REPAIR SPECIFICATION FOR LOW VOLTAGE
POLYPHASE INDUCTION MOTORS
INTENDED FOR PWM INVERTER APPLICATION
2nd Edition
Book 2 of the MotorDoc™ Series
Originally:
A Final Project
Presented to the
Faculty of the
School of Engineering
Kennedy-Western University
by
Howard W. Penrose, Ph.D.
Old Saybrook, CT
Disclaimer
Use of this Specification or the information contained in this study does not imply
or infer warranty or guaranties in any form.
MotorDoc™ E-Book
2nd Book of the MotorDoc™ Series
©1997, 2001
Howard W. Penrose, Ph.D.
SUCCESS by DESIGN
5 Dogwood Lane
Old Saybrook, CT 06475
e-mail: howard@motordoc.net
All Rights Reserved
$99.95
Repair Specification i
Abstract
REPAIR SPECIFICATION FOR LOW VOLTAGE
POLYPHASE INDUCTION MOTORS
INTENDED FOR PWM INVERTER APPLICATIONS
by
Howard W. Penrose, Ph.D.
SUCCESS by DESIGN
Purpose of the Study
With the increasing use of new Variable Frequency Drive (VFD) technologies new types of electric
motor failures have been discovered. These failures are the result of reflective wave, harmonics, motor
cooling, shaft currents, and others, which come from the types of outputs from Pulse Width Modulated
(PWM) inverters. While electric motor and drive manufacturers have been researching and modifying
new motors and drives to reduce these challenges in new applications, very little has been done in the
area of electric motor repair to withstand some VFD applications.
Repairing low voltage polyphase induction motors for potential VFD applications while maintaining
the original characteristics and efficiency of the motor is a possibility. Traditional electric motor
Repair Specification ii
repair practices have a tendency to damage the motor, reducing its original characteristics and
efficiency. In addition, the damage reduces the motor's ability to withstand VFD applications.
The purpose of this study is to determine the best electric motor repair practices to improve induction
motor survivability in potential inverter applications while maintaining the original efficiency rating of
the motor. The result will be a written electric motor repair specification to be used by electric motor
repair centers and end users for the repair of low voltage induction motors for VFD applications. The
specification is to help reduce the billions of dollars lost, per year, in unnecessary energy costs,
downtime, and repairs. The specification should also be utilized for the repair of all low voltage
induction motors to set the standard for the highest of quality repairs.
Method
Research data was collected for the purposes of the study. The format for the collection of the data
was to separate it into four stages. These four stages included a Previous Research Review, an Electric
Motor Repair Study, an Inverter Effect Study, and a Modified Repair Specification. The research has
determined that a specification, which has additional benefits, is both cost effective and justifiable. It
also has determined that there are additional areas for research which will have significant impacts in
the industry.
The study consisted of reviews of existing and ongoing research in the areas of inverter effects and
motor repair as well as an experiment with a motor and inverter. Research revealed that not only is a
repair standard feasible, but that existing equipment is available to meet a motor repair specification of
Repair Specification iii
this type. Additional work will continue in this area to improve repair methods and determine new
ways to reduce motor - VFD electro-mechanical challenges.
2nd Edition
The second edition includes additional information developed from research since its original
publication in 1996. These areas include the mechanical impacts of repair practices, the impact on
newer electric motor designs, and new test methods.
Repair Specification 1
Chapter 1
INTRODUCTION
Present Situation
Traditional electric motor repair practices are insufficient for electric motors in modern electrical
environments. This situation has been further aggravated by the use of electronic variable frequency
drives (VFD's) in commercial and industrial settings for energy conservation and manufacturing
processes. It is necessary to investigate and present a standard repair specification for low voltage
induction motor repair to reduce premature breakdown following the repair of an electric motor for
VFD environments.
Since 1990 the population of VFD's in industrial and commercial motor systems has
increased due to reduced cost, energy consciousness, and process improvement.
With the introduction of Pulse Width Modulated (PWM) technology using Insulated
Gate Bipolar Transistors (IGBT), the cost and size of modern VFD's have decreased
dramatically. This is compounded with the increased understanding by end users that
modern VFD's can usually pay for themselves, with energy savings, in a very short
period of time, in most cases under two to three years. Many industrial firms have
discovered that modern VFD's can help increase production while reducing
Repair Specification 2
downtime and maintenance costs by removing many of the cumbersome or
undependable mechanical systems which were used to vary speed or pressure. Over
the past decade the realized versus potential applications, in the United States, has
increased from 5% to over 14%.
Figure 1-1:Sine Wave vs. PWM
Pulse Width Modulated drives have a number of inherent potential affects on modern low voltage
induction motors. Harmonic distortion of the current waveform to the electric motor causes increased
motor losses, which shows in the form of excess heat, reducing electric motor efficiency and insulation
life. Partial Discharge causes ozone to occur in small voids in insulation systems which breaks down
the insulation. Reflective Wave phenomenon failures occur when the steep fronted pulses from the
inverter exceed the withstand voltage and rise time of the electric motor insulation resulting in coil to
coil or turn to turn shorts in the windings, usually following a period of partial discharge. Shaft
currents may occur due to erratic air gap magnetic flux effects on the rotor shaft causing currents
which pass through the motor bearings, reducing bearing life dramatically. Any given electric motor
Repair Specification 3
has a minimum operating speed for cooling purposes which, if the speed falls below this value, will
cause it to overload.
Figure 1-2: VFD Basics
Traditional induction electric motor repair leaves much to be desired for the harsh electrical
environment imposed by modern VFD's. In most cases, electric motor repair centers reduce costs,
wherever they can, in order to be competitive. Some of the short cuts include reducing wire size,
reducing insulation, machining short cuts, and shortening oven curing times in dip and bake processes.
Equipment costs are reduced through the use of burnout ovens, manual coil winding machines, and
varnish tanks. Traditional practices tend to:
1
Increase various motor losses resulting in higher operating temperatures and hot spots which
reduce insulation life.
2
Increase winding impedances resulting in greater susceptibility to reflective wave.
3
Poor machining practices which result in a greater chance that shaft currents will affect motor
bearings.
Repair Specification 4
4
Reduced insulation resulting in a greater chance that the motor will fail from reflective wave,
increased operating temperatures, or insulation imperfections.
5
Reduce motor operating efficiencies due to increased losses.
Purpose of Study
The purpose of this study is to determine the best electric motor repair practices to improve induction
motor survivability in potential inverter applications while maintaining the original efficiency rating of
the motor. The result will be a written electric motor repair specification to be used by electric motor
repair centers and end users for the repair of low voltage induction motors for VFD applications. The
specification is to help reduce the billions of dollars lost, per year, in unnecessary energy costs,
downtime, and repairs. The specification should also be utilized for the repair of all low voltage
induction motors to set the standard for the highest of quality repairs.
Definitions
Following are a series of definitions necessary for the purposes of this project:
1
2
End user - A term used for the owner of the electric motor and equipment or their
representative.
Repair Center - A business whose purpose is to repair and rewind electric motors.
3
Electric Motor - A device for converting electrical energy to mechanical torque. A basic
electric motor consists of a stator, endshields, rotor, bearings, and, depending on the
enclosure, a fan and fancover (Figure 1-3). The stator houses the stator core and windings
and acts as a shield from foreign material. The endshields house the bearings and center the
Repair Specification 5
rotor within the stator. The rotor consists of the rotor windings and core. Bearings are used
to allow for the low friction turning of the rotor.
Figure 1-3: Electric Motor Cutaway (GE Motor)
4
Stator and Rotor Core - The core of the stator and rotor consists of many layers of laminated
low carbon annealed, or silicon in newer motors, steel which is used as the medium for
magnetic fields. Laminations are often 0.019 to 0.045 inches thick to reduce eddy currents
(0.019 to 0.039 in energy efficient electric motors). The chemical makeup of the material is
such that it reduces the effects of hysterisis.
5
Stator Windings - Are made of many turns of insulated copper wire which are laid in slots
within the stator core. The basic winding styles are concentric and lap. Concentric windings
are often used for machine insertion at the factory or repair center. Lap windings are often
used when hand winding for mechanical and electrical strength.
Repair Specification 6
6
Rotor Windings - Are mounted within slots in the rotor laminations. They are often made up
of a copper or aluminum alloy with aluminum being the most common in motors under 600
VAC. The windings are in the form of solid bars through the rotor with shorting rings at either
end. The windings are energized as the result of magnetic fields cutting through the rotor
windings from the stator. A current flows through the rotor windings and interacts with the
magnetic fields in the airgap of the motor (The space between the rotor and stator
laminations), and a torque is developed.
7
Motor Losses - The energy that is consumed by electric motors is accounted for in heat losses
(Watts). These losses are broken down as core losses (eddy current and hysterisis), friction
and windage losses, stator losses, rotor losses, and stray load losses.
Figure 1-4: Motor Losses
4
3.5
3
Stray
Rotor
Stator
Friction
Core
2.5
2
1.5
1
0.5
0
25%
8
50%
75%
100%
125%
Core Losses - A combination of eddy current and hysterisis losses within the stator core.
Accounts for 15 to 25 percent of the overall losses.
9
Friction and Windage - Mechanical losses which occur due to air movement and bearing
friction. Accounts for 5 to 15 percent of the overall losses.
Repair Specification 7
10
Stator Losses - The I2R losses within the stator windings. Accounts for 25 to 40 percent of
the overall losses.
11
Rotor Losses - The I2R losses within the rotor windings. Accounts for 15 to 25 percent of the
overall losses.
12
Stray Load Losses - All the other losses not otherwise accounted for. Accounts for 10 to 20
percent of the overall losses.
13
Full Load Torque - The full load torque of a motor is the torque necessary to produce its
rated horsepower at full load speed. In pounds at foot radius, it is equal to the horsepower
times 5250 divided by full load speed.
14
Locked Rotor Torque - The locked rotor torque of a motor is the minimum torque which will
develop at rest for all angular positions of the rotor, with rated voltage applied at rated
frequency.
15
Pull-Up Torque - The pull-up torque of an alternating current motor is the minimum torque
developed by the motor during the period of acceleration from rest to the speed at which
breakdown torque occurs. For motors which do not have a defined breakdown torque, the
pull-up torque is the minimum torque developed up to rated speed.
16
Breakdown Torque - The breakdown torque of a motor is the maximum torque that will
develop with rated voltage applied at rated frequency, without an abrupt change in speed.
