1077-2618/09/$25.002009 IEEE

BY T IMOTHY WAYNE PASCHALL ,BRYAN K . OAKES , & GARY DONNER

THE COMMITTEE ASSOCI-

ated with the development of

the American Petroleum In-

stitutes (API) Standard 541,

Form-Wound Squirrel Cage Induction Motors

500 Horsepower and Larger, fourth edition,

realizes that many individuals working with

this standard may not have experience with

or knowledge of motor theory, operation, and

testing. For this reason, the standard includes

a design guide to assist with the decision-

making process. This article further addresses

the testing requirements of the standard by

explaining when and why these tests are

required. The sequence of topics in this arti-

cle follows the paragraph numbering for API

541, fourth edition.

API Standard 541 Fourth Edition

API 541, fourth edition [4], covers the mini-

mum requirements for the production and

purchase of all form-wound squirrel cage

Digital Object Identifier 10.1109/MIAS.2008.930896

Navigating the test requirements of API 541 fourth edition

STOCKBYTE

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induction motors above National Electrical ManufacturersAssociation (NEMA) 440 frame sizes (nominally 500 hpand larger) for use in petroleum industry services. Becauseof its success, API 541 has become the recognized standardfor purchasing high-quality, high-performance motors usedby most large industrial and utility purchasers throughoutthe world. The standard may be applied to adjustable-speedmotors and induction generators with appropriate attentionto the specific requirements of such applications. It alsogives information on electrical and mechanical design fea-tures such as insulation systems and accessories. By usingthe information in this article, users should be able to makemore informed decisions about specifying the testing fortheir API 541 fourth edition motors.

Test Procedures from API 541 Fourth Edition

Balance Check with Half Coupling(API 541 Fourth Edition Paragraph 2.4.6.3.3)This test is typically specified for two-pole motors of2,000 hp and larger. The test demonstrates that the couplingdoes not introduce additional unbalance into the rotatingassembly. This procedure is completed in the same time frameas the residual unbalance test.When selected on the data sheet,the vendor must determine if this testing is practical for themotor facility. If it is practical to install the coupling on theshaft in the balancemachine, then this test will provide impor-tant data on the complete balance of the rotating assembly. Ifthere is an increase in the unbalance because of the addition ofthe half coupling, these data will be reported to the purchaser.Corrections to the balancing of the rotating assembly and/orthe half coupling must bemutually agreed upon.

Residual UnbalanceVerification Test(API 541 Fourth EditionParagraph 2.4.6.3.6)This balance verificationprocedure is recommendedfor all motors with two orfour poles and for six-polemotors above 3,000 hp.This test is not necessary formotors having eight polesor more. The purpose of thistest is to demonstrate thatthe rotating assembly hasbeen correctly balanced.This test defines the amountof unbalance remaining inthe rotating assembly aftercompleting the balancingprocedures. A known trialweight (2 Ub) is added tothe rotating assembly at aspecific location and movedin six or 12 equally spacedlocations along the sameradius to determine theresidual unbalance in oneplane. Refer to Appendix D

of API 541, which shows the calculationmethod to verify thatthe residual unbalance amount is within the tolerance ofUb.

Ub 4Wr=Nmc, (1)where Ub is the residual unbalance (oz-in), Wr is theweight (lb) on each bearing journal (1/2 of the rotatingassembly weight), and Nmc is the motor rotational speedin revolutions per minute (not the rotating assembly bal-ancing speed).

The residual unbalance can be plotted on a polar graphto show the position of the unbalance that consists of anamplitude and angle. If the residual unbalance for anyplane is exceeded, then the rotor should be totally reba-lanced, and the residual unbalance testing should berepeated in all planes. The testing shows the quality ofbalancing of the rotating assembly before the motor isbuilt and confirms that the balancing process is completedproperly. This procedure is a time-consuming process butcan give important information on critical service motors.The balance may change over time, but the amount ofchange can be calculated if the residual unbalance is docu-mented during the building process. In Figure 1, a resid-ual unbalance report shows the position of the residualunbalance, balance tolerances, and actual residual unbal-ance and specifies the balance plane being measured.

Running Tests with Half Coupling(API 541 Fourth Edition Paragraph 4.3.1.6.2)This is a recommended test for all two-pole and four-polemotors operating above their first lateral critical speed.

