JM02011 analysis of AC motors

Vibration Monitoring and Current Analysis of AC Motors Using Motor Current and Vibration Analysis to Detect AC Motor Problems Summary This article contains an extensive summary, and

Vibration Monitoring and

Current Analysis of AC Motors

Using Motor Current and Vibration Analysis to

Detect AC Motor Problems

Summary

This article contains an extensive summary, and several examples, of analyzing AC motors using vibrational data and motor current analysis The first example references a large pulper motor in a paper mill operation, in which a shortened stator was causing the motor to fail The second example is in reference to an electric motor manufacturer in Israel that uses motor current analysis and vibration analysis to test electric motors in field applications The third example considers the monitoring of electric motors used in pumping stations The fourth example deals with a company using motor current analysis as a quality control, or testing process to establish the health of the motors being manufactured In the fifth case, a few more examples of pole pass frequencies are shown Finally, a case of motor excentricity is provided

Vibration Monitoring and Current Analysis of AC Motors

Introduction

Apart from vibration analysis, motor current

signature analysis (MCSA) is a powerful

monitoring tool for electric-motor-driven

equipment It provides a non-intrusive means

for detecting the presence of mechanical and

electrical abnormalities in motor-end driven

equipment, including altered conditions in the

process that may be downstream of the motor

driven equipment MCSA is based upon the

recognition that a conventional electric motor

powering a machine also inevitably acts as a

transducer of variations in the driven

mechanical load, as the latter are converted

into electric current variations that are

transmitted along motor power cables These

current variations, though very small in

relation to the average current drawn by the

motor, can be extracted reliably and

non-intrusively at a location remote, and processed

to provide indicators of condition (signatures)

These signatures may be trended over time to

give early warning of performance

degradation or process alteration

Although MCSA technology was developed

for the specific task of determining the effects

of aging and service wear on motor-operated

values used in nuclear power plant safety

systems, it is recognized as applicable to a

much broader range of machinery MCSA is

used to analyze pumps of various design,

blowers, compressors, and air-conditioning

systems powered by AC and DC motors

The use of devices, such as a data analyzer

(the instrument on the left in Figure 1), and

current clamp (the instrument on the right in

Figure 1) are common to collect and analyze

vibration and motor current data from an AC

motor These types of devices obtain data in a

non-invasive manner, which is important to

the operating process The use of a single

split-jaw current probe placed on one power

lead is sufficient to obtain data Since there is

no electrical connection being made or

minimal The resulting raw current signal is amplified, filtered, and further processed to provide a sensitive and selective means for extracting motor current noise information that reflects instantaneous load variation within the drive train and the ultimate load

Figure 1 SKF Microlog and AC/DC Current Clamp

This article provides an extensive summary of several case studies that relate to the use of motor current signature analysis and vibration analysis The reader is also referred to the

article entitled An Introduction into Motor

Current Spectrum Analysis (MCSA), JM02010

on the @ptitudeXchange website

Summary of AC Motor Monitoring

Mechanical Vibration

Using a standard accelerometer placed on the bearing cap detects several unique mechanical vibration signals generated by electrical faults

in the motor circuits One of the more common faults produced is a signal at twice line frequency (2FL or 2 F.L.) If the line frequency is 60 Hz, this signal is at 120 Hz or

7200 CPM If the line frequency is 50 Hz, the signal is at 100 Hz or 6000 CPM Care must

Vibration Monitoring and Current Analysis of AC Motors RPM or 3000 RPM) to ensure the signal is not

twice rotation speed of the shaft, rather than

twice line frequency

This two times line frequency signal is created

by any of the following faults:

