On radio frequency

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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: DOI10.1109/TIA.2013.2296626, IEEE Transactions on Industry Applications

On Radio-Frequency Based Detection of High-Frequency Circulating

Bearing Current Flow

Annette Muetze Ville Niskanen Jero Ahola

Graz University of Technology Lappeenranta University Lappeenranta University

8010 Graz, Austria 53850 Lappeenranta, Finland 53850 Lappeenranta, Finland

[email protected] [email protected] [email protected]

Abstract—The possibility of bearing damage caused byinverter-induced bearing currents in modern variable-speed drivesystems has been well recognised today. Further research isneeded to develop appropriate non-intrusive methods for detec-tion and monitoring of such currents. A radio-frequency basednon-destructive method has been applied to detect dischargebearing currents. The method is understood to work on theenergy that is radiated in the electric field during the bearingdischarge event. We show that the method is also applicable tohigh-frequency circulating bearing currents that have so far beenassociated with ohmic bearing characteristics and no dischargesoccurring. The analysis and understanding of the applicabilityof the method to detect such currents also contributes to furtherunderstanding of the electric characteristics of the bearing,notably the moment the current conduction begins.

NOMENCLATURE

CM Common mode.

DE Drive-end.

HF High frequency.

HV High voltage.

LV Low voltage.

MV Medium voltage.

NDE Nondrive-end.

PD Partial discharge.

PE Protective earth.

RF Radio frequency.

I. MOTIVATION

A. Bearing Current Research, Monitoring, and Diagnosis

The possibility of bearing damage caused by inverter-

induced bearing currents in modern variable-speed drive sys-

tems 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 preven-

tative measures to avoid bearing failure. Common approaches

include different types of filters and chokes, inverter modu-

lation 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

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 in-

sulation 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. Further-

more, 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.

B. Non-intrusive Detection of Inverter-Induced Bearing Cur-

rents Under all Operating Conditions

A radio-frequency (RF) based non-intrusive method to de-

tect 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



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.

II. CONTRIBUTION AND ORGANIZATION OF PAPER

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 in-

creases, the bearing lubrication film cannot maintain electri-

cally 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 considera-

tions 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).

III. REVIEW OF HF BEARING CURRENTS AND RF BASED

BEARING CURRENT DETECTION

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 amperes) when

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.

vCM

iCM

ib

vb

-0.6

3

-0.6

3

-0.6

3

-0.6

3

Discharge of voltageacross the bearing

i ib mirrors CM

Discharge BearingCurrents

HF circ. BearingCurrents

Fig. 1. Comparison of discharge and HF circulating bearing current mecha-nisms: discharge of capacitively coupled CM voltage across the bearing versusinductive coupling; vCM: CM voltage, iCM: CM current, vb: voltage acrossbearing, 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 half-

period 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 frame

rotor core

stator winding

stator core

end shield

F0

bearingib

shaft

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, framesize 400 mm, 500 kW rated power, motor speed n = 3000 rpm, bearingtemperature θb ≈ 70°C [22].



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 de-

termine 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 90−400MHz;

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 10mV within our laboratory

tests with a 50Ω 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. TEST SETUP

A. Drive Systems

Two drives with two different power levels and thus frame

sizes are used for the experiments. Both are 230/400V, 50Hz,

∆-connected, 4-pole induction motors operated by three-phase

400V, 50Hz 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 160mm frame

size 15 kW, MB-75 a 280mm frame size 75 kW machine. The

two 400V inverters are rated at 14.8A and 82A 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 5mm

thickness applied around the outer bearing race that was short-

ened with a short wire. As discussed above, this technique is

intrusive and will slightly alter the HF current flow when com-

pared 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 200MHz to 2GHz 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 1GHz and a maximum sam-

pling rate of 10GS/s, and an RF bandpass filter Mini-Circuits

BHP100+ with a bandwidth of 90−400MHz (input impedance

set to 50Ω). The HF bearing currents on the nondrive-end

(NDE) and drive-end (DE) sides were measured either with

Tektronix TCP202 50MHz passive (measurements shown in

Figs. 6(a), 6(b), 7(a), 7(b), 9–12) or R&S ZC20 50MHz

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 50MHz high voltage differential probe

(Figs. 4 and 5). The HF CM currents were measured using an

R&D EZ-17 100MHz passive current probe. Bearing temper-

atures 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 circulat-

ing 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. (Sec-

tion 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



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 pass-

ing 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 15MHz signal generator: The NDE bearing

insulation was left open: In this configuration, any cur-

rent flow across the NDE bearing is negligible. The DE

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. RESULTS I: HF CIRCULATING BEARING CURRENT

FLOW “AS IS”

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 measure-

ment results obtained with this machine are not discussed in

this section.