1

Actual Residual Unbalance: 0.091 (oz-in)Allowable Residual Unbalance: 0.894 (oz-in)0

(0.42, 322)

(0.02, 12)

(0.42, 18)

(0.41, 77)

(0.38, 136)

(0.40, 201)

(0.38, 268)

Balance Plane: Drive End

90

180

270

Residual Unbalance Plot

Residual unbalance plot.27

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The purpose for this test is to check the motor vibrationwith the coupling half installed on the motor shaft. If it isnot practical to install the coupling half on the shaft in thebalance machine, then this test should be reviewed as analternative. Balancing or testing with the half coupling canbe selected on the data sheets that are provided to thevendor. Vibration readings are recorded at one-fourth volt-age with and without the coupling installed. This showshow the rotating assembly is affected by the prebalancedcoupling. The magnitude of the vector change in the 1xvibration on the shaft and bearing housing shall not exceed10% of the vibration limits per API 541 fourth edition.Once the data at one-fourth voltage are recorded and veri-fied, the voltage is increased to the rated voltage of themotor. A full set of vibration readings are recorded to verifythat the data are within the API 541 fourth edition limitslisted in Table 1. If either test exceeds the limits specified byAPI 541 fourth edition, then the vendor and the purchasershall mutually agree on the appropriate corrective actions.

Also, when selected on the data sheets, the couplingmustbe mounted for the unbalance response testing. This itemcan be selected on the data sheets to specify the proper test-ing along with the selection for the unbalance response test.

Slow Roll Runout Measurement(API 541 Fourth Edition Paragraph 4.3.3.1)This test is recommended for motors having sleeve bearingsand proximity probes or provisions for probes. Slow roll run-out measurements are obtained using noncontact proximityprobes that are mounted in each bearing cap. There are twoprobes in each cap mounted 45 from vertical. Proximityprobes operate on the eddy current principle and measureshaft vibration and position relative to the bearing housing.

The slow roll runout measurement includes bothmechanical and electrical sources of vibration. Measure-ments are recorded at a low speed range so that the influ-ence of dynamic vibration is eliminated. The mechanicalrunout may be attributed to nonconcentric surfaces, bows,and surface imperfections, whereas the electrical runoutmay develop as a result of residual magnetism, residualstress concentrations, precipitation hardening, and metal-lurgical segregation.

The value of the slow roll runout data can be seen whencompensating the shaft vibration for runout. Slow roll vec-tors may add or subtract from the overall vibration levels.This is of importance when determining levels for motoralarm and shutdown. If the slow roll vectors subtract fromthe overall vibration, then the motor may be operating athigher vibration levels than desired. If the slow roll vector

adds to the overall vibration, the user may experienceunnecessary vibration trips of the motor.

API 541 fourth edition requires slow roll runout measure-ment documentation at two steps during the manufacturingprocess. The first measurement occurs with the rotor sup-ported in v-blocks at the journal centers prior tomotor assem-bly. The runout data are obtained by using proximity probesand dial indicators to measure at the center line of each probetrack while slowly rotating the rotor assembly a full 360.The total mechanical and electrical runout at this inspectionpoint should not exceed 25% of the allowed peak-to-peakunfiltered vibration amplitude. A motor operating at 3,600rev/min has a peak to peak vibration amplitude limit of 1.50mil; hence, the slow roll limit in v-blocks is 0.375mil.

The slow roll runout levels are also measured in theassembled machine with the rotor operating at speedsbetween 200 and 300 rev/min. The total mechanical andelectrical runout at this inspection point should not exceed30% of the allowed peak-to-peak unfiltered vibrationamplitude. A motor operating at 3,600 rev/min has apeak-to-peak vibration amplitude limit of 1.50 mil, so theslow roll limit at test is 0.45 mil.

AC High-Potential Test(API 541 Fourth Edition Paragraph 4.3.2.1.c)The ac high-potential test consists of the application of avoltage higher than the rated voltage for a specified timeto determine the adequacy against any breakdown of insu-lation materials and spacing under normal conditions(NEMA 3.1.2 [3]). Voltage is applied from all the leads ofthe winding to ground (stator core) or phase to phase. If ashort occurs through the insulation, excessive leakage cur-rent flows and stops the test. If the test is stopped becauseof excessive leakage current, the test fails and the windingis damaged and will need repair. The test is typicallyconducted using 60-Hz ac voltage. The standard high-potential voltage is twice rated plus 1,000 V rms for 1min as a final check per NEMA [3]. This test is used toverify the integrity of the ground wall insulation. This isconsidered a destructive test that stresses the insulationand damages insulation if the test is failed. The conserva-tive approach is to first perform a megger test. If that testis good, then it very gradually increases the value of the achigh potential while watching for a current runaway. Inreality, an ac high-potential test can do less damage to awinding because it places the stress across the capacitiveportion of the winding. The dc test places the stress acrossthe resistive portion of the winding and can cause moredamage very quickly.

Polarization Index Test(API 541 Fourth Edition Paragraph 4.3.2.1.d)The polarization index test is beneficial for determiningthe ground wall insulation condition. It is the ratio of the10-min to 1-min insulation resistance test using a dchigh-potential source

Polarization index Resistance after 10 minResistance after 1 min

:

Trending the polarization index of a motor provides anindication of the condition of the winding. The recommended

TABLE 1. API 541 FOURTH EDITION VIBRATION LIMITS.

