Analysis of motor/pump vibration

Vibration in a pump can be due to

1.Electrical imbalance

2.Mechanical unbalance – motor, coupling, or driven equipment

3.Mechanical effects – looseness, rubbing, bearings, etc.

4.External effects – base, driven equipment, misalignment, etc.

5.Resonance, critical speeds, reed critical etc

To solve a vibration problem one must differentiate between cause and effect 

Is the vibratory force the cause of the high levels of vibration or is there a resonance that amplifies the vibratory response.May be the support structure is just not good enough to minimize the displacement.

SOURCES OF VIBRATION

There are many different forces and interactions as a result of the power source and the interactions between the stator and rotor.

fig1

These can be classified as (frequency vibration oversize, symmetry, load, and unbalance)

Can be remembered as “FOULS” in a pump/motor

“F” stands for frequency of vibration like twice line frequency vibration,

“O” stands for oversize coupling and parts

“U” stands for unbalance like motor, thermal, driven machine etc

“L” stands for load related vibration

“S” stands for Symmetry of rotor/stator

 

“F” category

1.Twice Line Frequency Vibration:

2.One Times Line Frequency Vibration:

3.Rotor Bar Passing Frequency vibration:

4.Mode Shapes and Natural Frequencies of core Vibration 

 

“O” category

5.Oversize Coupling and parts: 

“U” category

6.Motor Unbalance.

7.Thermal Unbalance

8.Coupling Unbalance:

9.Driven Machine Unbalance: 

“L” category

10.Load Related Magnetic Force Frequencies and Mode Shapes 

“S” category

11.Elliptical stator due to Fundamental Flux:

12.Non Symmetrical Air-gap:

13.Eccentric Rotor:

14.Broken Rotor Bar:

15.Maintaining Balance in the Field:

16.Forcing Frequency Response Vibration

 

A brief explanation is as follows

1.Twice Line Frequency Vibration:

The power source of a pump is a sinusoidal voltage that varies from positive to negative peak voltage in each cycle. Many different problems either electrical or mechanical in nature can cause vibration at the same or similar frequencies

A power supply produces an electromagnetic attracting force between the stator and rotor which is at a maximum when the magnetizing current flowing in the stator is at a maximum either positive or negative at that instant of time.As a result there will be 2 peak electromagnetic forces (causes vibration here) during each cycle of the voltage or current wave reducing to zero at the point in time when the current and fundamental flux wave pass through zero

fig2

This will result in a frequency of vibration equal to 2 times the frequency of the power source (twice line frequency vibration). This particular vibration is extremely sensitive to the motor’s foot flatness, frame and base stiffness and how consistent the air gap is between the stator and rotor, around the stator bore. It is also influenced by the eccentricity of the rotor.

2.One Times Line Frequency Vibration:

Although not nearly as prevalent as twice line frequency vibration, one times line frequency vibration can exist. Unbalanced magnetic pull may result in vibration at line frequency (one times line frequency) as well as the usual twice line frequency vibration. If the rotor or stator moves from side to side, the point of minimum air gap may move from one side of the motor to the other. When the frequency of this motion corresponds to the frequency of the travelling flux wave, the unbalanced magnetic pull will shift from side to side with the point of minimum gap, resulting in vibration at line frequency. This line frequency vibration is normally very small or non-existent, but if the stator or rotor system has a resonance at, or near, line frequency, the vibration may be large.

3.Rotor Bar Passing Frequency Vibration:

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 which are applied to the stator teeth create vibration of the stator core and teeth. This source of vibration is at a frequency which is much greater than frequencies normally measured during normal vibration tests. Due to the extremely high frequencies, even very low displacements can cause high velocities if the frequency range under test is opened up to include these frequencies. Though these levels and frequencies can be picked up on the motor frame and bearing housings, significant levels of vibration at these higher frequencies will not be seen between shaft and bearing housing where they could be damaging.

fig3

4.Mode Shapes and Natural Frequencies of Core Vibration:

Under the applied magnetic forces (due to phase current) the stator core is set into vibration in the same manner that a ring of steel would respond if struck. Depending upon the modal pattern and frequencies of the exciting force, as described above, the stator would vibrate in one or more of its flexural modes m of vibration.

fig8

5.Oversize Coupling:

One consideration in coupling selection is coupling size. The coupling should be large enough to handle the application, including the required service factor, but should not be exceptionally large. Potential results of oversize couplings are:

*Increased motor vibration due to increased coupling unbalance and/or a change in the critical speed or rotor response due to increased weight. This is particularly true for flexible shaft machines.