1 Uneven air gap between rotor / stator

As the motor poles pass the narrow gap, the

magnetic pull is greater v 180 degrees on the

opposite side where the gap is the widest The

number of poles (motor speed) does not

change the results: an uneven air gap results in

a signal at twice line frequency

The cause of this uneven air gap is often due

to an uneven base plate under the machine’s

foundation, and is referred to as a soft foot

Some empirical data seems to indicate that the

twice line frequency signal appears when the

gap clearance exceeds 10% variance

Loosening and tightening one bolt at a time on

the foundation, with the motor running, while

observing the spectrum on the data analyzer,

can confirm soft foot When the soft foot is

loosened, the signal decreases, and then

increases as the nut is tightened If soft foot

occurs the foot should be shimmed during the

next shutdown to the same plane as the others

2 Damage stator windings or insulation

There are numerous causes of damage to the

stator: poor manufacturing, poor environment,

flaws in the insulation, etc Any damage to the

stator again creates an uneven magnetic field

around the rotor This uneven field generates

an uneven pull on the rotor, regardless of

motor speed, and causes a mechanical

vibration at twice line frequency

It is often possible to locate the area of

damage with either an infrared or thermal

detector (refer to JM02008 - Thermography

on the @ptitudeXchange website) Usually

there is an area on the motor housing where

the surface temperature is elevated 20 to 30 degrees

A damaged stator also generates a mechanical vibration signal at a frequency equal to the number of rotor bars, multiplied by rotation speed Again, in the area of stator damage, the magnetic field is weakened, and stronger 180 degrees away As each rotor bar passes this area of higher strength, the bar is

mechanically pulled in that direction

Typically, rotor bars have between 45-55 bars

in the rotor, but this can vary depending on the manufacturer For this reason, it is very

important to set the Fmax at least 100 times rotation speed when troubleshooting motor vibration Please note that this Fmax value is for troubleshooting purposes only

Since the number of rotor bars can vary greatly, it is most important to establish a procedure that states that at anytime a motor is down for repair, a count of the actual number

of rotor bars should be made and recorded It

is also important to record the bearing nomenclature so that the bearing frequencies can be accurately determined when analyzing for bearing degradation

The user can verify that the vibration is electrically induced by shutting off the motor while observing the frequency spectrum in the analyzer mode The moment the power is removed, the distorted magnetic field is instantly collapsed and the twice line signal disappears If the signal does not disappear, but rather slowly degrades, then the user knows there is some type of mechanical problem When setting up an analyzer, use

100 lines, 0 averages, and an Fmax of 2000 Hz

to provide a fast cycle time

If the data analyzer has enveloping circuits, and these two conditions are met, then the signal is also seen in any enveloped acceleration spectrums and will most certainly

Vibration Monitoring and Current Analysis of AC Motors generate harmonics of the fundamental

frequency

There is not an agreed upon amplitude of

concern if the twice line frequency signal is

present It is generally agreed that the

presence of 2x line frequency is not desirable

Generally accepted limits are between 0.04 -

0.06 IPS in the velocity spectrum at twice line

frequency As an example of the amplitude

levels of 2FL, a trend of an enveloped

acceleration reading, in which twice line

frequency was taken over a six months period,

shows an increase from 0.4 gE to 1.6 gE

When the motor reached a vibration level of

1.6 gE, the motor failed However, after the

motor was repaired, the trend was

reestablished, and the amplitude of that trend

consistently remained at 0.8 gE It is suspected

that the first failure was due to a damaged

stator In addition to the stator problem, soft

foot concerning the foundation was also

present After repairing the motor, the soft

foot contribution is still present, although it

exhibits itself differently This is most likely

due to torque on the mounting bolts in

addition to machine placement and

configuration

Sidebands

As with most vibration signals, the presence

of sidebands around fundamental frequencies

is a measure of an increase in severity As the

sidebands increase in number and amplitude,

so does the severity of the problem Some of

the sideband energy is pole pass frequency

and slip

Pole Pass Frequency =(number of poles)(slip)

Slip = (nominal speed - actual speed)

Around the rotor bar pass frequency it is

possible to see sidebands of twice line

frequency In troubleshooting, the data analyst

may find it necessary to increase resolution to either 1600 or 3200 lines to separate these sidebands and verify the existence of this energy

Analysis of AC Motor Current

The effectiveness of evaluating motor condition by performing an FFT of the motor current is verified many times over in the analysis of motors And, although it is often referred to as a method of detecting broken rotor bars, it is actually detecting abnormally high resistance in the rotor circuit (bad solder joints, loose connections, and damaged rotor bars)

At this point, a review of basic spectrum components is necessary to ensure a clear understanding of vibration analysis If there is

a fault in the rotor circuit, then the spectrum has two prominent features when displayed Using a logarithmic scale for clarity

concerning amplitude peaks with respect to the Y-axis, the display at 60 Hz or line frequency contains a large spike To the left,

at a distance equal to the rotor slip, times the number of poles, pole pass frequency is another spike of energy These spikes can be labeled A for line frequency, and B for pole pass frequency Note that the amplitudes of peaks A and B have to be obtained using cursor overlays, as it is necessary to use amplitudes to four decimal places, and most data analyzers only read the value to three places To determine the health of the machine, perform the following calculation:

Log (A/B) times (20) = amplitude (dB)