Speed Unfiltered 1/2 x 1x 2x

Bracket (ips) 3,600

1,800 0.10 0.10 0.10 0.10

1,200

Shaft (mil) 3,600

1,800 1.50 0.30 1.20 0.50

1,20028

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minimum value of polarization index for motors is 2.0, if thevalue is greater than 4.0, indicating that the insulation is inexcellent condition.

However, todays insulation systems have improved soit is now possible to get a 1-min reading that is above5,000 MX and does not change during the remainder ofthe test time. When this occurs, it is recommended thatthe phases be separated at the winding neutral when acces-sible and retested. During the retest, if each phase archives5,000 MX in 1 min or less and remains unchanged duringthe remaining test time and it has a resistance value that iswithin a few percentage points of each other, then thepolarization index should be disregarded. If there is a largedifference between the polarization index readings of theindividual phases, additional high-potential testingshould be done to find the problem.

API 541 fourth edition requires the stator winding polar-ization index values be measured before and after the high-potential test of the stator winding. These values provide anexcellent baseline for future trending and comparison. SeeIEEE Standard 43 [7] for additional details.

The polarization index is useful in evaluating windingsfor the following:

n buildup of dirt or moisturen gradual deterioration of the insulation (by compar-ing results of tests made earlier on the samemachine)

n fitness for over potential testsn suitability for operation.

Bearing Insulation(API 541 Fourth Edition Paragraph 4.3.2.1.g)Motors specified to API 541 fourth edition have two insu-lated bearings with one removable ground strap, usuallyon the drive end bearing. Insulating the bearings isrequired to prevent small voltages generated in the motorshaft from producing enough current flow in the bearingsresulting in erosion of the bearing metal.

A ground strap supplied with insulated bearings mustbe disengaged prior to testing the integrity of the insula-tion. The bearing insulation test is a routine test and per-formed on all motors.

The sources of shaft voltages are as follows:n magnetism in the rotor, shaft, bearings, endbrackets, etc.

n a closed, low reluctance magnetic circuit thatincludes the bearings

n capacitive coupling of voltages from invertersn stator core dissymmetry.Circulating bearing currents can quickly damage a

bearing by causing the surface of the bearing or shaft thatthe bearing supports to become fluted, as seen in Figure 2.If left unchecked, circulating shaft currents can destroy abearing in less than 24 hours of operation. The most reli-able way to inspect the bearing insulation is to have themotor at rest and uncoupled from the driven equipment.

IEEE Standard 112 [2] provides three methods forchecking bearing insulation resistance. The methods arethe light bulb method, the low voltage ohmmetermethod, and the 500-V megohmmeter methods.

The light bulb method requires an isolation of the bear-ing from parallel current paths by isolating the shaft from

the bearing by placing a sheet of insulation paper betweenthe shaft and the bearing. Use a filament light bulb con-nected in series with a standard 110 V ac power supply,and place one lead to the insulated bearing and the otherlead to the frame or bracket. If the bulb does not light, theinsulation is considered acceptable.

The low-voltage ohmmeter method requires isolatingthe bearing from parallel current paths by insulating theshaft from the bearing and attaching one lead from theohmmeter to the shaft and the other to the frame tomeasurethe bearing insulation resistance. With the meter on maxi-mum ohm scale, the circuit must be shown as open.

The megohmmeter method also requires isolating thebearing from the shaft and then attaching leads of a 500 Vdc megohmmeter between the shaft and the bearingbracket. Energize the megohmmeter to determine thebearing insulation resistance. With the meter on maxi-mum ohm scale, the circuit must be shown as open.

Bearing Temperature Rise(API 541 Fourth Edition Paragraph 4.3.2.1.h)The bearing temperature rise test is a routine test and per-formed on all machines. Depending on the bearing type,its purpose is to determine the following:

n The bearing is correctly seated.n There is appropriate contact surface between theshaft and bearing (Figure 3).

n The bearing is able to produce an appropriatelylarge enough oil wedge to support the shaft on oilat running speed.

The temperature rise test is performed by measuringthe total temperature rise of the bearing metal, at regularintervals, until the temperature stabilizes. On sleeve bear-ings, the temperature measurement is taken in the bottomof the bearing shell at a location that is not more than0.5 in from the minimum oil film. For antifriction bear-ings, the temperature measurement is taken at the outerrace of the bearing. Thermometers, thermocouples, resist-ance temperature devices (RTDs), or other temperaturedetectors may be used. Under no-load conditions, the

2Bearing damage due to shaft currents.

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lower the temperature rise that the bearing stabilizes at,the better. The maximum temperature rise, at full load,that is allowed at rated operating conditions is 93 C perparagraph 2.4.7.1.2 of API 541 fourth edition.

Bearing Dimensional or Alignment Checks(API 541 Fourth Edition Paragraphs 4.3.2.1.i.j. and k)The dimensional and alignment check for sleeve bearingsis recommended for all motors. A bearing inspection isrecommended before and after all running tests. Somechoose not to conduct a final bearing inspection as thereare risks involved in taking apart a motor that is operatingwith stable bearing temperatures. The risk of removingthe bearings and then reinstalling is that without anothervibration test one cannot confirm the motor is still operat-ing the same as it was prior to the bearing removal.

