Current [A] Voltage [V] The maximum amplitudes of the HF bearing currents at very low rotational speed do not increase as the NDE bearing is shortened, indicating that the influence of the bearing impedance at this speed, and once HF circulating bearing currents are flowing, is negligible. However, for a given temperature level, the maximum speed up to which HF circulating bearing currents were found increased almost by a factor of two, supporting the understanding that the role of the bearing impedance increases with increasing motor speed as the thickness of the lubricating film increases. Even as the occurrence of the HF circulating bearing currents themselves does increase towards higher rotational speed and lower bearing temperatures, the detection of these currents through the RF based method is more difficult with the NDE bearing shortened. The lack of contribution of radiated energy from the NDE bearing towards the radiated signal may be interpreted as one factor contributing to this observed behavior. 3 4 5 6 Time (us) 7 8 9 10 Unde Ude 0 -5 2 -3 x 10 2 3 4 5 6 Time (us) 7 8 9 10 RF 0 -2 3 4 5 6 Time (us) 7 8 9 10 1 0 -1 2 Inde Ide 3 4 5 6 Time (us) 7 8 9 10 5 Unde Ude 0 -5 2 2 3 x 10 4 5 6 Time (us) 7 8 9 10 -3 RF 0 -2 2 3 4 5 6 Time (us) 7 8 9 10 1 Inde Ide 0 -1 1 2 3 4 5 6 7 8 Time (us) 2 0 -2 1 Unde Ude 2 3 4 5 6 7 8 Time (us) Voltage [V] C. Analysis -1 2 5 (b) 300 rpm rotational speed and ≈ 30°C bearing temperature. x 10 2 0 -2 1 -3 RF 2 3 4 5 6 7 8 Time (us) (c) 900 rpm rotational speed and ≈ 50°C bearing temperature. Current [A] Maximum amplitudes of the HF circulating bearing currents reached again up to 1.2 A for 100 rpm and lower bearing temperatures in the range of some (20 . . . 30)°C. Also, very small HF circulating bearing currents of a few hundred milliamp` res e could be observed up to even 3000 rpm for elevated bearing temperatures above 60°C. Figs. 7(a)–7(d) show RF based detections of HF circulating bearing currents 150, 300, 900, and 2000 rpm respectively. Again, the measured HF bearing currents through the NDE and DE bearings have the same waveforms and amplitudes and opposite signs, and an RF current pulse is detected the moment the currents start to flow. Inde Ide 0 (a) 150 rpm rotational speed and ≈ 30°C bearing temperature. Voltage [V] B. Experimental Results with Motor MB-75 1 2 Voltage [V] However, these currents are much more rare with the small machine, and the energies released were often found to be so low that RF based detection was difficult. They will thus not be discussed any further. However, we would like to point out that this limitation does not impede on the practicability of the proposed method: With HF circulating bearing currents typically not occurring with machines with small frame sizes, but if suffering from HF bearing currents, being put at risk due to discharge bearing currents, there is no need to detect HF circulating bearing currents with such machines, and detection of discharge bearing currents through the RF method has been well proven. Current [A] This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TIA.2013.2296626, IEEE Transactions on Industry Applications 1 Inde Ide 0 -1 0 1 2 0 -2 0 x 10 2 0 -2 0 2 3 4 Time (us) 5 6 7 8 Unde Ude 1 2 3 4 Time (us) 5 6 7 8 -3 RF 1 2 3 4 Time (us) 5 6 7 8 (d) 2000 rpm rotational speed and ≈ 30°C bearing temperature. Fig. 7. Motor MB-75 and inverter IB-75, NDE bearing shortened: HF circulating bearing current detected through non-intrusive RF based method; measured currents, voltages and RF signals. 0093-9994 (c) 2013 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

