MOTOR CIRCUIT ANALYSIS MANUAL
ALL-TEST Pro, LLC
MCA ANALYSIS MANUAL
MCA ANALYSIS MANUAL
ALL-TEST Pro, LLC 166 Main St Old Saybrook, CT 06475 www.alltestpro.com Phone 860-395-2988 • Fax 860-399-3180Alpine Components
Telephone: 01424 437000 E-mail: info@alpine-components.co.uk
Table of Contents
INTRODUCTION ...1
MCA ...1
Use of this Manual...1
ALL-TEST Pro Instrument Philosophy ...2
ALL-TEST Pro Analysis Philosophy...2
Motor Diagnostic Theory ...3
Basic Electrical Theory...3
Applying MCA...5
Manual Testing...6
AUTO Testing ...6
Motor Testing Procedure...6
Initial Readings...7
Motor/Winding Analysis...7
Sample Test Results ...8
Condemning Criteria ...8
Data Analysis Tips...9
Condemning Priority...9
Condemning Tips ...10
Unassembled Motor Analysis...10
Data Interpretation Basic Rules and Tolerances ...12
Assembled Motor Analysis ...12
ANALYZING MEASUREMENTS AND CHANGE ...14
Troubleshooting Rules ...14
Shorted Windings: ...14
Winding Contamination and Rotor Position...14
Rotor Reposition Test:...14
Insulation Resistance: ...15
Insulation Resistance Readings ...15
Loose Connections...15
PREDICTIVE MAINTENANCE ...15
Predictive Maintenance Hints...17
Data Collection and Test Result Issues...17
Bad Results...17
Waiting ...18
Incorrect Readings ...18
Data Collection Procedure ...19
TROUBLESHOOTING MOTORS...21
Rotor Compensated Test ...21
Rotor Reposition Test...21
ROTOR TESTING...22
DATA COLLECTION...22
ANALYSIS...23
APPENDIX 1 Winding Alarm Scenarios ...24
Winding Alarm Scenarios ...24
Scenario 1: ALL readings are balanced...24
Scenario 2: Fi or I/F in Red all other measurements balanced ...24
Scenario 3: Fi or I/F in Red; L & Z in Alarm...25
Scenario 4: R > 5% Unbalance...26
Scenario 5: Fi or I/F balanced; L & Z in Alarm but similar pattern...26
Scenario 6: Fi or I/F balanced; L & Z in Alarm but non-similar pattern...27
Scenario 7: Fi & I/F balanced; L & Z in Alarm but non-similar pattern plus a frequency shift of the impedance measurement...28
Scenario 8: All measurements are balanced but insulation to ground measurement is low28 Scenario 9: Fi & I/F balanced; L & Z in Alarm, readings are Truncated...29
Scenario 10: Other readings...29
Scenario 11: Zero Values...30
APPENDIX 2: Test Methods for Determining the Impact of Motor
Condition on Motor Efficiency and Reliability...31
APPENDIX 3: Multiple Technology Approach ...40
APPENDIX 4: Single and Three Phase Transformer Testing; Using
Static Motor Circuit Analysis Techniques...50
APPENDIX 5: Synchronous Machine Testing With All-Test
Instrumentation...61
APPENDIX 6: Servo Motor Testing, Evaluating Servo Motors with
MCA...69
APPENDIX 7: Electrical Diagnostics for Generators ...70
APPENDIX 8: DC Current Motor Electrical Evaluation Using Motor
Circuit Analysis ...94
APPENDIX 9: Case Study - Phase Unbalance at Vermont Yankee
Nuclear ...102
APPENDIX 10: Case Study: Dynamometers 012002...104
APPENDIX 11: Utilizing the ALL-TEST PRO 31™ To Detect Pin-Hole
Shorts ...107
DISCLAIMER, COPYRIGHT & TRADEMARKS...110 Copyright: ...111Understanding Motor
Diagnostics using Motor
Circuit Analysis (MCA)
INTRODUCTION
MCA is a very simple and safe method to test electrical windings while the winding is de-energized. The basic premise of MCA:
In equipment with three phase windings all phases should be identical (same # turns, same wire size, coil diameter & etc.). Consequently, all characteristics of the windings should also be similar. If a change occurs in any one of these characteristics, the change is never for the better, (windings do not fix themselves) as degradation is taking place. By analyzing the amount and
relationships of the change it is possible to identify the cause of the degradation. Once the cause and the severity of the degradation is known it is now possible to determine the necessary action.
Use of this Manual
The purpose of this manual is to provide users of MCA technology tips and guidelines to follow when performing Motor Diagnostics using the ALL-TEST ProTM line of MCA instruments. This
manual is intended primarily for use with the ALL-TEST IV PRO 2000TM (ATIV). However,
some of the information is applicable to the ALL-TEST PRO 31TM (AT31). If the user doesn’t
have both an ALL-TEST PRO 31TM and ALL-TEST IV PRO 2000TM an ohm meter and
ALL-TEST Pro Instrument Philosophy
Knowing that the response of a motor to any type of measuring or test is dependent on not only the size and voltage of a motor, but in particular the design and construction, our philosophy is to make instruments that:
• are easy and safe to use • light weight and portable • shows fault(s) immediately • shows any change over time
• makes several different measurements for analysis
ALL-TEST Pro Analysis Philosophy
Testing and analysis of electrical motors, transformers or any winding is not “black magic art”. In reality it is very straightforward, if sufficient and proper measurements are performed.
It has been the experience at ALL-TEST ProTM that the most accurate and comprehensive method
of determining the true condition of motor windings is by controlling the testing parameters while the motor is de-energized. This method removes errors and conflicting results from stray or unknown sources used by other methods.
By injecting a known, low voltage AC sinusoidal, non-destructive signal through the motor windings, winding faults or weaknesses are not driven to failure. In many cases, potentially destructive faults can be easily corrected before total winding failure occurs.
Winding faults are indicated by variances in the response to the injected signal through the
windings. These variances cause unbalances in the measured response to the injected signal. Using MCA faults appear the same regardless the size or type of the winding. Motors as small as
automobile windshield wiper motors as well as 300 Megawatt Generator windings have been successfully tested.
Through additional investigation and measurement of these variances in the winding, faults can be quickly and easily identified, and corrected prior to energizing the winding which could lead to total destruction of the motor.
Examples:
A deteriorating Insulation to Ground situation, will normally require immediate attention as this fault can be very dangerous and lead to safety and immediate machine failure.
voltage motors, generally degrade over a longer period of time and provide the opportunity to correct the fault before it becomes a catastrophic failure requiring a complete rebuild or costly replacement.
MCA injects an AC signal through the windings, and measures the response of this signal to
identify any unbalances in the windings that indicate either a current or potential fault.
Motor Diagnostic Theory
ALL-TEST ProTM MCA instruments are based on proven electrical theory. The motor
system can be represented by developing the basic motor circuit, which is nothing more than a basic RCL circuit. This circuit represents the various components of the motor system. Each basic circuit represents one phase of the three phase motor system. Since each phase of the motor system is identical, each basic circuit should respond the same to an applied signal.