*A greatly oversize coupling can result in greatly severe shaft bending, excessive vibration, and, heavy rubbing of seals, ultimately resulting in catastrophic shaft failure.

6.Motor Unbalance:

Balancing is required on all types of rotating machinery, including motors, to obtain a smooth running machine. This is performed in the factory in a balance machine at a level of precision determined by the motor speed, size, and vibration requirements. Fundamental requirements for precision balance on any machine are:

*Parts must be precision manufactured for close concentric and minimal unbalance individually.

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

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

7.Thermal Unbalance:

Thermal unbalance is a special form of unbalance. It is caused by uneven rotor heating or uneven bending due to rotor heating. The proper solution is to determine the reason for uneven heating affecting shaft straightness, and fix the rotor.All rotors will have some change in vibration in transitioning from a cold state to a hot one.However, if the application is one of continuous duty, and, vibration levels are not excessive during start-up (i.e. motor cold), it is permissible to allow more change cold to hot without any damage to the motor. In these situations if the lowest vibration levels are desired at operating conditions, a hot trim balancing procedure can be performed. To perform this procedure, run the motor until all conditions thermally stabilize, and quickly perform a trim balance. If necessary,run the motor again after the initial trial weights have been installed and let the motor thermally stabilize before taking additional vibration measurements for final weight correction

8.Coupling Unbalance:

Use of a proper key and a balanced coupling leaves the machine alignment and mounting and the driven equipment balance as the remaining major factor in system vibration.

9.Driven Machine Unbalance:

Under normal circumstances, the unbalance of the driven machine should not significantly affect the motor vibration. However, if the unbalance is severe, or if a rigid coupling is being used, then the unbalance of the driven machine may be transmitted to the motor

10.Load Related Magnetic Force Frequencies and Mode Shapes 

The frequencies of the load related magnetic forces applied to the stator teeth and core equal the passing frequency of the rotor bars. A magnetic force is generated at the passing frequency of the rotor slot (FQR), which is motor speed in revolution per second times the number of rotor slots The forces applied to the stator teeth are not evenly distributed to every tooth at any instant in time; they are applied with different magnitudes at different teeth, depending upon the relative rotor-and stator-tooth location. This results in force waves over the stator circumference. The mode shape of these magnetic force waves is a result of the difference between the number of rotor and stator slots

fig8

11.Elliptical stator due to Fundamental Flux:

For 2-pole motors the Electro-mechanical force will attempt to deflect the stator into an elliptical shape. The primary resistance to movement is the strength of the core back iron and the stiffness of the housing around the stator core, which is restraining the core’s movement.

fig3

12.Non Symmetrical Air-gap:

Twice line frequency vibration levels can significantly increase when the air gap is not symmetrical between the stator and rotor.

fig4

13.Eccentric Rotor:

An eccentric rotor, which means the rotor core OD is not concentric with the bearing journals, creates a point of minimum air gap which rotates with the rotor at one times rotational frequency. Associated with this there will be a net balanced 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.This net unbalance force will rotate at rotational frequency, with the minimum air gap, causing vibration at one time rotational frequency.The flux causing the magnetic force is the fundamental flux wave, which rotates around the stator at the synchronous speed of the motor. The rotor attempts to keep up with the rotating flux wave of the stator, but the rotor slips behind the stator field as needed to create the necessary torque for the load. When the high point of the rotor (point of minimum air gap) aligns with the high point (maximum) of the stator flux, the force will be a maximum, and then it will decrease, becoming small under a point of minimum flux. Thus, an unbalance force is created which rotates at rotational speed and changes in magnitude with slip. The end result is a one times rotational speed vibration, which modulates in amplitude with slip. This condition occurs at no load or full load.

fig5

14.Broken Rotor Bar:

If a broken rotor bar or open braze joint exists, no current will flow in the rotor bar.As a result the field in the rotor around that particular bar will not exist. Therefore the force applied to that side of the rotor would be different from that on the other side of the rotor again creating an unbalanced magnetic force that rotates at one times rotational speed and modulates at a frequency equal to slip frequency times the number of poles. If one of the rotor bars has a different resistivity a similar phenomenon (as in the case of a broken rotor bar) can exist.Broken rotor bars or a variation in bar resistivity will cause a variation in heating around the rotor.This in turn can bow the rotor, creating an eccentric rotor, causing basic rotor unbalance and a greater unbalanced magnetic pull.

