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

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© 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 do 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

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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 1 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 do
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

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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 2 for housing vibration and Table 3 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 1. 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 2. 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 2 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
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,
Continuous record4,1.6 mil 6þp ing of data for 15
min 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
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2.5 mil 6p
NEMA MG1: From
1993 Rev 1 to
MG 1-2006
API RP541 1972

2.0 mil 4p
2.5 mil 6þp
API 541, second
edition 1987 1

IEC 60034 14 Ed
3.1 2007-03 2
IEEE 841, 2001
[16]

60 min of tape
recording after
heat run

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

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 3. COMPARISON OF SHAFT VIBRATION LIMITS.

Unfiltered

Filtered 13 r/min

Filtered 23 r/min

<13

NEMA MG1: 1987

N/A

N/A

N/A

N/A

NEMA MG1: From
1993 Rev 1 to
2006

2.8 mil 2p

N/A

N/A

N/A

N/A

N/A

N/A

N/A

Standard

IEC 60034-14 Ed
3.1 2007-032

3.5 mil 4þp
2.5 mil 2p
3.5 mil
4þP

API RP541 1972

N/A

N/A

N/A

API 541 second
edition 1987

2.0 mil 2p

1.5 mil 2p

1.0 mil 2p

2.5 mil 4p 3.0 mil
6þp1

2.0 mil 4p 2.4 mil
6þp1

1.5 mil 4p 2.0 mil
6þp1

API 541 third
edition 1995

1.5 mil 2,4,6þp

1.2 mil 2,4,6þp3

0.5 mil 2,4,6þp

0.1 mil or 20% of
unfiltered whichever is greater

API 541 fourth
edition 2006

1.5 mil 2,4,6þp

1.2 mil 2,4,6þp3

0.5 mil 2,4,6þp

0.1 mil or 20% of
unfiltered whichever 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.

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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.

41


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42

The fans may be individually balprecision designed and manufactured
anced before assembly on the rotor, but
with appropriate thermal stability and/
ADC ROTORS ARE
any additional balance weights at that
or excellent cooling of the rotor system
point must be added to the fan to
parts. The rotor core/laminations must
EASIER TO
ensure all balancing is done at the
be precision manufactured and have an
source of the imbalance as per API [8].
adequate (but not excessive) shrink fit
MANUFACTURE
The constant speed applications are

on the shaft that is maintained at all
typically satisfied with either a stiff
operating speeds and temperatures.
AND MORE
shaft design for smaller and slower
The rotor core must be able to expand
ECONOMICAL
speed machines or a flexible shaft design
and contract on the shaft without bindfor larger and high-speed motors. A stiff
ing and bending the shaft, a cause of
THAN CUBAR
shaft design is one that operates below
thermal vibration problems. When
its first lateral critical speed, while a
end connectors require retaining rings,
ROTORS.
flexible shaft design operates above the
the rings should be designed with a
first lateral critical speed. When the
high-strength material and a proper
rotor is precision designed and manuinterference fit. The retaining ring
material should be nonmagnetic and not susceptible to factured as described above, a two-plane balance making
stress corrosion. The rotor bars are typically shimmed and/ weight corrections at the rotor ends will usually suffice even
or swaged so they are tight in the slots. API does require, for the flexible rotors. The rotors operating at speeds in
for reasons of good heat transfer and to limit the vibration excess of the first actual lateral critical speed may be baland fatigue of bars, that all bars shall be maintained tight anced in at least three planes, including center plane at or
in their slots (swaged, center locked, or pinned). The end near the axial geometric center of the rotor assembly. The
connectors should be induction brazed or by some other flexible rotors may require a three-plane balancing to limit
means symmetrically heated to make the connection to the vibration as the machine passes through its critical speed
bars. This helps to eliminate variations in balance due to during run-up or coast-down if the critical speed is not
thermal change. The shaft and assembled rotor should be highly damped. This is accomplished by also making

precision machined or manufactured to maintain slow-roll weight corrections at the rotor center plane as well as at the
vibration levels within 0.00025–0.0005 in. It is important two ends. API defines the critical speed as highly damped if
to note that these limits are not defined by API. The rotor the amplification factor is 2.5. The amplification factor is
is prebalanced without fans, the fans are then assembled, the measure of a rotor bearing system’s vibration sensitivity
and the entire assembly is final balanced on the rotor fans. to unbalance when operated in the vicinity of one of its
The rotating assemblies for two- and four-pole machines, lateral critical speeds [12].
Per API 541 second, third, and fourth editions, the shaft
and when specified for slower speed machines, should be
extension keyway must be completely filled with a crowned,
component balanced per the following sequence:
contoured half key for balancing and no load tested at the
n The shaft/rotor core assembly should be balanced
manufacturer. The load testing can be carried out with the
in two or more planes.
motor mounted on a massive, rigid base, accurately aligned
n After the addition of a single component or two
identical components mounted symmetrically oppo- to a dynamometer and coupled to it with a precision balsite to the above-balanced assembly, balance correc- anced coupling and proper key. API also allows a dual
tions should be made only to the components added. 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.
Yoke
Stator

Twice-Line Frequency Vibration

Shaft
Rotor

(4) Mounting
Feet
The stator and rotor.