To allow the user to obtain the most benefits from of the ALL-TEST ProTM instruments and
this manual, a brief review of electrical theory as applied to Motor Diagnostics is presented below. A quick review of this section will help in understanding the results obtained from the ALL-TEST ProTM instruments. Additional information is available in any electrical text
book. ALL-TEST ProTM also offers on-site or public training courses to further the user’s
knowledge. For dates and locations of these courses please visit us on our website www.alltestpro.com.
Basic Electrical Theory
R -Resistance is the Direct Current, resistance measured in Ohms. The resistance should be
the same across all phases or fields. Any difference indicates a problem. Difference can be due to “over-winding”, corrosion, bad connections, & etc.
Note: If testing large coils or motors with 100 milli-ohm resistance or less, the operator
should use extreme caution to obtain “valid” readings. It may be necessary to clean the contacts or connectors to obtain good or “valid” measurements. It is recommended to Remeasure the phases several times to verify these values. *If accurate low resistance measurements are necessary use a low-resistance ohm meter.
Z- Impedance is the Direct Current and Alternating Current Resistance in a coil or winding.
Impedance includes DC Resistance, inductive reactance and capacitive reactance. Impedance is measured in Ohms.
Zero impedance of a winding indicates “a shorted” winding. See also I/F below.
Note: A coil or winding can have a severe turn-to-turn or coil to-coil fault, but show “good”
when “meggering”. It can also show a severe Ground Insulation fault but show perfectly OK turn-to-turn.
L- Inductance is the property of a changing magnetic flux to create (or induce) a voltage in a
circuit. Inductance is dependent on the number of turns and of the material in a spool or coil’s core. Inductance opposes any change in the current flow through a conductor. The value is a measurement of the ability of a coil to store a magnetic field. It is measured in Henrys or milli-Henrys.
Self-Inductance is the property of a circuit where a change of current in the circuit
creates (induces) a voltage in the same circuit.
Mutual Inductance is the concept that current flow through one conductor or
circuit can induce a voltage into a nearby circuit or conductor.
Note: In a three phase induction motor with the rotor in place, inductance unbalances can
be the result of unbalanced mutual inductance due to the rotor angular orientation (More commonly referred to as rotor position).
Phase Angle is a relative measurement that indicates the angular difference between two
waveforms of the same frequency. The results are expressed in degrees angular difference ( 0 – 900). In the electrical circuit the phase angle expresses the relationship of the AC current
to the applied voltage. This test is included in IEEE Std 1415™-2006 sec 4.3.20 as an effective method to identify winding shorts.
Basic Electrical theory states that:
In a purely Resistive circuit, current & voltage are in-phase. I.e. they both reach the same point in the waveform at the same time.
In a purely Inductive circuit voltage leads current by 90 degrees. I.e. Voltage reaches it maximum & minimum value 90 degrees before current.
In a purely Capacitive circuit current leads voltage by 90 degrees. I.e. Current reaches it maximum & minimum values 90 degrees before voltage.
If the Voltage leads the current the phase angle is positive, if the voltage lags the current the phase angle is negative.
In MCA the phase angle expresses the relationship of the measured current to the AC voltage applied by the ALL-TEST ProTM instruments.
Note: Phase angle should not be confused with the 120 degrees electrical separation
between electrical phases in a three phase system.
C- Capacitance is the capability of a body, system, circuit, or device for storing electric charge. It is a measure of the amount of electrical charge stored for a given applied potential. The unit of capacitance is the Farad (F). The capacitance of a circuit opposes any change in
The capacitance of a circuit is dependent on the geometry of the system and the material of the dielectric.
Any capacitors in the motor circuit should be tested separately from the motor.
INS- Insulation (to Ground) Test. Measured in Meg-Ohms.
A motor can have a good insulation to ground but fail other phase-to-phase tests and vice versa.
I/F- Current/Frequency Response is a test designed primarily to test for coil-to-coil or
turn-to-turn faults. This test is included in IEEE Std 1415™-2006 sec 4.3.33 as an effective method to identify winding shorts.
For the I/F test the low voltage AC signal is applied to the connected winding/windings, at a specific frequency and the resultant current is measured. Then the frequency of the applied AC signal is then doubled and the resultant current is again measured.
The I/F reading is the ratio of the current at the doubled frequency to the current at the original frequency. This result is displayed as a ratio. I.e. An I/F reading of
-50 indicates that the current at the doubled frequency is 50% lower than the current at the original frequency.
For a single phase winding, the I/F reading should be between -50 and -15 for a good coil. For an open winding the reading will be 0 (zero).
Note: Small unbalances (≤ 1) of the average in the three phase windings can be caused by
the rotor position, or display limitations of the instrument in Auto Mode. These can be verified by conducting a rotor compensated test or take manual measurement.
Applying MCA
ALL-TEST ProTM has two instruments designed to perform MCA.
ALL-TEST PRO 31™ (AT31) is a trouble-shooting tool that will test a wide variety of motors and some transformers. It will test most AC motors under 600V and can also be used to test higher voltage motors, depending upon their resistance, inductance, and impedance. The AT31 should be used in conjunction with an ohm meter, as it does not measure resistance (it does perform an insulation to ground resistance test).
ALL-TEST IV PRO 2000TM (ATIV) is an analytical instrument that will test virtually any
motor, generator, transformer, or coil based device. It is used not only for incoming inspection and trouble-shooting, but also for PdM or CBM testing purposes (data trending and time to failure estimation). The ATIV will not only detect most types of motor electrical faults, but it can also help the user determine how long it will run.
Manual Testing
Testing Motors has become a routine task in plants that recognize the importance maximizing plant reliability. Testing motors has become mandatory as acceptance criteria for inventory control. De-energized testing of new and rebuilt motors identifies faults before they are placed in the storeroom for extended periods.
Manually testing motors is done using either the AT31 or the ATIV. The motors can be tested either directly from the motor connection box or from the motor line leads located in the motor controller. Testing the motor remotely from the motor controller has the additional advantage of testing all of the cabling and connectors in the motor system as well.
Manual Testing requires connecting the ALL-TEST PRO instrument to the winding to be tested, manually selecting the parameters and measurements to be made. The results are either simply viewed or viewed & recorded. These measurements can then be analyzed and interpreted based on the guidelines provided in this manual. The measurements taken with the AT31 can also be entered into the ALL-TEST PRO Condition Calculator 4.0TM
, for analysis, storage and report generation (3-phase AC motors only).
The AT31 provides additional diagnostic capabilities, since it has the ability to perform tests at different test frequencies and provides real time display of the value tested.
Testing can also be performed using the ATIV in the manual mode (see ALL-TEST IV PRO 2000TM manual for specific details). The manual mode of the ATIV provides improved analysis
over the AUTO MODE for inductance or impedance measurements below 10, since the
measured values are displayed with decimal places instead of just whole numbers. AUTO Testing
In the AUTO mode:
1) The ATIV automatically tests the condition of the motor windings.
2) Automatically sets the measurement parameters and allows the measured data to be stored in the non-volatile memory.
