Troubleshooter’s Guide to Vibration (September 01, 2007)

The application of vibration analysis should be part of any proactive total equipment management program. A regularly scheduled vibration analysis program can be carried out by in-house vibration technicians or by local contractors who are familiar with the plant’s equipment and its processes. Various instruments can be used to determine vibration frequencies and pinpoint machinery problems.

There are several common vibration frequencies, and we deal with them in this Troubleshooter’s Guide to Vibration. (Part 1 appeared in MRO’s Feb. 2007 issue, Part 2 in April and Part 3 in June. Archives of these articles can be found at www.mromagazine.com.)

Frequencies discussed in this four-part series include rotor frequencies, rolling element bearing fault frequencies, sleeve bearing fault frequencies, gear drive problems, belt drives, fan blades and pump impellers, chain pass, and resonant frequencies. Part 4 is the concluding section and focuses on frequencies related to electrical equipment.

9. Electrical Equipment Frequencies: Electrical fault frequencies are caused by mechanical problems in electrical components, but these frequencies and their causes deserve some further explanation. Since motors use electromagnetic forces, in addition to mechanical forces, they exhibit some characteristics that differ from purely mechanical rotating machinery. For example, a motor shaft may bow or bend due to excessive localized heating from shorted laminations. The spectrum of this condition will appear as unbalance, but balancing the shaft and attached components will not solve the problem.

Also, unlike a pure mechanical machine, which for vibrational studies is a set of springs and masses, a motor also has electromagnetic ‘springs,’ which are functions of electrical loads, air gap variations, electromagnetic force unbalance, etc. As a result, rotating electrical machines produce a multitude of frequencies that are generated by the electromagnetic forces inherent in the machine.

The magnetic flux produced by current-carrying conductors in AC machines alternates at line frequency (FL). Therefore all AC motors produce a 2 X line frequency vibration. In power systems that operate at 60 Hz, the vibration frequency is 120 Hz.

In an induction motor, the poles in the stator winding produce a rotating magnetic force that acts across the air gap between stator and rotor. This produces a 120 Hz vibration of the stator and the mode shape of this vibration is determined by the number of poles in the winding. The number of poles and the line frequency also determine the synchronous speed of the rotating magnetic field and the running speed of the motor.

Therefore, speed in rpm = 7,200 / # of poles, so:

– a 2-pole winding = 3,600 rpm motor

– a 4-pole winding = 1,800 rpm motor

– a 6-pole winding = 1,200 rpm motor

– an 8-pole winding = 900 rpm motor

– a 14-pole winding = 514 rpm motor.

The rotors of induction motors rotate at less than the synchronous speed of the rotating magnetic field. This difference in speed is referred to as slip. For example, if a 4-pole motor is operating at 1,770 rpm on a 60 Hz system, the slip is 1,800-1,770 or 30 rpm. As a result, the pole pass frequency can be calculated as follows:

Slip rpm X number of poles

= pole pass frequency

The presence of this frequency on a vibration spectrum indicates a potential problem. The pole pass frequency will appear as sidebands in the spectrum. The number of rotor bars X actual rotor rpm and harmonics of the rotor bar passing frequency may indicate a problem.

An AC motor may have unequal magnetic forces causing unbalance, which in turn can be caused by variations in the current in the stator or rotor, or air gap variations between the rotor and stator, or a combination of these conditions. These variations in AC current may be caused by loose iron (weak or loose stator support), shorted or loose stator laminations, shorted or open windings, electrical unbalance between two consecutive conductor coils or a coil on the opposite side, or unbalanced resistance between any of the three current phases. These variations will affect the vibration spectra whether or not the motor is loaded. Any or all of these defects will appear on the spectrum analyzer as a high amplitude peak at 2 X line frequency with the absence of side bands around the 7,200 cycles per minute (cpm) frequency.

Unacceptable vibration amplitudes

For motors in the 50 to 1,000 hp range, the following vibration amplitudes of inches per second peak (ips) may be considered unacceptable.

* In service motors — amplitudes over 0.100 ips

* New or rebuilt motors — 0.050 ips

* Motors used for precision tools — 0.025 ips.

Broken or loose wires or connectors between the substation and main supply, between the supply and the stator, or between any of the three phase leads may cause a loose connection. These conditions may be seen as sidebands at 1/3 X line frequency on each side of the 7,200 cpm peak.

Electromagnetic force unbalance, due to variations in rotor current, is commonly caused by broken or cracked rotor bars, broken, cracked or poorly brazed end- ring joints, high-resistance end-ring joints or shorted or loose rotor laminations. These conditions will most often occur on the spectrum analyzer when the motor has at least a 60% load. (Keep in mind that this frequency may occur at 1 X rpm, which may be mistaken for unbalance).

When pole passing frequency side bands are present at three or four X running speed harmonics and have amplitudes higher than 0.0125 ips to 0.0150 ips peak, there is a problem with the motor.

