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Electrical Drives Juha Pyrhnen, LUT, Department of Electrical Engineering14.1

14. MOTOR BEARING CURRENTS IN INVERTER DRIVE ................................................... 1

14.1 Bearing Damage Caused by Electrical Current - Failure Mechanism ................................. 2

14.2 Non-Circulating Bearing Currents ....................................................................................... 4

14.3 Potential Rise Caused by the Motor Parasitic Capacitances ................................................ 6

14.4 Circulating Bearing Currents ............................................................................................... 6

14.5 Reduction of Bearing Currents ............................................................................................ 8

14.5.1 Grounding .................................................................................................................... 814.5.2 Output Choke ............................................................................................................... 8

14.5.3 Insulation of the Bearing .............................................................................................. 9

14.5.4 Shaft Grounding ........................................................................................................... 9

14.5.5 Galvanic Separation of the Power Tool and the Motor Shaft ...................................... 9

14.5.6 Using Conductive Bearing Lubricant .......................................................................... 9

14.5.7 Ceramic Ball Bearings ................................................................................................. 9

References ............................................................................................................................................ 9

14. MOTOR BEARING CURRENTS IN INVERTER DRIVE

Bearing currents have been a debated subject for the past hundred years. Bearing currents are thus

not a phenomenon brought by the modern power electronics, but shaft voltages and bearing currents

caused by them were investigated for instance by F. Punga and W. Hess in their article Eine Er-

scheinung an Wechsel- und Drehstromgeneratoren inElektrotechnik und Maschinenbau already in

1907; also P. Alger and H. Samson lectured on Shaft currents in electric machines at the A.I.R.E

Conference in 1924, etc. Those days, the source to the problems were not power electronics appli-

cations, but chiefly the inaccuracies involved in the manufacture of the motor. Some reasons for

bearing currents were for instance discharges through insulations, as well as shaft voltages induced

by the dissymmetries of magnetic structures.

In addition to the improvement in operating and control properties, the development of power elec-tronics brought also bearing currents that occurred in the motor systems in a completely new way;

however, the arising problem could not first be connected to power electronics. The most important

sources for bearing currents in the present inverter drives are the common-mode voltage fed by the

PWM-type inverter to the motor, a high switching frequency, poor cabling between the inverter and

motor, and the parasitic capacitances of the windings. Figure 14.1 illustrates a failure of the race of

a ball bearing caused by bearing currents (seven times enlarged image).

Figure 14.1 Damaged race of a ball bearing; 7 times enlarged (Haring ABB).

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An U.S. company Shaft Grounding Systems analyzed 1150 AC motors, 1000 of which were in-

verter drives, and the rest operated direct-on-line (Boyanton 1995). The analysis showed that 25 %

of the motors fed by inverter had a bearing failure of some degree already after 18 months of opera-

tion. Further, 65 % of the motors that had been operating for more than 18 months, the average be-

ing 2 years, had some kind of a bearing failure. Only one per cent of the DOL motors had expe-

rienced a bearing failure. However, the survey does not explain the reasons for determining the

bearing failures as caused by bearing current in particular, and not for instance by manufacturing ormounting faults. Furthermore, the company itself manufactures devices for shaft grounding.

14.1 Bearing Damage Caused by Electrical Current - Failure Mechanism

There are several occurrence mechanisms for bearing currents; however, the current flow through

the bearing is identical for different bearing current types. A ball bearing is comprised of two roll-

ing surfaces (races), between which a lubricated metal ball rotates. When the voltage is switched on

between these races, while the machine or device is either non-rotating or in a slow motion, there is

a low-resistance contact between the rolling surfaces through the ball, and thus the current circuit is

closed, and a small current flows in the circuit.

As the rotation speed increases, the lubrication film separates the ball from the race, and thus the

impedance between the two races starts to increase. However, the current flow will continue in the

circuit, if the voltage is high enough (Skibinski 1996). According to Skibinski, voltage withstanding

of a cold bearing to the pulsating voltage generated by a PWM inverter is about 35 V, whereas the

corresponding value for a warm bearing is ca. 610 V. For sinusoidal voltage, the corresponding

value for a warm bearing is of the scale of 0.21 V (Skibinski 1997).

Voltage withstanding is directly proportional to the quality and temperature of the lubricant, as well

as on the surface roughness of different parts of the bearing. When the electric circuit breaks, two

capacitors are created between the ball and the bearing races, the charge of which starts to increase.The equivalent circuit of the bearing is illustrated in Figure 14.2. When the voltage level between

the two ball-bearing cups increases high enough, the capacitors are discharged. The discharge cur-

rent flows through a very small contact surface, which implies a high current density. The phe-

nomenon resembles electric machining EDM , and causes for instance fluting in the bearing race as

shown in Figure 14.1.