3) Stores the data which can then be uploaded to the companion TREND/EMCAT
PRO 2005TM software for analysis, trending, report & work order generation.
Motor Testing Procedure
It is important to note that a “perfect” 3-phase motor is very difficult to find based upon manufacturing tolerances & etc. In other instances, motors may have a particular difference in design in order to meet special applications. In both cases, with the rotor mounted in the motor, it may be necessary to perform additional steps to isolate rotor or stator faults.
Initial Readings
In a few cases, some motors may exhibit unusual levels of unbalance. This can occur for a number of reasons, including:
1) Stator winding to rotor bar ratio and position. 2) Stator winding design (lap versus concentric coil).
3) Other tolerance-related issues including rotor casting voids.
If a series of unusual readings are detected with the ATIV, there are several ways of isolating the cause. One method requires the use of an AT31 or an inductance meter in concert with the ATIV (See Rotor Compensated Test in the Motor Troubleshooting Section), the other method involves a short series of additional tests using the ATIV (Rotor Reposition Test in the Motor Troubleshooting Section).
Performing Motor Diagnostics
using MCA
Motor/Winding Analysis
Performing motor/winding analysis has been greatly simplified with the development of advanced diagnostic tools such as the AT31 and ATIV, Condition Calculator 4.0TM, &
TREND/EMCAT PRO 2005TM, however, even as good as these tools are, additional information
and testing may be required before the final condition of the machine can be accurately assessed. To help maximize the data obtained from the ALL-TEST PROTM tools, the next few sections are
going to provide the analyst the procedures, techniques, suggestions and methods necessary to help the analyst properly and accurately diagnose most winding faults using Motor Circuit Analysis (MCA).
The basic rule for MCA is: If the data indicates a good winding then the winding is generally good. However, if MCA indicates a fault then additional testing should be performed before condemning a winding.
1) The test leads that are supplied with the ATIV instrument are very good leads for most applications, however, they are not good for every application. Any 4mm shielded, banana jack type instrument test leads can be used.Note: The repeatability of the
resistance readings can be improved by using a small wire brush to clean surface oxidation from connection points and by lightly squeezing the test jaws together while lightly twisting the clips on the connection point to ensure as solid a connection as possible. If high accuracy of the winding resistance measurements is required it is recommended that a
Note: Extreme care should be taken during the comp. lead test portion of the AUTO
measurement mode to improve resistance measurement accuracy and repeatability. 2) The ATIV uses a two-wire method for making the DC resistance measurement and is not be as accurate as a low resistance ohm meter, which uses either 3 or 4 wire method, when measuring very low resistance. The accuracy of the ATIV is +-1% from 1 to 999 ohms, therefore, when testing below 1 ohm the accuracy of the instrument’s measured value will degrade. Note: For MCA purposes the DC resistance measurements are used to detect problems related to connections and are not used to detect winding faults (Fi and I/F are a much better indication of winding faults).
3) The ATIV in the AUTO mode measures and displays the values of inductance (L) and impedance (Z) as whole numbers. Ex-2.9 Ω becomes 2 Ω , 2.1 Ω will also be displayed as 2 Ω. This may cause misinterpretation in the TREND/EMCAT PRO 2005TM software
with impedance (Z) and inductance (L) matching. Winding contamination is generally suspected if the impedance does not follow inductance from phase to phase. The analysis software algorithm evaluates the phase difference of both inductance and impedance. When the software evaluates the measurements of Z & L with values less then 10, because the ATIV truncates decimals below 10, the software may incorrectly diagnose contamination or overheated windings. Always verify the L & Z measurements by taking the measurements manually. The manual measurement of the ATIV measures and displays the decimals.
Sample Test Results
Reading T1-T2 T1-T3 T2-T3 Resistance .272 .273 .272 Impedance 47 53 58 Inductance 9 10 11 Phase Angle 73 72 71 I/F -43 -42 -41 Condemning Criteria
The original assumptions of MCA were based on that most industrial motors are 3 phase, AC Induction Motors with squirrel cage rotors. The rules and limitations presented below are based on these assumptions. However, that does not prevent MCA from being successfully applied to other motor/rotor types. In fact, MCA has been successfully applied to DC motors, single phase motors, both permanent magnet and externally excited synchronous motors, generators, distribution and transmission transformers. Also, in almost all applications the size of the winding is not an issue. The following criteria still apply.
or tests maybe required. Many of these are presented in the Appendix of this guideline. Data Analysis Tips
When a motor testing program is first implemented it is expected that between 20- 40% of the motor systems tested may exhibit some alarm condition when evaluated with the TREND/EMCAT PRO 2005TM software (the software provides color coded alarms for easy
interpretation). When a motor is in an alarm state, this does not necessarily mean that the motor will fail or should not be operated, but that the measured values have exceeded pre-determined limits established for most common motors.
The software alarm limits of the TREND/EMCAT PRO 2005TM are established for standard
3-phase squirrel-cage induction motors. Some motors may have a special design, which cause the measured values to be normally outside of these standard limits. In fact, many new motors will have an unbalance in inductance and impedance, due to rotor bar winding ratio. Therefore, it is virtually impossible to establish limits for all possible design configurations. Of these cases, it is necessary for the analyst to evaluate these readings on a case by case basis. The software flags any motor exceeding these limits to inform the analyst that they have exceeded normal limits. The following analysis tips provide a method of more closely evaluating these special circumstances. Following the Analysis tips are various Scenarios using actual readings. Reviewing these scenarios will provide additional insight into determining how to evaluate the test data.
Condemning Priority
The TREND/EMCAT PRO 2005TM software simply generates the alarms when the measured
values exceed pre-determined limits. However, not all faults are the same. The guidelines provided below will help the analyst place priority upon the alarms generated by the TREND/EMCAT
PRO 2005TM software.
One of the first considerations regarding winding faults should always be motor criticality. Obviously, the most critical motors should be afforded a higher priority than less critical motors. The second consideration is the type and location of the fault. Additional considerations include availability of spares, maintenance schedules and other plant operations.These priorities assume that the test data is valid and good connections are made. Bad test lead connection can negatively impact all readings.
1) Winding shorts are generally more severe than contamination or rotor faults, therefore, motors with unbalances in I/F & Fi alone should be evaluated first, to determine the condition of the winding.
2) Motors with alarms in Fi & I/F as well as inductance and or impedance should be evaluated next. It may be necessary to perform a rotor reposition or rotor compensated test to separate rotor from winding faults.
Condemning Tips
1) Never condemn a motor from the Motor Control Center. Faults in the cabling or connections between the test point and the motor itself can cause unbalanced readings. Before condemning a winding always perform a confirming test at the motor with the motor leads disconnected from the supply cabling.
A) To determine whether the fault is in the motor or the cabling retest the motor at the next connection point between the motor and the starter.
B) A rotor reposition test may be necessary to separate rotor from winding faults. (Refer to the Troubleshooting section of this manual for the Rotor Reposition Test)
2) Generally, never condemn a motor based on an unbalanced inductance or impedance alone (may require additional testing). The Rotor Bar/Winding Ratio can cause a large unbalance in mutual inductance as well assmall unbalances in the I/F & Fi readings. 3) Always verify the reading before condemning a motor. Stored energy in a motor system can corrupt the data set. Remember, it is much easier to take readings again than it is to remove the motor.
