Motor Testing and Reliability Based Improvement

By Moye Wicks

Introduction

A motor monitoring program is being developed as a service to offer customers of GHX Power Systems.  This summary will briefly explain the scope and intent of the motor circuit evaluation and current signature analysis programs.  A motor testing program has many advantages, but is primarily designed to identify early problems and to promote process reliability.  It increases knowledge of the present condition of customer identified motors, provides maintenance management the data needed for planning necessary equipment outages, and allows plant personnel to remain focused on their primary duties.  A comprehensive motor monitoring program is one that includes both off line motor circuit evaluation and on line motor current signature analysis.

Reliability History

Machine reliability is not a new concept, but has become a focus of various industries, especially in process plants, more significantly so in the last decade.  Reliability – based improvement programs have many advantages, but are primarily designed to increase machine availability for service by reducing or preventing catastrophic failures.  Motor reliability is now greatly improved through the collection and analysis of data using modern computer based analyzers.  Correctly identifying problems and tracking degradation of a motor can be accomplished using a time based approach to analysis of its condition while integrating time based and condition based maintenance.  Reliability Engineering has been described as collecting accurate data and having the ability to forecast asset – based problems using the trended data as a guide.

The term Reliability itself means the probability that equipment (a certain machine) will perform its required or intended function (i.e. pump or fan drive) satisfactorily under designed operating conditions, within a certain time period (its life expectancy). Most equipment owners know that they will have to spend money on maintenance of equipment, both expected and unexpected.  But what most wind up actually doing is spending more money than necessary on unneeded time- based disassembly, or simple reactive maintenance.  Studies have shown that companies desire to move from a reactive approach to maintenance, to one that is proactive, technical, comprehensive, and predictive in nature.  Equipment will degrade over time, and a proactive testing program helps monitor degradation of equipment.  A consistent proactive approach to maintenance should provide a plan that contains components of both operating and predictive test data.

Goals of the Program

The ultimate goal of a predictive program is to determine equipment condition and provide data needed to identify and allow repair of faults before catastrophic failure occurs, gaining the longest life possible from each piece of equipment.  A predictive program has the ability to reduce maintenance costs and at the same time increase equipment availability.  By allowing an outside company to provide services, a plant’s maintenance staff will have the option to spend more time actually helping to create the plant’s end product, and less time worrying about unnecessary equipment inspections, testing and maintenance.  Customers processes will become more profitable as technology improves the quality and quantity of data needed to make critical decisions on when to perform maintenance on the correct system component.

Condition Based Monitoring Program

 The benefits of a Condition Based Monitoring program include:

·        Reduction in unexpected motor failures

·        Improved machine availability for service

·        Less forced outages on electric motors

·        No need to remove the motor from the base before testing

·        Reduction in new and repaired motor costs

·        Improved quality assurance

·        Data can be used for a Root Cause Analysis

·        Advanced level data analysis and recommendations

·        Professional reports in hard copy and / or .pdf format

·        Better inventory control of spare parts such as bearings

·        Less scrap material

The analyzer we utilize is manufactured by the PdMA Corporation, of Tampa , Florida .  The software categorizes the analysis of each subject motor into what are known as “Fault Zones” which are areas of a motor where problems are likely to occur.  The fault zones are described as follows:

·        Power Quality >On Line

·        Power Circuit > On Line and Off Line

·        Insulation >         Off Line

·        Stator >              On Line and Off Line

·        Rotor >              On Line and Off Line

·        Air Gap >           On Line and Off Line

Off Line            Fault Zone                  On Line

Fault Zone Analysis – A breakdown of the general modes of failure of a motor:

One method of analysis of motor circuit data is to use an approach that “breaks down” areas of a motor into areas where faults are typically observed.  There are six fault zones of analysis used:

1.  Power Quality Fault Zone analyzes:

Ø      Voltage and current values and imbalance %

Ø      Voltage and current Total Harmonic Distortion %

Ø      Power Supply Harmonics

Ø      Voltage and Current Crest Factor (Peak / RMS)

 2.  Power Circuit Fault Zone Analyzes:

Ø      Connection based problems such as poor mechanical torque

Ø      Corrosion build – up on connections increasing the resistance at:

§         The motor starting contactor or breaker

§         A local disconnect near the motor

§         Inside the motor leads junction box

Ø      Defective power factor correction capacitors

Ø      Condition of motor power cabling from the starter     

3.  Insulation Fault Zone Analyzes:

Ø      Insulation deterioration caused by aging and cyclic fatigue

Ø      Damaged insulation caused by overheating stator windings

Ø      Poor quality of insulation in motor power cabling jacket

Ø      Contaminant build – up on stator insulation

Ø      Overall dielectric condition

 4.  Stator Fault Zone Analyzes:

Ø      Condition of the stator winding resistance value and imbalance %

Ø      Condition of the stator coil inductance value and imbalance %

Ø      Can analyze various types of insulation (i.e. Epoxy, Mica, Fiberglass, others)

Ø      Can determine phase to phase and turn to turn problems

Ø      Can test resistance to ground from 250V DC to 5,000V DC.

