25386404 IEEE 115 Measure Shaft Voltage

”Safeguarding Machinery Worldwide “www.gaussbusters.com

Bearing Insulation Testingin

Rotating Machineryby

Paul I. Nippes, PEand Beth Galano

Magnetic Products and Services, Inc.Holmdel, NJ 07733


Table of ContentsThe value and problems of bearing insulation are addressed.

Examples of damage are provided where bearing insulation had been shorted out, or was ineffective.

Examples for insulating bearings and couplings are shown.

Simplified diagrams for shaft current flow are presented.

An example of un-measureable, circulating bearing current from homo-polar magnetic flux is presented

Safety during testing is stressed.

Methods for determining the integrity of bearing insulation are provided in the form of IEEE Standards and MPS practices for testing bearing insulation.

Insulation testing methods with the unit operating are addressed.


WHY INSULATE BEARINGS ?

First insulated to block stray currents from destroying bearings in electric motors and generators in the early 1900’s.Bearing insulation continues to be installed at the outboard end of electrical machines.

expensive to install and maintain lowers the bearing support stiffness, adding complication to design and vibration analysiswhen OE is also a drive, electrical insulation is needed, either at the coupling, and/or at the driven equipment as well.

In recent years, simple insulation installation is proving to beinadequate to block bearing currents where electronic switching is prevalent. Two vital considerations:

resistance of the insulationit’s capacitive impedance


DRIVEN-LOAD: PUMP,COMPRESSOR, ETC.

ELECTRIC MOTOR, SYNCHRONOUS, INDUCTION OR DIRECT CURRENT

OI L FILMBEARING

ROTOR

STATOR ELECTRICAL INSULATION

Elementary representation of a motor driving a Load

Elementary diagram for considering shaft currents in typical motor - load arrangement


Elementary diagram for considering shaft currents in typical turbine - generator arrangement


“One method of insulating bracket-type machine to prevent flow of shaft currents” in article “Bearing Currents --- Their Origin and Prevention” by C. T. Pearce, of Westinghouse Electric, 1927


Modern method for insulating bracket type machines in article “Shaft Voltages – Shaft/Earth Voltages – Bearing Currents”Article “AR 170-220” by Siemens Field Services and Workshop Operations 04/1991


Modern open method to insulate pedestal bearings from article “Insulating Rotating Machinery Against Stray Electrical Currents” by Michael M. Calistrat, Paul I. Nippes and Ned Zeman, Preventative Maintenance Technology National Conference -1997.


Modern hidden method to insulate pedestal bearings from article “Insulating Rotating Machinery Against Stray Electrical Currents” by Michael M. Calistrat, Paul I. Nippes and Ned Zeman, Preventative Maintenance Technology National Conference -1997.


Four different methods to insulate shaft couplings from article “Insulating Rotating Machinery Against Stray Electrical Currents” by Michael M. Calistrat, Paul I. Nippes and Ned Zeman, Preventative Maintenance Technology National Conference -1997.


VOLTAGE & RESISTANCE MEASUREMENTS ELEMENTARY DIAGRAM FOR STRAY SHAFT CURRENT CIRCUIT MEASUREMENTS

BEARING OIL FILM

BEARING

INSULATION

DRIVEN LOAD SHAFT COUPLING MOTOR SHAFT

STRAY POWER SOURCEAC/DC VOLTS


VOLTAGE & RESISTANCE MEASUREMENTS ELEMENTARY DIAGRAM FOR STRAY SHAFT CURRENT CIRCUIT

MEASUREMENTS

BEARING OIL FILM

BEARING

INSULATION

INSULATION

DRIVEN LOAD SHAFT COUPLING MOTOR SHAFT

STRAY POWER SOURCEAC/DC VOLTS


The actual shaft voltage circuits of a boiler feed pump installation in a large power plant

Insulation needed here


Boiler Feed Pump from a large Generator


ELECTRICAL CURRENTDISCHARGE DAMAGE

PITTING, FROSTING

SPARK TRACKS


ELECTRICAL SPARK TRACKS


Example of localized circulating current due to through axial, or homopolar, magnetic flux in the rotor, from an article “Bearing Currents --- Their Origin and Prevention” by C. T. Pearce, of Westinghouse Electric, 1927

Measurement of current in the operating machine is impossible.