4) Winding shorts are first indicated by unbalances in Current/Frequency response (I/F) and Phase Angle (Fi).
5) Loose connections are indicated by unbalances in winding resistance measurements. 6) Winding contamination or overheated windings are indicated when Impedance Pattern (Z) does not follow Inductance Pattern (L).
7) Never condemn a motor if the readings are not repeatable. EMI or the shaft turning will also give inconsistent readings.
Unassembled Motor Analysis
If the rotor is removed from the stator the mutual inductance created by the stator magnetic field inducing a voltage into the rotor will no longer create an inductance unbalance. Therefore, the only part of the basic motor circuit that is responding to the injected AC signal from the instrument is the stator winding and stator iron. Any unbalances caused by rotor bar/winding ratio error are eliminated. Therefore, the fault criteria are much tighter for motors tested when the rotor is removed. Below are the tolerances for motors stators only (motors with the rotor removed).
Test Result Tolerance
Resistance (R) <5%
Impedance (Z) <3%
Inductance (Z) <5%
Phase Angle (Fi) +/- 0
Current frequency Response (I/F) +/- 0
Insulation Resistance < 600 Volts > 5 Megohms
Insulation Resistance >600 Volts > 100Megohms
Data Interpretation Basic Rules and Tolerances
There are specific rules that encompass virtually all test applications of three phase motors in which the motors are assembled and a rotor is installed. The common method for testing is from an MCC or disconnect with the rotor stationary.
Assembled Motor Analysis
If the rotor is installed in the stator the mutual inductance of the rotor may cause large inductance unbalances which will result in a large impedance unbalance. The Rotor Bar/Winding ratio may also cause small unbalances in I/F and Fi.
Test Result Tolerance Detail
Resistance (R) <5% Possible Loose connections
Impedance (Z)
and Inductance (L) Impedance follows Inductance Any unbalance is most likely due to rotor position or motor design Impedance (Z)
and Inductance (L) Impedance does not follows Inductance Possible winding contamination or overheated windings Phase Angle (Fi) +/- 1 digit (degree)
from average Indicates a winding short: 74, 75, 76 OK; 74, 74, 76 suspect; 73, 73, 76 failed
I/F +/- 2 digits (%) from
average Indicates a winding short: -44, -45, -46 OK; -44, -46, -46 suspect; -42, -45, -45 failed Insulation
Resistance Supply voltage < 600V >5 Megohms Indicates poor insulation to ground (I.e. ground fault) Insulation
Resistance
>100 Megohms Supply voltage > 600V
Indicates poor insulation to ground (I.e. ground fault)
Resistance (R): Unbalances in resistance are an indicator of loose connections, pitted contactors,
cold solder joints, and etc. In some cases the resistance unbalance has been the result of poor connections at the test leads. Always retake the resistance measurements if a resistance unbalance exists. Changes in the resistance measurements with repetitive readings indicate test lead or test lead connection issues. Attempt to clean the connection location and then retake the resistance readings. If the readings were taken at the motor control center taking reading progressively closer to the motor will normally locate the high resistance connection(s).
Inductance Unbalance (L): When a squirrel cage rotor is installed in the motor, inductance
unbalances are possible, especially on the smaller less expensive motors. If this does occur these unbalances is usually the result of the unbalanced mutual inductance created by the unequal rotor bar/winding ratios resulting by the rotor’s position. To verify that this unbalance is the result of the rotor position, the rotor reposition test should be performed. (See section on Rotor reposition test).
contributor to impedance, if the rotor position creates an unbalanced in the winding measurements then this should create an unbalance in impedance measurements as well, however, the unbalance in impedance should follow the unbalance in inductance. For example: a motor inductance unbalance may be 10, 14, 8, or a pattern of Medium, High, Low, the impedance should follow with a Medium, High, Low pattern, such as 120, 133, 115. In this case impedance follows inductance. However, if the impedance was 133, 115, 120, now the impedance pattern is High, Low, Medium and a fault would be indicated. See Impedance (Z) does not follow
Inductance (L) below.
Impedance (Z) does not follow Inductance (L) Changes to impedance such that it does not
follow inductance is normally caused by changes in the insulation system. The insulation system is a large dielectric. A change to the material condition of the insulation system will be reflected as a change in the capacitance of the motor circuit. A change in the capacitance of the system will change the capacitive reactance (Xc), which will normally cause the Impedance (Z) to not follow the inductance (L). The fact that impedance does not follow inductance (Z) is a good indicator of the insulation system changes, such as winding contamination, burned windings (overheated), very large phase unbalances or very poor rotor bar condition.
Phase Angle (Fi): The amount of the lag of the current to the applied voltage in the basic motor
circuit is one of the most sensitive of measurements in the basic motor circuit. Fi is usually one of the first measurements to change when the insulation system degrades (Winding short). Unbalances of >1 degree from the average indicate a winding short.
Current Frequency Response (I/F): Degraded winding insulation systems respond differently
at different frequencies. The I/F measurement are also one of the first indications of winding system degradation. The I/F readings should be between -15 to -50. All I/F reading should be balanced within 2 digits (per cent). Unbalances of >2 per cent from the average indicate winding shorts. A spread of >4 digits between maximum and minimum I/F measurements, also indicates winding faults. These readings are for non-compensated rotor position at the motor. However, if a winding fault is indicated additional testing may be necessary to verify the winding fault.
ANALYZING MEASUREMENTS AND CHANGE
Data interpretation of collected ALL-TEST IV PRO 2000TM data can be performed through
the TREND 2005/EMCAT PRO 2005TMsoftware. Troubleshooting Rules
Following are the basic rules for troubleshooting with the ATIV: Shorted Windings:
Developing windings faults, as well as shorted windings, are evaluated by viewing the Fi and I/F readings of similar coils or between phases:
Phase Angle (Fi) – The phase angle should be within 1 digit of the average reading. For
example, a reading of 77/75/76 would be good because the average reading is 76. A reading of 74/77/77 would be bad.
Current Frequency Response (I/F) – The current frequency response should be
within 2 digits of the average reading or <4 digits between maximum and minimum I/F measurements. For example, a reading of –44/45/46 would be good. A reading of -40/-44/-44 would be bad. However, a reading such as -42/-44/-44 should be considered suspect.
Winding Contamination and Rotor Position
The position of the rotor within the electric motor may cause a normal phase unbalance. Winding contamination may also cause phase unbalances. The difference between the two can be evaluated quickly by looking at the pattern of impedance and inductance.
Rotor Reposition Test: To verify the Impedance (Z) and/or Inductance (L) unbalance is the result of rotor bar/winding ratio unbalances it is necessary to evaluate the relationship of the Z & L unbalances. If the unbalances are related to the rotor, they will change relationship by changing the rotor position. For example, if there are inductances of 17/18/19 and values of impedances 24/26/29 with the rotor at its existing position. Rotate the shaft approximately 90 degrees (1/4 turn) the values should change relationship, such as inductances of 16/19/17 and impedance values of 23/30/25. This indicates that the unbalances are due to rotor position.
