Induction motor vibrations

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: be

From the point of view of API 541 fourth edition

B Y R A J E N D R A M I S T R Y ,

W I L L I A M R F I N L E Y , & S C O T T K R E I T Z E R

G OODdepends on the electrical and mechanicalMOTOR PERFORMANCE

design, as well as on motor operating

condi-tions Sound mechanical design reduces the

vibration levels and extends the life of the machine Over

the years, the demand continues to grow for motors with

greater reliability When done properly, a high degree of reliability can be achieved while keeping economics in mind This article discusses induction motor vibration, how the American Petroleum Institute (API) 541 views it, and what it means to the customer and manufacturer It also discusses the evolution of the standards commonly used today and how the various requirements attack different vibration concerns Any reference to API vibration in this Digital Object Identifier 10.1109/MIAS.2010.938396

© FOTOSEARCH

37

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,

draw-backs and performance

n motor vibration: what it means,

magnitude, phase angle, and

frequencies

n factors affecting motor vibration

Vibration, Frequency, and Phase

Vibration is the periodic back-and-forth motion of the object

Because of the internal and external forces, machines such as

motor also vibrate These vibrations are so small that sensitive

measuring equipment is needed to detect it

Frequency is the repetition rate of vibration per unit of

time It can be determined by measuring the amount of time

it takes to complete one cycle of vibration Several terms are

used in the industries to describe the frequency: synchronous

or 13; nonsynchronous, subsynchronous, or less than 13;

and super synchronous or greater than 13

The phase is the timing difference between vibration events

The timing difference between the root cause and its effect of

rotor behavior to find the possible root causes gives us a tool for

the diagnosis of rotating machinery [17] The quality or level of

motor vibration is an indicator of how well the motor is

designed, manufactured, installed, maintained, and operated

The vibration magnitudes, frequencies, and phase angles

indi-cate what possible sources of vibrations are being seen When

considering induction motor vibration, one is referring to

vibra-tion levels measured on the bearing housing and shaft The

housing readings are taken in the horizontal, vertical, and axial

direction or as close as possible to these locations The shaft

readings are taken with noncontacting eddy-current probes

mounted on the bearing housing and measure the relative

movement between the housing and shaft In North America,

housing readings are normally taken as velocity in inches per

sec-ond, zero to peak The shaft readings are taken as peak-to-peak

displacement in mil, as defined by API and National Electrical

Manufacturers Association (NEMA) MG1 [1], [7], [8]

Per the International Electrotechnical Commission (IEC)

60034-14 [10], the criterion for bearing housing vibration

magnitude at the machine bearings is the broadband root

mean square (rms) The standard measurement units are

defined as follows: displacement in micrometers, velocity in

millimeters per second, and acceleration in meter per second

squared [9] The criterion for the relative shaft vibration

magnitude is the peak-to-peak displacement in the direction

of the measurement per the International Standard

Organiza-tion (ISO) 7919-1 [14]

Motor Construction

Rotor Construction

To understand induction motor vibration and its effects, it is

first necessary to know the motor construction The motor is

comprised of a frame, stator, rotor, bear-ing housbear-ings, and main terminal box Typically, the frame material is cast iron or fabricated steel The stator is constructed from steel laminations with electrical windings inserted into axial slots

Four types of rotor construction exist today: the aluminum die cast (ADC), copper die cast, fabricated alu-minum bars (AlBar), and fabricated copper or copper alloy bars (CuBar) Although each type of rotor con-struction has advantages and disadvan-tages, this article will discuss the most common: ADC, fabricated copper bars (CuBar), and fabricated AlBar, with respect to vibration Typically, the ADC rotors are easier to manufacture and more economical than the CuBar rotors The aluminum rotor bars have approximately one-third the density of steel and 2.3 times the specific heat of copper Additionally, the coefficient of thermal expansion for a given temperature change is 31% greater for aluminum over copper More-over, aluminum has a lower yield strength than do copper

