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Squirrel Cage Rotor Testing - Pump Magazine

San CageRotor TestingEASA Convention 2003 Moscone Convention CenterSan Francisco, CAJune 30, 2003 Presented byTom BishopTechnical Support SpecialistElectrical Apparatus Service Association, Louis, MO1 Squirrel cage Rotor TESTINGByTom BishopTechnical Support SpecialistElectrical Apparatus Service Association, Louis, MOINTRODUCTIOND etermining whether or not a Squirrel cage Rotor is de-fective is an issue that is a challenge to every servicecenter as there is often no simple way to determine theintegrity of a Rotor . There are a wide variety of Rotor teststhat can be applied both in the service center and at theend user site that can aid in assessing Rotor , there are tests that can be performed with themotor assembled, and others that require main purpose of the information that will be pre-sented here is to describe many of the available teststhat can be utilized under these different addition to conventional Squirrel cage Rotor testingmethods such as the growler test, also covered will betechniques such as the use of a core loss tester, highcurrent excitation, and spectrum analysis of (Figure 1), with the bars and end rings being cast inone machine operation.

1 SQUIRREL CAGE ROTOR TESTING By Tom Bishop Technical Support Specialist Electrical Apparatus Service Association, Inc. St. Louis, MO INTRODUCTION Determining whether or not a squirrel cage rotor is de-

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Transcription of Squirrel Cage Rotor Testing - Pump Magazine

1 San CageRotor TestingEASA Convention 2003 Moscone Convention CenterSan Francisco, CAJune 30, 2003 Presented byTom BishopTechnical Support SpecialistElectrical Apparatus Service Association, Louis, MO1 Squirrel cage Rotor TESTINGByTom BishopTechnical Support SpecialistElectrical Apparatus Service Association, Louis, MOINTRODUCTIOND etermining whether or not a Squirrel cage Rotor is de-fective is an issue that is a challenge to every servicecenter as there is often no simple way to determine theintegrity of a Rotor . There are a wide variety of Rotor teststhat can be applied both in the service center and at theend user site that can aid in assessing Rotor , there are tests that can be performed with themotor assembled, and others that require main purpose of the information that will be pre-sented here is to describe many of the available teststhat can be utilized under these different addition to conventional Squirrel cage Rotor testingmethods such as the growler test, also covered will betechniques such as the use of a core loss tester, highcurrent excitation, and spectrum analysis of (Figure 1), with the bars and end rings being cast inone machine operation.

2 Larger motors, typically aboveNEMA frame size, may use fabricated aluminum rotorsthat have the bars (usually made by extruding) weldedto the end rings. In general, the following discussionapplies to both fabricated and die cast Rotor construc-tion, unless indicated PRINCIPLEST esting of a Squirrel cage Rotor requires some under-standing of how the Rotor functions. The Rotor of aninduction motor is like the secondary winding of a trans-former, with the motor stator being the primary. This iseasiest to visualize at motor startup, when the Rotor isnot yet turning. Currents and voltages are induced in thebars and end rings, which make up the cage , of the Rotor (Figure 2). The Rotor cage is similar in appearance to petrodent exercise wheels from over a century ago, thusthe name Squirrel cage . There are other types of rotorsused in AC motors such as synchronous and wound ro-tor, however, the focus here will be on the Squirrel cageinduction bars in a Squirrel cage Rotor form parallel paths,joined at their ends by end rings.

3 The stator winding polesRotor laminationRotor barEnd ringFanFanShaftFIGURE 1: TYPICAL DIE CAST Squirrel CAGEINDUCTION ROTORA lmost all Squirrel cage rotors have bars and end ringsmade of alloys of either aluminum or copper, or purecopper. The Rotor cage consists of the bars and the endrings. Copper or copper alloy rotors are usually of fabri-cated design. That is, the bars and end rings arefabricated prior to assembly into the Rotor , and thenbrazed or welded together. Far less common are copperrotors with cast bars that were manufactured over 50years ago, although there is new technology that maymake these commercially available in the near rotors are predominantly of die-cast construc-FIGURE 2: Squirrel cage ROTORThe Squirrel cage Rotor consists of the bars and the Rotor bars into parallel circuits equal to thenumber of stator poles.

4 The number of Rotor poles isalways equal to the number of stator poles. A 2-polewinding divides the Rotor into 2 parallel circuits that con-tinuously move around the Rotor cage as the rotorrevolves. The greater the number of poles, the greaterthe number of Rotor circuits. The end rings completethese circuits, thus a 2-pole winding end ring will besubject to more current than with a higher number ofpoles in the winding. This factor makes end ring integ-rity more critical as the number of poles decrease (andspeed increases).The current conducted through the Rotor bars is essen-tially proportional to the number of poles in a windingfor a given motor. For example, a 2-pole windingspreads the poles across about half the bars, while a4-pole winding divides the bars into quarters (quad-rants). This makes it possible to use the same Rotor barshape and size for a number of winding designs withdifferent numbers of poles.