Figure 1-5: Motor Design and Torque
Repair Specification 8
17
Variable Frequency Drive (VFD) - A variable frequency drive is a device used to vary
voltage and frequency to a polyphase induction motor to vary the speed of the motor. The
purposes vary from energy efficiency applications to precise control of machinery.
18
VFD Input Section - Consists of a rectifier and filter. The purpose is to transform the
incoming AC voltage to DC voltage, also known as the bus voltage.
Figure 1-6: Input Section of a Variable Frequency Drive
Repair Specification 9
19
VFD Control Section - Consists of a control board and firing circuits. The control section
accepts real world input and performs the required operations through the use of a
microprocessor.
20
VFD Output Section - The output section includes the base drive circuits and the inverter.
The base drive sends signals which tell the inverter which IGBT's (or other types of power
transistors) to turn on and in which order. There is a diode in parallel with the IGBT, known
as a free wheeling diode, which allows energy stored in the collapsing motor fields to return
to the circuit when the IGBT turns off.
Figure 1-7: Inverter Section
21
Harmonics - Harmonics, generated by variable frequency drives, generally are fed back into
the power system and towards the induction motor. Voltage and current harmonics tend to
create alternate fields within motors and rotors, cause transformers to overheat, and interfere
with other electronic systems. Odd harmonics of the fundamental frequency are generally
found in power electronic systems. VFD's and three phase electronic systems generate 5th
and 7th harmonic distortion. Multiples of the 5th harmonic cause counter rotating fields in an
Repair Specification 10
electric motor, while multiples of the 7th harmonic cause positive rotating fields in an electric
motor. Combined 5th and 7th harmonics generate a 6th rotor harmonic. Harmonics generate
heat within the motor core, increase losses, and, if large enough, can cause the motor to
overload.
22
Reflective Wave Phenomenon - An interesting phenomenon which occurs in PWM drives is
known as reflective wave. This is a case where the fast rise time (dV/dt) of each pulse causes
a steep voltage wavefront that is directed towards the electric motor. As a result of a
combination of voltage overshoot on the switching portion of the IGBT, the switching
frequency, dV/dt, and the impedance of the motor cables and windings, voltage levels at the
motor terminals can exceed 1600 VAC in under 0.2 microseconds in a 480 VAC application.
This means that, at certain critical cable distances and high carrier frequencies, motor
winding conductors, which are generally rated for 500 Volts per micro-second withstand, will
short causing catastrophic motor failure.
23
Partial Discharge – Occurs in small voids in insulation between conductors and conductors
and ground. These discharges result in ozone, which damages the insulation system until the
winding faults.
24
Shaft Voltages - The type of voltage and current entering an electric motor from a VFD cause
imperfect currents to flow within the motor rotor. In some cases, this will cause eddy
currents to flow within the rotor shaft. These currents will want to flow out one motor
bearing into the motor housing and back through the other bearing. The bearings act like
capacitors, with energy collecting on the inner race of the bearing until it discharges through
the balls and outer race. The grease within the bearing acts as an electrolyte, and discharges
tend to occur where bearing surface imperfections cause high spots on the bearing housings
Repair Specification 11
and balls to come in close contact with each other. These discharges cause minute pits to
form on the machined surfaces causing early bearing failure.
Repair Specification 16
Chapter 2
REVIEW OF RELATED LITERATURE
Most Discussed Topics
Current literature surrounding the topics of electric motors and VFD's cover both energy efficiency and
motor failure due to inverter operation. Energy efficiency is a major topic as motors used in variable
torque applications can have paybacks well under one year. The topic of VFD effects on electric
motors tend to focus on five major areas:
1
Harmonic Distortion and how it effects the motor and other electrical equipment.
2
Reflective wave and steep fronted wave phenomenon and reduced insulation life.
3
Shaft voltages and reduced bearing life.
4
Increased temperature rise.
5
Minimum operating speeds and motor cooling.
Traditional electric motor repair practices are inadequate for electric motors which are being applied
with VFD's. If repaired electric motors are to be used with modern VFD inverters, the types of failures
must be addressed and corrected for during the repair.
Motors, Drives, and Energy Efficiency
With the advent of the Energy Policy Act of 1992 (EPACT), the US Department of Energy (US DOE)
set the Office of Industrial Technologies (OIT) to educate end users and distributors on the benefits of
Repair Specification 17
energy efficient motors. OIT assigned members of the US DOE Motor Challenge Group to pursue this
task and create cooperation between the government, industry, distributors, and manufacturers in order
to reduce energy consumption in the area of electric motors. While it was first determined that electric
motors were consumers of electrical energy, it was later discovered that electric motors were
converters of energy, rivaled only by transformers in efficiency. A group of individuals, representing
all aspects of the motor industry, met to put together Motor Challenge '95 in Chicago and determined
that electric motors were only part of an "electric motor system."
Electric motor systems account for 20% of all energy consumed in North America, with 57% of all
electrical energy generated. It was also found that roughly 70% of all electrical energy consumed by
manufacturers and industry, 48% of commercial electrical consumption, and 43% of home energy
consumption. When studied, it was found that a motor system is broken down into several
components: Incoming power; Motor control; Electric Motor; Coupling; Load; and Process. It was
also found that each component had an average opportunity for energy improvement (USDOE, 1994):
1
Power Quality Improvement - 8%
2
Motor Control (including VFD's) - 44%
3
Retrofitting with energy efficient electric motors - 18%
4
Coupling through Process - 33%
For example, a 250 horsepower (hp), 1800 Revolutions Per Minute (RPM), 92% efficient electric
motor versus a 250 hp, 1800 RPM, 94% motor would have a payback of $1,035/year (Formula 2-1) at
4000 hours, full load, and $0.06/kWh. A VFD in a variable torque application operating for 4000
hours per year, the energy efficient electric motor, 1000 hrs/yr at 100% speed, 1500 hrs/yr at 75%
Repair Specification 18
speed, and 1500 hrs/yr at 50% speed, would yield a payback of $25,999/yr (Formula 2-2). As it can
be seen, VFD's can yield a much larger payback due to pump and fan affinity laws (hp varies by the
cube of the operating speed).
Formula 2-1: Energy Payback of Energy Efficient Motors
250hp x .746kW/hp x 1.0 Load Factor x $0.06/kWh x 4000hrs x
[(100/92) - (100/94)] = $1,035/ year
Formula 2-2: Energy Payback for VFD's
100% Speed = 250 hp; 75% Speed = 105 hp; 50% Speed = 31 hp
[4000hrs x .746 kW/hp x (250hp/.94)] - {[1000 hrs x .746 kW/hp x (250hp/.94)] +
[1500 hrs x .746 kW/hp x (105hp/.94)] + [1500 hrs x .746 kW/hp x (31hp/.94)]}
= 433,314 kWh less energy/yr x $0.06/kWh = $25,999 / yr
As it can be seen, in the above example, the application of VFD's carry a tremendous financial benefit
in Variable Torque applications. However, with the benefits come a potential problem, how modern
VFD's affect the life of the electric motor and motor system.
Variable Frequency Drive Effects On Electric Motors
Variable Frequency Drives directly effect the electrical, mechanical, and thermal capacity of an
electric motor (EMCW '96, 1996, p. 109). These effects are in the form of harmonic distortion,
reflective wave, shaft currents, temperature rise, and minimum operating speeds.
Repair Specification 19
"A major effect of harmonic voltages and currents in rotating machinery is increased heating due to
iron and copper losses at the harmonic frequencies. The harmonic components thus effect the machine
efficiency, and can also affect the torque developed." (IEEE 519-1992, p. 35) The 5th and 7th
harmonics present in the inverter side of the VFD create a 6th rotor harmonic, which causes excessive
rotor heating, especially in areas where the laminations are smeared or shorted. The 5th and 7th also
cause excessive heating in the stator core where laminations are shorted or smeared. "Harmonics
create excessive heat in motors. Most motors are cooled by an internal fan driven by the motor. As
speed is decreased, the cooling capacity of the motor is decreased. The amount of heat the motor can
withstand depends also on the insulation of the windings." (Bonneville Power Administration, 1990,
p. 20) Harmonic distortion also causes uneven flux distribution in the air gap causing a situation know
as cogging (IEEE 519-1992, p.35). Combined, harmonic distortion problems from VFD's cause
additional heating and other electro-mechanical effects in electric motors.
"Fast rising voltage pulses cause non-linear voltage distribution in the winding (ie: uneven voltages
between coils and between turns). The shorter the rise time, the higher are the voltages between coils
and turns." (Voltage Reflection, 1994, p.7) This phenomenon is known as voltage reflection, which is
a case where the terminal voltage at the electric motor can exceed 1.9 times the VFD's DC bus voltage
(ie: 480 VAC application = 1250 VDC in less than 0.2 microseconds). The interturn wire insulation is
designed to withstand between 1000 and 1500 VDC in 2 microseconds, or greater. Fortunately, this
phenomenon occurs only at critical distances based upon the frequency of PWM pulses (carrier
frequency) and the impedance of the cable and motor. "... the electrical stress is limited to 60 Hz and
600 V rms AC in accordance with IEEE Standards. Unfortunately, these standards do not specify the
Repair Specification 20
maximum repetitive voltage transients or rate of rise (dV/dt) that the winding can safely withstand and
still meet the life expectations." (Industry Applications, 1996, p. 386).
"Recent experience suggests that PWM voltage sources with steep wavefronts especially increase the
magnitude of the above electrical problems, leading to motor bearing material erosion and early
mechanical failure." (Industry Applications, 1996, p. 250) Penrose (EMCW '96, 1996, p. 111) states
that bearing life is affected by electrical current flowing through the shaft, bearings, and motor
housings causing pitting in the bearing balls and surfaces. This has the effect of drastically reducing
the mechanical life of the electric motor. Values greater than 0.4 V may develop harmful bearing
currents while values over 2.0 V are considered catastrophic. (Industry Applications, 1996, p.252)
Traditional Motor Repair and VFD's
Penrose (Electrical Insulation, 1997) explains that peening and metalizing can cause mechanical
stresses in motor bearings, reducing bearing life. This stress is primarily caused by reduced clearances
within the bearing between the ball surfaces and the races. This increases the opportunities for shaft
currents to discharge as well as increasing friction, reducing efficiency.