 5.  Rotor Fault Zone Analyzes:

Ø      Overall condition of the rotor

Ø      Broken, cracked, or high resistance end joints of the rotor bars

Ø      Off line and On line detailed analysis

Ø      Wound rotor insulation and bar condition

Ø      Field condition of synchronous motors

Ø      In – Rush / Start up test and Rotor Influence Checks

 6.  Air Gap Fault Zone:  (Concentricity should be within 10% of measured air gap mean value)

Ø      Dynamic and Static Eccentricity analysis

Ø      Air Gap inconsistency caused by anomalies such as: loose or misaligned end bells, and warped or bowed rotor shaft

Ø      Stator winding mis-shaping due to overheating or soft foot  

The included Advanced Spectral Analysis test analyzes the current spectrum by removing the 60 Hz band, and plotting torque – based variations at frequencies from 0 to 60 Hz.

Typical Technologies

 The typical technologies used in analysis of most motors that can be included with a motor reliability program include these areas:   

1. Motor Circuit Analysis – Off line monitoring of the motor circuit, which can be conducted from a motor starter or directly at the motor leads.  This examination of the motor circuit evaluates all off line parameters of the motor including:
 
1. Capacitance
2. Resistance and % Imbalance
3. Inductance and % Imbalance
4. Polarization Index Test, lasting 10 minutes with constant test voltage.  Measures resistance to ground value and absorption ratio.
5. Step Voltage Test, lasting 9 – 12 minutes with increasing steps of test voltage, measures leakage current.  
6. Rotor Influence Test, a method of plotting inductance with degrees of manual rotation of the motor rotor across more than one stator pole.  This is not necessary on every motor and requires another person.
2. Motor current signature analysis – On line monitoring of the motor while running.  Measures parameters of current and voltage (if available) to analyze motor under loaded conditions.  This important technology allows the motor to remain in service during the analysis.  This allows the study of the response to current, voltage, and variables affecting the motor windings.  Motors operating at 460 volts can be measured across the line, and medium voltage motors are measured using the Current and Potential transformer secondaries.  On line parameters include but are not limited to:
1. Measurement of current and its imbalance
2. Measurement of voltage (if available) and its imbalance
3. Rotor bar condition in response to current (should be >70% N.P. amps)
4. Analysis of power quality of incoming power to motor circuit
5. Rotor eccentricity indications

3. Vibration Analysis – used to measure mechanical parameters of a loaded, running motor.  Mechanical driven equipment is not routinely studied.

4. Infra Red Analysis and Ultra Sonic Analysis (Customers should pre-arrange with GHX if these are desired.)  Infra Red Analysis is used to detect various problems in running motors and other plant equipment, such as High voltage corona, insulator sparking, deteriorated electrical connections producing uneven conductor heating, and other problems that tend to generate a thermal gradient across surfaces.  Ultrasonic Analysis is used to detect electrical problems as well as compressed air passing through an orifice (usually unknown), as well as air compressor performance studies, and general leak detection.

Motor Testing Disclaimer:

A one time test is a snapshot in time of the health of a motor. The data obtained from the single test are compared to industry standards and a determination is made based on these standards which guide the recommendations of the analyst.  A one time test will not predict future failures of the motor or give information related to the life expectancy of the motor.  To predict potential future failures or the life expectancy of equipment analyzed, multiple test data must be collected over specific time intervals, and the results analyzed and trended.   

Even with trending, all potential failures will not be detected and failures attributed to external factors such as voltage spikes, increased ambient temperatures, overloads, etc) will not be detected.  The goal of the program is to reduce the number of unexpected motor failures, which results in financial savings and gives increased awareness of the condition of analyzed equipment. 

            Interested parties may contact GHX Power Systems for problem consultation or services desired.  GHX is located 3 miles south of downtown Houston , Texas and can be contracted for work anywhere in the U.S.A.  

Contacts: 

Ø      Moye Wicks, Motor Reliability Specialist

Ø      Gary DuBroc, General Manager

Address:

Ø      GHX Power Systems, a Division of GHX Inc.

Ø      7111 Ardmore St. Building H

Ø      Houston , TX 77054-3500

Ø      Phone:

§         (713) 748-0702  ( Main )

§         (713) 749-9620  (Fax)

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