2 3 4 Time (us) 5 6 7 8 2 3 4 Time (us) 5 6 7 8 1 2 3 4 Time (us) 5 6 7 8 20 0 -20 0 2 3 4 Time (us) Ib [A] 1 10 0 -10 0 5 6 7 8 RF [mV] 10 0 -10 0 1 2 2 3 3 4 Time (us) 5 4 Time (us) 5 6 6 7 7 8 |Zb| [ohm] Icom [A] 10 0 -10 0 1 60 80 100 120 Time (us) 140 160 180 200 20 40 60 80 100 120 Time (us) 140 160 180 200 20 40 60 80 100 120 Time (us) 140 160 180 200 (a) Measured bearing current, bearing voltage, and RF signal. 8 10 2 0 20 40 60 80 100 120 Time (us) 140 160 180 200 20 40 60 80 100 120 Time (us) 140 160 180 200 100 1 1 2 2 3 3 4 Time (us) 5 4 Time (us) 5 6 6 7 7 8 8 Angle of Zb [deg] Vb [V] Ib [A] 10 0 -10 0 40 0 -10 0 (a) 900 rpm rotational speed and ≈ 35°C bearing temperature. 10 0 -10 0 20 0 -2 0 10 RF [mV] 10 0 -10 0 1 0 -10 0 2 Ib [A] 1 Vb [V] 10 Icom [A] 10 0 -10 0 RF [mV] Vb [V] This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TIA.2013.2296626, IEEE Transactions on Industry Applications 0 -100 0 (b) Computed bearing impedance. 2 3 4 Time (us) 5 6 7 8 1 2 3 4 Time (us) 5 6 7 Fig. 9. Motor MA-15 and inverter IA-15, external HF voltage supply of DE bearing: measured currents, voltages and RF signals and computed bearing impedance for 300 kHz HF supply voltage, 210 rpm rotational speed, and ≈ 30°C bearing temperature; computed bearing capacitance during capacitive behavior Cb = 0.21 nF. 8 Ib [A] 1 10 0 -10 0 Icom [A] 5 0 -5 0 10 0 -10 0 1 2 3 4 Time (us) 5 6 7 8 RF [mV] Vb [V] (b) 1800 rpm rotational speed and ≈ 50°C bearing temperature. 10 0 -10 0 1 2 3 4 Time (us) 5 6 7 8 (c) 3000 rpm rotational speed and ≈ 50°C bearing temperature. Fig. 8. Motor MA-15 and inverter IA-15: Bearing currents due to rotor ground currents detected through non-intrusive RF based method; measured currents, voltages, and RF signals. technique, as the current flow is preceded by the establishment of a current conducting path that comes along with a change of the current conducting properties of the bearing and the emission of an RF pulse. VIII. R ESULTS IV: E XTERNAL S UPPLY C URRENTS OF HF B EARING A. Reasoning For the larger machine, too, bearing currents due to rotor ground currents with waveforms equivalent to HF circulating bearing currents could be obtained and detected with the RF based method. In these cases, the total HF CM current of about 10 A would flow through the DE bearing. (Additional figures would not add more value and are thus omitted for reasons of space.) These tests were carried out to further understand the reason for the observed behavior. Emphasis was placed on operation at low rotational speed of a few hundred revolutions per minute where HF circulating bearing currents are more prevalent and electrically insulating behavior of the bearing followed by a discharge occurring within the bearing is less expected. Note also that even at 1 MHz, the impedance provided by the 5 mm think electrically insulating layer is in the order of several kiloohms which is at least by a factor of 103 larger than the one of the bearing current path. Thus, any current flow across the NDE bearing is negligible. C. Analysis B. Experimental Results with Motor MA-15 With respect to the current conduction mechanism, bearing currents due to rotor ground currents and HF circulating bearing currents are very similar, since both are directly related to the waveform of the HF CM current. Thus, the currents flowing in this type of test can, too, be detected through the RF The bearing may form a capacitive film even at low rotational speed. However, the capacitive behavior does not exist constantly but changes to ohmic behavior. For example, Figs. 9 and 10 show the measured DE bearing currents and voltages, the detected RF signals, as well as the computed B. Experimental Results with Motor MB-75 0093-9994 (c) 2013 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TIA.2013.2296626, IEEE Transactions on Industry Applications 10 0 Ib [A] -10 0 1 100 150 200 Time (us) 250 300 350 400 0 -10 0 0 100 150 200 Time (us) 250 300 350 50 100 150 200 Time (us) 250 300 350 400 -0.2 0 |Zb| [ohm] |Zb| [ohm] 50 100 150 200 Time (us) 250 300 350 250 300 350 400 50 100 150 200 Time [µs] 250 300 350 400 10 2 400 0 50 100 150 200 Time (µs) 250 300 350 400 50 100 150 200 Time (µs) 250 300 350 400 100 50 0 -100 0 50 100 150 200 Time (us) 250 300 350 400 (b) Computed bearing impedance. Angle of Zb [deg] Angle of Zb [deg] 200 Time [µs] (a) Measured bearing current and bearing voltage. 