Ru

Ci

Rk

Ci

Rs

Civ Z

rpm

shaft

Figure 14.2. Bearing and its equivalent circuit

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Notations in the figure:

Ci is the capacitance F between the ball and the race in the ball bearing

Civ is the capacitance F between the races of the ball bearing

Rk is the resistance of the ball of the ball bearing

Rs is the resistance of the inner race of the ball bearing

Ru is the resistance of the outer race of the ball bearing

Z is the nonlinear impedance of the ball bearing

The nonlinear impedanceZchanges as a function of speed, varying from the low resistance of the

non-rotating machine to the impedance of megaohms at the rated speed.

Bearing currents may cause an outage of the rotated equipment already after a few weeks opera-

tion, or, on the other hand, it may take several years before a bearing failure is created. If the rotat-

ing machine rotates at constant speed, there occur flutes that are transverse to the direction of mo-

tion of the ball, since the dielectric breakdowns occur at regular intervals. If the rotation speed of

the motor varies, the above mentioned fluting does not take place, but the ball race corrodes in the

entire length of the raceway.

The function of the inverter is to switch the phase conductors timely either to the positive or to the

negative DC voltage. In the commonly used PWM technology, the positions of the power switches

are changed for instance by comparing the three-phase sine wave of the desired frequency to the

higher-frequency triangle wave. By investigating for instance the potential of the star point (N

point) to the ground in the case of Figure 14.3, we see that the potential of the star point is other

than zero.

If for instance the phases Uand Vare connected to Udc, and the phase Wto 0, the windings of the

phases Uand Vwill be connected in parallel, and thus their impedance is halved. As a result, the

voltage of theNpoint is thus 2/3Udc. In Figure 14.3, the zero level of the voltage is selected to bethe potential of the negative DC voltage. We see thus that the potential of the Npoint pulsates at the

switching frequency of the power switches as shown in Figure 14.4.

If the motor winding is connected in delta, the analysis becomes more complicated; however, the

potential of the winding relative to the frame can be shown to vary between 0 and Udc. If all the

phases are connected to Udc, the whole winding is in the potential of Udc, and correspondingly,

when connected to 0, there is no potential difference.

Bearing currents can be divided into two categories: circulating and non-circulating bearing cur-

rents. The damages caused by both current types are similar, but their elimination or reduction is

implemented by different methods.

U

V

W

N

Figure 14.3 Star-connected motorwinding.

UDC

00 t Figure 14.4 Typical common-mode voltage produced by PWM.

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14.2 Non-Circulating Bearing Currents

Non-circulating currents are created by an increase in the potential differences between stator wind-

ings and the rotor, between stator winding and the frame, and between the frame and ground.

The function of the supply cable is to transfer the power fed by the inverter to the motor with a suf-

ficiently low power loss. According to the EMC regulations, the cable may not cause disturbancesto the environment, and furthermore, the cable should be adequately immune to the disturbances

from the environment. The electrical safety regulations also set limits to the cross-sections of the

phase and protective conductors. In order for the cable between the inverter and motor not to pro-

duce disturbances to the environment, the cable has to be shielded with a well-conductive material;

this holds for both AC and DC drives.

If asymmetric supply cables are employed against the recommendations of frequency converter

manufacturers (cf. Figure 14.5), a notably high voltage will be induced to the PE conductor.

L1

L2

L3

PE

L1

L2

L3

PE

L1

L2

L3

PE

armouring

Figure 14.5 Asymmetric supply cables

The induced voltage is caused by the combined-mode voltage fed by the inverter as well as by thevoltages of the phase conductors including very high du/dtvalues. For instance, the rise time of fast

IGBT switches may be below 100 ns, which implies that the spectrum includes frequencies above

10 MHz. When connecting for instance a PE conductor and the armouring to the inverter frame, the

potential of the motor relative to ground increases. If the armouring is a too-large-impedance path

for the high-frequency current, the current starts to flow also through the bearings of the motor and

the power tool to ground as shown in Figure 14.6.

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rpm

Motor Frame

BearingCl

Zl

Rl

Rotor Shaft

Inverter

Zpe

ZarmMotor Driven machine

rpm

Bearing

Cl Zl

Rl

IpeI

l

Iarm

Il

Figure 14.6 Bearing current path when using an inappropriate motor cable

Notations of Figure 14.6

Cl is the capacitance between the ball and the ball-bearing cups F

Iarm is the current flowing in the armouring of the cable A

Il is the current flowing through the bearings A

Ipe is the current flowing in the PE conductorA

Rl is the resistance of the ball-bearing cup

Vpe is the voltage induced to the PE conductorVZl is the non-linear impedance of the ball-bearing cup

Bearing current may also easily flow through the bearings of the power tool. This happens often for

instance in paper machines, roller mill drives, and in other drives including solid metal structures.

