Induction motor vibrations

From the point of view of API 541 fourth edition BY RAJENDRA MISTRY, WILLIAM R FINLEY, & SCOTT KREITZER PERFORMANCE greater reliability When done properly, a high degree of depends on the electrical and mechanical reliability can be achieved while keeping economics in design, as well as on motor operating condi- mind This article discusses induction motor vibration, tions Sound mechanical design reduces the how the American Petroleum Institute (API) 541 views it, vibration levels and extends the life of the machine Over and what it means to the customer and manufacturer It also the years, the demand continues to grow for motors with discusses the evolution of the standards commonly used G OOD MOTOR IEEE INDUSTRY APPLICATIONS MAGAZINE  NOV j DEC 2010  WWW.IEEE.ORG/IAS © FOTOSEARCH today and how the various requirements attack different Digital Object Identifier 10.1109/MIAS.2010.938396 vibration concerns Any reference to API vibration in this 1077-2618/10/$26.00©2010 IEEE 37 comprised of a frame, stator, rotor, bearing housings, and main terminal box THE MOTOR IS Typically, the frame material is cast iron or fabricated steel The stator is COMPRISED OF A constructed from steel laminations with electrical windings inserted into FRAME, STATOR, axial slots Four types of rotor construction ROTOR, BEARING exist today: the aluminum die cast HOUSINGS, AND (ADC), copper die cast, fabricated aluminum bars (AlBar), and fabricated MAIN TERMINAL copper or copper alloy bars (CuBar) Although each type of rotor conBOX struction has advantages and disadvantages, this article will discuss the most common: ADC, fabricated copper bars (CuBar), and fabricated AlBar, with respect to vibration Vibration, Frequency, and Phase Typically, the ADC rotors are easier to manufacture and Vibration is the periodic back-and-forth motion of the object Because of the internal and external forces, machines such as more economical than the CuBar rotors The aluminum motor also vibrate These vibrations are so small that sensitive rotor bars have approximately one-third the density of steel and 2.3 times the specific heat of copper Additionally, the measuring equipment is needed to detect it Frequency is the repetition rate of vibration per unit of coefficient of thermal expansion for a given temperature time It can be determined by measuring the amount of time change is 31% greater for aluminum over copper Moreit takes to complete one cycle of vibration Several terms are over, aluminum has a lower yield strength than copper used in the industries to describe the frequency: synchronous As a result of these material density and specific heat differor 13; nonsynchronous, subsynchronous, or less than 13; ences, the AlBar will become much hotter, expand further, and generate much higher stresses while accelerating the and super synchronous or greater than 13 The phase is the timing difference between vibration events same load inertia (WK2) Porosity may also be present The timing difference between the root cause and its effect of in die cast rotors because of trapped gases during the castrotor behavior to find the possible root causes gives us a tool for ing process or uneven shrinkage during cooling All of the diagnosis of rotating machinery [17] The quality or level of these factors can contribute to a higher vibration over a motor vibration is an indicator of how well the motor is CuBar construction At present, most manufacturers maindesigned, manufactured, installed, maintained, and operated tain good control over these processes, eliminating most of The vibration magnitudes, frequencies, and phase angles indi- the concern Despite this benefit, copper bar rotors are gencate what possible sources of vibrations are being seen When erally preferred for API motors because of their ease of repconsidering induction motor vibration, one is referring to vibra- arability As a result, a damaged copper bar motor can be tion levels measured on the bearing housing and shaft The repaired and placed back into service much faster housing readings are taken in the horizontal, vertical, and axial A fabricated aluminum rotor bar has a cost advantage direction or as close as possible to these locations The shaft over a fabricated copper bar and a manufacturing advantreadings are taken with noncontacting eddy-current probes age over ADC, which has various limiting factors, such as mounted on the bearing housing and measure the relative tooling and size movement between the housing and shaft In North America, Another key difference is that the end connector of an housing readings are normally taken as velocity