2 0 150 0 (a) Measured bearing current, bearing voltage, and RF signal. 10 100 400 0 -10 0 50 0.2 50 Ib [A] -1 0 10 RF [mV] 50 Ub [V] Vb [V] 10 0 -50 -100 0 (b) Computed bearing impedance. Fig. 10. Motor MA-15 and inverter IA-15, external HF voltage supply of DE bearing: measured currents, voltages and RF signals and computed bearing impedance for 300 kHz HF supply voltage, 300 rpm rotational speed, and ≈ 30°C bearing temperature; computed bearing capacitance during capacitive behavior Cb = 0.38 nF. bearing impedances for 300 kHz HF supply voltage and 210 as well as 300 rpm rotational speed: Over a certain time of some tens of microseconds, the bearing impedance increases slightly, before it turns mainly capacitive again. Fig. 10 also shows how such a mode change can occur for only a very short moment: Following the discharge and change to ohmic behavior at 30 µs, the impedance increases and turns capacitive again at around 100 µs, but returns to resistive mode already after a short time of a few µs. C. Experimental Results with Motor MB-75 Similar results are obtained for the larger machine as for the smaller machine: Again, the bearing may form a capacitive film even at low rotational speed. As in the case of the smaller machine, the capacitive behavior does not exist constantly and also changes to ohmic behavior. In contrast to the case of the smaller machine, states with less pronounced behavior, i.e. capacitive or resistive, in which phase angles in the order of -45° are observed, too. Figs. 11 and 12 show again the measured DE bearing currents and voltages as well as the computed bearing impedances for 300 kHz HF supply voltage and 210 as well as 300 rpm rotational speed, now for the larger machine. Fig. 11. Motor MB-75 and inverter IB-75, external HF voltage supply of DE bearing: measured currents and voltages and computed bearing impedance for 300 kHz HF supply voltage, 210 rpm rotational speed, and ≈ 30°C bearing temperature; computed bearing capacitance during capacitive behavior Cb = 1.7 nF. to this capacitive mode, the resistive mode expected from the conventional understanding occurs. The transitions are not always instantaneous (when compared to the time scale, i.e. taking place within a few microseconds.) A change from resistive to capacitive mode may for example be preceded by a slight increase in the bearing impedance before a rather steep change to capacitive behavior occurs. The transitions are significantly less distinct for the larger machine. Bearing temperature as well as voltage applied across the bearing are expected to influence the bearing impedance behavior, too. Their analysis is subject of further research and not within the scope of this paper. Further results on the bearing impedance properties–notably the observed “mode change behavior”–have been presented in [35]. While such further research is required to fully understand the observed behavior of the bearing impedance, the results shown in this paper illustrate that some capacitive mode may exist the moment HF circulating bearing currents begin to flow. This will eventually allow such currents too, to be detected through the RF based method. This is in contrast to the common understanding that only in the case of discharge bearing currents discharges occur within the bearing. IX. C ONCLUSIONS D. Analysis For certain short time intervals, a bearing is observed to form a capacitive film even at low rotational speed. In addition Interpreting our observations in the context of HF circulating bearing currents, we postulate