Examples are included in Figures 2-4

Vibration Monitoring and Current Analysis of AC Motors

6 30-36 32-63 Multiple Rotor Bars Broken, Slip Ring and Joint Problems Overhaul

7 <30 <32 Severe Problems Throughout Overhaul / Replace

Table 1 Motor Current Analysis Severity and Recommended Corrective Action Chart

Vibration Monitoring and Current Analysis of AC Motors

Figure 2 This is the spectrum from a damaged compressor motor that had 5 broken rotor bars, and a damaged end ring Log 0.018572/1.0571 X 20 = 35.1 dB Refer to Table 1: category 6 severity

Figure 3 Motor in lab with four cut rotor bars and a broken end ring Log 0.0908/8.777 X 20 = 39.7 dB Refer to

Table 1: dB severity level

Figure 4 Motor in lab with no damage Log 0.003079/1.704 X 20 = 54.9 dB Refer to Table 1: dB severity level

Vibration Monitoring and Current Analysis of AC Motors Note that the chart applies to rotor circuit

damage, and that the motor must be under at

least 75% load The amplitude of the pole pass

frequency is not linear with respect to reduced

loads If the amplitudes that correspond to

reduced loads are used, the results will not be

accurate The examples in the following

section help illustration good and bad motor

circuit spectra

Observations of Other Motor Problems

High efficiency motors obtain their efficiency

and use less electricity by employing two

methods: use of a smaller air gap, and a layer

of thinner insulation on the windings If the

owner installs these motors on the same

transformer circuit as a DC motor, it is

possible for the DC motor’s Silicon Circuit

Rectifiers (SCR) to provide feedback onto the

AC circuit and induce high voltage spikes into

the motors The reduced insulation rapidly

deteriorates and leads to a reduced motor life

Field results have shown as much as a 50%

reduction in the life of the motor due to this

type of problem DC motor problems can be

seen at the SCR firing frequency

SCR Frequency = (6)(Line Frequency)

Check connections, SCRs, control cards, and

fuses if you experience this frequency

Enveloped AC Motor Current

The principal for an enveloped AC motor current operation is as follows When the motor current with a damaged rotor circuit is enveloped, the resulting spectrum shows energy at the actual pole pass frequency (e.g

at 0.8 Hz), not as a sideband of the 60 Hz signal or 59.2 Hz Initial investigation shows a

relationship between the pole pass frequency

amplitude as a ratio to the overall amplitude

of an FFT spectrum taken with an Fmax of 25

Hz Typically, in a good motor this is a very low amplitude signal and is not seen in an enveloped spectrum So, the frequency must

be calculated to locate it

Initial data shows that a good motor has a ratio

of 5% or less but as damage increases the percentage also increases Additionally, harmonics of pole pass frequency are an indication of damage Initial testing shows that this method is sensitive to the condition of the motor and will detects very early stages of degradation in the rotor circuit

Figure 5 shows an example of an enveloped current spectrum with no sidebands around the 2x line frequency Zooming into the 25 Hz area, Figure 6 shows clearly the pole pass frequency The ratio between this pole pass frequency peak and the overall amplitude is 63%, indicating a possible problem

Figure 5 Twice line frequency (119.792 Hz) with harmonics (239.583 Hz, #59.375 Hz,…) using Enveloped Gs (gE) Running speed is 4792.4 RPM

Vibration Monitoring and Current Analysis of AC Motors

Figure 6 Enveloped AC motor current, pole pass frequency of 0.8125 Hz generated by 5 broken rotor bars and a damaged end ring Ratio of pole pass frequency amplitude (0.81) to overall amplitude (1.29) is 63%, indicating possible damage to the motor

Case 1: Pulper Motor with

Shorted Stator

Background

This case study was developed from the

analysis of a large pulper motor in a paper mill

in the southern United States It was common

practice for the paper mill to replace the

pulper motor routinely during the plant

shutdown over the winter holiday This

shutdown occurred during the end of

December, and into the beginning of January

The paper manufacturing facility uses

predictive maintenance technologies, such as

vibration analysis and motor current analysis,

to help assess machinery health In this case,

after the winter holiday, the newly installed

pulper motor began operation Based upon

data obtained shortly after the refurbished

motor was replaced while the mill was

operating at full capacity, the pulper motor

began to exhibit an abnormally high level of

vibration In an instance when machinery is

not monitored using predictive maintenance

technology, a refurbished motor would

probably not be considered as a “bad actor” or

a poorly performing machine in the plant The

results of this case study point directly to the

refurbished motor as the problem The

following measurements are related to a

specific instance in which MCSA and

vibration data were used to determine a problem affecting a pulper motor in the plant Vibration data was obtained on the motor in two specific areas These areas exhibited the greatest response to the condition of the motor The two areas, A and B, are diagramed

in the illustration below

Figure 7 An illustration of the electric motor on which analysis data was collected The data was collected in areas A an B disignated in the illustration