NOTE: Since the ATIV stores Z & L as whole numbers only when the measured values are <
10, lower measurement values of L & Z may be confusing. For example: This may be the case if the inductances are 5/5/5 and the impedances are 8/9/8. To confirm the analysis, manually measure the Impedance and Inductance of all three windings using the Manual Mode of the Instrument.
Insulation Resistance: The insulation resistance (Meg-Ohm readings) will show a breakdown of insulation between the winding conductors and ground. The latest edition of the IEEE Std. 43-2000 states that the insulation resistance of windings should fall in the following ranges:
Insulation Resistance Readings
Type of Insulation
Insulation Resistance Values
All insulation systems prior to 1974
1 Meg-Ohm + 1 Meg-Ohm/kV
Random Wound Motors
> 5 Meg-Ohms
Form Wound Motors and DC
Armatures
> 100 Meg-Ohms
Loose Connections
Loose connections or glazing on contacts will show as resistance unbalances. The maximum resistance unbalance should be 5%.
PREDICTIVE MAINTENANCE
As early as the 1960’s, many companies realized that by routinely monitoring the operating condition of rotating equipment it is possible to obtain an advanced warning of operational or other problems that would impact continued efficient operation. This early warning provides time to remove the machine from operation and affect minor repairs and adjustments before catastrophic failures occur.
This maintenance philosophy, referred to as Predictive Maintenance (PdM), has escalated since the early 1980’s, with the introduction of micro-processor based data-collectors. Many of the machines’s operating characteristics, such as temperature, pressure, oil condition, vibration and performance can be trended to identify changes. However, one of the glaring holes in predictive maintenance has been the inability to easily & accurately identify faults within electrical equipment, such as motors, transformers, solenoids and other like
equipment. One of the main reasons for this was the lack of available easy-to-use predictive maintenance instruments for testing motors or other electrical equipment.
Predictive maintenance instruments should be : 1) Hand Held
2) Easy to use
Implementing Predictive Maintenance
Implementing a successful predictive maintenance program requires more than purchasing an instrument and taking data. Predictive maintenance programs when successfully
implemented require a complete understanding of the PdM process.
Successful predictive maintenance consists of three phases- Detection, Analysis and Correction. Each one of these phases is important in its own aspect. Problems are created when short cuts are taken and phases are skipped or combined.
Detection: The detection phase involves periodically monitoring the operating
characteristics of the selected equipment. These values are trended, compared to previously recorded data from that machine or similar machines, then compared against pre-determined or published standards and/or reviewed for any change. During the detection phase, the data collection process should be done quickly and carefully, with the intent of monitoring as many machines as possible. When a change is detected, additional data may be necessary, to determine the cause of the machine’s condition change. This is done during the analysis phase. In most cases, the MCA data taken during the detection phase may be sufficient to identify developing shorts or other winding issues. But at times, additional data or testing needs to be performed to more accurately identify the problem.
It is usually a waste of time to perform these test for a more detailed analysis during the detection process, as it slows down the detection process. Most experienced predictive maintenance departments have recognized the importance of separating these two processes.
Analysis: The analysis process involves taking additional and perhaps different types
of tests than the detection process. This additional testing may require disconnecting the motor from the load, turning the shaft or separating the motor leads and requires more time to take the data. Since usually only a few machines during the detection inspection exhibit any significant change, it is usually more time effective to only take the data necessary to identify a change during the detection process, and then go back for a more detailed look, once a change is detected.
However, if the plant site is remote or has other access limitations, these may justify more detailed data be taken during the detection process.
Correction: The correction phase involves correcting and eliminating the problem
that triggered the analysis. This may require cleaning a motor, tightening
connections, or a complete motor rewind. The exact type of correction and repairs are determined by the analysis. Details for correcting and eliminating these problem are outside the scope of this manual.
Predictive Maintenance Hints
The following recommendations are from over 20 years experience with MCA using the ALL-TEST ProTM line of motor testers. It should be remembered that these are
recommendations only and are designed as suggestions to provide for the optimum
program. It may not be possible to implement each and every suggestion on all applications: 1) When performing Predictive Maintenance (PdM) on three phase electric motors, try to place the rotor in the same position each time (I.e. Place the shaft key at the 12 o’clock position) this will minimize changes from data collection to data collection due to rotor position. (You may also use the AT31 in conjunction with the ATIV to do a
rotor compensated test). This step is recommended for long term trending,
2) When the initial test is performed, if variations in the inductance measurements of 5 to 15% between phases, perform a rotor reposition test in order to determine if the variation is due to the rotor bar/winding ratio or if there is a rotor fault. These variations may be normal or the unbalance may be due to rotor position. Review the rotor position section of this manual for further clarification.
3) The upper limit of equipment size that can be successfully tested is primarily dependent on the DC resistance of the windings. For the ATIV, the winding’s DC resistance needs to be between 0 .001 and 999 ohm in each phase. The windings capacitance & inductance, plus cable length can also impact the range of equipment that can be tested.
4) Always mark your motor leads and always collect data from 1-2, 1-3, and 2-3. This is the order in which the TREND 2005/EMCAT PRO 2005TMsoftware records and displays the data. Establish a pattern when numbering the leads for example left to right, front to back, or top to bottom. This also provides consistency in readings. This consistency also helps identify in which phase the fault occurred.
I.e. If an increase in resistance in 1-2 and 1-3 occurs, then you should check the phase 1 connections.
Data Collection and Test Result Issues
There are issues inherent with faulty motors and with data collection process of motors in an industrial environment. Following are some of those issues when collecting data with the ATIV and their resolution:
Bad Results
1) EMI (Electro-Magnetic Induction) can cause problems with resistance readings. This usually only occurs when testing from the MCC. EMI can come from heavily loaded cables located directly next to the cables from the motor that is being tested. The values are normally in the milli-Volt range and not always detectable with a multi-meter or volt meter.
feature of the AT31 to measure the EMI level. If the EMI cannot be removed the motor must be tested from the motor itself.
2) Motor Rotor Turning If the shaft in the machine being tested is rotating, it will induce a
voltage into the basic motor circuit. This will affect all of the readings.
Note: To determine whether the bad result is caused by EMI or rotating shaft, use the
Rotor feature of the AT31. If the shaft is rotating the bar graph on the screen will move back and forth across the display screen of the AT31. If the bar graph moves erratically across the screen check the EMI level using the EMI feature of the AT31.
3) Low Resistance If the winding resistance is less than 0.001 Ω, installing a small resistor (approximately 0.25 Ω) in series with the test leads may increase the resistance measurement into the allowable measurement range.
Waiting
1) Test Leads in wrong ports The most common reason for the ATIV screen to display waiting is the test leads were left in the insulation resistance test ports after measuring the insulation resistance to ground.
2) Open Circuit If the test leads are in the correct port then the signal from the ATIV does not
have a complete path and an open circuit exists. Test closer to the motor to find the open circuit. The open circuit may be in the test leads. Check them for continuity.