Measurement of axial magnetic flux difficult, if not impossible.

Corrections possible:

1) Degauss if cause is a residual magnetism source.

2) Install a significant non-magnetic member into the flux path.


Example of Through Magnetism generating Eddy Currents


Example of Through Magnetism generating Eddy Currents –

currents discharge at outside edges passing through the babbitt


IMPEDANCE EFFECT OF DIFFERENT FREQUENCY CURRENTS THROUGH BEARING, SEAL AND/OR COUPLING INSULATION

Of major concern is the ineffectiveness of bearing insulation when subjected to high frequency voltages and more particularly the voltage spikes associated with electronic wave generation and switching. The exact bearing insulation impedance is difficult todetermine. A simple calculation of the resistive and capacitive components of the bearing insulation reveals a significantly lowered capacitive impedance compared to the ideal resistive component and thus controls, along with the bearing oil film, the amount of current through the bearing. Insulation is reduced inverse to the ratio of the frequencies. The resistive component will decrease over time due to oil impurities and insulation deterioration over time .

Impedance vs. Frequency

100

1000

10000

100000

1E+06

1E+07

1E+08

1E+09

1E+10

1E+11

1E+12

1E+13

10 60 110 160 210 260 310 360

Freq (Hz)

Z (ohm)

0.02119

0.042354

0.063518

0.084682

0.105846

0.12701

0.148174

0.169338

0.211667

l/A Ratios

2.12E+12

2.12E+11Limits of

Resistance


Impedance vs. Frequency

0

50

100

150

200

250

300

0 500 1000 1500 2000 2500 3000 3500

Freq (Hz)

Z (ohm)

0.02119

0.042354

0.063518

0.084682

0.105846

0.12701

0.148174

0.169338

0.211667

l/A Ratios

Expanded capacitive impedance for different frequency currents through bearing, seal and/or coupling insulation demonstrating the effect of the ratio of insulation thickness (i) to the insulation effective area (a) or a/i


SAFETY FIRST WHEN MAKING SHAFT VOLTAGE MEASUREMENTS

Wear hard hat, face shield, safety shoes.Place voltage measuring brush onto the shaft only after you are certain that no rotating element, such as a keyway or bolt is in the zone of brush contact. Move measuring brush to the shaft very slowly and stand to the side.


Same as IEEE 112, basically



PART I – ACCEPTANCE AND PERFORMANCE TESTING IEEEStd 115-1995



BEARING INSULATION CONDITION BY MEASUREMENTS

Comparative evaluation of the bearing insulation is possible on electrical machines if there is access to shaft, the bearing sleeve, its housing or pedestal.

Open access to insulated pedestals and bearing housings make it convenient for voltmeter, ohmmeter or other reliable and safe measurements.

A disadvantage of open access is the ease for shorting around the insulation by instrumentation, shielded cables, un-insulated bolts and dowels etc.

No access, such as completely isolated bearings, both electrically and physically, protects from accidental shorting, and is generally safe from insulation shorting, except for wires run internal to the isolated bearing, such as temperature detectors.

A disadvantage of no access is the inability to make insulation measurements. However, this can be overcome by installation of an isolated metallic separator in the bearing insulation making a or sandwich having an insulated wire brought out for making measurements.


AN ALTERNATE PROCEDURE TO FOLLOW:

READ AND RECORD VOLTAGE VALUES, WITH THE UNIT OPERATING USING A HAND-HELD BRUSH AND A RELIABLE METER.

RECORD SPEED AND LOAD AND IF THERE IS SHAFT GROUNDING,

PHOTOGRAPH AND DESCRIBE THE SHAFT GROUNDING,

HOW IT IS CONNECTED ELECTRICALLY AND

WHERE IT IS LOCATED ON THE UNIT.