"Fan replacement should also be considered, when the original fan has been damaged. The
replacement fan should be original, as well. If a fan is replaced by a larger fan, or one with more fins,
the motor efficiency will be reduced. If a fan is replaced by a smaller fan, or one with fewer fins,
cooling will be reduced, reducing the life of the motor." (Electrical Insulation, 1997) With the
increased heating inherent with VFD's, reduced fan size will reduce the life of the motor further.
Repair Specification 21
"The magnetic properties of the stator cores are damaged during burnout. The high temperatures
break down the lamination insulation along with the winding insulation creating paths for eddy
currents and hot spots... Often the stator core warps, causing irregular magnetic field paths and in turn
causing odd rotor currents." (E/EIC / EMCW '95, 1995, p. 458) These problems are aggravated by
the type of output of a variable frequency drive increasing shaft voltages and increasing hot spot
temperatures.
"Each coil's length may be different [in traditional coil winding] due to operator controlled tensioning,
causing unbalanced impedance values... If the turns are not layered correctly, coil insertion times will
increase and the chances of damaging the wire insulation are greater." (E/EIC / EMCW '95, 1995, p.
458) In addition, there may be a great number of wire crossovers, creating high potential points where
voltage can flash across, shorting the wire and failing the motor.
"... the varnish coating in the stator is wasted and acts as a thermal insulator, increasing internal
temperatures and stator and rotor I2R losses, and may clog cooling paths through the core. Dip and
bake provides moderate resistance to humidity due to voids in the winding insulation, particularly in
the stator slots." (E/EIC / EMCW '95, 1995, p. 458) In addition to increased heat, the voids in the
insulation system create weak spots which can flash over in VFD environments.
Summary
As it can be seen, there are a number of areas within traditional electric motor repair which can be
improved through alternate motor repair practices and motor repair standards for inverter application.
Repair Specification 22
These methods would maintain motor efficiency, reduce repair turnaround, and maintain acceptable
motor life in inverter applications.
Repair Specification 25
Chapter 3
METHOD
Project Approach
This research project shall consist of four stages with the final conclusion consisting of a repair
specification for three phase induction motors under 600 VAC. The four stages consist of the
following:
1
Previous Research Review
2
Electric Motor Repair Study
3
Inverter Effect Study
4
Modified Repair Standard
Stage 1: Previous Research Review
The first portion of the project shall consist of a review of previous research studies of inverter
applications and electric motor repair. The purpose is to further review research in these areas in order
to determine the necessary procedural changes to existing electric motor repair standards and practices.
Sources include the US Department of Energy (US DOE), Canadian Electrical Association (CEA),
Trade Journals, and The Institute of Electrical and Electronics Engineers, Inc.'s (IEEE) publications.
In addition, ongoing research by the Dreisilker Electric Motors, Inc.'s Research and Development
Department are reviewed. Notes, articles, and research papers are kept on file for the duration of the
project.
Repair Specification 26
Stage 2: Electric Motor Repair Study
Motor repair processes shall be analyzed through observation and research. The types of insulation,
stripping processes, machining and machine tolerances, winding methods, testing, and varnishing
processes shall be reviewed. A review of existing repair standards published by the Electrical
Apparatus Service Association (EASA) and IEEE will be compared to the existing practices. The
effect of repair on induction motors, which may be used in inverter applications, shall be determined
by reviewing collected data and the materials and practices observed.
Stage 3: Inverter Effect Study
A Pulse Width Modulated (PWM) Variable Frequency Drive (VFD) was set up to allow possible
maximum reflection pulses. The drive is to be connected for 480 VAC with a carrier frequency of
approximately 16 kHz or better then to a 480 VAC, 1800 RPM motor set at about twice the
recommended distance, per the maintenance manual. Both the motor and drive are to be matched.
The following is to be collected using a Fluke 97 Digital Storage Oscilloscope and a Fluke 41B Digital
Storage Harmonics Analyzer.
1
PWM Voltage output at the inverter (1)
2
PWM Current output at the inverter (1)
3
Harmonic Content (Voltage and Current) at the inverter output (2)
4
PWM Voltage input to the motor measured at the motor terminals (3)
5
PWM Current input to the motor measured at the motor terminals (3)
Repair Specification 27
The data is to be collected and stored on a Toshiba, Pentium, 120 MHz, Laptop for inclusion into
Chapter 4 of this project. The output of both instruments can be captured and the output imported into
WordPerfect 6.0 for Windows(tm) files.
Motor Nameplate:
1
General Electric
2
Model 5K6184BX205B
3
5 horsepower
4
1745 RPM
5
230 / 460 VAC
6
3 Phase
7
60 Hz
8
12.8 / 6.4 Amps
9
83.5 % efficiency
10
40oC Ambient
11
Insulation Class B
12
184T Frame
13
Type KS
Variable Frequency Drive:
1
General Electric
2
Model 6KAF343005E$-A1
3
5 horsepower
4
460 VAC
5
3 Phase
Repair Specification 28
6
Serial Number 615236R078
The critical distance for the VFD is 60 feet, the drive to motor cable distance is to be 110 feet and the
carrier frequency set at 16 kHz. The calculated distance would be as figured in Formula 3-1, 2, 3 and
4.
Formula 3-1: Speed of Pulse Propagation (v)
v = 1 / _(I * C)
I = Inductance per Meter; C = Capacitance per Meter
Formula 3-2: Cable Characteristic Impedance (ZC)
ZC = _(I / C)
The Motor Characteristic Impedance (ZM)
Formula 3-3: Reflection Factor at Motor (RM)
RM = (ZM - ZC) / (ZM + ZC)
Formula 3-4: Critical Cable Length
LCR = (v * tr) / 2
tr = the rise time of the pulse
Figure 3-1: Test Points
Repair Specification 29
Stage 4: Modified Repair Standard
Through observation and experiment, the summary of this study consists of a Repair Specification for
Low Voltage Polyphase Induction Motors Intended for PWM Inverter Application. The Specification
is meant to reduce the effects of inverter application on repaired motors while maintaining relatively
low repair costs, improving repair quality, and maintaining the efficiency value of the motor. The
standard also includes quality test sheets for maintaining records of motor testing associated with the
standard.
Repair Specification 32
Chapter 4
RESEARCH ANALYSIS
Stage 1: Previous Research Review
Continued study of the effects of PWM waveforms from IGBT inverters on induction motors is
outlined at this stage. Harmonic distortion, reflective wave, shaft currents, temperature rise, and
minimum operating speeds are covered through the review of previous research. Not surprisingly,
there is a great deal of information on the problems associated with this type of application, but few
published papers discuss solutions.
It is clear that the output waveforms of modern Variable Frequency Drives have a direct bearing on
electric motor life and premature failure. While the failures tend to be electro-mechanical in nature, it
is readily apparent that most engineers focus on either the electrical or mechanical effects depending
on their particular discipline. However, to truly understand the implications of VFD's on modern
induction motors, both the electrical and mechanical causes of failure must be understood.
Steep fronted surges and pulses created through the use of IGBT's are the primary culprits of electromechanical failure. These are usually combined with improper application of the motor and drive (ie:
the installer did not read the instructions). Factors influencing the PWM effects are (Persson, 1992, p.
1095):
1
RMS input voltage to the inverter.
Repair Specification 33
2
The rise time of the inverter output voltage.
3
Cable length from the inverter to the motor.
4
Motor insulation system, including wire, slot, and varnish insulation.
5
Stator winding design.
After reviewing many of the new studies, it is apparent that most of them focus on the interturn failures
of the winding conductors. In a few cases, it is noted that the entire insulation system must be
considered when viewing winding insulation failures. As noted in "Failure Mechanism of the Interturn
Insulation of Low Voltage Electric Machines Fed by Pulse Controlled Inverters," (Electrical
Insulation, 1996, p. 9) the causes of failure are due to partial discharges between conductors of
different potentials. This is further supported by Les Manz in "Motor Insulation System Quality for
IGBT Drives." (Industry Applications Magazine, 1997, p. 51) It should also be noted that the
switching frequency of the inverter plays a major part as it affects the distribution of the surges and
standing waves in the electric motor (Persson, 1992, p. 1095). The greater the frequency, the less the
effects are able to penetrate deep into the motor windings. This led to the original belief that standing
wave and steep fronted surge failures would be found as shorted or burned insulation in the first few
turns of each phase.
Interturn conductor failures were soon found to occur in other areas of electric motors. Phase to phase,
coil to coil, and deep turn to turn shorts had been found. However, these failures were not identified as
inverter related failures as few individuals understood how to identify them. It was later determined
that the winding design played an important part in the survival of electric motors in Variable
Frequency Drive applications. Concentric wound motors were found to be more likely to fail due to
high potential between the conductors in the coils of each phase. When the conductors are allowed to
Repair Specification 34
touch between coils of the same phase, the high potential voltage between the conductors and the
repetitive voltage pulses from the VFD, cause partial discharges across the insulation, resulting in
failure. Lap wound stators have a lower potential between conductors in each phase, coil to coil, and
have a higher resistance to coil to coil failures. Phase to phase failures occur where the phases are not
properly insulated from each other. This will be the result when a manufacturer will cut costs by
reducing the amount of insulation in the motor, creating a very high potential between conductors of
two separate phases. If the coils are not properly rewound and inserted in the stator, there is the
potential of a first and last turn touching or one conductor crossing several others. This creates an
opportunity for partial discharge between the conductors ending with a turn to turn short in a coil. It is
also noted that these failures occur primarily in motors operating on 460 + voltage systems, at critical
distances, and without added inductance between the motor and drive.
Motor speed is another consideration when operating an induction motor with a Variable Frequency
Drive. The concept behind the use of a VFD is to vary the speed of the electric motor. A new barrier
is introduced: What is the minimum operating speed of the motor at which point it will no longer
effectively cool itself and overload? This is compounded by the fact that VFD's cause the electric
motor to operate much warmer than it would normally. This is the result of a waveform which appears
"dirty", harmonic distortion, and electrical stresses from the voltage pulses. On average, most motors
can drop to around 50% speed without requiring additional cooling from an externally operated fan.
Many manufacturers now use Class F (Table 4-1) insulation, as opposed to class B, in order to allow
the electric motors to operate at higher temperatures.
Repair Specification 35
Table 4-1: Insulation Class Rating
System Class
Temperature, oC
Temperature, oF
A
105
221
B
130
266
F
155
311
H
180
356
Another result of the output of a Variable Frequency Drive on an electric motor is the potential for
increased shaft currents in the electric motor.