Incorrect Readings
1) Capacitors or Lightening Arrestors in the motor circuit will filter the test results and create incorrect readings which could provide false positives or false negative results. Always disconnect any capacitors or lightening arrestors connected to the motor circuit. Note: The manual mode of the
ATIV provides a convenient method of testing for faulty capacitors.
2) Non-Repeatable Resistance Measurements EMI interference will cause non-repeatable resistance values (see Bad Results above). Contamination in the motor or cabling can cause non-repeatable resistance readings. Also, carbon buildup or heavy contamination in cabling.
Disconnect the motor leads at the motor connection box and retest.
Poor connections at the test clips will normally affect the resistance only, but it may affect other readings also, depending upon severity. If a resistance unbalance exists always retake readings before condemning motor. If resistance phase measurements are repeatable from one test to another the resistance unbalance is most likely in the motor or cabling. If resistance measurements do change from one test to another the fault is most likely the result of poor connections at the test leads. Clean the connections thoroughly until repeatable readings are obtained.
2) Servo-motors, Machine tool Motors Some machine tool and servo-motors have permanent
magnet rotors which may affect the I/F & Fi measurements. The I/F & Fi readings will be repeatable but outside standard tolerances. Testing permanent magnet rotors may require special
readings will provide additional indication of winding degradation. Rotor faults can also cause the
difference between the I/F & Fi readings to increase, it is suggested that a rotor position test be performed to verify a rotor fault.
3) Synchronous Motor Testing Synchronous motors windings will also test for shorts in the
rotor windings. If the Auto test indicates winding shorts, the short could be either in the stator or the rotor windings. To determine which winding is shorted rotate the shaft approximately 900 and
retake the readings. If the fault remains in the same phase as the original test the short is most likely in the stator winding. If the fault shifts to another phase the fault is most likely in the rotor. The synchronous motor rotor winding are a single winding, evaluating them is done by trending or comparing current reading to a baseline.
4) Wound Rotor Motor Testing wound rotor induction motors will also identify shorts in the
rotor windings. If the Auto test indicates winding shorts, the shorts could be either in the stator or the rotor windings. The windings of a wound rotor motor are tested using the slip rings. The same tolerances apply to the 3 phase rotor windings as to the stator. If the rotor tests good the fault is in the stator.
Data Collection Procedure
Using the ALL-TEST IVPRO 2000TM
1. Check the charge on the ATIV. Five stars is fully charged, three stars is near the end of the useful charge, two stars provides a battery alert and then will shut the unit off.
2. Select the Insulation Resistance Test Voltage for the motor under test. (Ref to the ALL-TEST IV PRO 2000TM manual)
3. De-energize the motor to be tested following the appropriate lock out/tag out (LOTO) procedures and verify the equipment is de-energized.
a. Remove one leg of each phase of any power factor correction capacitors that may be present in the circuit.
b. If performing an insulation-to-ground test from a variable frequency drive or if other electronics are in the circuit, disconnect leads from the device being tested. Failure to do so could result in serious or catastrophic damage to the VFD or electronics from the applied test voltage of 500 or 1000 VDC.
4. Check connection points to ensure that they are not coated with any foreign substance or that the leads are not burned and oxidized. A dirty contact point may result in poor readings.
5. Turn on the instrument and backlight, as necessary. 6. Select AUTO MODE and follow the instructions:
a. Short leads together to set up the resistance test
b. Connect to windings as per message on the screen. Use T1 - T2; T1 – T3; and,
T2 – T3 as Winding 1, Winding 2 and Winding 3.
7. Between each winding test the options ‘OK’ and ‘REMEASURE’ are shown. (Note: It is recommended to ‘REMEASURE’ after the first winding readings to discharge any
readings repeat then: Select ‘OK’ to move on to the next winding, if the readings do not repeat:
a. If the ‘REMEASURE’ readings vary significantly, check the test lead connections before completing the REMEASURE.
b. If the ATIV flashes “Waiting …” or “BAD RESULT:”
i. If the instrument flashes “Waiting …” check to ensure that the test leads are connected to the ATIV test ports and to a good winding at the test point. If the test leads are in the proper ports, then the winding is open. To verify an open winding, test a known good winding.
ii. If the instrument states “BAD RESULT” this indicates that either the test winding has a resistance value below 0.001 Ohm or low level current (EMI) is present in the cables. If EMI is suspected, test as close to the motor as possible. If other windings read above 0.001, the winding or conductors are directly shorted.
iii. It could also be that the shaft is turning, for example a damper on a fan may allow the fan to rotate so the motor rotor is not completely at rest.
8. Continue until all three windings have been successfully tested. After the completion of all three winding tests, the user is asked whether or not to perform an
“INS TEST”. To perform the “INS TEST” (Meg-Ohm test) select ‘Yes’; and then move the test leads to insulation test ports. To skip the “INS TEST” select ‘No’.
9. To verify that both test leads are properly grounded it is recommended to perform one “INS TEST” between 2 known ground points where there is no paint (i.e.: bolts, on motor, etc.). This measurement should result in a “zero” ground reading and verifies that both test leads are properly grounded.
a. Remove the red test lead and connect it to any of the motor leads. (Caution: Do
not remove both test leads)
b. Select ‘REMEASURE’ and test between any of the motor leads and the point of ground.
10. When testing is completed, some of the test readings will be displayed on the screen. To view other data scroll up/down using the up/down buttons on the keypad. NOTE: ALWAYS SAVE YOUR READINGS BEFORE GOING ANY FARTHER! (Refer to ALL-TEST IV PRO 2000TM Operating Manual for detailed steps to save.)
11. When saving, name the data test set with up to 11 alpha-numeric characters (minimum 3), save phases and horsepower (up to 3-digits). Horse Power is not needed and does not affect the analysis of the machine.
12. Before re-energizing equipment, quick-check readings (Reference the following section or User Manual). Normal method: Check Fi and I/F readings for winding shorts, impedance and inductance pattern matching for winding contamination or overheated windings, resistance for loose or broken connections and insulation to ground for grounded windings.
13. If no dangerous condition or motor fault exists, return equipment to service using LOTO procedures.
TROUBLESHOOTING MOTORS
Rotor Compensated Test
1. Connect the AT31 (an Inductance tester may be used in substitute for the AT 31) to the first set of windings (T1-T2) and rotate the shaft until the highest impedance reading is obtained.
2. Measure the first winding using the AUTO MODE of the ATIV reading then stop. 3. Connect the AT31 to the second set of windings (T1-T3) and rotate the shaft until the
highest impedance reading is obtained.
4. Measure the second winding using the AUTO MODE of the ATIV reading (second winding) then stop.
5. Repeat for your third winding. If the final results still show an unbalance, the stator should be suspect, if they are balanced; the rotor should be checked using the rotor testing steps for either the AT31 or ATIV. If both the stator and rotor test good, then the unbalance is due to the motor design and should be noted.