This inspection is invaluable and is recommended espe-cially for two-pole motors operating above their first reso-nant speed (lateral critical speed) to ensure the bearingswere not damaged when the rotor speed passed throughthe resonance.

The inspection first consists of a sketch or photographof the bearing contact pattern (Figure 3). The contactbetween the shaft journal and the bearing bore shouldcover a minimum of 80% of the axial length and besymmetrical with no edge loading.

The second inspection consists of measuring the journalto bearing clearance and the bearing shell to bearing capcrush. Measurements are made by using a special extruded

plastic thread material that has a graduated scale printed onthe envelope. Measuring bearing clearances and crush withthe extruded plastic thread requires the removal of the bear-ing cap and top section of the bearing. For bearing clearan-ces, strips of the extruded plastic thread are laid across thejournal diameter of the shaft. The top bearing section isbolted to the bottom bearing section. When the bearingbolts are tightened, the pressure causes the plastic thread tobe flattened. Then, the bearing cap is installed and tight-ened, which will provide the proper clearance that the bear-ing will see during the operation. The less clearance thereis, the greater the flattening and the wider the plastic threadstrips. The numbers on the graduated scale indicate bearingclearance in thousandths of an inch, as shown in Figure 4.

Measuring the bearing housing to the bearing outer diam-eter (OD) occurs by placing plastic thread strips on the bear-ing OD. Then a shim is placed between the bearing cap andbearing housing to provide a known clearance between thehousing the bearing OD. The bearing cap is tightened downto crush the plastic thread. The cap is then removed to viewthe width of the plastic thread on the OD of the bearing. Theplastic thread is visible on the bearing OD as shown in Fig-ure 5 and can then be measured for a direct comparison withthe graduated scale on the envelope. The value that is mea-sured on the bearing OD is then subtracted by the knownshim thickness to determine the crush on the bearing OD.

At an ambient temperature, the crush fit between the out-side of the bearing shell and the bearing housing should havezero clearance to an interference fit of 0.002 in. The journal-to-bearing clearance will vary based on the speed of themotorand bearing size, but a general rule is 0.0010.0015 in of dia-metric clearance per inch of the journal diameter.

No Load Vibration Test(API 541 Fourth Edition Paragraph 4.3.3)This test is a standard vibration test for all API 541 motors toverify the motor vibration is within the limits of API 541 astested on a seismic mass. A seismic mass is a massive founda-tion with a mounting plate mounted on springs and damper,which allows for the motor to be isolated from the environ-ment and allow for the evaluation of the motor performance.One indication that a foundation is massive is that the vibra-tion amplitudes of the foundation (in any direction) near themachine feet or base frame are less than 30% of the amplitudes

4Clearance measurements on the shaft OD.

5Clearance measurement on the bearing OD.

3Sleeve bearing wear pattern inspection.

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that could be measured at the adjacent bearing housing in anydirection as defined in API 541 fourth edition.

The motor is placed on the seismic mass, and a check forsoft feet is performed with a dial indicator to ensure that eachfoot does not deflect greater than 0.001 in, as it is loosenedand then retightened. If deflection exceeds this value, shim-ming will be required to minimize the foot deflection to theabove level. Once the soft foot is confirmed and acceptable,the motor is tested at full voltage to simulate the motor inservice but at no-load conditions. The stator winding andbearing temperatures are monitored to verify that they do notexceed the design criteria. The vibration is recorded duringthe testing until the bearing temperatures are stable to within1 C per 30-min readings. After the bearing temperaturestabilization point is determined, the motor operates for 1 hwhile recording the vibration and temperatures. This testingalso demonstrates the bearing performance with respect toexcessive noise or lubrication leaks. The vibration criteria aredefined in API 541 fourth edition and are listed in Table 1.

In some cases, only the bracket vibration is recordedbecause noncontacting probes or provisions for noncontact-ing probes were not specified on the motor. The vibrationmust be recorded in the horizontal, vertical, and axial direc-tions on each of the bearing housings. The vibration datashould include unfiltered amplitudes and filtered ampli-tudes at these defined frequencies: one-half running speed,one-time running speed, one-time running speed phaseangle, two-time running speed, one-time line frequency,and two-time line frequency. The temperature and vibra-tion data should be in 30-min intervals during this test toverify the quality of the motor. For two-pole motors, a spe-cial set of data is required to be recorded for 15 min. Thisvibration is recorded continuously for 15 min in 1-minincrements to capture the modulation of the vibration andcannot exceed the limits of API 541 fourth edition. Onrequest, the customer can obtain spectrum plots for eachvibration point that is recorded during the mechanical run.