As a result of these material density and specific heat differ-ences, the AlBar will become much hotter, expand further, and generate much higher stresses while accelerating the same load inertia (WK2) Porosity may also be present

in die cast rotors because of trapped gases during the cast-ing process or uneven shrinkage durcast-ing coolcast-ing All of these factors can contribute to a higher vibration over a CuBar construction At present, most manufacturers main-tain good control over these processes, eliminating most of the concern Despite this benefit, copper bar rotors are gen-erally preferred for API motors because of their ease of rep-arability As a result, a damaged copper bar motor can be repaired and placed back into service much faster

A fabricated aluminum rotor bar has a cost advantage over a fabricated copper bar and a manufacturing advant-age over ADC, which has various limiting factors, such as tooling and size

Another key difference is that the end connector of an AlBar rotor is welded to the rotor bars as opposed to brazed Additionally, the end connectors of the AlBar rotor clamp the rotor punchings, as opposed to the use of sepa-rate end heads in the CuBar construction [11]

In conclusion, all types of rotor constructions can be designed and manufactured to ensure low vibration In general, a copper-fabricated rotor should be more robust and can be visually inspected for flaws during manufactur-ing to ensure a high-quality product Although this type

of construction has the ability of being more easily repaired

in the field, if impractically designed and manufactured, these advantages would not be guaranteed Finally, the design must take into account the relative movement dur-ing motor startdur-ing so that the motor still continues to per-form after multiple starts

Bearing Types The most common type of bearing used today is the anti-friction bearing (AFB) In comparison to a sleeve bearing,

an AFB can be less reliable, have a limited life, and will not

THE MOTOR IS COMPRISED OF A FRAME, STATOR, ROTOR, BEARING HOUSINGS, AND MAIN TERMINAL

BOX.

38

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 1 lists the

gen-eral 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

manu-factured and the motor installation A sleeve bearing

will have good damping, while an AFB will provide very

lit-tle 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

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

four-pole and slower machines Eventually, it was determined

that these levels were too loose and did not provide the

nec-essary reliability that was required or could easily be

achieved In 1993, NEMA changed the method of

measure-ment 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

require-ments for many of the construction features but did not

modify or lower the vibrations levels The vibration levels

are shown in Table 2 for housing vibration and Table 3 for

shaft vibration

At the same time, IEC standard 60034-14 is

establish-ing newer and lower levels than what was published

previ-ously; however, these new values are still higher than API

limits In addition, NEMA is presently working on

estab-lishing various levels of vibration based on the criticality of

the application Ideally, all standards should agree on

simi-lar values that demand cost-effective designs while

ensur-ing good reliability However, there is a point of diminishensur-ing

returns where lower vibration levels become extremely

difficult and costly but will not return substantial benefits

in reliability

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

n produce different movements on different components

n be applied in different directions

n 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 measure-ments to different causes and thereby establish performance and design requirements intended to minimize these vibra-tions This section will explain how different vibration lim-its 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 fre-quency that may be less than, equal to, or greater than the synchronous frequency of the rotor

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 signif-icant changes to balance readings Additionally, not per-forming 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 do not define how to manufacture the motor Instead, these specifications establish limits and allow the motor manufac-turers 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 Regard-less, many of these requirements are easily achieved and therefore good reliability additions It is the requirements

TABLE 1 GENERAL CRITERIA FOR BEARING SELECTION.

39

that add little value and have higher

costs that need to be reviewed in future

editions of API 541

API balance requirements are more

important with respect to the vibration

limits at operating speed API requires

residual unbalance not exceeding 4W/N

oz-in at each journal, where W is one half

the weight of the rotor and N is the

maxi-mum operating speed of the machine In

SI units, this permitted unbalance level is

6,350W/N g-mm, where W is the

weight per journal in kilograms and N is

the maximum operating speed [7], [8] This permitted

unbal-ance level corresponds to about G 0.70 in the ISO 1940-1

[15] system Balance is more critical and also more difficult to

perform on two-pole motors API 541 does provide the option

to check the unbalance response at oper-ating speed Additionally, balancing at a speed lower than operating speed could create an unbalance value too small for the sensitivity of the balancing machine The assembled motors are then tested to confirm that vibration requirements are met in operation in the actual machine API does not allow trim balancing to compensate for the thermal bow of the assembled motor This compensation may be performed by many motor man-ufacturers today, but this exception to the specification should be done in the cold condition and should be approved by the customer

For the adjustable speed drive (ASD) applications, the vibration limits are the same as for fixed speed units The

TABLE 2 COMPARISON OF HOUSING VIBRATION LIMITS.