In roller mill drives, an additional problem is caused by the constantly varying shaft grounding im-

pedance, as the machined workpiece connects and disconnects the shafts of the motor.

Using a lubricant with better insulating capacity in the bearings of the power tool than in the motor

bearings may destroy the bearings of the power tool. The reason for this is that the voltage with-

standing of the lubricant with better insulating capacity is higher, and consequently, dielectric

breakdown results in a higher current density, which puts stresses particularly on the power tool.

This is slightly paradoxical, since the bearings that are exposed to large bearing currents, will have

a longer lifetime when using a lubricant with poorer quality than vice versa. Figure 14.6 shows that

Ipe =Iarm +Il . (14.1)

If the armouring provides a sufficiently low-impedance return path forIpe, thenIpeIarm.

There are notable differences also between symmetrical cables. Analyses have shown that there

may be a 13-fold difference in the voltage induced by the supply cable to the armouring when com-

paring poor and good supply cables. Correspondingly, 56-fold differences in the noise conductionfrom cable to cable were reported between the cables; these results show clearly the significance of

cabling in the occurrence of bearing currents (Bentley 1996).

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14.3 Potential Rise Caused by the Motor Parasitic CapacitancesDue to the high-frequency components of the voltage fed to the motor, the traditional motor model,

which is comprised of concentrated resistances and inductances, has to be left aside. As there is a

high-frequency voltage acting in the stator windings, a current starts to flow in the parasitic capacit-ances between the motor frame and the stator winding, and between the stator winding and the ro-

tor. Due to the symmetry of the motor, the parasitic capacitances that occur in every electrical mo-

tor, can be considered to be distributed evenly in the whole stator winding, and their values increase

considerably together with the motor size. In the case of differential mode voltage, corresponding

currents start to flow also through the parasitic capacitances between the stator phase windings.

The common-mode voltage tends to raise both the potential of the motor frame relative to ground,

and the potential of the rotor relative to the frame. Figure 14.7 illustrates an approximate equivalent

circuit of the motor for a voltage including high-frequency components. A simplified equivalent

circuit of the bearing is included in the illustration to show the closure of the rotor circuit (Chen

1996a).

Ru

Ci

Rk

Ci

Rs

Civ Zrpm

Motor frame

bearing

rotor

statorCkrt

Ckrk

Figure 14.7 Circuit of the non-circulating currents

Notations in Figure 14.7:

Ckrkis the parasitic capacitance between the stator winding and the motor frameCkrt is the parasitic capacitance between the stator winding and the rotor

Cl is the capacitance between the ball and ball-bearing cups

Rl is the resistance of the ball-bearing cup

Zl is the non-linear impedance of the ball-bearing cup

Bearing current may also in this case flow through the power tool, as presented above.

14.4 Circulating Bearing Currents

If the current flowing in and out from the stator winding are equal, the sum of the magnetic flux

produced by the winding becomes zero. As an effect of the parasitic capacitances, the incoming and

outgoing currents are not equal, as can be seen in Figure 14.8.I+ nI indicates the current flowing

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into the motor winding a, andI- nIthe current going out through a. At each parasitic capacitance,

a coupling current of magnitude In leaves the phase winding. When investigating the situation at

section AA, we see that the sum current is other than zero.

I-I

I-2I

I-(n-2)I

I-(n-1)I

I-nI

I+I

I+2I

I+(n-2)I

I+(n-1)I

I+nI

aa'

I

A A

I

I

I

I

I

I

I

I

I

Figure 14.8 Winding current in one loop of one phase

When investigating the cross-section of Figure 14.9 from the location A-A of Figure 14.8, now in-

cluding all the three phases, we notice a net flux linkage surrounding the phase windings; it is not

cancelled due to the aforementioned current unbalance (Chen 1996b).

a' a

Figure 14.9 The fluxes produced by the winding due to the combined-mode voltage in a three-phase machine at the

cross-section A-A of Figure 14.8.

According to Faradays induction law, the change in the magnetic flux density produces an electricfield circulating it (Pyrhnen 1996). This creates a voltage difference between the N and D ends of

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the shaft. The only circuit that could cancel the varying magnetic field, closes according to Figure

14.10 through both bearings, the shaft, and the frame of the motor.

Figure 14.10. Circuit of a circulating bearing current

A high-frequency current flowing in the stator windings induces a voltage to the motor shaft, themagnitude of which may be even 20 times the voltage of a motor operating direct-on-line.