in inches per sec- AlBar rotor is welded to the rotor bars as opposed to ond, zero to peak The shaft readings are taken as peak-to-peak brazed Additionally, the end connectors of the AlBar rotor displacement in mil, as defined by API and National Electrical clamp the rotor punchings, as opposed to the use of sepaManufacturers Association (NEMA) MG1 [1], [7], [8] rate end heads in the CuBar construction [11] Per the International Electrotechnical Commission (IEC) In conclusion, all types of rotor constructions can be 60034-14 [10], the criterion for bearing housing vibration designed and manufactured to ensure low vibration In magnitude at the machine bearings is the broadband root general, a copper-fabricated rotor should be more robust mean square (rms) The standard measurement units are and can be visually inspected for flaws during manufacturdefined as follows: displacement in micrometers, velocity in ing to ensure a high-quality product Although this type millimeters per second, and acceleration in meter per second of construction has the ability of being more easily repaired squared [9] The criterion for the relative shaft vibration in the field, if impractically designed and manufactured, magnitude is the peak-to-peak displacement in the direction these advantages would not be guaranteed Finally, the of the measurement per the International Standard Organiza- design must take into account the relative movement durtion (ISO) 7919-1 [14] ing motor starting so that the motor still continues to perform after multiple starts Motor Construction IEEE INDUSTRY APPLICATIONS MAGAZINE  NOV j DEC 2010  WWW.IEEE.ORG/IAS article refers to API 541 fourth edition, unless otherwise stated [8] To follow and understand API 541 specification, this article will discuss the following topics in detail: n overall motor construction as it relates to vibration n rotor construction: benefits and drawbacks n bearing types: benefits, drawbacks and performance n motor vibration: what it means, magnitude, phase angle, and frequencies n factors affecting motor vibration Bearing Types Rotor Construction 38 To understand induction motor vibration and its effects, it is first necessary to know the motor construction The motor is The most common type of bearing used today is the antifriction bearing (AFB) In comparison to a sleeve bearing, an AFB can be less reliable, have a limited life, and will not provide a prior indication of immanent failure However, the AFBs are less expensive and can handle axial thrust if the application so requires Additionally, the AFBs may be preferred on smaller, slower speed machines where they are more reliable Although the selection of bearing type for a particular machine can be somewhat subjective, Table lists the general selection criteria [11] Unfortunately, the proper selection of bearing type can be much more complicated than the simple guideline mentioned earlier Once the type of bearing is chosen, the method of lubrication must be established Within the same application and comparing the same feature or characteristic, arguments can be made for either bearing design The vibration levels depend on the quality of rotor manufactured and the motor installation A sleeve bearing will have good damping, while an AFB will provide very little damping This increased damping in sleeve bearings reduces the amplification factor but slightly alters the actual critical speed For this reason, the motors with AFBs can never run near a rotor resonance, while those with sleeve bearings can run on a critical speed as long as it is highly damped However, when properly designed, both types of bearings will allow low vibration Vibration Sources There are many electrical and mechanical forces present in the induction motors that can cause excessive vibration These forces can n result from different sources Characteristics Bearing Type Long life Sleeve Availability AFB Maintenance Sleeve Repair Sleeve Quietness Sleeve Application flexibility AFB Thrust load AFB Belt drive AFB Compactness AFB Indication of failure Sleeve Cost AFB produce different movements on different components be applied in different directions produce movements that are not the same for all components or seen in all directions As a result, it is possible to tie certain vibration measurements to different causes and thereby establish performance and design requirements intended to minimize these vibrations This section will explain how different vibration limits or frequencies of vibration can affect the design and how a motor could be designed to minimize this specific vibration Several definitions as defined by API 541 