MPS Shaft Voltage Measurements

Measure and record the voltages, AC and DC separately if using a standard voltmeter, or as displayed if using a VCM and/or an oscilloscope :

a) Shaft end to end voltages across the driver (motor), and then across the load,.b) Shaft voltages combined overall end to end. Should equal the sum of voltages in a)

and indicates that the measuring brush and ground interconnections are continuous. c) Shaft voltages to ground at the OB and IB shaft locations, First, open circuit, then

with 1k Ohm and 1 Ohm resistor in parallel across the voltmeter leads. Voltage drops indicates: w/ 1 K Ohm = electrostatic source; w/ 1 Ohm = residual magnetic source.

d) Repeat item c) for the load shaft voltages for the OB and IB shaft locations. e) With voltages of reasonable values in a)-c), it is expected that the highest shaft

voltage measured is across the motor/generator shaft outboard end to ground. A low voltage indicates either low source voltage or shorting of the shaft to ground

f) If individual voltages measured in a) thru d) are basically equal to zero, voltage measurements will not suffice to evaluate the condition of the bearing insulation and it will be necessary to conduct the same readings using an ohmmeter, but without the patallelresistors. A decision to do nothing at this stage may be justified because of the low voltages measured, however this may not be a permanent situation and bearing resistance remains unchecked. Unit shut-down measurement of the insulation may be needed following the procedures of IEEE 115, 112 and 113.

g) To continuously track the shaft grounding current/voltage install an MPS Voltage-Current monitor and reliable shaft grounding and voltage sending brushes.


Measuring DevicesHand-held VCM readings Storage Oscilloscope Readings


EARLY WARNING PROVIDED BY SHAFT GROUNDING AND VOLTAGE SENSING BRUSH SIGNALS TO THE MPS VCM


CONFIGURATION # TGDUAL GROUNDING BRUSHES, DUAL VCM’S IN ONE BOX, DUAL VOLTAGE SENSING

HP TURB. IP TURB.

LP TURB. GENERATOR

EXCITER

VCM-E/NSIIEASY ACCESS FOR ALARM SETTINGS.

SHAFT CURRENT AND VOLTAGE SIGNALS-OUT FOR CONTINUOUS TRENDING AND ALARMING

DUAL SHAFTGROUNDINGBRUSHES

SHAFT VOLTAGESENSING BRUSH SB # 2

VOLTAGE SPIKEAND TRANSIENTSABSORBER

TAPPED RESISTORSOR CURRENT SHUNTS

BONDING TO LOWER BEARING CASINGAND FROM THERE TO GROUND

SHAFT VOLTAGESENSING BRUSH SB # 1


Single/Standard VCM’s Shown

VCM-E FEATURES:1.Attaches to both an insulated shaft grounding brush for current measurement, and to a shaft voltage sensing brush for shaft voltage measurement.2.Continuously monitors and outputs 4-20 ma signals for shaft grounding currents and shaft voltages.3.Alarms when shaft riding brushes are worn or need adjustment.4.Alarms on high grounding brush current or high shaft voltage.5.Reduces costly parts replacement.6.Ideal for critical, large or high speed rotating machinery requiring uninterrupted shaft grounding, while providing reliable unit condition monitoring data.

Shaft Condition VCM and grounding brushes that provide early warning of problems via monitoring shaft voltages and grounding currents


Summary and ConclusionsConsider bearing insulation employed and comparative impedance values using the characteristic graphs previously presented showing bearing insulation ratios of thickness to gross area (t/A) as a function of different shaft voltage frequencies.

Familiarize and follow bearing insulation testing standards per IEEE, both for operating units and those at standstill.

Routinely perform bearing tests with the unit running, per MPS’recommendations and most importantly stress safety during testing.

Educate personnel to be “on-the-watch” for damage where the bearing insulation had either been shorted out, or is ineffective.