Assumptions:

2.0 mil 4p 2.5 mil 6p NEMA MG1: From

1993 Rev 1 to

MG 1-2006

2.0 mil 4p 2.5 mil 6þp API 541, second

recording after heat run

0.05 in/s 6p

0.074 in/s 4p 0.065 in/s 6p

2,4,6þp

API 541, third

edition 1995

record-ing of data for 15 min for two-pole motors

API 541, fourth

edition 2006

0.1 in/s 2,4,1.

6 mil 6þp

0.1 in/s 2,4, 1.6 mil 6þp

0.1 in/s 2,4, 1.6 mil 6þp

0.1 in/s 2, 4,1.6 mil 6þp

Continuous record-ing of data for 15 min for two-pole motors

IEC 60034 14 Ed

IEEE 841, 2001

[16]

1 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.

2 Vibration for standard grade A and shaft height greater than 280 mm (11 in).

VIBRATION IS THE PERIODIC BACK-AND-FORTH MOTION OF THE OBJECT.

40

limits need to be met at all supply

fre-quencies in the operating range Most

medium-to-large motors are used for

constant speed applications, but the

number of ASD motors is increasing

considerably for many reasons,

espe-cially to increase efficiency Constant

speed motors only need to be precision

balanced at operating speed, while

adjustable speed applications require

that acceptable rotor balance be

main-tained throughout the operating speed

range It is also critical that all the

components remain tight and not

change unbalance throughout the

entire temperature and speed range for

the ASD motors

Rotor balance involves the entire rotor structure that is

made up of a multitude of parts, including the shaft, rotor

laminations, end heads, rotor bars, end connectors,

retain-ing rretain-ings (where required), and fans All of these items

must be addressed in the design and manufacture to

achieve a stable precision balance

API does not define how to ensure the rotor bars are to be

maintained tight in the slot nor does it describe the

concentricity limits of the rotor core or end connector However, it does require that actions be taken to assure concen-tricity and rotor component security Good mechanical slow roll indicates good concentricity with the bearing journal diameter and minimizes oil film instability in the bearing

API makes the following state-ments regarding good manufacturing practices

n The slow-roll acceptance criteria for an assembled motor rotating between 200 and 300 r/min should not exceed 30% of the allowed peak-to-peak unfiltered vibration amplitude or 0.25 mil (6 lm), whichever is greater

n Looseness of parts, which can result in shifting during operation, causing a change in balance, must be avoided or minimized

n Balance correction weights should be added at or near the points of unbalance

API 541 fourth edition defines vibration acceptance val-ues at operating temperature, requiring the product be

TABLE 3 COMPARISON OF SHAFT VIBRATION LIMITS.

Assumptions:

NEMA MG1: From

1993 Rev 1 to

2006

3.5 mil 4þp IEC 60034-14 Ed

3.5 mil 4þP

API 541 second

edition 1987

2.5 mil 4p 3.0 mil

2.0 mil 4p 2.4 mil

1.5 mil 4p 2.0 mil

API 541 third

edition 1995

unfiltered which-ever is greater API 541 fourth

edition 2006

unfiltered which-ever is greater

1 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 sensitivity criteria.

2 Vibration for standard grade A and shaft height greater than 280 mm (11 in).

3 Run out compensated.

API 541 DOES PROVIDE THE OPTION TO CHECK THE UNBALANCE RESPONSE AT OPERATING SPEED.