Electromechanical
Force Between
the Stator and Rotor

1

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 displacement 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 maximum, either positive or negative at that instant in time. As
a result, two peak forces exist during each cycle of the


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 eccentricity. This allows the motor designer to minimize vibration through other design or construction 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.

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

0

180
Force–Force Between a Stator and Rotor

on a Two-Pole Motor

0

180

360

2

One-period flux wave and magnetic force wave.

Min-Gap-Maximum 90
Force
F1

180

Rotational
Force

F2
Stator

Rotor

13 Rotation Vibration: Eccentric Rotor

Rotor eccentricity occurs when the rotor core outer diameter 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

360

Max-Gap-Minimum
Force

270
Exaggerated View of Eccentric Rotor

3
The eccentric rotor.

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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 particular vibration is extremely sensitive to the motor’s foot flatness, frame and base stiffness, 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 vibration, 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 vibration 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 modulation 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 stator from the bearing housings will all influence this vibration 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 benefit of consistent results when the motor is placed in different locations. Although the design methods can vary,
achieving lower levels of vibration is the primary objective.

43


44

1) good, stable shaft material
2) proper rotor core to shaft fit
3) no loose parts that change unbalTHE VIBRATION
ance during operation and speed
Weak Motor Base
LEVELS DEPEND
change
If the motor is kept on a weak fabricated
4) end connector symmetrically brazed
steel base, such as a pedestal, slide rail
ON THE QUALITY
(depending on rotor construction)
base, or pump stand, then the possibil5) low run out
ity exists that the vibration, which is
OF ROTOR
measured at the motor, is greatly influn bearing journals
enced by a base that itself is vibrating.
n probe fits
MANUFACTURED
Ideally, the base should be stiff enough
n shaft extension
to meet the “Massive Foundation” criten rotor core outer diameter

AND THE MOTOR
ria defined by API 541 [5]–[9]. Essen6) no resonant frequencies near the
tially, this specification requires that the
operating speed or known forcing
INSTALLATION.
vibration near the motor feet be less
frequencies
than 30% of the vibration measured at
7) no degradation of the above items
the motor bearing. If the base vibration
due to multiple restarts
exceeds this limit, then API level of vibration for the motor
8) proper rotor construction for the application: copmay not be achieved.
per, ADC, etc.
9) proper bearing selection for the application: sleeve,
AFB, etc.
Misalignment
10) stiff frame construction with proper foot flatness
The motor should be coupled to the driven equipment
11) no resonances in frame or bearing housing that can
such that the vibration should not increase beyond the
cause excessive vibration at known forcing frequencies.
vibration limit specified as a coupled unit. The coupling
As discussed earlier, it is also critical to keep the relative
should not be considered as a vibration damping device
and should be aligned per the coupling manufacturer’s run out between the bearing journals and rotor core outer
specification. Good alignment in the cold and hot condi- diameter between 0.001 and 0.002 in, as higher levels can
tion reduces the stresses on the shaft and bearings and min- cause vibration problems. It is important to note that API
does not define the limits of mechanical and electrical run
imizes vibration.

out but define only the resulting vibration. However, API
does state that the total run out between the bearing jourResonance
Resonant bases on either horizontally or vertically mounted nal and the noncontacting eddy-current probe fit should
machines can increase the vibration levels five to ten times be less than 0.45 mil to minimize the effects of run out on
over vibration levels on a rigid base. Any base resonant the total vibration. Although it is possible to correct vibrafrequency should be removed 15% from the motor operat- tion for noncontacting eddy-current probe slow roll, this is
not yet included in API.
ing speed or any other source of vibration.
To achieve good vibration levels over the entire speed
range and also from ambient temperature to operating
Manufacturing Requirements
Table 4 lists the manufacturing requirements of the differ- temperature, the copper rotor bars should be tightly
ent standards to achieve good motor vibration. Many of installed in the core. Swaging, shimming, or pinning of the
these manufacturing requirements were discussed earlier rotor bars are several ways to accomplish this requirement.
in the article. However, in summary, the design and manufacturing requirements needed to ensure low vibration and Cost Versus Return
Along with lower vibration levels, there is a motor cost
reliability are as follows:
increase associated with more controlled manufacturing
processes, higher tolerances and better raw materials.
Table 5 compares the requirements for lower vibration
Stator
Slot
levels with respect to three motor construction characFr
teristics: rotor construction, bearing type, and shaft
construction.
The noncontacting eddy-current probes require special
Ft
shaft material and additional manufacturing processes,
while the mounting of velocity sensors or accelerometers
(to measure housing vibration) is relatively simple. The
Rotor

cost of the probes increases as the power output decreases.
View of Tooth and Forces
Tooth

