Squirrel Cage Rotor Testing

San Francisco . 2003 Squirrel Cage Rotor Testing EASA Convention 2003 Moscone Convention Center San Francisco, CA June 30, 2003 Presented by Tom Bishop Technical Support Specialist Electrical Apparatus Service Association, Inc. St. Louis, MO SQUIRREL CAGE ROTOR TESTING By Tom Bishop Technical Support Specialist Electrical Apparatus Service Association, Inc. St. Louis, MO INTRODUCTION tion (Figure 1), with the bars and end rings being cast in one machine operation. Larger motors, typically above Determining whether or not a squirrel cage rotor is de- NEMA frame size, may use fabricated aluminum rotors fective is an issue that is a challenge to every service that have the bars (usually made by extruding) welded center as there is often no simple way to determine the to the end rings. In general, the following discussion integrity of a rotor. There are a wide variety of rotor tests applies to both fabricated and die cast rotor construc- that can be applied both in the service center and at the tion, unless indicated otherwise. end user site that can aid in assessing rotor condition. Further, there are tests that can be performed with the motor assembled, and others that require disassembly. ROTOR PRINCIPLES Testing of a squirrel cage rotor requires some under- The main purpose of the information that will be pre- standing of how the rotor functions. The rotor of an sented here is to describe many of the available tests induction motor is like the secondary winding of a trans- that can be utilized under these different circumstances. former, with the motor stator being the primary. This is In addition to conventional squirrel cage rotor testing easiest to visualize at motor startup, when the rotor is methods such as the growler test, also covered will be not yet turning. Currents and voltages are induced in the techniques such as the use of a core loss tester, high bars and end rings, which make up the cage, of the rotor current excitation, and spectrum analysis of vibration. (Figure 2). The rotor cage is similar in appearance to pet rodent exercise wheels from over a century ago, thus FIGURE 1: TYPICAL DIE CAST SQUIRREL CAGE the name “squirrel cage”. There are other types of rotors INDUCTION ROTOR used in AC motors such as synchronous and wound rotor, however, the focus here will be on the squirrel cage Rotor lamination induction rotor. Fan Rotor bar End ring The bars in a squirrel cage rotor form parallel paths, Fan joined at their ends by end rings. The stator winding poles Shaft FIGURE 2: SQUIRREL CAGE ROTOR Almost all squirrel cage rotors have bars and end rings made of alloys of either aluminum or copper, or pure copper. The rotor cage consists of the bars and the end rings. Copper or copper alloy rotors are usually of fabricated design. That is, the bars and end rings are fabricated prior to assembly into the rotor, and then brazed or welded together. Far less common are copper rotors with cast bars that were manufactured over 50 years ago, although there is new technology that may make these commercially available in the near future. The squirrel cage rotor consists of the bars and end Aluminum rotors are predominantly of die-cast construc- rings. 1 divide the rotor bars into parallel circuits equal to the FIGURE 3: FRACTURES number of stator poles. The number of rotor poles is always equal to the number of stator poles. A 2-pole winding divides the rotor into 2 parallel circuits that con- tinuously move around the rotor cage as the rotor revolves. The greater the number of poles, the greater the number of rotor circuits. The end rings complete these circuits, thus a 2-pole winding end ring will be subject to more current than with a higher number of poles in the winding. This factor makes end ring integrity more critical as the number of poles decrease (and speed increases). The current conducted through the rotor bars is essen- tially proportional to the number of poles in a winding for a given motor. For example, a 2-pole winding spreads the poles across about half the bars, while a Fractures at the bar-to-end ring connection are com- 4-pole winding divides the bars into quarters (quad- mon in fabricated rotors. rants). This makes it possible to use the same rotor bar shape and size for a number of winding designs with different numbers of poles. Regardless of the number fatigue due to reaching the end of normal life. The bars of poles, a single open rotor bar can reduce motor torque that remain intact are then subjected to higher than and cause other problems such as vibration. The cause normal currents, leading to increased risk of fracture. of the torque disturbance and