41

precision designed and manufactured

with appropriate thermal stability and/

or excellent cooling of the rotor system

parts The rotor core/laminations must

be precision manufactured and have an

adequate (but not excessive) shrink fit

on the shaft that is maintained at all

operating speeds and temperatures

The rotor core must be able to expand

and contract on the shaft without

bind-ing and bendbind-ing the shaft, a cause of

thermal vibration problems When

end connectors require retaining rings,

the rings should be designed with a

high-strength material and a proper

interference fit The retaining ring

material should be nonmagnetic and not susceptible to

stress corrosion The rotor bars are typically shimmed and/

or swaged so they are tight in the slots API does require,

for reasons of good heat transfer and to limit the vibration

and fatigue of bars, that all bars shall be maintained tight

in their slots (swaged, center locked, or pinned) The end

connectors should be induction brazed or by some other

means symmetrically heated to make the connection to the

bars This helps to eliminate variations in balance due to

thermal change The shaft and assembled rotor should be

precision machined or manufactured to maintain slow-roll

vibration levels within 0.00025–0.0005 in It is important

to note that these limits are not defined by API The rotor

is prebalanced without fans, the fans are then assembled,

and the entire assembly is final balanced on the rotor fans

The rotating assemblies for two- and four-pole machines,

and when specified for slower speed machines, should be

component balanced per the following sequence:

n The shaft/rotor core assembly should be balanced

in two or more planes

n After the addition of a single component or two

identical components mounted symmetrically

oppo-site to the above-balanced assembly, balance

correc-tions should be made only to the components added

The fans may be individually bal-anced before assembly on the rotor, but any additional balance weights at that point must be added to the fan to ensure all balancing is done at the source of the imbalance as per API [8] The constant speed applications are typically satisfied with either a stiff shaft design for smaller and slower speed machines or a flexible shaft design for larger and high-speed motors A stiff shaft design is one that operates below its first lateral critical speed, while a flexible shaft design operates above the first lateral critical speed When the rotor is precision designed and manu-factured as described above, a two-plane balance making weight corrections at the rotor ends will usually suffice even for the flexible rotors The rotors operating at speeds in excess of the first actual lateral critical speed may be bal-anced in at least three planes, including center plane at or near the axial geometric center of the rotor assembly The flexible rotors may require a three-plane balancing to limit vibration as the machine passes through its critical speed during run-up or coast-down if the critical speed is not highly damped This is accomplished by also making weight corrections at the rotor center plane as well as at the two ends API defines the critical speed as highly damped if the amplification factor is 2.5 The amplification factor is the measure of a rotor bearing system’s vibration sensitivity

to unbalance when operated in the vicinity of one of its lateral critical speeds [12]

Per API 541 second, third, and fourth editions, the shaft extension keyway must be completely filled with a crowned, contoured half key for balancing and no load tested at the manufacturer The load testing can be carried out with the motor mounted on a massive, rigid base, accurately aligned

to a dynamometer and coupled to it with a precision bal-anced coupling and proper key API also allows a dual frequency heat run test per IEEE 112 Additionally, if the motor exceeds the vibration limits during coupled, full load, steady-state operation, API provides a correction procedure based on uncoupled vibration readings taken under hot and cold conditions on the same foundation

Twice-Line Frequency Vibration Twice-line frequency vibration can also be a significant portion of the overall vibration in induction machines For machines at speeds up to 1,200 r/min, the filtered and unfiltered vibration limit is 1.6 mil peak-to-peak displace-ment and 0.1 in/s true peak velocity for rated speeds above 1,200 r/min The source of this vibration is dependent on various parameters within the machine

The power source is a sinusoidal voltage that varies from positive to negative peak voltage in each cycle The power supply applied to the stator produces a rotating magnetic field developing an electromagnetic attractive force between the stator and rotor (Figure 1)

This force reaches its maximum magnitude when the magnetizing current flowing in the stator is at a maxi-mum, either positive or negative at that instant in time As

a result, two peak forces exist during each cycle of the

Yoke

Stator

Shaft

Rotor

(4) Mounting

Feet

Electromechanical Force Between the Stator and Rotor

1

The stator and rotor.