fig6

15.Maintaining Balance in the Field:

When a finely balanced high speed motor is installed in the field, its balance must be maintained when the motor is mated to the remainder of the system. In addition to using a balanced coupling, the proper key must be used.One way to achieve a proper key is to have the shaft key way completely filled, with a full key through the hub of the coupling and the entire key outside the coupling crowned to match the shaft diameter. A second approach is to use a rectangular key of just the right length so that the part extending beyond the coupling hub toward the motor just replaced the unbalance of the extended open key-way. This length can be calculated if the coupling hub length and key-way dimensions are known.An improper key can result in a significant system unbalance, which can cause the vibration to be above acceptable limits

16.Forcing Frequency Response Vibration

*Weak Motor Base:

If the motor is sitting on a fabricated steel base, such as a slide base, then the possibility exists that the vibration which is measured at the motor is greatly influenced by a base which itself is vibrating. Essentially, this requires that support vibration near the motor feet to be less than 30% of the vibration measured at the motor bearing. To test for a weak base, measure and plot horizontal vibration at ground level, at bottom, middle, and top of the base, and at the motor bearing. If the motor is on a rigid base, the vibration at the bearing will closer to .25 mils but if support is not rigid it will be showing 2.50 mils as shown below

fig16

A weak motor base usually results in high 1x vibration, usually in the horizontal direction as shown above. However, it may also result in high 2X (twice rotational frequency) or 2f (twice line frequency) vibration, which also is a common vibration frequency in motors. The support posts must be tied together and heavily stiffened with the intention to meet the criteria for a “massive foundation.”Even where resonance of the base is not a factor, heavy stiffening of a light support structure can greatly reduce vibration.

*Reed Critical Base Issues:

A vertical motor’s reed critical frequency is a function of its mass, distribution of mass, and base geometry. The reed critical should not be confused with the motor rotor’s lateral critical speed. If the motor’s operating speed (or any other frequency at which a forcing function is present) coincides with the reed critical, great amplification in the vibration amplitude will occur Machine weight, center of gravity location, and static deflection. Bases found in typical installations are not as stiff, and correspondingly, the reed critical frequency will be lowered. If the reed critical drops into a frequency at which there is a forcing function present (most commonly the operational speed), the reed critical frequency will have to be changed. Usually, this is not difficult to do, and is most commonly accomplished by either changing the stiffness of the base, or by changing the weight of the base/motor. Where the reed critical drops below the operational speed to about 40% to 50% of running speed, this can result in sub-harmonic vibration at the system resonant speed in motors with sleeve guide bearings.

*Resonant Base:

If the motor’s operating speed (or any other frequency at which a forcing function is present) coincides with the base resonant frequency, great amplification in the vibration amplitude will occur. The only solution to this problem is to change the resonant frequency of the base. Usually, this is not difficult to do, and is most commonly accomplished by either changing the stiffness of the base, or by changing the weight of the base/motor.

*Bearing Related Vibration:

Sleeve bearing machines may occasionally experience “Oil Whirl” vibration, which occurs at a frequency of approximately 45% of running speed. This may be quite large, particularly if there is a critical speed at or just below 45% of running speed, which is referred to as an “oil whip” condition. Other than basic bearing design considerations which will not be dealt with here, a common cause is high oil viscosity due to low oil temperature in flood lubricated motors operating in cold ambient conditions.Similar sub-harmonic vibration, but low in amplitude, may occur in ring lubricated bearings, probably due to marginal lubrication. Other causes of vibration are journal out of roundness or bearing misalignment.

IDENTIFICATION OF CAUSE OF VIBRATION PROBLEM

Can be done by

*Vibration Data Gathering/Analysis:

Today, the most common units are displacement for shaft vibration measurement, and velocity for housing vibration measurement. Vibration can be measured in units of displacement (peak to peak, mils), units of velocity (zero to peak, inches per

Second), or units of acceleration (zero to peak, g’s).Acceleration emphasizes high frequencies, displacement emphasizes low frequencies, and velocity gives equal emphasis to all frequencies.