Trending

One key elements in assessing mechanical and electrical health of a machine is to acquire data related to the machine on a regular basis

or interval Once several data intervals have been collected a trend can be established In Figure 8, the trend is shown The overall trend

of the vibration data is increasing This increasing level indicates that a possible motor problem

Vibration Monitoring and Current Analysis of AC Motors

Figure 8 This is the trend data of the large pulper motor over a six-month period The data is taken in Velocity (IPS) and is plotted versus Time An increase from ~0.05IPS to < 0.3 IPS can be seen in the trend This increase shows a higher level of vibration of the pulper motor, which usually indicates a problem with the system.

Specific Measurements

After several months of data collection, a

variation in the trend was apparent In Figure

9, a spectral peak at 7200 RPM or 120 Hz can

be seen This peak is twice the line frequency

of the motor, or twice the frequency at which

the power supply is operating (60 Hz in the

It is important to point out that electrical or magnetic defects quickly disappear once the power supply is disconnected Additionally, most electrical problems only occur when the motor is loaded This can help differentiate mechanical problems from electrical problems

in spectrum analysis

Vibration Monitoring and Current Analysis of AC Motors

Figure 9 Individual Vibration Spectrum of the failing pulper motor The failure was due to a shortened stator indicated by a peak at 7200 RPM Running speed of the motor happens to be 700 RPM - indicated by the vertical line on the left-hand side of the spectrum.

2x Line Frequency

One of the more common problems exhibited

at 2x line frequency occurs from a shortened

stator inside the motor In Figure 9, the 2x line

frequency displayed is produced by such a

problem This shortened stator creates an

uneven air gap between the motor and the

stator When a motor rotates at 3600 RPM, the

magnetic pull towards the closest pin rises and

falls from 0 to maximum twice during the

displacement of the rotor with respect to the

stator Since this occurs twice per rotation it

produces 2x line frequency, or:

2F L = 2 x 60Hz = 120Hz (U.S.A.)

2F L = 2 x 3600CPM = 7200CPM

If an uneven air gap is left undetected or

un-repaired, the motor will fail

Other typical problems that exhibit 2x line

frequency are eccentric rotors, loose or open

rotors, arching between loose rotor bars and end rings, and phase problems due to loose or broken connectors All of these problems will exhibit various levels of 2x line frequency As stated before, any peak at 2x line frequency is suggested as being a poor condition for the motor

Conclusion

Based upon the increase in the trend in Figure

8, and the specific measurement from that increase, the reconditioned motor was disassembled When the motor was examined,

it was determined that the stator in the motor was damaged The motor was replaced and operations continued as schedule with very little interruption to the process Figure 10 shows measurements taken after the stator problem was resolved Operating levels of the motor returned to normal

Vibration Monitoring and Current Analysis of AC Motors

Figure 10 Overall trend of the pulper motor after the shortened stator problem was resolved The overall values returned to normal operating levels Indicated by the marker towards the right-hand side of the trend.

Case 2: Broken Rotor Bar

Background

This case study relates to the testing of electric

motors in air fans The testing is conducted on

50 Hz motors containing four (4) poles and

running at a speed of 1483 RPM – 1490 RPM

(average 1487 RPM) In addition to vibration

data, Motor Current Signature Analysis

(MCSA) is also used as an additional test to

validate the motors and aid in the confirmation

of vibration data

Specific Measurements

An example of test results from a good motor

and a bad motor are contained in Figures 11

and 12 An evaluation of peaks to either side

of the center frequency, helps determine the

state of the motor When considering sideband

data, the following calculations are used

2

2

F xL Sync

Slip x

sidebands

F xL Sync

Sact Sync x

Sact = Actual Rotation

With 4 poles, we can calculate the following:

Sync = 60(50 Hz)/(4/2) = 1500 Hz Sact = 1487 RPM (mean RPM Figures 6 & 7) Slip = 1500 RPM – 1487 RPM = 13 RPM

Sync

Slip x sidebands =

Hz x RPM

RPM x

1500

132