ADC ROTORS ARE EASIER TO MANUFACTURE AND MORE ECONOMICAL

ROTORS.

42

voltage or current wave, reducing to zero at the point in

time when the current and fundamental flux wave pass

through zero (Figure 2) This results in a frequency of

vibration equal to two times the frequency of the power

source (twice-line frequency vibration) [12] This

particu-lar vibration is extremely sensitive to the motor’s foot

flat-ness, frame and base stiffflat-ness, and the consistency of the air

gap between the stator and rotor It can also be influenced

by the eccentricity of the rotor API 541 fourth edition

requires the motor feet to fall within 0.005 in of a common

horizontal plane Additionally, it limits the foot flatness to

0.0005 in/ft and requires that different mounting planes

be parallel to each other within 0.002 in/ft

The basic forces are independent of load current and are

nearly the same at both no load and full load This is

because the main component of twice-line frequency

vibra-tion, created by an unbalanced magnetic pull due to air

gap dissymmetry, does not change with load

For the two-pole motors, the twice-line frequency

vibra-tion level will appear to modulate over time due to its close

relationship with two times rotational vibration The

motors with problems, such as a rub, loose parts, a bent

shaft extension or elliptical bearing journals, can cause

vibration at two times rotational frequency Because of its

closeness in frequency to twice-line frequency vibration,

the two levels will add together when they are in phase and

subtract when they are out of phase This modulation will

repeat at a frequency of two times the slip on the two-pole

motors Slip occurs in induction motors due to the rotor

trying to stay in phase with the rotating field around the

stator The rotor falls behind the stator field by a certain

number of revolutions per minute (slip speed) depending

upon the load Even at no-load, twice rotation vibration on

the two-pole motors will vary from 7,200 cycles/min (120

Hz) due to slip Since there is some slip on induction

motors, although small at no-load, it may take 5–15 min

to slip one rotation A larger load will produce a greater

slip speed Slip is typically 1% of rated speed at full load

and decreases to near 0% slip at no-load Since vibration

levels are not constant over time, API requires measuring

vibration to perform a modulation test In a vibration

mod-ulation test, the motor is allowed to run for a period 15

min, and vibration is recorded continuously to allow the

maximum and minimum to be established Other

standards require only a vibration snapshot, which may not

reveal the peak vibration over a period of time

In general, the methods used to reduce this level of

vibration are the responsibility of the motor manufacturer

The frame stiffness, flux densities, and isolation of the

sta-tor from the bearing housings will all influence this

vibra-tion level, but only foot flatness and parallelism is defined

by API The remaining design parameters are left to the

motor manufacturer Good foot flatness has the added

ben-efit of consistent results when the motor is placed in

differ-ent locations Although the design methods can vary,

achieving lower levels of vibration is the primary objective

Rotor eccentricity occurs when the rotor core outer

diame-ter is not concentric with the bearing journals, creating a

point of minimum air gap that rotates with the rotor at

13 rotational frequency An eccentric rotor will have a net

unbalanced magnetic force acting at the point of minimum air gap, since the force acting at the minimum gap is greater than the force at the maximum gap, as illustrated

in Figure 3 This net unbalance force will rotate with the minimum air gap, causing vibration at 13 rotational frequency API has no defined requirement that limits this concentricity; instead, the specification defines a limit for vibration modulation resulting from this excessive eccen-tricity This allows the motor designer to minimize vibra-tion through other design or construcvibra-tion features Other parameters, such as bearing and rotor stiffness and levels of magnetic field, also influence this vibration With low vibration as the goal, the motor designer is free to use his own method to meet the end requirement

Rotor Bar Passing Frequency Vibration The high frequency, load-related magnetic vibration at or near rotor slot passing frequency is generated in the motor stator when current is induced into the rotor bars under load The magnitude of this vibration varies with load, increasing as load increases The electrical current in the bars creates a magnetic field around the bars that applies

an attracting force to the stator teeth These radial and tangential forces that are applied to the stator teeth, as seen

in Figure 4, create vibration of the stator core and teeth

This source of vibration is at a frequency that is much greater than frequencies normally measured during normal vibration tests As a result, this normally does not come into play in any of the vibration tests

90

270

180 F2

F1

Min-Gap-Maximum Force

Max-Gap-Minimum Force

Rotational Force

Stator Rotor

Exaggerated View of Eccentric Rotor

3

The eccentric rotor.