The erratic currents within the stator create
imperfections within the magnetic fields in the air gap. This impresses currents within the rotor shaft
which travel through the bearings, reducing the life of the bearings dramatically (Transactions on
Industry Applications, 1996, p.250). The currents also occur as a direct result of capacitive coupling
between the conductors, stator frame, and rotor.
As noted in "Those Pesky Inverter Drives," (Kevin Jones, 1996, p. 25) Kevin Jones notes that most
electric motor repair shops are just realizing the necessity to improve electrical insulation in repaired
motors to avoid inverter failure. Wire manufacturers are taking advantage of the present publicity by
developing wire insulation meant for inverter duty. This, however, takes away from the view of the
total motor insulation system and its importance over just one component (wire insulation). In effect,
Repair Specification 36
the total insulation system from wire to slot and phase insulation, not to mention varnishing methods,
must be observed.
Summary of Inverter Effect and Varnishing Systems
It has been suggested that motors repaired for inverter applications would withstand Variable
Frequency Drive effects through the use of Vacuum Pressure Impregnation (See Stage 2) versus dip
and bake or trickle varnish processes. In order to review this area, the effects of inverter output on
induction motors shall be summarized as well as the results of the three varnishing processes.
Modern Variable Frequency Drives affect motor insulation systems in a very specific manner. The
newer VFD's use a technology known as "Pulse Width Modulation (PWM)." The input of the VFD
uses a diode rectifier to create a constant DC Bus voltage filtered with a capacitor bank and inductors.
The inverter section consists of six (normally) power transistors known as Insulated-Gate Bipolar
Transistors (IGBT's). These are fired in a specific sequence through internal drive logic (main and
driver boards) too create a series of pulses at DC Bus levels at varying duration in either the positive or
negative direction. The result is a nearly sinusoidal current to the electric motor, although it will
actually appear kind of ragged. The greater the number of pulses (generated by the control, or
switching, frequency) and how they are fired, the smoother the current waveform. The "dirtier" the
current waveform, the more heat is generated in the electric motor due to increased heat loss.
The pulses in a PWM inverter will operate at switching frequencies from 2 kHz to 20 kHz. The lower
the frequency, the more "noise" the motor makes. This noise resembles that of bearing failure, but is
actually caused by expanding and collapsing magnetic fields in the airgap of the motor, but is not
Repair Specification 37
harmful. So, especially in commercial applications, the switching frequency may be set towards the
higher end in order to reduce noise (the motor rotor bar design also has an effect). IGBT's also have a
very fast turn on rate, as little as 0.1 microseconds.
When the IGBT turns on, there is a little overshoot, due to the drive capacitors discharging, and
ringing at the edge of each pulse. This creates a high, steep, travelling wavefront approaching the
motor, which, at short distances, has limited effects. At greater distances, line transmission effects
come into play, as well as voltage reflection as the wavefront approaches the high impedance of the
motor through low impedance cable. These can cause a voltage doubling at the motor terminals with a
very steep wavefront which can penetrate deep into the motor windings, depending on the frequency of
the pulses (the lower the frequency, the deeper into the windings). The weakest point in the insulation
system will usually fail.
The failures normally occur in 460 V + applications which are installed incorrectly. This is usually a
motor and drive which are installed at a critical distance without installing line reactors to reduce the
voltage and rise time of the pulses. The other cause is the motor construction. Areas within the coils
where a first and last turn touch (sloppy winding), concentric coils in a delta wound motor that has the
coils touching, reduced motor insulation (including varnish voids), and sharp turns in the coil ends. A
number of things have been done, or claimed, in order to reduce "inverter failures," which are actually
quite rare, to include VPI, "inverter duty wire," etc. Each, alone, will not eliminate these challenges,
but have to be considered as a group.
Repair Specification 38
There are three types of varnish application systems which have their pro's and con's for this type of
application:
1
Dip and Bake (Usually epoxy varnish): An inexpensive and common varnishing system.
Most motor shops and manufacturers can afford the equipment. When done correctly, two
dips and bakes without shortcuts, and with correct application of the rest of the insulation
system, Dip and Bake can be acceptable for inverter use. However, another type of failure
may result - Partial Discharge (PD). At 60 Hz PD will normally occur in systems over 6000
VAC, at higher frequencies PD may occur at much lower voltages. PD occurs where voids
between conductors exist, ie: bubbles in the dip varnish within a coil. Charges, much like
those in a capacitor, occur in the gasses within the voids and discharge, reducing the
insulation life. These voids may also capture moisture which, when the inception voltage of
the wire insulation is overcome, will cause a short.
2
Vacuum Pressure Impregnation (VPI): Very expensive equipment and varnish. The
difference between this system and dip and bake is the cost, it does not eliminate voids in
windings which increase the chance for failure due to Partial Discharge. While the concept is
reasonable, it is not realistic. The Medium Voltage motors which go through the VPI process
have taped windings which holds the varnish in, low voltage induction motors do not. The
low voltage motor is allowed to drain before it is placed in a curing oven. Voids occur in the
windings as the motor drains as the varnish does not cure at this point.
3
Trickle Varnishing (Normally Polyester):
A relatively inexpensive system, falling
somewhere between dip and bake and VPI. Curing occurs as the process is being applied and
produces a relatively low amount of waste varnish. The varnish flows through the windings
Repair Specification 39
due to gravity and capillary action, removing any voids. The final result is equivalent to
three dips and bakes, minus voids.
Rewind Studies
In 1991, a study was published by Ontario Hydro on electric motor rewind. The purpose was to
determine the effects of rewind repair on electric motor efficiency by failing nine of ten twenty
horsepower which were then randomly sent to electric motor rewind shops. The motors were of a
standard efficiency design and used an older annealed steel core. The average loss of efficiency was in
the area of 1.1% with the greatest reduction around 3.4%, the average was 2.2%. Compounded by the
loss in efficiency, the motors were found to have had conductor cross sectional area reduction, core
damage resulting in hot spots, and reduced insulation systems. The stators were all burned out in
burnout ovens and the stators dipped and baked. In inverter applications, the cost saving shortcuts and
stripping methods will reduce the life of the motor further.
In 1993, a similar study was performed by BC Hydro on ten of eleven energy efficient motors. The
results were slightly different than the Ontario Hydro study as the average loss was 0.5%. The core
steels were of silicone steel manufacture and had little or no increase in core losses. The greatest
losses were found to be in increased bearing losses due to friction. The one motor which had the least
reduction in efficiency failed in a later inverter test due to improper coil insertion and scratches on the
wire insulation due to improper insertion. The losses due to bearing replacement indicate that lower
quality bearings were used. The lower quality bearings would have a much reduced life due to shaft
currents.
Repair Specification 40
The Bonneville Power Administration's US Department of Energy sponsored "Industrial Motor Repair
in the United States," while studying the effects of repair on energy efficiency, did not focus on motor
repair. Additionally, the study focused on surveys and not on site analysis of motor repair. The
resulting "Electric Motor Model Repair Specifications" utilized the repair specifications of one repair
shop and overlooked many accepted repair methods. The repair methods, however, represent the
majority of repair methods of an average medium to high quality repair shop.
In 1995, the Canadian Electrical Association (CEA) performed a controlled electric motor repair
study. Demand Side Energy of Vancouver, BC, in conjunction with Hydro Quebec, a motor repair
industry organization, and Dreisilker Electric Motors, Inc. repaired several motors using a burnout
oven and the Dreisilker Thumm method. All of the motors in the program underwent the same repair
process, including dip and bake. It was found that in controlled repair processes there is little or no
reduction in efficiency after three rewinds.
In 1998, Dreisilker Electric Motors, Inc. and a Senior Research Engineer from the University of
Illinois at Chicago’s Energy Resources Center performed a study on the mechanical impact of
stripping temperatures on the electric motor. It was found that temperatures above 600oF would cause
the stator frames to distort, generating soft-foot conditions (0.002 to 0.050 inches) in materials ranging
from cast iron to aluminum (aluminum had greatest impact, cast iron had the least).
Repair Specification 44
Stage 2: Electric Motor Repair Study
Electric motor repair is normally not considered by most until a motor actually fails and causes process
or environmental inconvenience. The failed motor would be pulled from the customer location and
sent to a motor repair shop for evaluation. Past operating histories are not normally made available to
the motor repair shop or even considered by either the shop or customer. The motor may end up being
repaired repeatedly before it is noticed that a cause of failure needs to be investigated, including
inverter related failures.
Following outlines the various traditional repair methods utilized for electric motor repair (Electrical
Insulation, 1997, p. 14):
Upon receipt of an electric motor by an electric motor shop, a number of tests are normally performed
including a Megger test, phase to phase continuity test, and no-load test run. The Megger test
measures leakage to ground by applying 500 VDC or 1000 VDC (for up to 575 Volt motors) in order to
determine if the motor is grounded. A reading of 1.5 Megohms is the absolute minimum with several
hundred Megohms recommended for safe operation. Either the Megger or a multimeter is used to
check for continuity between phases in order to detect opens. If both sets of readings are acceptable,
most repair shops will test run the motor at full voltage and no-load. The voltage, speed, and current
are measured and recorded for future reference. At this stage, many of the motor defects may be
detected and noted.
Repair Specification 45
The motor is disassembled after marking the endshields and attachments for location and position. All
of the parts are removed, bearings pulled, and rotor removed from the stator. Visible internal defects
are observed and noted. The parts are cleaned and all windings are baked at 290oF for at least eight
hours.
Winding and mechanical tests are performed in order to evaluate the condition of the parts for
quotation purposes. A Megger test is performed at 500 or 1000 VDC with a minimum of (1 Megohm +
1 Megohm / Volt) for safety and several hundred Megohms recommended. An AC or DC Hi-Potential
test is performed at a voltage as found in formula 4-1. The AC test is a pass / fail test as, if the
winding fails the test
Formula 4-1: Hi-Potential Voltage Calculation
AC Potential = 0.65 * (2Em + 1000 V)
DC Potential = 0.65 * (2Em + 1000 V) * 1.7
Em = Motor Voltage Rating
a path to ground is formed through the motor insulation. A DC test is more forgiving, as it involves a
series of voltage increases (steps) up to the calculated level.