the following: For “sufficiently” large HF circulating bearing currents and energy 0093-9994 (c) 2013 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TIA.2013.2296626, IEEE Transactions on Industry Applications understanding–the occurrence of some discharge. Ub [V] 5 0 A PPENDIX -5 50 100 150 200 Time [µs] 250 300 350 400 50 100 150 200 Time [µs] 250 300 350 400 Ib [A] 0.2 0 -0.2 0 |Zb| [ohm] 2 Angle of Zb [deg] 0 50 100 150 200 Time (µs) 250 300 Ctot = Crf + Cb,NDE + Cb,DE . With the voltage across the bearing before the breakdown vb , the energy stored in these capacitances and the maximum energy released during the discharge, Ec , is given by ([20], eq. (3)) 1 2 Ec = Ctot vb . 2 (a) Measured bearing current and bearing voltage. 10 Before a discharge occurs, energy is stored within the two bearing capacitances, Cb,DE and Cb,NDE , and the rotorto-frame capacitance, Crf that are all connected in parallel. These capacitances can be summarized in an equivalent total capacitance, Ctot ([20], eq. (2)), 350 400 50 ACKNOWLEDGEMENT Funding for the Open Access Publication was provided by the Government of Styria. 0 -50 -100 0 R EFERENCES 50 100 150 200 Time (µs) 250 300 350 400 (b) Computed bearing impedance. Fig. 12. Motor MB-75 and inverter IB-75, external HF voltage supply of DE bearing: measured currents and voltages and computed bearing impedance for 300 kHz HF supply voltage, 300 rpm rotational speed, and ≈ 30°C bearing temperature; computed bearing capacitance during capacitive behavior Cb = 8.5 nF. stored in the circuit before these currents start to flow, the occurrence of these bearing currents, too, can to some extent be detected using the proposed non-intrusive RF based method. The energy release during a very short time–translating into the radiating power–occurs as the current–driven by the HF voltage in the loop of the HF circulating bearing current– paths through the formerly electrically insulating lubricating film of the bearing. This form of “penetration” brings along some energy conversion as well as dissipation that radiates via the machine acting as a transmitting antenna and can be detected through the receiving antenna. When compared with discharge bearing currents, the detected RF signals have lower amplitudes – a few millivolts instead of some tens of volts. This may be attributed by the smaller amount of energy stored in the capacitances (two bearing capacitances and one rotorto-frame capacitance) before the breakdown occurs. Note also that these energies are below the threshold level for bearing current detection set in the commercially available device determined in our laboratory (10 mV) ([34]). It is also observed that–for certain short time intervals–a bearing may form a capacitive film even at low rotational speed; and that transitions between this capacitive mode and the resistive mode expected from the conventional understanding occur. These modes and transitions might further explain the possibility to detect HF circulating bearing currents through the RF based method that requires some form of energy to be radiated as a result of–as per the current [1] S. Chen and T.A. Lipo, “Source of induction motor bearing currents caused by PWM inverters,” IEEE Trans. En. Conv., vol. 11, no. 1, pp. 2532, Jan./Feb. 1996. [2] J. Erdman, R. Kerkman, and D. Schlegel, “Effect of PWM inverters on AC motor bearing currents and shaft voltages,” IEEE Trans. Ind. Appl., vol. 32, no. 2, pp. 250-259, Mar./Apr. 1996. [3] S. Chen, T.A. Lipo, and D. Fitzgerald, “Modeling of bearing currents in inverter drives,” IEEE Trans. Ind. App., vol. 32, no. 6, pp. 1365-1370, Nov./Dec. 1996. [4] D. Busse, J. Erdman, R. Kerkman, and D. Schlegel, “Bearing currents and their relationship to PWM drives,” IEEE Trans. Power Electron., vol. 12, no. 2, pp. 243252, Mar. 1997. [5] D. Busse, J. Erdman, R. Kerkman, D. Schlegel, and