Flux–Flux Around a Stator on a Two-Pole Motor

Force–Force Between a Stator and Rotor

on a Two-Pole Motor

180

180

2

One-period flux wave and magnetic force wave.

43

Requirements for Field

Installation

Weak Motor Base

If the motor is kept on a weak fabricated

steel base, such as a pedestal, slide rail

base, or pump stand, then the

possibil-ity exists that the vibration, which is

measured at the motor, is greatly

influ-enced by a base that itself is vibrating

Ideally, the base should be stiff enough

to meet the “Massive Foundation”

crite-ria defined by API 541 [5]–[9]

Essen-tially, this specification requires that the

vibration near the motor feet be less

than 30% of the vibration measured at

the motor bearing If the base vibration

exceeds this limit, then API level of vibration for the motor

may not be achieved

Misalignment

The motor should be coupled to the driven equipment

such that the vibration should not increase beyond the

vibration limit specified as a coupled unit The coupling

should not be considered as a vibration damping device

and should be aligned per the coupling manufacturer’s

specification Good alignment in the cold and hot

condi-tion reduces the stresses on the shaft and bearings and

min-imizes vibration

Resonance

Resonant bases on either horizontally or vertically mounted

machines can increase the vibration levels five to ten times

over vibration levels on a rigid base Any base resonant

frequency should be removed 15% from the motor

operat-ing speed or any other source of vibration

Manufacturing Requirements

Table 4 lists the manufacturing requirements of the

differ-ent standards to achieve good motor vibration Many of

these manufacturing requirements were discussed earlier

in the article However, in summary, the design and

manu-facturing requirements needed to ensure low vibration and

reliability are as follows:

1) good, stable shaft material 2) proper rotor core to shaft fit 3) no loose parts that change unbal-ance during operation and speed change

4) end connector symmetrically brazed (depending on rotor construction) 5) low run out

nbearing journals

nprobe fits

nshaft extension

nrotor core outer diameter 6) no resonant frequencies near the operating speed or known forcing frequencies

7) no degradation of the above items due to multiple restarts

8) proper rotor construction for the application: cop-per, ADC, etc

9) proper bearing selection for the application: sleeve, AFB, etc

10) stiff frame construction with proper foot flatness 11) no resonances in frame or bearing housing that can cause excessive vibration at known forcing frequencies

As discussed earlier, it is also critical to keep the relative run out between the bearing journals and rotor core outer diameter between 0.001 and 0.002 in, as higher levels can cause vibration problems It is important to note that API does not define the limits of mechanical and electrical run out but define only the resulting vibration However, API does state that the total run out between the bearing jour-nal and the noncontacting eddy-current probe fit should

be less than 0.45 mil to minimize the effects of run out on the total vibration Although it is possible to correct vibra-tion for noncontacting eddy-current probe slow roll, this is not yet included in API

To achieve good vibration levels over the entire speed range and also from ambient temperature to operating temperature, the copper rotor bars should be tightly installed in the core Swaging, shimming, or pinning of the rotor bars are several ways to accomplish this requirement Cost Versus Return

Along with lower vibration levels, there is a motor cost increase associated with more controlled manufacturing processes, higher tolerances and better raw materials Table 5 compares the requirements for lower vibration levels with respect to three motor construction charac-teristics: rotor construction, bearing type, and shaft construction

The noncontacting eddy-current probes require special shaft material and additional manufacturing processes, while the mounting of velocity sensors or accelerometers (to measure housing vibration) is relatively simple The cost of the probes increases as the power output decreases Conclusions

The vibration requirements of various international standards and the design considerations and manufactur-ing processes required to achieve these low vibration levels were discussed in this article Depending on the criticality

of the application, the end user must decide what values of

THE VIBRATION LEVELS DEPEND

ON THE QUALITY

OF ROTOR MANUFACTURED AND THE MOTOR INSTALLATION.