Repair Specification 46
Table 4-2: Bearing Shaft Fits (k5)
Bearing Number
Average Bore (mm)
Bearing Fit Tolerance (inches)
Minimum
Maximum
03
17
0.6693
0.6697
04
20
0.7875
0.7878
05
25
0.9844
0.9847
06
30
1.1812
1.1815
07
35
1.3781
1.3785
08
40
1.5749
1.5753
09
45
1.7718
1.7722
10
50
1.9686
1.9690
11
55
2.1655
2.1660
12
60
2.3623
2.3628
13
65
2.5592
2.5597
14
70
2.7560
2.7565
15
75
2.9529
2.9534
Repair Specification 47
16
80
3.1497
3.1502
If there is a sudden increase in the measured leakage, then the winding has failed. A surge
comparison test is performed at a level calculated as the Hi-Potential test.
Table 4-3: Bearing Housing Fits (H6)
Bearing No.
200 Series Housing Bore
300 Series Housing Bore
ODmm
Mininch
Maxinch
ODmm
Mininch
Maxinch
03
40
1.5748
1.5754
47
1.8504
1.8510
04
47
1.8504
1.8510
52
2.0472
2.0479
05
52
2.0472
2.0479
62
2.4409
2.4416
06
62
2.4409
2.4416
72
2.8346
2.8353
07
72
2.8346
2.8353
80
3.1496
3.1503
08
80
3.1496
3.1503
90
3.5433
3.5442
09
85
3.3465
3.3474
100
3.9370
3.9379
10
90
3.5433
3.5442
110
4.3307
4.3316
11
100
3.9370
3.9379
120
4.7244
4.7253
12
110
4.3307
4.3316
130
5.1181
5.1191
Repair Specification 48
13
120
4.7244
4.7253
140
5.5118
5.5128
14
125
4.9213
4.9223
150
5.9055
5.9065
15
130
5.1181
5.1191
160
6.2992
6.3002
16
140
5.5118
5.5128
170
6.6929
6.6939
The winding waveforms determine the phase to phase, coil to coil, or turn to turn condition of the
windings. Mechanical tests are performed with inside and outside micrometers. Acceptable
measurements are found in Tables 4-2 and 3.
Assuming machining and rewinding is required, certain steps are taken in traditional electric motor
repair. Through the rest of this stage of the study many types of low voltage repair processes shall be
reviewed.
There are a number of ways to machine bearing fits, including (Electrical Insulation, 1997, p. 15):
1
Peening: Is the practice of punching or marring mechanical fits to create a tighter fit. This
practice is not recommended for repair as it is uncontrolled. The marring of the metal
deformed the surface creating high spots meant to hold the bearing solidly in place. The
force used to mar the surface determines the tightness of the fits and the number of marks in a
given spot determines the surface area of the new fit. Wear on the bearing surface, the
amount and concentricity, will affect the internal forces within the bearing, usually increased
bearing friction and reduced internal clearances. This has the multiple effect of reducing
Repair Specification 49
motor efficiency, bearing life, and increasing the opportunity for bearing currents to damage
the bearings.
2
Metallizing: Consists of a one- or two-part spray process that requires metal to be removed
first. This process is susceptible to separation from the material to which it is attached in
instances of non-symmetrical pressure or when the surfaces have not been properly prepared.
When the material does separate, it creates uneven pressure on the bearing, having the same
effect as peening.
3
Welding: Similar to metallizing; However, it creates a stronger metal to metal bond, when
properly applied. If a repair requires adding metal, this is the preferred method. Machining
does create excessive wear and tear on the cutting tools, however.
4
Sleeving: The process of returning fits by machining and sleeving a motor shaft or housing.
This is the recommended method of motor repair, as it is more controlled. The bearing
housing is machined open concentrically to allow for insertion of a new sleeve, or the shaft is
turned down to allow a sleeve to be sweated on. Both methods are press fits and the sleeves
are normally .250 inches thick per side, allowing the part to be recentered on the lathe and
enough material to be removed to ensure a proper fit and concentricity.
5
Refabrication: While expensive, this method is the best for machining severely worn motor
parts, shafts in particular.
In order to rewind the electric motor, the windings must be removed. All processes begin with
removing one coil end. Following are the traditional methods for coil removal (Electrical Insulation,
1997, p. 16):
Repair Specification 50
1
Direct Flame: A flame from a torch or other source is directed into the core of the motor,
also includes placing the stator in a bonfire. The temperature is uncontrolled and severe
damage to the core will occur. Damage to the frame occurs causing warping, uneven stator
air gap, and soft foot. The winding is reduced to ash, and the windings removed.
2
Chemical Stripping: The core is lowered into a chlorinated solvent bath and kept submerged
until the varnish is dissolved enough for coil removal. Chemical stripping is ineffective in
many cases, such as overloaded stators. The chlorinated solvent presents potential health,
environmental, and disposal problems. In some cases, the solvent is not completely removed
when the stator is rewound, and the solvent works against the new motor insulation.
3
Burnout: The stator is placed into a burnout oven that is set for a recommended temperature
of 650oF (345oC). It is kept at this temperature until all the varnish and insulating materials
are turned to ash (eight hours or more). If the temperature exceeds this level, and often does,
damage to the stator core and frame will result, reducing motor efficiency and mechanical
reliability. Gasses and other byproducts are exhausted through a smoke stack into the
atmosphere.
4
Mechanical Stripping (Dreisilker / Thumm Method): Using a heat source, such as gas jets, a
distance away from the core, the back iron and insulation is warmed until the windings
become soft and pliable (approximately 10oC above the insulation class of the varnish
insulation). The coils and insulation are removed using a slow steady hydraulic pull.
Temperatures remain low, stripping times extremely fast (ie: 2.5 hours for a 350 hp motor),
and there are no airborne byproducts or disposal problems. Attempts at duplicating this
process using pneumatic pulling methods have resulted in core laminations being pulled
apart. Therefore, pneumatic machines of this type should be avoided.
Repair Specification 51
5
Mechanical Stripping (Water Blasting): A high-pressure stream of water is used to blast the
coils out of the stator slots. This is a fast method of coil removal. Personal injury due to
high water pressure and mechanical damage can be avoided by experienced personnel and
safety devices.
6
Mechanical Stripping (Hot vapor process chemical stripping): A stator is submerged in a
bath of non-chlorinated petroleum based solvent at a temperature of 370oF for a short period
of time. It is then removed and the coils are removed with high pressure air. The solvent has
an oily smell, which must be masked, and is difficult to dispose of. Personal injury and
mechanical damage can be avoided by experienced personnel and safety devices.
Direct Flame and burnout ovens cause hot spots and warping of the stator core. The concern over
these types of damage include reduced operating efficiency and reduced insulation life in normal
operating conditions.
In VFD operation, the hot spot temperature and core losses increase
dramatically reducing the motor's ability to withstand VFD operation dramatically. Warping causes
uneven magnetic fields within the stator airgap, which increases the possibility of harmful shaft
currents.
Once the stator has been stripped and cleaned, the coils must be replaced. The first step is to insulate
the stator slots with Class F or H insulating paper. Some electric motor shops will not fully insulate
windings, will reduce wire sizes for easier installation, and / or change winding design (Industrial
Motor Repair In the United States, BPA, 1995). The insulation rating of the motor is determined by
the lowest insulation class any portion of the electric Motor (ie: Class B lead wire insulation will
Repair Specification 52
result in a Class B insulation rating for the motor). The wire insulation is normally of a double bonded
type with a voltage withstand of 500V per microsecond (Table 4-4).
Table 4-4: Properties of Conventional Wires and Survival at
2 kV, 20 kHz, 90oC, 83 kV/ microsecond
Std Wire 1
Std Wire 2
Std Wire 3
18 H APTz
18 T APTz
18 H APTz-DDg
Wire Diameter
0.0434-0.0436"
0.0443-0.0444"
0.0539-0.0543"
Bare Wire Dia.
0.0404 inch
0.0401 inch
0.0398-0.0400"
Insulation Build
0.0030-0.0032"
0.0042-0.0043"
0.0141-0.0143"
13607 volts
16012 volts
10246 volts
4389 volts/mil
3766 volts/mil
722 volts/mil
0.09
0.08
N/A
580 seconds
1198 seconds
N/A
Dielectric
Breakdown
Dissipation Factor @
1kHz
Time to failure
There are several types of winding methods (Electrical Insulation, 1997, p. 17):
Repair Specification 53
1
Hand Winding: Is performed by a tower-type coil winding machine and mechanical counter.
The winding technician must try to maintain correct tension and layering of the coils, or the
coils will be difficult to lay in the stator slots. In the worst case, there will be wires crossing,
which will increase the turn to turn potential in the wire, creating an area that may short under
certain operating conditions, including VFD's.
2
Automatic Coil Winding Machines: Maintain constant tension and proper count of the coils.
This process still requires a technician to observe operation, but still succeed in reducing
labor time.
3
Computerized Coil Winding Machines: The process of winding is fully automated, leaving
the technician free to perform other tasks while the machine winds the stator coils. Proper
tension, layering, and turn counts are maintained. Layering of the coils allows for nick-free
insertion of the coils, and lower potential between conductors.
The coils are then inserted by hand or machine. Once the coils have been inserted, the coil ends are
insulated and connected. The coil ends are tied down for mechanical strength. Care must be taken not
to pull up the phase insulation, if any.
Winding tests are performed before the motor is varnished. A 500VDC Megger test is performed with
1.5 Megohms as a minimum, with 500 Megohms recommended. A hi-potential test is performed at
values calculated at formula 4-2, as well as a surge comparison test at the same level.
Formula 4-2: Hi-Potential Calculation
VAC = (2Em + 1000)
VDC = (2Em + 1000) * 1.7
Repair Specification 54
The final step is to insulate the windings with varnish. As with the slot insulation, it is common
practice to use Class F or H varnish on the windings. There are several different varnish methods
(Electrical Insulation, 1997, p. 17):
1
Dip and Bake (Usually Epoxy): An inexpensive and common system. Most motor shops and
manufactures can afford the equipment. When done correctly, two dips and bakes without
shortcuts, and with correct application of the rest of the insulation system, can be acceptable
for inverter duty use. However, another type of failure may result - Partial Discharge. At 60
Hz, PD will normally occur in systems over 6000 VAC, at higher frequencies PD will occur at
much lower voltages. PD occurs where voids between conductors exist, ie: bubbles in the
dip varnish within a coil. Charges, much like those in a capacitor, occur in the gasses within
the voids and discharge, reducing the insulation life. Requires approximately 20 hours for
the complete the dip and bake process.