G. Skibinski, “System electrical parameters and their influence effect on bearing currents,” IEEE Trans. Ind. Appl., vol. 33, no. 2, pp. 577584, Mar./Apr. 1997. [6] P. Link, “Minimizing electric bearing currents in ASD systems,” IEEE Ind. Appl. Mag., vol. 5, pp. 55-66, July/Aug. 1999. [7] H.E. Boyanton and G. Hodges, “Bearing fluting,” IEEE Ind. Appl. Mag., vol. 8, pp. 53-57, Sep./Oct. 2002. [8] R.F. Schiferl and M.J. Melfi, “Bearing current remediation options,” IEEE Ind. Appl. Mag., vol. 10 , pp. 40-50, July/Aug. 2004. [9] H. Akagi and T. Doumoto, “An approach to eliminating high-frequency shaft voltage and leakage current from an inverter-driven motor,” IEEE Trans. Ind. Appl., vol. 40, no. 4, pp. 1162-1169, July/Aug. 2004. [10] A. Muetze and A. Binder, “Don’t lose your bearings - mitigation techniques for bearing currents in inverter-supplied drive systems,” IEEE Mag. Ind. Appl., vol. 12, no. 4, pp. 22-31, July/Aug. 2006. [11] A. Muetze and A. Binder, “Practical rules for assessment of inverterinduced bearing currents in inverter-fed AC motors up to 500 kW,” IEEE Trans. Ind. Electron., vol. 54, no. 3, pp. 1614-1622, Apr. 2007. [12] D.F. Busse, J.M. Erdman, R.J. Kerkman, D.W. Schlegel, and G.L. Skibinski, “An evaluation of the electrostatic shielded induction motor: a solution for rotor shaft voltage buildup and bearing current,” IEEE Trans. Ind. Appl., vol. 33, no. 6, pp. 1563–1570, Nov./Dec. 1997. [13] D. Hyypio, “Mitigation of bearing electro-erosion of inverter-fed motors through passive common-mode voltage suppression,” IEEE Trans. Ind. Appl., vol. 41, no. 2, pp. 576–583, Mar./Apr. 2005. [14] D. Dahl, D. Sosnowski, D. Schlegel, R.J. Kerkman, and M. Pennings, “Gear up your bearings,” IEEE Ind. Appl. Mag., vol. 14, no. 4, pp. 45– 53, Jul./Aug. 2008. [15] A. Muetze and H.W. Oh, “Design aspects of conductive microfiber rings for shaft-grounding purposes,” IEEE Trans. Ind. Appl., vol. 44, no. 6, pp. 1749–1757, Nov./Dec. 2008. [16] F.J.T.E. Ferreira, M.V. Cistelecan, and A.T. De Almeida, “Evaluation of slot-embedded partial electrostatic shield for high-frequency bearing current mitigation in inverter-fed induction motors,” IEEE Trans. En. Conv., vol. 27, no. 2, pp. 382–390, June 2012. 0093-9994 (c) 2013 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TIA.2013.2296626, IEEE Transactions on Industry Applications [17] T. Maetani, Y. Isomura, A. Watanabe, K. Iimori, and S. Morimoto, “Suppressing bearing voltage in an inverter-fed ungrounded brushless DC motor,” IEEE Trans. Ind. Electr., vol. 60, no. 11, pp. 4861–4868, Nov. 2013. [18] A. Muetze and A. Binder, “Techniques for measurement of parameters related to inverter-induced bearing currents,” IEEE Trans. Ind. App., vol. 43, no. 5, pp. 1274-1283, Sep./Oct. 2007. [19] V. S¨ rkim¨ ki, Radio frequency method for detecting bearing currents in a a induction motors, PhD Thesis, Lappeenranta University of Technology, Finland, 2009. [20] J. Ahola, V. S¨ rkim¨ ki, A. Muetze, and J.A. Tamminen, “Radioa a frequency-based detection of electrical discharge machining bearing currents” IET Electric Power App., pp. 386-392, vol. 5, no. 4, Apr. 2011. [21] A. Muetze, J. Tamminen, and J. Ahola, “Influence of motor operating parameters on discharge bearing current activity,” IEEE Trans. Ind. App., vol. 47, no. 4, pp. 1767-177, Jul./Aug. 2011. [22] A. Muetze and A. Binder, “Calculation of circulating bearing currents in machines of inverter-based drive systems,” IEEE Trans. Ind. Electron., vol. 54, no. 2, pp. 932-938, Apr. 2007. [23] G. Stone, E.A. Boulter, I. Culbert, and H. Dhirani, Electrical Insulation for Rotating Machines: Design, Evaluation, Aging, Testing, and Repair, IEEE Press Series on Power Engineering, ISBN-13: 978-0471445067, 2004, 392 pages. [24] Accessed on October 10, 2013. [Online]. Available: http://www.mps-systeme.de/mpscms/index.php?id=home&L=1 [25] Accessed on October 10, 2013. [Online]. Available: https://www.omicron.at/en/products/powertransformer/diagnosis/partial-discharge-analysis/ [26] Accessed on October 10, 2013. [Online]. Available: http://www.hvpd.co.uk/products/ [27] Accessed on October 10, 2013. [Online]. Available: http://www.eatechnology.com/instruments/partial-dischargeinstruments/pd-monitor [28] D. Bogh, J. Coffee, G. Stone, and J. Custodio, “Partial-dischargeinception testing on low-voltage motors,” IEEE Trans. Ind. Appl., vol. 42, no. 1, pp. 148–154, Jan./Feb. 2006. [29] A. Cavallini, E. Lindell, G.C.. Montanari, and M. Tozzi, “Off-line PD testing of converter-fed wire-wound motors: when IEC TS 60034-18-41 may fail?,” IEEE Trans. Diel. and El. Ins., vol. 17, no. 5, pp. 1385–1395, Oct. 2010. [30] M. Tozzi, A. Cavallini, and G.C. Montanari, “Monitoring off-line and on-line PD under impulsive voltage on induction motors - Part 1: standard procedure,” IEEE El. Ins. Mag., vol. 26, no. 4, pp. 16–26, Jul./Aug. 2010. [31] M. Tozzi, A. Cavallini, and G.C. Montanari, “Monitoring off-line and on-line PD under impulsive voltage on induction motors - Part 2: testing,” IEEE El. Ins. Mag., vol. 27, no. 1, pp. 14–21, Jan./Feb. 2011. [32] M. Tozzi, A. Cavallini, and G.C. Montanari, “Monitoring off-line and on-line PD under impulsive voltage on induction motors - Part 3: criticality,” IEEE El. Ins. Mag., vol. 27, no. 4, pp. 26–33, Jul./Aug. 2011. [33] V. Niskanen, A. Muetze, and J. Ahola, “On the role of the shaft end and the influence of frame size and load coupling on the RF emission characteristics of induction motors,” EPE Journal. [34] Accessed on October 9, 2013. [Online]. Available: http://www.skf.com/group/products/condition-monitoring/basiccondition-monitoring-products/electrical-discharge-detector/index.html. [35] V. Niskanen, A. Muetze, and J. Ahola, “Study on bearing impedance properties at several hundred kilohertz for different electric machine operating parameters,” Proc. 5th IEEE Energy Conversion Conference and Exhibition, pp. 4460-4467, Denver, CO, September 15-19, 2013. Annette Muetze (S’03-M’04-SM’09) is a full professor at Graz University of Technology in Graz, Austria, where she heads the Electric Drives and Machines Institute. She received the Dipl.-Ing. degree in electrical engineering from Darmstadt University of Technology, Darmstadt, Germany and the degree in general engineering from the Ecole Centrale de Lyon, Ecully, France, both in 1999, and the Dr. Tech. degree in electrical engineering from Darmstadt University of Technology in 2004. Prior to joining Graz, she worked as an Assistant Professor at the Electrical and Computer Engineering Department, University of Wisconsin-Madison, Madison, US, and as an Associate Professor at the School of Engineering of the University of Warwick in the UK. Ville Niskanen was born in Sotkamo, Finland in 1984. He received the M.Sc. degree in electrical engineering from Lappeenranta University of Technology (LUT), Finland, in 2010. He currently works as a junior researcher for proactive maintenance of electrical equipment at the Department of Electrical Engineering at Lappeenranta University of Technology. His main research interests are the diagnostics of electrical motor driven systems. Jero Ahola was born in Lappeenranta, Finland in 1974. He received the M.Sc. and D.Sc. degrees in electrical engineering from Lappeenranta University of Technology, in Finland, in 1999 and 200. He currently works as a professor for energy efficiency and preventive maintenance of electrical equipment at the Department of Electrical Engineering in Lappeenranta University of Technology. His main research interests are diagnostics of electrical drive systems and power line communications. 0093-9994 (c) 2013 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

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