vibration is that current Rotor faults commonly cause torque pulsations, speed in the affected bar will be less than in adjacent bars. fluctuations, vibration, and changes of the frequency The affected bar therefore will contribute less torque components in the supply current and magnetic fields. when it passes the stator winding poles, with the torque Other phenomena that may occur include increased disturbance creating vibration. noise, overheating, and arcing in the rotor along with damaged rotor laminations. The faults that occur also There is a great deal of misunderstanding as to “how serve to provide information that can be analyzed by many broken bars a motor can operate with”. As Table 1 performing rotor testing. illustrates, the answer varies. For example, a 4-pole motor with 48 stator slots and 57 rotor bars could de- DISASSEMBLED MOTOR ROTOR TESTING velop a cusp with only one open bar, whereas the same motor with 59 bars might not develop a cusp until 3 For the purposes of this discussion, there are two basic bars have failed. This explains how a motor could “run considerations for rotor testing. The motor can either for years with broken rotor bars,” possibly performing be tested while disassembled or assembled. The dis- worse or better as more bars fail. assembled testing techniques are most applicable to the service center environment and will be considered Fabricated rotor faults are primarily caused by fractures first. in the joints between bars and end rings (Figure 3). The faults in die cast rotors most often relate to porosity in Visual inspection either the bars or end rings, or both. The faults often Inspection of a rotor after it has been removed from the develop, or become worse, as a result of a pulsating stator may reveal obvious faults, such as a failed bar to load, too many starts or too frequent starting, or simply end ring joint. A high concentration of balance weights TABLE 1: STATOR/ROTOR SLOT COMBINATIONS Peoles Ngois Cpoggin Cus 24±41, ±2, ±3, ± ±06, ±12, ±18, ±2 ±2, -4, -1 46±01, ±2, ±3, ±4, ±5, ± ±012, ±24, ±48, ±6 ±4, -8, -2 68±21, ±2, ±4, ±5, ±7, ± ±018, ±36, ±54, ±7 ±6, -12, -3 80±21, ±2, ±6, ±7, ±9, ±1 ±024, ±48, ±7 ±8, -16, -4 2 in one area may be an indicator of a void. If the initial should be inspected for evidence of porosity. Skewed inspection does not detect any flaws, thoroughly clean laminations sometimes shift in the manufacturing pro- the rotor, but do not grit blast it. Repeat the inspection cess, partially or completely closing a rotor slot. process. Closely inspect for signs of localized heating along the In particular, look for cracked end rings, a die-cast end rotor bars. If the rotor has previously been painted, over- ring that has separated from the laminations (indicat- heated areas will often appear as blackened arc spots ing broken bars), and signs that the rotor has heated to that have “broken through” the painted finish. These the point that the alloy forming the squirrel cage has burn marks indicate that there is either a high resis- liquefied and been thrown out of the rotor slots. Inspect tance joint, or a break, in the rotor bars. A completely the end rings and fins of die cast rotors for evidence of broken bar will sometimes cause burning of a section porosity or casting flaws. Cooling fins with splits on cast- of laminations as current passes from the broken bar to ing parting lines can indicate flaws due to the casting adjacent bars through the laminations. In severe cases, process. with fabricated rotors, the arcing from this current may cause the broken bar to burn through the top of its slot, The inner diameter of the end ring at the laminations resulting in a rotor bar to stator core rub. Figures 4 FIGURE 4: OPEN ROTOR BARS Open bars in a fabricated aluminum rotor. FIGURE 5: OPEN ROTOR BARS Fabricated copper alloy rotor with open bars that have worn through the tops of the slots. 3 the stator as well as the rotor whenever evidence of FIGURE 6: OPEN ROTOR BARS rotor surface heating is detected. Tap test Broken fabricated rotor bars may be detected by tap- ping on the bars from one end ring to the other with a hammer and screwdriver. Loose or broken bars will re- spond much differently from tight sound bars. This method works best with two people performing it. One person taps the bars and the other monitors bar movement. The bar movement can be sensed by holding a second screwdriver on the bar about 3 to 4” (75 to 100 mm) from the location being tapped. Figure 8 illustrates a rotor bar crack that could have been detected These rotor bars have come out of the slots and by tap testing.

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