Hot-to-Cold Vibration Test(API 541 Fourth Edition Paragraph 4.3.3.10)Formotors that are tested on a dynamometer, the vibration thatis recorded may not meet the requirement provided by API541 fourth edition because of the setup and nonisolated equip-ment around the testing area. These points require an alternatemethod that must be approved by the purchaser for evaluatingthe performance and reliability of the motor. The hot-to-coldvibration change provides information about the vibrationcharacteristics of the motor from a hot condition to a cold con-dition. This method takes a hot uncoupled point of vibrationand is divided by a cold uncoupled point of vibration at thesame location to determine the change in vibration due to heat-ing. This ratio is then multiplied by the no-load vibrationrecorded during the no-load testing on the seismic mass. Thisis called the responsive amplification factor and must meet thevibration criteria provided byAPI 541 fourth edition.

Also new to API 541 fourth edition, the vector changebetween the cold uncoupled value and the hot uncoupledvalue must not exceed 0.6 mil on the shaft and 0.05 in persecond (ips) on the bracket. This vector change is only eval-uated on the filtered 1X vibration. Appendix E shows exam-ples of the method to determine the resultant vector changeand acceptability criteria. If the motor does not pass the

resultant vector change, an alternate method shall be to coolthe motor back to ambient temperature, repeat the colduncoupled, repeat the heat run, and repeat the hot uncoupled.The variation between the first and second hot uncoupledvibration readings must be within 10% of the allowable lim-its of the API 541 fourth edition shaft vibration.

The hot-to-cold change is completed on most motorsthat undergo dynamometer testing due to the manufactur-ing setup and testing equipment limitations. This proce-dure is important to verify that the motor vibration is notaffected by the change in temperature.

Stator Core Test(API 541 Fourth Edition Paragraph 4.3.4.1)This test is a quality test for the stator core plate insulationand ensures that the core plate insulation is not damaged. Thetest is conducted on the stator prior to the insertion of thestator coils. Rated flux is maintained on the stator core for aminimum of 30 min while continuously monitoring statortemperatures. Deficiencies are defined as any core location thatis 5 C above the adjacent core temperature. This test shouldbe specified by the user only for unspared applications or wherethemotor will be inaccessible for easy repair or replacement.

Surge Comparison Test(API 541 Fourth Edition Paragraph 4.3.4.2)This recommended test is performed to determine the wind-ing insulation condition. This test detects turn-to-turn, coil-to-coil, and phase-to-phase insulation defects. The risk of notdoing the test is that any marginal turn-to-turn insulation inthe windingmay not fail during the running tests butmay failin operation when the motor is subjected to mild power sys-tem surges. The test voltage should be agreed upon with thevendor to avoid a premature failure. Surge comparison testingis based on the principle that in a stator with no defects allthree-phase windings are identical. Each phase is tested againstthe others and compared to ensure that they are identical.

Special Surge Test of Coils(API 541 Fourth Edition Paragraph 4.3.4.2.1)This test is recommended for windings rated 6,600 V andabove.When specified, two extra coils are processed alongwiththe complete wound stator windings for testing. The test forone coil consists of three successive applications of an impulsevoltage with a 1.2-ls rise time, a 50-ls tail time and a crestvalue of 5 PU. This impulse voltage is applied to both termi-nals of the coil conductor while the conducting surfaces of thesimulated slot portions of the coil are connected to ground.The remaining coil receives a test of the turn insulation withno less than three voltage impulses within 1 min appliedbetween the coil terminations. The impulse voltage will have arise time of 0.10.2 ls with voltage values of 2.0, 3.5, and 5.0PU. The crest value is gradually increased until the point ofinsulation failure is reached. If a failure occurs at less than 5PU, the cause of failure needs to be understood to determinethe impact to the stator. A special warranty agreement betweenthe purchaser and the vendormay need to be reached.

Power Factor Tip-Up Test(API 541 Fourth Edition Paragraph 4.3.4.3)This test is recommended for motors that operate at6,600 V and above, which provides a baseline for later

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maintenance tests to detect corona discharge within theinsulation system. This test may also be performed onspecified sacrificial coils for development testing, but anacceptance criterion needs to be agreed upon between theuser and the supplier if this test is going to be performedon a purchasers motor. The relative difference (tip-up)may be used as an acceptance or baseline parameter.

The power factor tip-up test is conducted across a voltagerange less than to greater than the rated nameplate voltage todetermine the general condition of the winding insulation.The difference in power factors between the measurementsmade at the lowest test voltage and at operating line-toground voltage is referred to as the power factor tip-up. Thetest results are plotted for each motor, and the percent devia-tion is calculated. When multiple motors of the same designare tested and compared, the test values should be similar.

Any motor that deviates significantly from the othermotors should be investigated further for any potential wind-ing or insulation problems. It is recommended that this test isperformed on a routine basis and trend the test results. Bytrending the test results, the users can determine when and ifan outage is required to overhaul the machine. In general, thelower the tip-up value, the better the quality of the insulation.