2
VPI: Very expensive equipment and varnish. The difference between this system and dip
and bake is the cost, it does not eliminate voids in windings which increase the chance for
failure due to PD. While the concept is reasonable, it is not realistic. The Medium Voltage
motors which go through the VPI process have taped windings which holds the varnish in,
low voltage induction motors do not. The low voltage motor is allowed to drain before
putting it in the oven. Voids occur in the windings as the motor drains as the varnish does
not cure at this point. Not much different in time from Dip and Bake and the Varnish is very
expensive.
Repair Specification 55
3
Trickle Varnishing (Usually Polyester): A relatively inexpensive system (falls somewhere
between dip and bake and VPI). Curing occurs as the process is being applied (ie: 2 hours
for a 350hp motor for the complete process, including curing) and produces a relatively low
amount of waste varnish. The varnish flows through the windings due to gravity and
capillary action, removing any voids. The final result is an equivalent of 3 dips and bakes,
minus voids.
In most cases, the rotor is balanced, before assembly, with all of the rotating components mounted.
The rotor is mounted on centers and spun at a multiple of the running speed. The amount and angle of
vibration is determined by the balancing machine and technician. Weight is added or removed in order
to reduce vibration in the rotor at running speed. Reducing vibration in the electric motor is important
as vibration causes reduced bearing life and can damage structures.
The motor bearings are mounted on the shaft bearing journals. Bearings are mounted using the
following methods:
1
Arbor Press: The bearings are placed on the shaft, then a sleeve is placed against the inner
race of the bearing. The assembly is placed in an arbor press and the bearings are pressed on.
If the bearing or sleeve is not mounted correctly, or if the bearing journal is too tight, this
type of installation may mar the shaft surface. If the surface becomes marred, it will cause
uneven pressure in the bearing as if the shaft surface was peened.
2
Induction Heater: The bearings are place on a laminated bar which is placed on a coil. The
coil is energized and induction causes the bearing to get hot. The bearing is allowed to heat
Repair Specification 56
to approximately 203oF which allows the inner race of the bearings to expand. The bearing is
then slid onto the journals and cooled down. If the bearings are allowed to overheat, the
inner race may deform the balls within the bearing and the bearing races themselves. If the
bearing is allowed to get hot enough, the metal may become brittle or crack.
3
Convection Oven: The bearings are placed in a convection oven set for 203oF long enough
for the bearings to reach temperature. The bearings are removed from the oven and mounted
on the shaft. This is a time consuming process as it may take several hours to get to
temperature.
4
Hot Oil Bath: The bearings are placed in a temperature controlled oil bath until they reach
temperature. They are then removed from the oil bath and mounted on the shaft. This
process does not take as long as the convection oven, but is messy.
Once the bearings have cooled the motor is assembled. The shaft is placed into the stator in a manner
not to damage the windings or laminations. The endshields are then placed onto the stator over the
shaft and bearings.
The motor is test run before placing other components, such as fans and fan covers, to determine that
there are no defects. This test run is normally performed unloaded for ten or fifteen minutes to
determine if there are unusual noises or if the bearing housings are overheating. Amp and voltage
readings are taken and recorded, the amperage should be approximately 30 to 50% of full load at rated
voltage for 1800 or 3600 RPM motors.
Repair Specification 57
All components are remounted on the motor depending on the marks placed on the motor during
disassembly. The motor is then run at all voltages and speeds. The motor is run for 30 minutes at the
customer operating voltage and speed. Voltage and Amperage readings are taken and recorded. In
some cases, bearing temperature and vibration readings are recorded.
The motor is painted and returned to the customer. While painting has the least effect on the operation
of the electric motor, it is what the end user sees. Therefore, the poorest repair job can appear to the
end user to be the best, based upon the outer appearance of the motor. Shipping an electric motor is
also a concern, as the motor must be shipped in the correct manner to avoid damage to bearings. For
example, a vertical pump motor should be shipped and handled vertically.
Repair Specification 60
Stage 3: Inverter Effect Study
As outlined in the Chapter 3, Stage 3 outline, an inverter effect study was performed as part of this
research paper. The study was designed to determine the electrical environment of an electric motor
and Variable Frequency Drive System.
The inverter effect study was performed on April 1, 1997, during the course of an 8-hour day. The
location, personnel, and equipment was provided by Dreisilker Electric Motors, Inc., 352 Roosevelt
Rd., Glen Ellyn, IL 60137, as part of the Field Service Division's Research and Development budget.
The personnel involved were:
1
Howard W. Penrose; Director, Field Service / R&D - Provided experiment outline, direction,
and verification.
2
Hao Zhong; Electronic Engineer, Field Service - Experiment design and performance as well
as data collection.
3
Jay Malo; Electronic Technician, Field Service - Assisted with the design and data collection
for the experiment.
The provided location was the Field Service / Electronics Department. All appropriate safety
measures were followed to ensure the safety of personnel both working and not working on the
experiment, as well as protective measures for the equipment. This included barriers and signs where
conductors crossed exit / entrances.
Repair Specification 61
The electric motor and Variable Frequency Drive were procured as outlined in Chapter 3. All
adjustments were made as outlined. It was determined, through review of the owners manual, that a
distance of five feet would be selected for the close readings, and 110 feet for the critical distance
readings. Three separate conductors of #16 gage, stranded wire, were selected and run around the
perimeter of the Electronic Department space completing one loop. The three conductors were kept
separate and at a height of approximately two feet from the ground. The motor and VFD were located
three feet from each other and were connected to a common ground. The conductors were connected
by twisting strands together to allow for data collection points. The motor was not loaded during
operation.
Motor and VFD at Five Feet
The motor, drive, Fluke 41B Power Analyzer, and Fluke 97 Digital Storage Oscilloscope were set up
for the first part of the experiment. The applied voltage was 480 VACRMS. The drive was started at 16
kHz switching frequency and a 20 second ramp-up time. The data collected were as shown in the
following Figures:
Figure 4-1: Inverter Output Voltage
Repair Specification 62
Figure 4-2: Current Waveform
As it can be seen, the captured waveforms are not "neat," which is an unfortunate idiosyncrasy of the
Fluke 97. The same points were double checked with a TekTronics 50 MHz oscilloscope with a CRT.
The data is interpreted from the TekTronics scope and the Fluke 41B Analyzer.
Repair Specification 63
Motor and VFD at 110 Feet
The motor and Variable Frequency Drive were reconnected with the 110 feet of wire. They were then
restarted as before. One of the most significant changes in operation was the sudden increase in
electrical noise from the electric motor. Following is the data collected by the Fluke 97:
Figure 4-3: Inverter Output Voltage
Figure 4-4: Current Waveform
Repair Specification 64
It is noticed that there is some change in the voltage waveform which is similar to data found in other
studies, but a much lower level. The waveforms were much more apparent with the Tektronics
oscilloscope.
Harmonic Analysis
There was very little significance between the short and long distance as far as harmonics are
concerned. At both distances, the levels remained the same. Following are the various waveform and
harmonic figures and the summary table produced by the Fluke 41B:
Figure 4-5: Voltage Waveform
Voltage
1000
500
Volts 1Ø
0
-500
-1000
.
2.08
4.17
6.25
8.34
mSec
10.42 12.51 14.59
Repair Specification 65
Figure 4-6: Current Waveform
Current
5.0
Amps
1Ø
2.5
0.0
-2.5
.
2.08
4.17
6.25
-5.0
8.34
10.42 12.51 14.59
mSec
Figure 4-7: Power Waveform
Power
5000
Watts
1Ø
2500
0
-2500
.
2.08
4.17
6.25
-5000
8.34
10.42 12.51 14.59
mSec
Figure 4-8: Voltage Harmonics
Voltage
500
400
Volts rms
1Ø
300
200
100
0
DC
2
4
1
6
3
8
5
10
7
12
9
14
11
13
16
15
18
20
17
19
22
21
24
23
26
25
28
27
30
29
31
Harmonic
Figure 4-9: Current Harmonics
Current
3.0
2.5
Amps
rms 1Ø
2.0
1.5
1.0
0.5
0.0
DC
2
4
1
6
3
8
5
10
7
9
12
11
14
13
16
15
18
17
20
19
22
21
24
23
26
25
28
27
30
29
31
Harmonic
Figure 4-10: Power Harmonics
Power
1
0.0
DC
3
2
5
4
7
6
9
8
11
10
13
12
15
14
17
16
19
18
-0.2
KW 1Ø
-0.4
-0.6
-0.8
-1.0
Harmonic
21
20
23
22
25
24
27
26
29
28
31
30
Repair Specification 66
The Fluke 41B Analysis was taken at the output of the VFD. However, it is observed that the
instrument converts the pulses back to a waveform. The harmonic content is fairly high as IEEE 519
only allows voltage Total Harmonic Distortion (THD) to be 5% and current THD to be 3%. In this
case it can be observed that the THD of the drive and motor system would contribute to motor heating.
Table 4-5: Power Analysis Summary
Summary
Frequency
Power
Voltage
Current
59.96
RMS
487.2
2.96
1 phase
Peak
678.5
4.17
KW
-0.88
DC Offset
0.1
-0.03
KVA
1.44
Crest
1.39
1.41
KVAR
1.14
THD Rms
1.72
2.41
Peak KW
-2.39
THD Fund
1.72
2.41
128o lead
HRMS
8.4
0.07
Total PF
-0.61
KFactor
DPF
-0.62
Phase
Stage 3 Conclusion
1.04
Repair Specification 67
Although the levels of reflective wave and steep wavefronts were not as significant as was expected,
the level of harmonic content was. When reviewing the difference between the experiment and an
actual application several distinct differences show:
1
Wire - In a true application, the wire would have been run through conduit. This would have
allowed capacitive coupling between the cable and ground. In many cases, the wire would
have been solid, which would have allowed for increased distortion. Also, the wires would
have also been immediately adjacent to each other, allowing for some additional noise at long
distances.
2
Motor Load - A motor load would have significantly changed the power quality and voltage
reflection. Most motor applications will be 50% loaded or better.
3
Frequency - Varying the frequency and speed would have had some effect on the motor and
drive in this application.