*Direction of Measurement: 

Measurements should be made in three planes (vertical, horizontal, and axial) on both bearing housings.

fig15

If the problem originates in the rotor (unbalance or oil whirl for instance), then shaft vibration data is preferable.If the problem originates in the housings or motor frame (twice line frequency vibration for instance), then housing vibration data is preferable. Housing vibration is generally obtained with magnetically mounted accelerometers. Shaft vibration can be obtained one of two ways: shaft stick or proximity probe. There is an important distinction between the two methods of obtaining shaft vibration data: the proximity probe will give vibration information of the shaft relative to the housings, whereas measurements obtained with a shaft stick yield vibration information with an absolute (i.e. inertial) reference. Housing vibration data is always obtained in terms of an absolute reference. If the motor has proximity probes then they should be used. If it does not, then proximity probes may be carefully set up with magnetic mounts. In this case it is important to have the tip of the proximity probe on a ground, uninterrupted surface. Even with this precaution taken, the electrical run out will be higher than in a motor specifically manufactured for use with proximity probes.

*Snap shot versus modulation graph

A snapshot refers to obtaining spectral vibration data at an instant in time. Details of amplitude vs. frequency are readily available in this format. A modulation refers to collecting vibration data for a period of time (typically ten or fifteen minutes), so that the variation in vibration as a function of time can be analyzed.

TROUBLE SHOOTING

Maintenance Items

-Check for loose bolts – mounting or other loose parts

-Keep motor clear of dirt or debris

-Check for proper cooling and inlet temperatures or obstructions such as rags, lint or other enclosures

-Check Bearing and stator Temperatures

-Lubricate as recommended

-Check proper oil levels

Check vibration periodically and record using a check list as shown below

*Are all bolts tight? Has soft foot been eliminated?

*Is hot alignment good? If it’s not possible to verify hot alignment, has cold alignment been verified (with appropriate thermal compensation for cold to hot)?

*Is any part, box top cover, piping vibrating excessively (i.e. are any parts attached to motor in resonance)?

*Is the foundation or frame the motor is mounted to vibrating more than 25% of motor vibration (i.e. is the motor base weak or resonant).

*Is there any looseness of any parts on motor or shaft?

*Integrity of fans and couplings – have any fan blades eroded/broken off, are any coupling bolts loose/missing, is coupling lubrication satisfactory? 

*Ideally, vibration measurements should be obtained with the motor operating under the following conditions:

*Loaded, Coupled, Full Voltage, All Conditions Stabilized

Normal operating conditions

* First measurement to be obtained.

*Represents state of machine in actual operation.

* May indicate which test should be taken next.

Unloaded, Coupled, Full Voltage:

* Removes load related vibration, while everything else remains the same.

*Not always possible to get to zero load, but some reduced load is usually possible.

Unloaded, Uncoupled, Full Voltage:

*Removes all effects of coupling and driven machine.

*Isolates motor/base system.

Unloaded, Uncoupled, Reduced Voltage (25% if possible):

*Effect of magnetic pullover forces minimized (most effective use is in comparison to vibration at full voltage,

*25% usually only possible at motor service shop or motor manufacturers facility. If motor is a Y-Δ

connected motor, then Y connection is effectively 57% voltage as compared to Δ connection at the same terminal voltage. A comparison of vibration under both connections will reveal voltage sensitivity of motor.

Unloaded, Uncoupled, Coast Down:

*Will make any resonance/critical speed problem apparent for entire motor/base/driven equipment system.

CONCLUSION

As motor ages, the vibration levels may slowly increase.There may be a multitude of reasons of why the levels may increase over time:

Degradation of the bearings (sleeve bearings) loosening of rotor bars

Accumulation of debris in the oil guards, between rotor and stator, etc.

Changes in mounting conditions: deterioration of grouted base, changes in alignment/soft foot, etc

Loosening of things mounted to the motor

The factor limiting the vibration limits at these levels is the motor bearings. Generally, sleeve bearings (as compared to anti friction bearing motors) are more restrictive in terms of vibration limits. Sleeve bearing motors can operate continually at one-half their diametrical bearing clearance, without any damage.They can operate at slightly higher levels for short periods of time as well, but these higher limits must be established with the motor manufacturers.If the motor is sitting on a weak base, higher housing vibration limits and shaft vibration limits (if measured by shaft stick and not by a proximity probe) can be tolerated.Vibration problems can vary from a mere nuisance to an indication of imminent motor failure. With solid knowledge of motor fundamentals and vibration analysis, it is possible to identify the root cause of the problem, and more significantly correct.

 

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