Sealed Winding Conformance Test(API 541 Fourth Edition Paragraph 4.3.4.4)This test may be applied to critical, special-purposemotors that have windings that will be exposed to weatheror wash-down. This test involves submerging the motorwinding or spraying it with a wetting solution to verifythat the winding is sealed as defined in NEMA Part 20.Results are obtained by measuring and recording the insu-lation resistance of the dry stator using a 500-V dcstandard megger. However, many users want a full-voltage, high-potential test. Next, the stator core is wet-ted either by immersion in a tank of wetting agent or byspraying the core using a wetting agent to thoroughly wetall surfaces. After wetting the stator core, another insula-tion resistance measurement is taken using a 500-V dcmegger. The value of resistance at the initial application ofvoltage at 1 min and at 10 min is recorded.

If the windings fail this test, the stator may be putthrough another vacuum pressure impregnation (VPI)cycle and retested. If the windings fail to pass this testafter a third attempt, a new stator winding may be manu-factured at the purchasers option.

Stator Inspection Prior to VPI(API 541 Fourth Edition Paragraph 4.3.4.5)This is a physical inspection of the wound stator before thewinding is put through the VPI process. Purchasers ofmotors that are critical, unspared machines or have voltageratings 6,600 V and above should select this physicalinspection. The inspection should be witnessed whenspecified on the data sheets.

Inspection points to consider are as follows:n is the entire winding assembly checked?n is it clean and free of contamination?n is it free of any visible damage?n does it look uniform and consistent?

n are the coil heads uniformly taped with adequatetension?

n is the coil bracing uniform? (the bracer rope shouldoverlap to provide the most mechanical integrity)

n are the lead connections secured for some lengthwithin the coil head before exiting?

n are the leads labeled in such a way that themanufacturer can keep track of which lead isthrough the VPI process?

n are RTDs properly located between top and bot-tom coils?

Complete Test(API 541 Fourth Edition Paragraph 4.3.5.1)This test is common for large or critical service machinesand two-pole units. There are several methods for an API541 fourth edition complete test to be specified and com-pleted that will show the motor performance and durabil-ity. In IEEE Standard 112 [2], there are several methods forperforming this test, but the most common are Method B(dynamometer testing) and Method F (equivalent circuitcalculation). Method B testing requires that the motor bemounted on a dynamometer while Method F testing can beperformed without any load applied to the motor. If themotor manufacturers facilities do not allow for Method Btesting, Method F is an alternate method.

Another part of the complete test is to determine thelocked rotor power factor. This value is obtained by block-ing the motor shaft such that it cannot rotate and record-ing motor voltage, current, and kilowatts in the lockedrotor condition. With these data, the locked rotor powerfactor can be calculated and provided in the test report.

The methods that are provided in IEEE Standard 112,Method B [2] allow for the full-load current and slip to bemeasured in a running condition. The other methodsallow for assumptions and calculated values to determinethe current and slip.

Within IEEE Standard 112 [2], there are different meth-ods of measuring locked rotor, accelerating and breakdowntorque, and current. These values are measured using one ofthe various methods supported in the specification. Addi-tional data can then be calculated depending on the IEEE testmethod selected by the motor manufacturer due to the testfacilitys capabilities. The test can be completed at reducedvoltage to minimize the test facilitys requirements, and cal-culations can be used to determine the motors torque andcurrent versus speed characteristics at rated voltage.

Another part of the API 541 fourth edition completemotor test is the heat run testing that is performed at ratedservice factor for a minimum of 4 h or until the bearing andwinding temperatures stabilize per IEEE Standard 112 [2](1 C or less change in temperature over a 30-min period).

The dual-frequency testing as described in IEEE Standard112 [2] details how the motor is heated in the absence of anymechanical loading at the shaft extension. The method used tobring themotor up to operating temperature is generally at thediscretion of the motor manufacturer and/or test facility. Thetest involves one power source that has a low-voltage auxiliarypower source of a different frequency superimposed on it. Theauxiliary power supply is usually 10Hz below the frequency ofthemain power supply and has the same rotation. The auxiliaryvoltage is adjusted so that the motor operates at rated current.The dual frequency heats the rotor very quickly, simulating theaffect of running themotor at full mechanical load.

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Noise Test(API 541 Fourth Edition Paragraph 4.3.5.1.1.g)The noise test is conducted in accordance with NEMAMG-1 Part 9 with the motor operating at no load, full volt-age, rated frequency, and sinusoidal power. NEMA MG-1Part 9 describes measuring noise in terms of sound powerlevels, but many motor manufacturers and specificationsrequire measurements recorded in sound pressure levels.

Sound power is the rate of energy transfer in a mediumand is neither room nor distance dependent and belongsstrictly to the sound source. Sound pressure is the force ofsound on a surface area perpendicular to the direction ofthe sound. Sound pressure is basically the change in thelocal ambient pressure due to the passage of a sound wave.