The harmonic distortion and voltage reflection would impact the motor application. Even unloaded
there was an audible change in the motor operation, in the form of electrical noise. The level of
harmonic distortion in this experiment was enough to create some additional heating in the motor, even
at full speed. Therefore, it is prudent to apply a specification to the repair of motors for inverter duty
applications.
Repair Specification 71
Stage 4: Low Voltage Induction Motor Repair Specification
for Motors In Inverter Applications
1.
INTRODUCTION
1.
Purpose: To provide a specification for use by electric motor repair facilities and
end users in order to present higher quality, more cost effective, and timely repairs
for electric motors in inverter applications. This specification shall also provide a
means for evaluating repairs and electric motor repair facilities.
2.
Scope: This specification covers the general recommendations for the repair of
electric motors above five horsepower and outlines the responsibilities and testing
requirements for world class, energy efficient, and environmentally friendly electric
motor repair for inverter applications. These recommendations apply to horizontal
and vertical motors, NEMA frame size 140 and above, having a voltage rating of
600 VAC or less. These specifications can also be applied to motors which are not
meant for inverter applications.
3.
References: This specification refers to all NEMA and IEEE specifications and
standards which relate to the equipment within the scope of this specification.
2.
PREREPAIR RESPONSIBILITIES:
1.
End User Responsibilities:
1.
Record Keeping: The more information which can be provided in any
repair, the better.
By keeping and making available a repair and
Repair Specification 72
maintenance history, recurring problems and other correctable situations
may be identified.
2.
Proactive Management: By remaining proactive with the repair facility
and providing authorized contact names, confusion can be avoided
through excellent communication.
Also ensure that the name(s) of
contacts at the repair facility are available to the authorized contacts.
3.
Pre-Evaluate the Repair Facility (Attachment 1):
(1)
Quality System: Ensure the repair facility has a recognized
quality control program equal to or exceeding ISO 9002
requirements. Also ensure that this program is available for end
user review and audit.
(2)
Pricing Structure:
Agreement as to costs, inspection,
rewinding, or reconditioning motors should be established in
advance. It should be clear that any changes to any agreements
are agreed to in writing before or during the repair process.
(3)
Warranty: All warranty and conditions should be in writing and
clearly understood by both parties before repairs are begun.
(4)
Customer Service: The repair facility should have a dedicated
customer service staff. The end user should also ensure that a
customer service representative is assigned to their account.
(5)
Field Servicing: The repair facility should have personnel
trained and in place to conduct field investigation and repair of
Repair Specification 73
the electric motor and associated controls. Services such as field
balancing, vibration analysis, and infrared inspection are a plus.
(6)
Pickup and Delivery: The repair facility should have a system
or vehicles in place to provide pickup and delivery services.
(7)
Lifting Equipment Capacity:
The facility should have
appropriate equipment of the correct capacity and condition to
adequately handle the motors and move them throughout the
repair shop.
(8)
Cleanliness: Facilities should be clean and orderly, and tools
and equipment in good repair.
The area, containers, and
equipment used to apply the insulation systems should be in
excellent condition.
(9)
Insulation Requirements: The necessary equipment to install
and test the insulation system should be available. The minimum
insulation system capability for rewound electric motors should
be Class F.
(10)
Winding Removal: Appropriate coil removal practices should
be in place. Mechanical stripping methods (ie: Dreisilker /
Thumm Method) should be used due to speed, environmental
cleanliness, and they result in no reduction in efficiency. Most
mechanical methods do not damage the electric motor. Burnout
and Chemical stripping methods are environmentally unfriendly
Repair Specification 74
and are proven to reduce electric motor efficiency as well as
damaging the motor stator.
(11)
Equipment (All equipment to be used for testing must be
calibrated):
(1)
Machining equipment capable of handling any
machining requirements of the electric motor.
(2)
Machine measuring tools:
Inside and outside
micrometers, dial indicators, depth gages, etc.
(3)
Balancing equipment capable of handling all provided
rotating parts of the equipment.
(4)
Vibration Analysis equipment and certified vibration
analysts.
The equipment should be capable of
producing vibration spectra which can be used for
reporting and troubleshooting.
(5)
Infrared Analysis capabilities for core testing stators.
(6)
Core test capabilities for before and after Watts per
Pound loss records.
(7)
Computer controlled or automatic rewind machines.
(8)
Equipment to perform dip and bake, trickle
impregnation, and Vacuum Pressure Impregnation
(VPI) of insulating varnish.
(9)
Growler.
(10)
Bearing Heaters: Induction, oven, oil bath, or arbor
press.
Repair Specification 75
(11)
Test voltage and power capabilities to test the motors
under full voltage and no load. Also an inverter test
stand designed to operate motors used in inverter
applications.
(12)
(12)
Ohmmeters and Milli-Ohmmeters.
(13)
High Voltage Meggers (up to 1000 VDC)
(14)
High Potential Testers (AC and DC)
(15)
Surge Comparison Testers.
Repair vs. Replace: A repair vs. replace policy should be
agreed between the end user and repair facility to ensure
unnecessary disassembly and inspection time is avoided.
2.
Repair Facility Responsibility:
1.
Traceability: A system for tracking repairs and paperwork must be in
place. All parts must be individually marked or in marked bins and
located. Job numbers should associate parts and paperwork and must be
located on the repaired motor.
2.
Record Keeping: All records maintained through the repair including Job
Tickets, Test Sheets, and Winding Data, should be kept for a determined
period of time.
3.
Personnel: The repair center shall maintain sufficient personnel of
suitable training and experience to perform repairs. Training records shall
be maintained. All training shall meet the repair center's quality program
requirements and all appropriate OSHA and other safety standards.
Repair Specification 76
3.
INCOMING INSPECTIONS
1.
Job Tickets (Attachment 2): Should have space for customer information,
nameplate information, and any additional notes and information.
1.
Record all nameplate information available. If the motor nameplate is
missing, the end user should be contacted to determine if the information
is available.
2.
The motor should be visually inspected upon receipt of the motor. Any
discrepancies should be noted on the Job Ticket and communicated to the
customer. For instances where unusual damage or conditions cannot be
described, pictures should be taken.
3.
The motor is evaluated for possible repair vs. replace. This may include
an evaluation determining that a certain amount of repair would merit
replacement (ie: OK for bearings, replace if rewind or machining).
2.
Initial Inspection (A Test Sheet (Attachment 3) is initiated):
1.
Physical Inspection: The shaft is inspected, including the keyway. The
shaft is then checked for freedom of rotation and looseness in the
endshields.
2.
Insulation Resistance Test: A megger test of 500 or 1000 VDC must be
performed for one minute. The minimum safe insulation level is 1
Megohm + 1 Megohm per kV of the motor with several hundred
Megohms being recommended. The measurement should be adjusted for
temperature (IEEE 43-1974, p. 9).
Repair Specification 77
3.
Motor Circuit Analysis: A Motor Circuit Analysis test should be
performed prior to test running the electric motor. Readings of resistance,
impedance, phase angle, inductance, and current/frequency should be
performed and compared, phase to phase, compensating for rotor position.
An ALL-TEST™ or equivalent instruments should be used.
4.
Test run the motor, noting any operating faults and noting voltage and
current readings.
4.
DISASSEMBLY AND INSPECTION
1.
Mechanical Disassembly:
1.
Punch mark all parts, including endshields and connection box, for
position identification. The standard is two punchmarks on the drive end
and one on the opposite drive end. Each set of punchmarks should be
unique from all others.
2.
Remove all bolts and parts. Look closely for cracks or physical defects in
the parts. Remove the rotor in a way that the laminations are not smeared
(some devices are available to assist with this process).
3.
Remove the bearings with bearing pullers. Observe the condition of the
bearing journal as the bearing is being removed. High shiny spots, rust,
flaking, or other obvious defects indicate potential machining.
4.
Inspect the windings for excessive grease, dirt, or moisture and indicate
any findings on the test sheet. Also observe for obvious signs of failure
such as overloaded, single phased, or shorted windings and record any
findings.
Repair Specification 78
5.
All parts should be cleaned with solvent or soap and water. The stator and
rotor should be placed in an oven set for 295oF for six to eight hours until
dry.
2.
Mechanical Tests and Measures:
1.
The bearing housings and journals are measured with inside and outside
micrometers.
The results are compared to Tables 4-2 and 3 for
acceptance.
2.
The shaft is placed on centers and the output shaft checked for straightness
with a dial indicator.
3.
Electrical Tests and Measures:
1.
The rotor windings are to be tested with a growler and iron filings. The
filings are placed on a sheet of paper and run across the surface of the
rotor. Any breaks in the line of iron filings indicate broken rotor bars.
The surface should be viewed for holes or pits.
2.
Stator Winding Tests:
(1)
Visually inspect stator windings looking for loose ties, broken
conductors, loose stator, discolored or brittle insulation, or other
visual defects.
(2)
Insulation resistance test at 500 VDC with the results to be 200
Megohms, or better.
(3)
An AC or DC hi potential test should be performed at a voltage
calculated in Formula 4-1. The AC hi-pot is a pass / fail test with
the fail damaging the insulation beyond repair. The DC hi-pot is
Repair Specification 79
more forgiving with a sudden increase in DC leakage indicates
failure.
(4)
4.
A motor circuit analysis test is performed.
Repair Evaluation:
1.
All test results are reviewed and repair recommendations and pricing are
determined, including availability of parts. Repair versus replace is again
reviewed.
5.
2.
The end user is contacted and a purchase order is obtained.
3.
The scope of work is outlined on the job ticket for reference.
Mechanical Repairs: The following methods are used to bring mechanical fits back to
original, or better. Reference the appropriate journal and bearing housing tables for correct
sizes.
1.
Sleeving the shaft or housing is preferred. The housing is opened up or the shaft is
turned down to accept the sleeve. The sleeve is pressed into the housing or heated
up and slipped onto the shaft. The part is then recentered back onto a lathe and the
sleeve is turned to the correct size.
2.
Welding may be used on parts which are direct connected as opposed to pulley
driven applications. The part is turned down, and weld is applied. The part is then
turned down to the correct size.
3.
The best method for returning fits to the correct size is to remanufacture the parts.
This method is less cost effective.
Repair Specification 80
6.
REWIND REPAIR:
1.
Stators shall be stripped using a mechanical stripping method.
1.
If mechanical processes are not available, the internal temperature of the
stator in a burnout oven must be guaranteed not to exceed 650oF and only
one stator may be burned out at a time.
2.