Rotor Reposition Test
Using the ATIV, a short series of tests may be performed in order to determine whether the unbalanced readings are due to the rotor, stator or design. The steps are straightforward:
1. Note the position of the rotor after saving the original readings. Rotate the shaft 90 degrees from its original position and remeasure the motor windings in the same order that they were originally taken.
2. Reference the previous reading and note if the unbalance has shifted with the rotor movement. If it has, or if the readings are inconclusive, retake the measurements at 180 degrees from the original position.
3. If the readings remain unbalanced in the original position, the stator windings are most likely faulted, if the readings shift with the rotor position, perform a rotor test as outlined in the motor troubleshooting guide.
Shifting Readings Example
T1-T2 T1-T3 T2-T3 Impedance 00 47 53 58 Inductance 00 9 10 11 Impedance 900 53 58 47 Inductance 900 10 11 9 Impedance 1800 58 47 53 Inductance 1800 11 9 10
Maintained Readings Example T1-T2 T1-T3 T2-T3 Impedance 00 47 53 58 Inductance 00 9 10 11 Impedance 900 47 53 58 Inductance 900 9 10 11 Impedance 1800 47 53 58 Inductance 1800 9 10 11
Note: The readings will not be exact, this is just an example
ROTOR TESTING
One of the important features of the ATIV is the ability to perform a complex analysis of a three phase motor rotor.
DATA COLLECTION
Three phase squirrel cage rotors can be tested with the ATIV Motor Circuit Analyzer using the manual inductance testing method.
Note: Where the MCA method could miss some rotor faults the ATPOL on-line tester is a very useful supplement detecting all types of rotor problems while the motor is running.
1) Verify the winding integrity by performing an Auto test of the motor. 2) Rotate the shaft so the shaft keyway is in the 12 O’clock position.
3) Manually measure the inductance of the first winding (T1-T2), record the measurement on a pre-labeled data sheet; continue with the other two windings (T1-T3) and (T2-(T1-T3) by connecting to the other windings selecting “Remeasure” from the menu bar on the manual inductance measurement screen. Record these values in the appropriate spaces on the data sheet.
Set up a sheet of paper labeled T1 through T3 across the top and 24, then 1-23 along the left side. See Appendix one for a sample data sheet
4) Rotate the shaft the specified number of degrees as recommended by the table below. A minimum of 24 shaft positions should be taken. However, on 4 pole motors or greater it is recommended that more positioned be taken. The more rotor positions taken, the more detail is available for analysis.
Number of Poles Minimum # of Readings Degrees Shaft Rotation 2 24 150 4 36 100 6 48 7.50 8 72 50 >10 72 50
5) Repeat the measurement procedures in step 4 above until the rotor has made one complete revolution.
NOTE: The more accurate the positioning of the rotor the more accurate the analysis
will be. It is recommended to attach a rotating protractor or use a piece of circular graph paper attached to the shaft to provide maximum accuracy.
6) When a complete set of data have been taken the results should be graphed using either the Rotor Test feature of the EMCAT PROTM 2005 software, or directly into a
spreadsheet program such as Microsoft Excel. ANALYSIS
1) The readings will not be identical but should result in a repeating pattern as the shaft is rotated. If the pattern varies, there are rotor, casting, or air gap problems. 2) Rotor and casting problems show as a sudden change in one location on the motor while air gap problems change consistently around the rotor.
3) The resulting waveform should be even and 120 degrees out of phase from each other. There are a number of cases where these readings will deviate:
A) Large deviations at the peak or valley of at least one waveform will identify high resistant points in copper rotor bars, possibly where the bar is welded to the shorting ring.
B) Similar deviations will indicate broken rotor bars or, in small inexpensive aluminum rotors, the rotor laminations may not be set properly, leaving variations in the resistance of each rotor bar (low quality motor).
C) A more common problem in many electric motors (some manufacturers have more challenges than others) is casting voids. This is usually found as a flat point at the incline or decline on at least two of three sine-waves.
D) Eccentric rotor problems are normally found when the inductance tapers off or the waveform moves higher or lower (arcs from right to left).
APPENDIX 1 Winding Alarm Scenarios
Winding Alarm Scenarios
Over 20 years experience conducting motor testing using MCA has identified 11 common scenarios created by the 6 measurements taken during the auto test mode using the ATIV. The section below provides a sample display and brief write up of each scenario.
Scenario 1: ALL readings are balanced
This is a good winding and additional testing should be performed at the normal test interval. Scenario 2: Fi or I/F in Red all other measurements balanced
All measurements are within acceptable limits except I/F, which is out by a differential of 6. This generally indicates a phase to phase fault. If this test was performed at the Motor Control Center (MCC) retest directly at the motor.
All measurements are within acceptable limits except Fi which is out by a differential of 3 and >1 from average. This generally indicates a coil to coil fault. If this test was performed at the Motor Control Center (MCC) retest directly at the motor before condemning the motor.
Scenario 3: Fi or I/F in Red; L & Z in Alarm
Fi is in alarm indicating a probable coil to coil winding fault. Impedance & Inductance are also both in alarm, but Impedance is following Inductance. This is most likely the result of rotor bar/winding ratio differences. Perform a rotor reposition test to verify this condition.
Scenario 4: R > 5% Unbalance
All measurements are within acceptable limits except R which is in alarm (> 5%). This indicates high resistance connections. If these readings were taken at the MCC disconnect at the next closest connection point and retest motor. If unbalance persists continue towards motor until resistance unbalance either clears or further isolation is not possible. It often happens that the resistance unbalance clears during the troubleshooting process, especially if the loose connections are tightened during the process.
Rotor Position 1
Rotor Repositioned
Fi & I/F measurements are in balance, however, both the L & the Z measurements are in alarm. This could be the result of rotor bar/winding ratio (Rb/W). In this scenario, impedance follows inductance in a Low-High–Medium (L,H,M) pattern. This unbalance is most likely the result of Rb/W differences. This is a common reading especially on smaller motors. To verify if this unbalance is the result of Rb/W perform rotor reposition test.
Scenario 6: Fi or I/F balanced; L & Z in Alarm but non-similar pattern
Fi & I/F measurements are in balance, however, both the L & the Z measurements are in alarm. Since the impedance unbalance does not follow the inductance the capacitance of the system has changed. This is most likely the result over the insulation becoming brittle due to overheating or the windings are contaminated.
Scenario 7: Fi & I/F balanced; L & Z in Alarm but non-similar pattern plus a frequency shift of the impedance measurement
Fi & I/F measurements are in balance, however, both the L & the Z measurements are in alarm. The impedance measurement of T2-T3 is almost double the measurements of the other two windings. When this occurs it is normally caused by a frequency shift of the instrument when making the impedance measurement for that winding. Experience has shown that this is a very early indication of winding or rotor issues.
Scenario 8: All measurements are balanced but insulation to ground measurement is low
All winding measurements are balanced, however, the insulation to ground readings are below recommended levels. Recommend clearing ground before operation.
Scenario 9: Fi & I/F balanced; L & Z in Alarm, readings are truncated
Fi & I/F are balanced, however, L & Z are in alarm, it is difficult to determine the L & Z pattern since the ATIV truncates any decimals in the auto mode and displays 0’s in place of the decimal. To determine if these patterns match manual readings of L & Z should be taken.