The sound pressure level can be directly calculatedfrom the sound power level per the following:

Lp LWA 10 log102prd2

So

, (2)

where Lp is the average sound pressure level in a free-fieldover a reflective plane on a hemispherical surface at 1 mdistance from the motor in decibels, Lwa is the sound powerlevel in decibels, rd 1.0 m 0.5 times the maximumlinear dimension of the motor in meters, and So 1.0 m2.

Noise levels are typically specified as 85 dBA at 3 ft(1 m) under no-load conditions however a purchaser canchose lower levels on the data sheet. Noise levels less than85 dBA should be discussed with the motor vendor todetermine impact on the design.

DC High Potential Test(API 541 Fourth Edition Paragraph 4.3.5.1.2)The dc high-potential test (sometimes referred to as the stepvoltage test) measures the dielectric strength of the insulationto some predetermined level. This test may aid in predictingthe condition of the stator windings. It may also allowapproximate prediction of the insulation breakdown voltage.

During the dc high-potential test, the voltage isincreased in a step function to the maximum voltage inTable 2. As the voltage is increased in steps, a microammeteris used to observe the leakage current while recording vol-tages and leakage currents at each step. The dc high-potential test is a when specified test that should only bespecified if the end user is planning to conduct a dc test inthe field for trending purposes. The recommended maxi-mum test dc voltage is listed in the Table 2.

Rated Rotor Temperature Test(API 541 Fourth Edition Paragraph 4.3.5.2)Specify this test if the motor is not going to have a com-plete API test, but it is still important to know that therotor is thermally stable. There are three methods avail-able to conduct this test:

1) loading the machine with a dynamometer or simi-larly calibrated load

2) the dual frequency method3) the forward stall short-circuit method.Loading the test motor with a dynamometer until

thermal stability has been achieved is the recommendedmethod to demonstrate that the rotor has reached its oper-ating temperature. This method is not always practical

because of test stand limitations; hence, the following testsare considered equivalent tests to the actual load test.

The dual-frequency testing was discussed in the Com-plete Test section and describes the method of testing tocreate the heat into the rotating assembly.

In the forward stall short-circuit method, the motor beingtested is driven at rated speed and rotation by an auxiliarydrive motor. The terminals of the motor under test are con-nected to a reduced voltage fixed frequency supply with nor-mal rotation. Generally, the supply frequency is 2025% lessthan the machine rated (nameplate) frequency. The auxiliarydrive motor should have a power rating of at least 10% of themotor under test. With the auxiliary drive motor driving thetest motor at rated speed, the voltage at the motor terminals isadjusted to rated current. Themachine under test is then oper-ating as an induction generator with a slip of approximately25%. Because the reduced voltage on the machine produceslower than normal stator iron losses, the test is repeated withtwo no-load temperature tests at rated frequency. The first no-load test is at the motors of rated voltage. The second test isrun at the voltage used during the forward stall test. The dif-ference between the stator temperature rises in these two testsis added to the temperature rise measured during the forwardstall test. The resultant temperature rise is used as the totaltemperature rise. Several additional tests may be required untilthe calculated total temperature rise is achieved.

Unbalance Response Test(API 541 Fourth Edition Paragraph 4.3.5.3)The unbalanced response test verifies that the motor doesnot generate excessive vibration when it passes through itsfirst resonant speed or critical speed. The test also finds orverifies the location of the resonant speed. This test is rec-ommended for all flexible shaft motors typically two-polemotors of 800 hp and larger and four-pole motors of5,000 hp and larger. For rigid shaft motors, this testing isimportant to locate and verify that the critical speed isabove the 15% separation margin that is required by API541 fourth edition. This test is also important for motorsintended to operate on variable frequency drives or thosewith starting capabilities that exceed API 541 fourthedition paragraph 2.2.4.1.

TABLE 2. DC HIGH-POTENTIAL TEST VOLTAGELEVELS FROM API 541.

Motors RatedVoltage (kV)

DC High-Potential TestVoltage (kV)

E (2E 1)(0.75)(1.75)2.3 7.4

2.4 7.6

4.0 11.8

4.16 12.2

6.6 18.6

6.9 19.4

13.2 36.0

13.8 37.533

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When flexible shaft motors are operated on an adjustable-speed drive below their base speed, care must be taken toensure that the speed range does not include the critical speed.To avoid this, the drive is typically set to block out or skipcontinuous operation at the speed range from 15% below to15% above critical speed. The same concerns exist when usinga variable frequency drive to operate a rigid shaft motor aboveits base speed possibly encroaching on the first lateral critical.

The unbalance response test is conducted by attaching aknown unbalance weight to both ends of the rotating assemblywith the weights in phase to excite the first resonant frequency.Rotating oneweight 180 out of phasewill excite higher-orderresonant frequencies. The minimum unbalance weight isdetermined by taking the allowable residual unbalance weight(Umax) of 4W/N andmultiplying that amount by four. There-fore, the unbalance response weight is equal to 4Umax.