B. Experimental Results with Motor MB-75

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 1.2A

for 100 rpm. For elevated bearing temperatures above 60°C,

very small HF circulating bearing currents of a few hundred

milliamperes could be observed up to even 3000 rpm. The

HF CM current was measured to approximately 10A. The

frequency of the HF CM and the bearing currents was in the

range of 1MHz. 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.



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(a) 100 rpm rotational speed and ≈ 30°C bearing temperature.

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(b) 100 rpm rotational speed and ≈ 30°C bearing temperature.

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(d) 1500 rpm rotational speed and ≈ 65°C bearing temperature.

Fig. 6. Motor MB-75 and inverter IB-75: HF circulating bearing currentdetected through non-intrusive RF based method; measured currents, voltagesand RF signals.

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 10mV, 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.

VI. RESULTS II: INCREASED HF CIRCULATING BEARING

CURRENT FLOW


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.



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.


Maximum amplitudes of the HF circulating bearing currents

reached again up to 1.2A for 100 rpm and lower bearing tem-

peratures in the range of some (20 . . . 30)°C. Also, very small

HF circulating bearing currents of a few hundred milliamperes

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.

C. Analysis

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 tem-

perature 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 cur-

rents 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.

VII. RESULTS III: ROTOR GROUND CURRENTS


With the machine grounded via the rotor instead of the

stator, and the NDE bearing fully insulated, the total HF CM

current of about 8A and close to 1.5MHz 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.

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(d) 2000 rpm rotational speed and ≈ 30°C bearing temperature.

Fig. 7. Motor MB-75 and inverter IB-75, NDE bearing shortened: HFcirculating bearing current detected through non-intrusive RF based method;measured currents, voltages and RF signals.



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Fig. 8. Motor MA-15 and inverter IA-15: Bearing currents due to rotorground currents detected through non-intrusive RF based method; measuredcurrents, voltages, and RF signals.


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

10A would flow through the DE bearing. (Additional figures

would not add more value and are thus omitted for reasons of

space.)

C. Analysis

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

0 20 40 60 80 100 120 140 160 180 200-10

0

10

Time (us)

Vb

[V]

0 20 40 60 80 100 120 140 160 180 200-2

0

2

Time (us)

I b[A

]

0 20 40 60 80 100 120 140 160 180 200-10

0

10

Time (us)

RF

[mV

]

(a) Measured bearing current, bearing voltage, and RF signal.

0 20 40 60 80 100 120 140 160 180 200

102

Time (us)

|Zb| [

oh

m]

0 20 40 60 80 100 120 140 160 180 200-100

0

100

Time (us)

An

gle

of

Z b[d

eg]

(b) Computed bearing impedance.

Fig. 9. Motor MA-15 and inverter IA-15, external HF voltage supply of DEbearing: measured currents, voltages and RF signals and computed bearingimpedance for 300 kHz HF supply voltage, 210 rpm rotational speed, and≈ 30°C bearing temperature; computed bearing capacitance during capacitivebehavior Cb = 0.21 nF.

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. RESULTS IV: EXTERNAL SUPPLY OF HF BEARING

CURRENTS

A. Reasoning

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 1MHz, the impedance provided by the 5mm

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.

B. Experimental Results with Motor MA-15

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



0 50 100 150 200 250 300 350 400-10

0

10

Time (us)

Vb

[V]

0 50 100 150 200 250 300 350 400-1

0

1

Time (us)

I b[A

]

0 50 100 150 200 250 300 350 400-10

0

10

Time (us)

RF

[mV

]

(a) Measured bearing current, bearing voltage, and RF signal.

0 50 100 150 200 250 300 350 400

102

Time (us)

|Zb| [

oh

m]

0 50 100 150 200 250 300 350 400-100

0

100

Time (us)

An

gle

of

Z b[d

eg]


Fig. 10. Motor MA-15 and inverter IA-15, external HF voltage supply of DEbearing: measured currents, voltages and RF signals and computed bearingimpedance for 300 kHz HF supply voltage, 300 rpm rotational speed, and≈ 30°C bearing temperature; computed bearing capacitance during capacitivebehavior Cb = 0.38 nF.

bearing impedances for 300 kHz HF supply voltage and 210as 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 30µs, the impedance increases and turns capacitive

again at around 100µs, 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.