LETTER OF INQUIRY ON BEARING INSULATION TESTING –PAGE 1

Mr. Nippes,

This is to request your opinion and thoughts on the following voltage measurement approach. The machine in question is a typical large STG with double insulation at the bearing and seal on the CE of the generator. A single test lead is connected from the metal between the insulation halves.

Thank you for the interesting e-mail. You are fortunate to have the metal member between insulation halves. However, as you discover, inconsistent readings are confusing

Often we may have a low megger reading between the test lead and ground (or possibly indication of a short circuit). When this condition exists we do not know for certain if both insulation halves are shorted and, if they are, if the oil film is or is not providing adequate protection against electrical discharge and pitting. Obviously, we would not intend to operate for an extended period of time with this condition but the question always arises concerning whether we might be experiencingdamage or not during short term operation until repairs are made. A viable and consistent test approach to make this determination would be very helpful.

Is the unit operating when you make megger readings? What is the applied megger voltage? Megger readings may be influenced by stray voltages on an operating unit. It is preferable to fires make voltage readings, AC and DC, or with an MPS hand-held VCM. If there is voltage, megger or resistance readings may be inconsistent , especially if done on the operating machine.

Have you had any experience with measuring the voltage drop between the test lead and ground and between the test lead and the shaft as a tool to determine if one or both insulation halves are shorted?

I believe the best way to test the insulation is to connect electrically the shaft to the metal separating the insulation. Make voltage readings from this interconnection to the lower bearing housing on the operating machine. Next remove the interconnection insulation and measure the voltage between the metal separating the insulation and the shaft, both AC and DC or with an MPS hand-held VCM. If a notable voltage is detected chances are that theintervening insulation, plus the resistance of the shaft oil films, for all bearings, is high and most likely protective.If, in the above two tests the voltage is zero, an ohmmeter or low voltage megger unit according to IEEE Testing Standards can be utilized to measure the combined insulation resistance of the oil films. If you use a 500 V megger, false readings can occur due to possible tracking impurities on the insulation, and in some cases, the 500 V megger will set up a tracking path. It is preferable to use the 115V with light bulb in series as listed in IEEE 112 and 115.


LETTER OF INQUIRY ON BEARING INSULATION TESTING –PAGE 2

Have you measured current flow in the ground path to determine if random discharges across the oil film might be occurring or is this approach only useful for a sustained current flow?

It is virtually impossible to intercept the current path for inserting a current shunt and it can be equally difficult to encircle the current path to ground for measure bearing AC currents. Bearing through current can be diverted from the bearing through a shaft grounding brush, however will not be equal to the through-bearing current. The brush current to ground can be measured using a current shunt signal to an MPS voltage current monitor, or VCM, providing both intermittent and sustained readings, alarms and 4-20ma signals.

Do you think the oil film alone (in the absence of a mechanical problem that minimized the normal clearances) would be sufficient to prevent electrical damage for voltages in the 10 to 20 volt range even if both insulation halves were shorted?

The answer is “YES”, and even higher voltages are possible for an ideal bearing oil film. You are correct to inquire if the oil film may have contact or conduction and this may occur at any bearing, affecting tests made on the shaft insulation. The oil film is an excellent isolator, but over time it is seldom without either contact or impurities. Very often bearing problems do not occur until long after initial installation. This indicates increased impurity deposits or mechanical distortion/wear would be the cause for bearing current damage. With the many variables involved, a simple, consistent test routine is not easy to set up. Possibilities include a logic chart based upon findings and combinations of. A satisfactory logic chart may eventually be available.

Are there other methods or approaches that you have found to be useful?

Experience is that shaft voltages, bearing contact and impurities in the oil create conditions leading to bearing/ seal and gear currents. Sustained current in bearings, seals etc. often come from electromagnetic asymmetries in electrical machines or from high residual magnetism. Damage is usually rapid and severe and, in absence of a shaft-voltage current monitor, first detection is an increase in vibration and temperature.2005. Your inquiry is well timed since I am currently composing a presentation for EPRI for delivery at the LEMUG session in New Haven, CT, August 15-17,

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25386404 IEEE 115 Measure Shaft Voltage