To perform the test, an adjustable-frequency drive is setup to run the motor at 120% of rated speed and then per-mitted to coast to rest. During this test, the shaft vibrationrelative to the bearing housing will be plotted against speed.The shaft displacement value for motors with a defined sepa-ration margin should fulfill (3) or 55% of the minimumdesign shaft-to-bearing and seal diametric running clearan-ces, whichever is smaller.

Ds 1:512,000

N

r, (3)

where Ds is the shaft displacement, in mils peak to peak,and N is the operating speed nearest the resonant speed ofconcern (revolutions per minute).

Consider the following example in which the operatingspeed is 3,600 rev/min and shaft-to-bearing clearance is6.00 mil. For this example, the shaft displacement Ds 2.74mil. From a shaft-to-bearing clearance approach the allow-able shaft displacement would equal 3.33 mil. The smallervalue is then the maximum shaft displacement the motor isallowed to experience outside of the resonant frequency zone.

Additional limits for the shaft displacement at anyspeed outside the operating speed range or separation-margin limits shall not exceed 80% of the minimumdesign shaft-to-bearing diametric running clearance.

Using 6.00 mil as the shaft-to-bearing diametric run-ning clearance provides an allowable shaft displacementlevel of 4.80 mil while going through the resonantfrequency. The unbalance response test should provide aresult showing the first resonant speed is at least 15%above or below the motors operating speed.

When a motor does not comply with the 15% separationmargin, a well-damped resonance (response) must be demon-strated. To do this, run the motor to 120% of its rated speedwith the unbalance weights attached and then allow the motorto coast to rest. The shaft displacement over the entire speedrange, from0% to 120%, shall not exceed the following value:

Ds 1:512,000

Nmax

r, (4)

whereDs is the shaft displacement, in mils peak to peak, andNmax is the maximum rated speed in revolutions per minute.

Also, when selected on the data sheets, the coupling mustbe mounted for the unbalance response testing. This item

can be selected on the data sheets to specify the proper testingalong with the selection for the unbalance response test.

Housing Natural Frequency Test(API 541 Fourth Edition Paragraph 4.3.5.4)This test is typically specified for the first motor manufac-tured of a certain frame size or a uniquely designed motor.The risk of not requiring this test is low for a previouslyestablished design due to the low bearing housing vibra-tion limits required by API 541 fourth edition.

The test results are obtained using an fast Fourier trans-form (FFT) analyzer. Impact testing is performed by usinga calibrated hammer to impact the bearing housing whilean accelerometer measures the response from the impact.The bracket response is acquired in the horizontal, vertical,and axial directions with the impact being made in thesame direction as the response being measured. To elimi-nate the interaction between the bearing housings, therotor shall be turned at a slow roll speed (200300 rev/min). The separation margin between the location of theresonant frequency and the operating speed must be at least20%. A 15% separation margin also applies to two timesoperating speed and also one and two times line frequen-cies. A significant resonance is defined as a peak that lieswithin 6 dB in amplitude of the fundamental bearinghousing resonance.

A bracket response test with a natural frequency loca-tion at 164 Hz is well above the operating speed of 60 Hz.Also, it provides more than a 15% separation margin fromthe 1X and 2X running speeds and line components.

ConclusionsThis article describes the types of testing for specificmotors per API 541 fourth edition. This article will helpspecify the proper motor testing for a API 541 fourthedition motor. Also, it serves as a quick reference guide forAPI 541 fourth edition paragraphs.

References[1] M. J. Costello, Shaft voltages and rotating machinery, IEEE Trans.

Ind. Applicat., vol. 29, no. 2, pp. 419426, Mar./Apr. 1993.[2] Standard Test Procedures for Polyphase Induction Motors and Generators,

IEEE Standard 112.

[3] Motors and Generators, NEMA MG-1.[4] Form Wound Squirrel Cage Induction Motors500 Horsepower and Larger,

API Standard 541, fourth ed.

[5] IEEE Guide for Testing Turn-to-Turn Insulation on Form-Wound StatorCoils for Alternating-Current Rotating Electric Machines, IEEE Standard522.

[6] EASA Electrical Engineering Handbook, Electrical Apparatus ServiceAssociation, Inc., St. Louis, MO, 1997.

[7] IEEE Recommended Practice for Testing Insulation Resistance of RotatingMachinery, IEEE Standard 43.

[8] D. Synder, How to make bearings last in electric motors, Mach.Design, Apr. 2006.

[9] H. Penrose, Keeping up resistance, Uptime Mag., Feb. 2007.

Timothy Wayne Paschall (wpaschall@baldor.com) and BryanK. Oakes are with Baldor Electric Company in Kings Moun-tain, North Carolina. Gary Donner retired from Shell OilProducts. Paschall is a Member of the IEEE. Oakes is aSenior Member of the IEEE. Donner is a Fellow of the IEEE.This article first appeared as Navigating the Test Require-ments of API 541 Fourth Edition at the 2007 Petroleumand Chemical Industry Conference.

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