Scenario 10: Other readings
Most readings are zero, the L & Z readings may be 0 due to the values being < 1. The ATIV will truncate readings of Z & L that are <1 to 0.0. Verify whether these values are actually 0 or if they are < 1 by taking manual measurements. However, since the Fi & I/F of 2 of the windings are also 0 this indicates a short circuit in the winding.
Scenario 11: Zero Values
APPENDIX 2: Test Methods for Determining the Impact
of Motor Condition on Motor Efficiency and Reliability
Dr Howard W. Penrose, Ph.D. Old Saybrook, CT 06475
Abstract
This article discusses the financial impact of motor condition on electric motor efficiency and reliability by reviewing a combination of Motor Circuit Analysis (MCA) and vibration techniques. Cost impacts on energy, production and maintenance will be outlined. The topic will surround a utility study and US Department of Energy market transformation success during 2000 and 2001. The primary areas of concern are phase balance, rotor bars, cleanliness and bearing issues.
Introduction
Electric motors are the prime mover of industry and our general comfort in commercial buildings. The motor systems consume 20% of all energy used in the United States and 59% of all electricity generated. Within each sector:
q 78% of electrical energy in industrial systems (>90% in process industries) q 43% of the electrical energy in commercial buildings
q 37% of the electrical energy in the home
There are well over 1.2 billion electric motors, of all types, used throughout the United States. However, electric motors are often ‘out-of-sight, out-of-mind,’ until production is down due to a burn-out or catastrophic bearing failure.
It is important to understand that equipment usually fails over time, reliability decreases and losses increase (efficiency decreases) over time prior to most catastrophic failures. Although some equipment faults are instantaneous, the larger majority of catastrophic faults that impact production are the result of a failure in the implementation of a maintenance program. This failure is primarily due to management not fully understanding that maintenance is an investment in the business and not an ‘expense of doing business.’ If you do not invest in materials, equipment and people, you do not have product to sell: If you do not invest in predictive maintenance practices (PM, TPM, RCM, or any other program), you do not have product to sell or less of it at a higher overall production cost.
Proper implementation of a maintenance program has been shown to reduce energy consumption in plants by as much as 10-14%,1,1 while also reducing unplanned production
Table 1: Estimations for Downtime Costs1
Industry Average Downtime Costs, per hour
Forest Products $7,000
Food Processing $30,000
Petroleum and Chemical $87,000
Metal Casting $100,000
Automotive $200,000
In a recent utility energy and reliability project, a group of electric motors from 5 to 200 horsepower were reviewed in several industries, including: Petroleum and Chemical; Forest Products; Food Processing; Mining (Quarry); and Pulp & Paper. The plants varied from having no existing planned maintenance program to full implementation, including an existing energy program. Of these motors, randomly evaluated, 80% were found to have at least one deficiency with 60% of those (48% of the original) found to be cost effective to replace. The plants without programs had the greatest number of defective motors, the plants with existing maintenance and energy programs had the least number of defective motors. Eight percent of the motors were evaluated to determine the types of faults and the potential cost avoidance with corrective action (repair or replace) by using vibration analysis and motor circuit analysis (MCA). Several had a combination of electrical and mechanical problems:
Table 2: Utility Energy Project Findings
Type of Test Percentage of Faults
Vibration Analysis 45% of motors tested
Motor Circuit Analysis 70% of motors tested
Insulation Resistance (Meg-Ohms) 5% of motors tested
Several motors had combined vibration and electrical faults. A few had winding faults combined with insulation resistance faults. Several had shorted windings that were continuing to cause production problems, but were written off as ‘nuisance’ trips (detected in the study by using MCA). “Findings of the advanced portion of the Motor PAT Tool demonstration project indicate that measuring for … phase unbalance of resistance, inductance, impedance, phase angle and I/F (current/frequency response) provided more useful results.”1 The combined incremental production cost avoidance of 20 of the defective
motors, from 5 to 250 horsepower, was $297,100, rendering implementation costs insignificant.
The purpose of this paper is to first provide information for determining cost avoidance through the application of a maintenance program on electric motors. This will be followed with a discussion of the implementation of motor circuit analysis (MCA) and vibration analysis.
Cost Avoidance Through Maintenance
There are a number of ways to determine cost avoidance through the implementation of maintenance programs. In this discussion, the focus will be on the methods introduced through the US Department of Energy’s Industrial Assessment Centers (IAC’s), which provide a very basic and conservative method. The PAT Tool Demonstration Project used a much more complex method1, which is outside the scope of this article. However, some
of the tools, such as MotorMaster Plus1, will be used to provide cost information for motor
repair costs.
“Utility representatives have indicated that in a survey of facilities with no preventive maintenance programs, motor rewinds represented 85% of the total number of motor repairs (on average). After preventive maintenance programs were established, the numbers of rewinds were reduced to about 20% of the total.”1 This statement has been found to hold
true through research projects including: Dreisilker’s Total Motor System Maintenance and Management Program (DTM2 ™), the PAT Tool Project, and others.
For the purpose of this discussion, we will consider a paperboard plant with 485 motors. There are two operating production lines that have a potential downtime cost of $6,575 each. An average of 3 motors were repaired per month, of which a majority (70%) required rewind replacement (normally caused by immersion, contamination or the motors became coated in material). The facility operated 8,000 hours per year with the catastrophic failures normally causing one line to fail at a time. Additional costs, not covered by this discussion, included cleaning of the system prior to re-starting the operation. No maintenance program in place.
Table 3: Breakdown of Motor Horsepower and Repair Costs
Motor Size Number of Motors Rewind Cost Recondition
Cost < 20 horsepower 347 (Replacement, not
repaired) - - 20 15 $660 $220 25 10 $760 $255 30 2 $880 $295 40 3 $1,020 $340 50 27 $1,295 $430 75 18 $1,500 $500 100 21 $1,610 $540 125 32 $1,820 $610 400 6 $3,400 $1,200 750 4 $7,735 $2,600
The first step is to calculate the unplanned production downtime costs: Equation 1: Unplanned Production Downtime Cost
PCDowntime = (MF/Yr) x (PLost/failure) x (PCost)
= (36 motors/yr) x (4 hrs/failure) x ($6,575/hr) = $946,800/year
Where PC is the annual cost of unplanned downtime, MF is the number of motor failures, P represents production
Step 2 is to calculate the average cost of rewinding equipment. In this case, we will concentrate on just 20 horsepower and larger.
Equation 2: Average Cost of Rewinding Motors Ravg = ((Nn1 x RWCn1) + … + (Nnn x RWCnn))/NT
= ((1520 x $66020) + (1025 x $76025) + … + (4750 x $7735750)) / 138 motors
= $1,650
Where Ravg is the average rewind cost, Nn is the number of motors for each horsepower, RWCn is the rewind
cost for each horsepower
The average cost for reconditioning the motors is calculated the same way, except the reconditioning cost is used instead of rewind costs. For this example, the average