5 Regardless of the numberof poles, a single open Rotor bar can reduce motor torqueand cause other problems such as vibration. The causeof the torque disturbance and vibration is that currentin the affected bar will be less than in adjacent affected bar therefore will contribute less torquewhen it passes the stator winding poles, with the torquedisturbance creating is a great deal of misunderstanding as to howmany broken bars a motor can operate with . As Table 1illustrates, the answer varies. For example, a 4-polemotor with 48 stator slots and 57 Rotor bars could de-velop a cusp with only one open bar, whereas the samemotor with 59 bars might not develop a cusp until 3bars have failed. This explains how a motor could runfor years with broken Rotor bars, possibly performingworse or better as more bars Rotor faults are primarily caused by fracturesin the joints between bars and end rings (Figure 3).

6 Thefaults in die cast rotors most often relate to porosity ineither the bars or end rings, or both. The faults oftendevelop, or become worse, as a result of a pulsatingload, too many starts or too frequent starting, or simplyfatigue due to reaching the end of normal life. The barsthat remain intact are then subjected to higher thannormal currents, leading to increased risk of faults commonly cause torque pulsations, speedfluctuations, vibration, and changes of the frequencycomponents in the supply current and magnetic phenomena that may occur include increasednoise, overheating, and arcing in the Rotor along withdamaged Rotor laminations. The faults that occur alsoserve to provide information that can be analyzed byperforming Rotor MOTOR Rotor TESTINGFor the purposes of this discussion, there are two basicconsiderations for Rotor Testing . The motor can eitherbe tested while disassembled or assembled.

7 The dis-assembled Testing techniques are most applicable tothe service center environment and will be inspectionInspection of a Rotor after it has been removed from thestator may reveal obvious faults, such as a failed bar toend ring joint. A high concentration of balance weightsFIGURE 3: FRACTURESF ractures at the bar-to-end ring connection are com-mon in fabricated ,3 ,2 ,1 42 ,81 ,21 ,6 01-,4-,2 46 ,5 ,4 ,3 ,2 ,1 06 ,84 ,42 ,21 02-,8-,4 68 ,7 ,5 ,4 ,2 ,1 27 ,45 ,63 ,81 03-,21-,6 801 ,9 ,7 ,6 ,2 ,1 27 ,84 ,42 04-,61-,8 TABLE 1: STATOR/ Rotor SLOT COMBINATIONS3in one area may be an indicator of a void. If the initialinspection does not detect any flaws, thoroughly cleanthe Rotor , but do not grit blast it. Repeat the particular, look for cracked end rings, a die-cast endring that has separated from the laminations (indicat-ing broken bars), and signs that the Rotor has heated tothe point that the alloy forming the Squirrel cage hasliquefied and been thrown out of the Rotor slots.

8 Inspectthe end rings and fins of die cast rotors for evidence ofporosity or casting flaws. Cooling fins with splits on cast-ing parting lines can indicate flaws due to the inner diameter of the end ring at the laminationsOpen bars in a fabricated aluminum 4: OPEN Rotor BARS should be inspected for evidence of porosity. Skewedlaminations sometimes shift in the manufacturing pro-cess, partially or completely closing a Rotor inspect for signs of localized heating along therotor bars. If the Rotor has previously been painted, over-heated areas will often appear as blackened arc spotsthat have broken through the painted finish. Theseburn marks indicate that there is either a high resis-tance joint, or a break, in the Rotor bars. A completelybroken bar will sometimes cause burning of a sectionof laminations as current passes from the broken bar toadjacent bars through the laminations.

9 In severe cases,with fabricated rotors , the arcing from this current maycause the broken bar to burn through the top of its slot,resulting in a Rotor bar to stator core rub. Figures 4 Fabricated copper alloy Rotor with open bars that haveworn through the tops of the 5: OPEN Rotor BARS4through 6 illustrate open bar faults in fabricated motor supply voltages can result in heat-ing of a Rotor core. Open Rotor bars can also cause rotorheating. If unbalanced voltages are the cause, the en-tire surface of the Rotor laminations will have evidenceof overheating, often displaying a blue-color motor manufacturers heat treat rotors after diecasting, to remove excess aluminum and to break thepotential bond between bars and laminations. The heattreating process results in a blue colored finish on therotor laminations. Inspect the Rotor closely for otherevidence of a fault before concluding that a blue colorfinish indicates a Rotor fault.

10 Localized heated surfaceareas likewise should be closely inspected, to deter-mine if a Rotor fault or something external to the rotorcaused them (Figure 7). A stator winding fault may re-sult in consequential damage to the Rotor core. Inspectthe stator as well as the Rotor whenever evidence ofrotor surface heating is testBroken fabricated Rotor bars may be detected by tap-ping on the bars from one end ring to the other with ahammer and screwdriver. Loose or broken bars will re-spond much differently from tight sound bars. Thismethod works best with two people performing it. Oneperson taps the bars and the other monitors bar move-ment. The bar movement can be sensed by holding asecond screwdriver on the bar about 3 to 4 (75 to100 mm) from the location being tapped. Figure 8 illus-trates a Rotor bar crack that could have been detectedby tap penetrant testIf the visual inspection does not reveal any defects inthe Rotor , and an open Rotor is suspected, an option isto perform dye penetrant inspection.


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