14.5 Reduction of Bearing Currents

Bearing currents cannot be completely eliminated in inverter drives; however, they can be reduced

to a level at which they are no longer harmful to the operation. There are several alternatives to this,

and the best results are achieved by a combination of several alternatives

14.5.1 Grounding

The rise in the potential of the motor frame relative to ground can be prevented or reduced only by

proper cabling. To provide efficient protection against radio-frequency interferences, in addition to

armouring, the cable has to be equipped with a well-conductive radiation shield without holes, as

in the armouring. The couplings of the armouring and the shield to the frame have to be very low-

inductive; this is achieved best by using a conductive sleeve around the conductor. The conductive

sleeve has to be galvanically connected to the cable shielding, around the entire cable, and the

sleeve itself has to be fixed by a conductive pad to the supply or consuming device. Finally, ar-

mouring and shielding are wired up to the PE bus as directly as possible. This way, a Faraday cage

is created all the way from the inverter to the consuming device.

14.5.2 Output Choke

The purpose of the output choke is to decrease the du/dt values occurring in the output voltage,

thereby eliminating the highest frequencies of the spectrum of the output voltage. This significantly

reduces the currents flowing through the parasitic capacitances of the motor, and consequently, the

current flowing to and from the winding are closer to balance, that is, the shaft voltage of the motor

decreases. Furthermore, the voltages induced to the motor cable decrease. An output choke has thus

effect on both circulating and non-circulating bearing currents. With more complicated couplings

comprised of inductances and capacitances, the output voltage can be made to resemble almost pure

sinewave, in which case the bearing currents approach the level of a DOL motor.

Recently, filters that suppress common-mode voltages in particular have been designed; with these

filters, bearing currents can be reduced considerably. A lossy magnetic core mounted around phase

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conductors is also an efficient method to filter common-mode current, yet it produces significant

losses.

14.5.3 Insulation of the Bearing

To eliminate non-circulating bearing current, both bearings should be insulated in order to prevent

the closed-loop circuit. This, however, does not completely eliminate the problem, since a rise inthe shaft potential easily leads in a bearing failure of the power tool. In the case of circulating bear-

ing current, sheathing one of the bearings suffices to cut the circuit, however, there is still a slight

chance that the insulated surface of the bearing creates quite a large capacitance together with the

ball-bearing cup, and thus allows a current flow in spite of the insulation.

14.5.4 Shaft Grounding

The most efficient method to prevent a bearing failure in the motor is to mount brushes on the mo-

tor shaft close to the bearings and to couple them low-inductively to the frame. This may sound

somewhat peculiar, as the initial target was to eliminate brushes in the AC machine. However, as

the brushes are in contact with a smooth shaft, and the current passing through the brushes is small,

the carbon dust is not a problem either.

14.5.5 Galvanic Separation of the Power Tool and the Motor Shaft

If the bearing currents are detected to flow through the motor shaft to the power tool, these devices

can be galvanically separated, thus breaking the circuit. However, finding an electrically insulating

switch between the power tool and the motor may prove difficult.

14.5.6 Using Conductive Bearing Lubricant

When using conductive bearing lubricant, the bearing current can flow through the bearing without

causing damages to the bearings. However, this has not been documented so far; Chen 5 only

mentions conductive bearing lubricant as an alternative to avoid shaft grounding with brushes.

14.5.7 Ceramic Ball Bearings

An almost certain way to avoid destruction of the ball bearings is to use bearings with ceramic

balls. The drawbacks of these balls are poorer withstanding to forces and high price when compared

with conventional ball bearings.

References

ABB Industry Oy. 1996. Grounding of the Drive System. [1]

Bentley, John M. 1996. Evaluation on Motor Power Cables for PWM AC Drives, IEEE Conference

record of 1996 Pulp & Paper Industry Technical Conference. 2

Boyanton, Hugh. 1995. Bearing Damage Due To Electric Discharge, Shaft Grounding Systems, pp.

129. 3

Punga, F. and Hess, W. 1907. Eine Erscheinung an Wechsel- und Drehstromgeneratoren. Elektro-

technik und Maschinenbau Vol. 25, pp. 615618.

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Pyrhnen, Juha. 1996. Shkmagnetismin luentojen runko v:lle 1996 (Lecture notes in Electromag-

netism 1996, in Finnish) 4

Shaotang, Chen. 1996a. Source of Induction Motor Bearing Currents Caused by PWM Inverters,

IEEE Transactions on Energy Conversion, Vol. 11, no.1, pp. 2532. 5

Shaotang, Chen. 1996b. Circulating Type Motor Bearing Current in Inverter Drives, IEEE -IAS

Annual Meeting 1996, Vol. 1., pp. 162167. 6

Skibinski, Gary. 1997. Bearing Currents and Their Relationship to PWM Drives, IEEE Transac-

tions on Power Electronics, Vol. 12, no.2, pp. 243251. 7

Skibinski, Gary. 1996. Effect of PWM Inverters on AC Motor Bearing Currents and Shaft Voltages,

IEEE Transactions on Industry Applications, Vol. 32, no.2, pp. 250259. 8

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