D. Analysis

For certain short time intervals, a bearing is observed to

form a capacitive film even at low rotational speed. In addition

0 50 100 150 200 250 300 350 400-10

0

10

Time [µs]

Ub

[V]

0 50 100 150 200 250 300 350 400-0.2

0

0.2

Time [µs]

I b[A

]

(a) Measured bearing current and bearing voltage.

0 50 100 150 200 250 300 350 400

102

Time (µs)

|Zb| [

oh

m]

0 50 100 150 200 250 300 350 400-100

-50

0

50

Time (µs)

An

gle

of

Zb

[deg

]


Fig. 11. Motor MB-75 and inverter IB-75, external HF voltage supply of DEbearing: measured currents and voltages and computed bearing impedance for300 kHz HF supply voltage, 210 rpm rotational speed, and ≈ 30°C bearingtemperature; 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. CONCLUSIONS

Interpreting our observations in the context of HF cir-

culating bearing currents, we postulate the following: For

“sufficiently” large HF circulating bearing currents and energy



50 100 150 200 250 300 350 400

-5

0

5

Time [µs]

Ub

[V]

0 50 100 150 200 250 300 350 400-0.2

0

0.2

Time [µs]

I b[A

]

(a) Measured bearing current and bearing voltage.

0 50 100 150 200 250 300 350 400

102

Time (µs)

|Zb| [

oh

m]

0 50 100 150 200 250 300 350 400-100

-50

0

50

Time (µs)

An

gle

of

Z b[d

eg]


Fig. 12. Motor MB-75 and inverter IB-75, external HF voltage supply of DEbearing: measured currents and voltages and computed bearing impedance for300 kHz HF supply voltage, 300 rpm rotational speed, and ≈ 30°C bearingtemperature; computed bearing capacitance during capacitive behavior Cb =

8.5 nF.

stored in the circuit before these currents start to flow, the oc-

currence 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 rotor-

to-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 (10mV) ([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 under-

standing occur. These modes and transitions might further

explain the possibility to detect HF circulating bearing cur-

rents through the RF based method that requires some form

of energy to be radiated as a result of–as per the current

understanding–the occurrence of some discharge.

APPENDIX

Before a discharge occurs, energy is stored within the

two bearing capacitances, Cb,DE and Cb,NDE, and the rotor-

to-frame capacitance, Crf that are all connected in parallel.

These capacitances can be summarized in an equivalent total

capacitance, Ctot ([20], eq. (2)),

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))

Ec =1

2Ctotv

2

b .

ACKNOWLEDGEMENT

Funding for the Open Access Publication was provided by

the Government of Styria.

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[5] D. Busse, J. Erdman, R. Kerkman, D. Schlegel, and G. Skibinski, “Sys-tem 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

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IEEE Ind. Appl. Mag., vol. 10 , pp. 40-50, July/Aug. 2004.[9] H. Akagi and T. Doumoto, “An approach to eliminating high-frequency

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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

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Mag. Ind. Appl., vol. 12, no. 4, pp. 22-31, July/Aug. 2006.[11] A. Muetze and A. Binder, “Practical rules for assessment of inverter-

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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. Skib-

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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 ringsfor 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, “Evaluationof slot-embedded partial electrostatic shield for high-frequency bearingcurrent mitigation in inverter-fed induction motors,” IEEE Trans. En.

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[17] T. Maetani, Y. Isomura, A. Watanabe, K. Iimori, and S. Morimoto,“Suppressing bearing voltage in an inverter-fed ungrounded brushlessDC motor,” IEEE Trans. Ind. Electr., vol. 60, no. 11, pp. 4861–4868,Nov. 2013.

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[33] V. Niskanen, A. Muetze, and J. Ahola, “On the role of the shaft endand the influence of frame size and load coupling on the RF emissioncharacteristics of induction motors,” EPE Journal.

[34] Accessed on October 9, 2013. [Online]. Available:http://www.skf.com/group/products/condition-monitoring/basic-condition-monitoring-products/electrical-discharge-detector/index.html.

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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 engineer-

ing 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 elec-

trical engineering from Lappeenranta Univer-

sity 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.

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