These problems may cause non-uniform heating, which may cause the rotor shaft to overheat and bend. This condition will cause the rotor to become thermally bowed and as the shaft bows, the electromagnetic unbalance increases, generating even more heat.

When this condition is suspected, confirm the motor’s phase with a stroboscope. The phase will be continually changing. Any rotor flexing will cause temperature increases which will show up as phase changes that can be seen with the stroboscope. (This is unlike misalignment, because vibration amplitude at 1 X rpm and phase will stabilize at a particular rpm if there is a misaligned condition). The condition may increase to a point where the bowed rotor will contact the stator causing catastrophic failure.

Air gap between rotor and stator affects the amount of induced rotor current. As a general rule, the air gap should be less than 5% of the total radial air gap between the stator and rotor and it can be measured with a feeler gauge.

Static eccentricity is a condition where the minimum air gap is fixed at one condition. This is commonly caused by such conditions as soft foot distortion, distorted stator core, worn sleeve bearings (where used), or non-concentric rolling element bearing housings in the motor end bells. This condition shows up as a high-amplitude peak at 2 X line frequency of 7,200 cpm, with harmonics but no side bands.

Dynamic eccentricity is a condition where the minimum air gap ‘moves around’ the stator bore as the rotor turns. The causes are:

* Eccentric rotor

* A rotor rotating eccentrically by a bent or thermally bowed shaft

* A rotor running at or near its critical speed

* An unbalanced overhung fan, which can induce rotor eccentricity

* Loose or worn bearings

* Misaligned condition

* Worn or misaligned coupling.

The vibration spectrum may show a high amplitude at 1 X rpm and may or may not have pole passing frequency side bands.

A problem may also be developing if the 2 X line frequency (7,200 cpm amplitude) exceeds the previous guidelines. These problems,
if present, may occur when the motor is operating at 60% to 70% of full load.

If the stator and rotor slot teeth are not equidistant, there will be a variation of reluctance, causing variations in magnetic forces, in turn causing motor torque variations. Torque pulses may excite loose or broken rotor bars or end rings, loose windings, laminations or supports in the stator. This will be indicated by 2 X line frequency (7,200 cpm) with harmonics and will appear as a mechanical looseness condition if multiples of the 2 X line frequency appear in the spectrum.

Loose stator coils in synchronous motors will generate fairly high vibration at coil pass frequency (cpf), which equals the number of coils X rpm (# coils X poles X # coils/pole). The cpf will be surrounded by 1 X rpm sidebands.

Synchronous motor problems may also be indicated by high amplitude peaks at about 60,000 to 90,000 cpm accompanied by 2 FL sidebands. Analysts should obtain at least one spectrum up to 90,000 cpm at each motor bearing housing.

Loose rotor bars

Loose or open rotor bars in AC induction motors are indicated by a 2 X line frequency (2 FL) sidebands surrounding rotor bar pass frequency and/or its harmonics (rotor bar pass frequency = number of bars X rpm). This often will cause high levels at 2 X RBPF (rotor bar frequency), with only a small amplitude at 1 X RBPF. Electrically induced arcing between loose rotor bars and end rings will often show high levels at 2 X RBPF (with 2 FL sidebands), but little or no increase in amplitudes at 1 X RBPF.

Electrical fault frequencies, including cracked or broken rotor bars, loose rotors, loose transformer laminations and eccentric stators or rotors are all non-synchronous (see Figures 15, 16, 17, and 18).

Other testing needed

It is highly recommended that in addition to vibration analysis, electrical equipment be subjected to periodic electrical testing such as motor current and circuit analysis and magnetic flux analysis. This additional electrical condition monitoring will prove highly beneficial in analyzing electrical equipment problems and may save much time and effort in making the correct diagnosis.

To conclude, vibration analysis, if utilized correctly and regularly and with proper interpretation, can provide a huge return on investment by providing early recognition of machine problems. Early recognition, in turn, will reduce repair costs and downtime, eliminate catastrophic failures, improve reliability and productivity, extend equipment life and will allow the plant to take advantage of sound planning and scheduling of necessary maintenance.

If used in conjunction with other predictive maintenance technologies, such as oil analysis, the return on investment (ROI) will be awesome!

Technical editor Lloyd (Tex) Leugner is the principal of Maintenance Technology International Inc. of Cochrane, Alta., a company that specializes in the resolution of maintenance problems and provides training for industry. He can be reached at 403-932-7620 or texleug@shaw.ca. Parts 1 through 4 of this article are archived on www.mromagazine.com for future reference.

Illustrations are courtesy of Maintenance Technology International Inc. of Cochrane, Alta., and Technical Associates of Charlotte, N.C., www.technicalassociates.net.

References

The Simplified Handbook of Vibration Analysis, Arthur R. Crawford.

Rotating Machinery Vibration, Maurice L. Adams, Jr.

Introduction To Machinery Analysis and Troubleshooting, 2nd Edition, John S. Mitchell.

Vibration Spectrum Analysis, 2nd Edition, Steve Goldman, P.E.