IEEE INDUSTRY APPLICATIONS MAGAZINE  NOV j DEC 2010  WWW.IEEE.ORG/IAS

Requirements for Field
Installation

Magnetic Field Around Rotor
Bar and Resulting Forces

4
The magnetic field around the rotor bar and the resulting
force on stator teeth.

Conclusions
The vibration requirements of various international
standards and the design considerations and manufacturing 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


Not mentioned

Not mentioned

Yes, at machine feet, vibration
<25% of max velocity at
bearing housings

Æ10% of 13 r/min, Æ5% of
23 r/min and 13 and 23 LF

Not mentioned

Hot-vibration measurement

Modulation

Mounted on a massive
foundation

Frame resonance

Balance with key

N ¼ frequency range (Hz); Nop ¼ operating frequency (Hz); n ¼ 1, 2, and 3.

Not mentioned

Trim-balancing allowed

1

Not mentioned

Not mentioned

Air-gap tolerance

Not mentioned

Foot flatness

Not mentioned

Not mentioned

Induction-brazed end rings

Full-speed balancing

Not mentioned

Center-locked rotor bars

Balance spec

Not mentioned

Tight rotor bars

Not mentioned

Not mentioned

Seamless end connectors

Not mentioned

Not mentioned

Forged shaft

Step balance

Not mentioned

Noncontacting eddycurrent probe fit run out

End rings symmetrically brazed

Not mentioned

Bearing journal run outs

NEMA MG1 2006

TABLE 4. COMPARISON OF MANUFACTURING REQUIREMENTS.

Yes, to limit the vibration values
25% of unfiltered p–p or 0.25 mil total
Yes, special requirements

Required for speed > 1,000 r/min

Yes, to limit the vibration values
30% of unfiltered p–p or
0.25 mil total

Yes, two pole and operating
speed > 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 min for two pole
Æ20% of 1 and 23 and 20% of 1 and 2 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 min for two pole
Yes, at machine feet, vibration
<30% of max velocity at
bearing housings
N ¼ nNop Æ 0.20Nop1
Yes

Same as API 541 if probes
required

Same as API 541 if probes
required

Not mentioned

N ¼ nNop Æ 0.25Nop1
Not mentioned

Not mentioned

Mounting planes parallel within 0.002 in/ft

Flatness of each foot within 0.0005 in/ft

Mounting planes parallel
within 0.002 in/ft

Each foot parallel to same
horizontal plane within 0.002 in/ft

Flatness of each foot within
0.0005 in/ft

4W/N

All feet in the same horizontal plane
within 0.005 in

Æ 10%

Æ 10%
All feet in the same horizontal
plane within 0.005 in

Not mentioned

No

Unbalance

4W/N

Unbalance
No

Yes
Yes

All feet in the same
horizontal plane within
0.005 in

Not mentioned

Not mentioned

Not mentioned

Yes

Yes, allowed and other methods too

Yes, allowed and other methods
too

Brazing material must be
phosphorous free

Yes

Yes

Yes

Not mentioned

Yes

Yes

Not mentioned

Not mentioned

Not mentioned

Not mentioned

API 541 Fourth Edition

API 541 Third Edition

Not mentioned

API 547

IEEE INDUSTRY APPLICATIONS MAGAZINE  NOV j DEC 2010  WWW.IEEE.ORG/IAS

45


TABLE 5. A COMPARISON OF THE REQUIREMENTS FOR LOWER VIBRATION LEVELS
WITH RESPECT TO MOTOR CONSTRUCTION CHARACTERISTICS.
Components

Requirements for Lower Vibration

Rotor, ADC

Better quality aluminum
More precise and accurate injection method
Close tolerance machining

Rotor, CuBar

Better quality copper
Good connecting process to end connectors
Close tolerance machining

Bearing, AFB

High-precision, quality bearing
Better fit and tighter tolerances between bearing and shaft and housing
Better lubrication

Bearing, sleeve

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

Shaft, solid

Homogeneous material for good slow roll and stability
Good stress relieved shaft material
Precision machining, tighter tolerances
Better fit and tighter tolerances between core and shaft
Better thermal stability

IEEE INDUSTRY APPLICATIONS MAGAZINE  NOV j DEC 2010  WWW.IEEE.ORG/IAS

Shaft, spider

46

Homogeneous material for good slow roll and 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

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 Horsepower 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 induction-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 Rotating 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 Machinery Diagnostics. Minden, NV: Bently Pressurized Bearing Press, 2002.

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