include: n Lateral critical speed: a shaft rotational speed at which the rotor-bearing support system is in a state of resonance n Forcing phenomena: a vibration with an exciting frequency that may be less than, equal to, or greater than the synchronous frequency of the rotor n n n Vibration Due to Unbalance at 13 Rotational Speed The most commonly considered and most easily understood source of vibration is the vibration due to unbalance Some standards define a maximum residual unbalance (e.g., API at 4W/N oz-in) to address this problem Although this is an important consideration, the total unbalance at operating speed is also critical The change from ambient temperature to the temperature at operating conditions may cause significant changes to balance readings Additionally, not performing the balance in a sleeve bearing similar to the production motor or with a bearing support system with stiffness different than the actual production machine may cause problems in the assembled motor It should be noted that NEMA and IEC in most cases not define how to manufacture the motor Instead, these specifications establish limits and allow the motor manufacturers to determine how to meet them API defines many more design and manufacturing requirements that may in some cases increase reliability but not in all cases Regardless, many of these requirements are easily achieved and therefore good reliability additions It is the requirements IEEE INDUSTRY APPLICATIONS MAGAZINE  NOV j DEC 2010  WWW.IEEE.ORG/IAS History of Vibration Requirements Before 1993, vibration levels were primarily defined by NEMA and were established at 1.0 mil on the housing for two-pole machines and 2.0 mil on the housing for fourpole and slower machines Eventually, it was determined that these levels were too loose and did not provide the necessary reliability that was required or could easily be achieved In 1993, NEMA changed the method of measurement to inches per second and lowered the level to 0.12 in/s on a massive base for most ratings (0.15 in/s on a resilient base) In 1972, API RP 541 was developed and defined vibration on an elastic and rigid mount Later in 1987, API 541 second edition introduced vibration levels in a graphical form API 541 third edition was introduced in 1995 and fourth edition in 2003 This version changed the requirements for many of the construction features but did not modify or lower the vibrations levels The vibration levels are shown in Table for housing vibration and Table for shaft vibration At the same time, IEC standard 60034-14 is establishing newer and lower levels than what was published previously; however, these new values are still higher than API limits In addition, NEMA is presently working on establishing various levels of vibration based on the criticality of the application Ideally, all standards should agree on similar values that demand cost-effective designs while ensuring good reliability However, there is a point of diminishing returns where lower vibration levels become extremely difficult and costly but will not return substantial benefits in reliability TABLE GENERAL CRITERIA FOR BEARING SELECTION 39 to check the unbalance response at operthat add little value and have higher ating speed Additionally, balancing at a costs that need to be reviewed in future VIBRATION IS THE speed lower than operating speed could editions of API 541 create an unbalance value too small for API balance requirements are more PERIODIC the sensitivity of the balancing machine important with respect to the vibration The assembled motors are then tested to limits at operating speed API requires BACK-AND-FORTH confirm that vibration requirements are residual unbalance not exceeding 4W/N met in operation in the actual machine oz-in at each journal, where W is one half MOTION OF THE API does not allow trim balancing to the weight of the rotor and N is the maxicompensate for the thermal bow of the mum operating speed of the machine In OBJECT assembled motor This compensation SI units, this permitted unbalance level is may be performed by many motor man6,350W/N g-mm, where W is the ufacturers today, but this exception to weight per journal in kilograms and N is the maximum operating speed [7], [8] This permitted unbal- the specification should be done in the cold condition and ance level corresponds to about G 0.70 in the ISO 1940-1 should be approved by the customer For the adjustable speed drive (ASD) applications, the [15] system Balance is more critical and also more difficult to perform on two-pole motors API 541 does provide the option vibration limits are the same as for fixed speed