Stator

Slot

Rotor

Ft

View of Tooth and Forces

Magnetic Field Around Rotor Bar and Resulting Forces

Fr

4

The magnetic field around the rotor bar and the resulting

force on stator teeth.

44

Nop

Nop

Nop

Nop

45

vibrations will provide a longer motor operating life and

the cost associated with obtaining these low levels

References

[1] Motors and Generators, NEMA MG 1-2006.

[2] Motors and Generators, NEMA MG 1-1998.

[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 Horsepower and Larger,

2nd ed., API 541, 1987.

[7] Form-Wound Squirrel-Cage Induction Motors—250 Horsepower and Larger,

3rd ed., API 541, Apr 1995.

[8] Form-Wound Squirrel-Cage Induction Motors—500 Horsepower and Larger,

4th ed., API 541, June 2004.

[9] General-Purpose Form-Wound Squirrel Cage Induction Motors—250

Horse-power and Larger, 1st ed., API 547, Jan 2005.

[10] Mechanical Vibration of Certain Machines with Shaft Heights 56 mm and

Higher: Measurement, evaluation and limits of Vibration severity, IEC

60034-14, 2003.

[11] M Hodowanec and W R Finley, “Copper versus aluminum

induc-tion-motors: Which construction is best?” IEEE Ind Applicat Mag.,

vol 8, no 4, pp 14–24, July/Aug 2002.

[12] W R Finley, M M Hodowanec, and W G Holter, “An analytical approach to solving motor vibration problems,” IEEE Trans Ind Applicat., vol 36, no 5, pp 1467–1480, Sept./Oct 2000.

[13] Machinery Protection System, 4th ed., API 670, Dec 2000.

[14] Mechanical Vibration of Non-reciprocating Machines: Measurements on Rotat-ing Shafts and Evaluation Criteria, ISO 7919-1, July 1996.

[15] Mechanical Vibration Balance Quality Requirements for Rotors in a Constant (rigid) State—Part 1: Specification and verification of balance tolerances, ISO 1940-1, Apr 2004.

[16] Petroleum and Chemical Industry—Severe Duty Totally Enclosed Fan-Cooled (TEFC) Squirrel Cage Induction Motors—Up to and Including 370 kW (500 hp), IEEE 841, Mar 2001.

[17] D Bently, C T Hatch, and B Grissom, Fundamentals of Rotating Machin-ery Diagnostics Minden, NV: Bently Pressurized Bearing Press, 2002.

Rajendra Mistry (rajendra.mistry@siemens.com), William R Finley, and Scott Kreitzer are with Siemens Energy and Auto-mation in Norwood, Ohio Mistry and Scott are Members of the IEEE Finley is a Senior Member of the IEEE This arti-cle first appeared as “Induction Motor Vibrations in View of the API 541—4th Edition” at the 2008 Petroleum and Chemical Industry Committee

TABLE 5 A COMPARISON OF THE REQUIREMENTS FOR LOWER VIBRATION LEVELS

WITH RESPECT TO MOTOR CONSTRUCTION CHARACTERISTICS.

More precise and accurate injection method Close tolerance machining

Good connecting process to end connectors Close tolerance machining

Better fit and tighter tolerances between bearing and shaft and housing Better lubrication

Better circulation of lubrication Better fit and tighter tolerances between shaft journal and bearing Better lubrication and better temperature control for better viscosity stability

Good stress relieved shaft material Precision machining, tighter tolerances Better fit and tighter tolerances between core and shaft Better thermal stability

Better process for uniform welding between various types of shaft and spider bar materials

Good stress relieved shaft material Precision machining

Better fit and tighter tolerances between core and shaft Better thermal stability

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