units The TABLE COMPARISON OF HOUSING VIBRATION LIMITS Assumptions: n rigid mounting base n zero-to-peak velocity n peak-to-peak displacement n vibration values listed are for two-, four-, and six-pole motors Standard Unfiltered NEMA MG1 1987 1.0 mil 2p Filtered 13 r/min Filtered 23 r/min Filtered LF Modulation N/A N/A N/A N/A 0.12 in/s 0.12 in/s 0.12 in/s 0.12 in/s N/A 2,4,6p 2,4,6p 2,4,6p 2,4,6p 1.0 mil 2p N/A N/A N/A 0.092 in/s 2,4p 0.074 in/s 2p 0.1 in/s 2p 0.1 in/s 0.07 in/s 6p 0.06 in/s 4p 0.05 in/s 6p 0.074 in/s 4p 0.065 in/s 6p 2,4,6þp API 541, third edition 1995 0.1 in/s 2,4,6p 0.1 in/s 2,4,6p 0.1 in/s 2,4,6p 0.1 in/s 2,4,6p Continuous recording of data for 15 for two-pole motors API 541, fourth edition 2006 0.1 in/s 2,4,1 mil 6þp 0.1 in/s 2,4, 1.6 mil 6þp 0.1 in/s 2,4, 1.6 mil 6þp 0.1 in/s 2, Continuous record4,1.6 mil 6þp ing of data for 15 for two-pole motors 0.128 in/s 2,4,6þp N/A N/A N/A N/A 0.08 in/s 2,4,6p N/A 0.05 in/s 0.05 in/s N/A 2.0 mil 4p IEEE INDUSTRY APPLICATIONS MAGAZINE  NOV j DEC 2010  WWW.IEEE.ORG/IAS 2.5 mil 6p NEMA MG1: From 1993 Rev to MG 1-2006 API RP541 1972 2.0 mil 4p 2.5 mil 6þp API 541, second edition 1987 IEC 60034 14 Ed 3.1 2007-03 IEEE 841, 2001 [16] 60 of tape recording after heat run Special purpose motor: Driving unspared equipment in critical service, motor rated over 1,000 hp, motors driving high inertia loads, vertical motors, motors requiring vibration sensitivity criteria 40 N/A Vibration for standard grade A and shaft height greater than 280 mm (11 in) concentricity limits of the rotor core or limits need to be met at all supply freend connector However, it does require quencies in the operating range Most API 541 DOES that actions be taken to assure concenmedium-to-large motors are used for tricity and rotor component security constant speed applications, but the PROVIDE THE Good mechanical slow roll indicates number of ASD motors is increasing OPTION TO good concentricity with the bearing considerably for many reasons, espejournal diameter and minimizes oil film cially to increase efficiency Constant CHECK THE instability in the bearing speed motors only need to be precision API makes the following statebalanced at operating speed, while UNBALANCE ments regarding good manufacturing adjustable speed applications require practices that acceptable rotor balance be mainRESPONSE AT tained throughout the operating speed n The slow-roll acceptance criteria range It is also critical that all the for an assembled motor rotating OPERATING components remain tight and not between 200 and 300 r/min SPEED change unbalance throughout the should not exceed 30% of the entire temperature and speed range for allowed peak-to-peak unfiltered the ASD motors vibration amplitude or 0.25 mil Rotor balance involves the entire rotor structure that is (6 lm), whichever is greater made up of a multitude of parts, including the shaft, rotor n Looseness of parts, which can result in shifting laminations, end heads, rotor bars, end connectors, retainduring operation, causing a change in balance, ing rings (where required), and fans All of these items must be avoided or minimized must be addressed in the design and manufacture to n Balance correction weights should be added at or achieve a stable precision balance near the points of unbalance API does not define how to ensure the rotor bars are to be API 541 fourth edition defines vibration acceptance valmaintained tight in the slot nor does it describe the ues at operating temperature, requiring the product be TABLE COMPARISON OF SHAFT VIBRATION LIMITS Unfiltered Filtered 13 r/min Filtered 23 r/min First Lateral critical speed Required for speed > 1,500 r/min 25% of unfiltered p–p or 0.25 mil total Yes, but not for thermal compensation No change of 13 >0.6 mil on shaft and 0.05 in/s on housing from hot to cold 15 for two pole Æ20% of and 23 and 20% of and LF N ¼ nNop Æ 0.15Nop1 Yes Yes, but not for thermal compensation No change >50% of the allowable limit from hot to cold 15 for two pole Yes, at machine feet, vibration ... [3] Motors and Generators, NEMA MG 1-1993 [4] Motors and Generators, NEMA MG 1-1987 [5] Form-Wound Squirrel Cage Induction Motors, API RP 541, 1972 [6] Form-Wound Squirrel-Cage Induction Motors—250... after heat run Special purpose motor: Driving unspared equipment in critical service, motor rated over 1,000 hp, motors driving high inertia loads, vertical motors, motors requiring vibration sensitivity... greater Special purpose motor: driving unspared equipment in critical service, motor rated more than 1,000 hp, motors driving high inertia loads, vertical motors, and motors requiring vibration