The stator must then be core loss tested and checked for concentricity,
straightness of laminations, and soft foot before and after the process.
2.
All electrical insulation must be of Class F, or better.
3.
All magnet wire must be double bonded, or better.
4.
Wire must be laid in the slots in a manner in which the wire may not be scratched or
crossed.
5.
Phase insulation and in-betweens must be used, regardless of motor horsepower or
size.
6.
Winding style, wire size, and connections must duplicate the original motor winding
style.
7.
Internal connections must be braized or silver soldered. Lugs must be crimped onto
all motor leads.
8.
The following rewind tests are required:
1.
AC or DC hi-potential test of a level determined by Formula 4-2.
2.
A motor circuit analysis.
3.
Impedance comparison test. Each phase must be +/- 3% phase unbalance
maximum.
Repair Specification 81
4.
A spin test. This test is performed by applying 10% full rated voltage to
the stator. A dummy rotor or ball bearing is spun in the inside of the stator
to check for reversed coils.
9.
Varnishing Methods:
1.
All varnish must be of Class F, or better.
2.
Trickle Varnishing (preferred).
(1)
Ensure that the stator winding heats evenly and that there is a
good coating of varnish on the end turns. Ensure that the varnish
has fully cured upon removal from the trickling machine, or
place in a bake oven at 295oF until cured.
(2)
Remove excess varnish with solvent or soft sanding wheel
ensuring that laminations are not smeared.
3.
Dip and Bake. Two full dip and bakes are required.
(1)
Cover all machined surfaces with masking to allow for ease of
varnish removal during the cleaning process.
(2)
Heat stator to 104oF in a bake oven.
(3)
Dip stator in epoxy varnish and wait until all escaping air from
the stator ends.
(4)
Remove from the dip tank and allow to drain.
(5)
Place in bake oven set at 295oF for eight hours, or until cured.
(6)
Repeat dip and bake steps a second time.
Repair Specification 82
(7)
Remove excess varnish from all machined surfaces and bolt
holes using scrapers, taps, and soft sanding wheels. Ensure that
lamination surfaces are not smeared.
7.
BALANCING:
1.
Rotor assemblies must be dynamically balanced to the nearest fraction of operating
speed. All fixed components must be attached.
2.
The amount and location of any balancing weights is to be recorded on the test
sheet.
3.
Balancing is to be performed to manufacturer's specifications or as referenced on
Table 4-7.
Table 4-7: Balancing Specifications (Austin Bonnet, 1993, p.19)
8.
Speed (RPM)
Displacement (inches)
Velocity (In/Sec)
3600
0.0010
0.188
1800
0.0015
0.141
1200
0.0020
0.126
900
0.0025
0.188
REASSEMBLE AND TEST:
Repair Specification 83
1.
The rotor is to be reassembled ensuring that the correct inside bearing caps are
placed on the shaft before the bearings. The bearings are placed on the shaft fully
up against the bearing shoulder.
2.
The rotor is replaced into the stator in a way that it does not drag across the
laminations.
3.
Bearing housings are one to two thirds full with grease then the housings are
installed.
4.
A 500 VDC Megger test is performed. The value should be greater than 1000
Megohms.
5.
The motor is test run at all voltages and speeds.
1.
Check to ensure rotor is not binding by turning it several revolutions by
hand.
2.
Operate motor at all voltages and speeds, measuring voltage and current.
If there is a current unbalance greater than three percent, rotate the phases
and rerun to determine if it is due to the motor or incoming power.
3.
Select the highest operating voltage and speed. Test run the motor for 30
minutes. Check for bearing temperature per Table 4-8.
Table 4-8: General Bearing Temperature Limits
Standard
Synthetic
Normal
80oC (176oF)
110oC (230oF)
High
90oC (194oF)
120oC (248oF)
Repair Specification 84
100oC (212oF)
Fail
6.
9.
130oC (266oF)
Recheck that all punch marks match and paperwork is complete.
SHIPPING AND RECORD KEEPING
1.
The motor is to be painted.
2.
All records are to be kept on file.
3.
Shipping is to be performed in a manner in which the motor and motor bearings will
not be damaged.
Repair Specification 90
Chapter 5
SUMMARY, DISCUSSION, AND
RECOMMENDATIONS
Summary of Study
The conclusions and motor repair specification for repairing induction electric motors for variable
frequency drive applications is the culmination of several years of research by the Dreisilker Electric
Motors, Inc.'s Field Service / Research and Development Department. The data consisted of
information from published research papers, North American research projects, motor and drive
manufacturers, and ongoing research by Dreisilker. The "Low Voltage Induction Motor Repair
Specification for Motors In Inverter Applications" represents the most cost effective method for motor
repair available.
As most research has focused on individual components of the electric motor, such as motor wire
insulation, the most obvious solutions have been overlooked. In troubleshooting any system it is best
to troubleshoot the entire system. Therefore, in the case of an electric motor, both the electrical and
mechanical, as well as their subcomponents, must be reviewed in cases such as the failures of motors
due to VFD application.
Variable frequency drives, when incorrectly applied, can reduce the life of a three phase induction
motor to minutes. These failures can be caused by harmonic distortion, reflective wave, steep fronted
Repair Specification 91
waves, shaft voltages, increased temperature rise, and motor speed versus cooling. While motor and
drive manufacturers are looking to how these effect electric motors and drives in new applications, few
are looking into how repaired electric motors are affected.
The original efficiency of the electric motor is affected through traditional motor repair practices.
Inadequacies in these repair practices also affect how the motor will survive through standard as well
as incorrect VFD applications. Therefore, it was necessary to develop a repair specification designed
for motors which may see VFD application which is designed to overcome these inadequacies.
Discussion and Recommendations
It is apparent that additional research is necessary in the future. This has been determined by the
results found in the Inverter Effect Study which left the following questions:
1
Is it possible that metal conduit helps generate, or aggravates, the conditions that already
exist due to capacitive coupling?
2
Is the level of reflective wave directly affected by motor loading or speed?
3
How much affect does the type of cable wire have on the electrical challenges presented by
VFD application?
The answers to these questions may directly impact future installations of VFD's. Future studies
should vary the motor, motor speed, distance, types of cable, types of conduit, drive, and motor
manufacturer. To date, the published studies related to these areas do not vary these variables.
Repair Specification 93
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2
Boglietti, Aldo (1996). "Influence of the Inverter Characteristics on the Iron Losses in
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3
Bonnett, Austin H. (1996). "Analysis of the Impact of Pulse-Width Modulated Inverter
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4
Bonneville Power Administration (1990). Adjustable Speed Drive Application Guidebook.
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5
Bonneville Power Administration (1995). Industrial Motor Repair In the United States.
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Bonneville Power Administration (1995). Electric Motor Model Repair Specifications.
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8
Brancato, Emanuel L. (1992). "Estimation of Lifetime expectancies of Motors." IEEE
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9
Douglas, John (1996). "Considerations in Burning Off Varnish for Winding Removal."
Electrical Manufacturing and Coil Winding '96, 73-74.
Repair Specification 94
10
Erdman, Jay M. (1996). "Effect of PWM Inverters on AC Motor Bearing Currents and
Shaft Voltages." IEEE Transactions on Industry Applications, Vol. 32, No. 2, 250-259.
11
EASA Engineering Committee (1992). Guidelines for Maintaining Motor Efficiency During
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12
EASA Engineering Committee (1992). Stator Core Testing. EASA Tech Note No. 17.
13
EASA Engineering Committee (1994). Cause and Analysis of Anti-Friction Bearing
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Fleming, Alex (1996). "Evaluation of Electric Motor Repair Procedures." Electrical
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Gray, James Will and Haydock, Frank J. (1996). "Industrial Power Quality Considerations
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Holtz, Joachim (1996). "On the Spatial Propagation of Transient Magnetic Fields in AC
Machines." IEEE Transactions on Industry Applications, Vol. 32, No. 4, 927-937.
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IEEE Standard Collection (1995). Electric Machinery: 1995 Edition. Piscataway, NY:
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Industry Applications Society and Power Engineering Society (1992). IEEE Recommended
Practices and Requirements for Harmonic Control in Electrical Power Systems. New York,
NY: Institute of Electrical and Electronics Engineers, Inc.
19
Jouanne, Annette von (1996). "filtering Techniques to Minimize the Effect of Long Motor
Leads on PWM Inverter-Fed AC Motor Drive Systems." IEEE Transactions on Industry
Applications, Vol. 32, No. 4, 919-926.
Repair Specification 95
20
Kaufhold, M. (1996). "Failure Mechanism of the Interturn Insulation of Low Voltage
Electric Machines Fed by Pulse Controlled Inverters." IEEE Electrical Insulation Magazine,
Vol. 12, No. 5, 9-16.
21
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Conference, Chicago '95, 457-460.
22
Penrose, Howard W. (1996). "Field Testing Existing Electric Motor Insulation for Inverter
Duty." Electrical Manufacturing and Coil Winding '96, 109-114.
23
Penrose, Howard W. (1997). "Anatomy of a Low Voltage AC Induction Motor Rewind."
IEEE Electrical Insulation Magazine, January / February 1997.
24
Persson, Erik (1992). "Transient Effects in Application of PWM INverters to Induction
Motors." IEEE Transactions on Industry Applications, Vol. 28, No. 5, 1095-1101.
25
Petroleum and Chemical Industry Committee (1990). IEEE Std. 1068-1990: IEEE
Recommended Practice for the Repair and Rewinding of Motors for the Petroleum and
Chemical Industry. New York, NY: IEEE Standards Board.
26
Rehder, R.H., Draper, R.E., and Moore, B.J. (1996). "How Good is Your Motor Insulation
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27
Rosenberg, Robert and Hand, August (1987). Electric Motor Repair, Third Edition, CBS
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28
Shipp, David D. and Vilcheck, William S. (1996). "Power Quality and Line Considerations
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2, 403-410.
Repair Specification 96
29
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30
Vagati, Alfredo (1996). "AC Motors for High-Performance Drives: A Design-Based
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Repair Specification A-1
Attachment 1
Evaluating A Motor Repair Center
Several Items must be considered before an electric motor repair is ever required:
Record Keeping
You should keep good records for each motor including:
•
___ Nameplate Information
•
___ Repair History
•
___ Maintenance History
Prequalification
You should prequalify several repair centers before a repair is needed. Questions to ask your repair
shop include, but not limited to:
•
___ What is the crane and lifting capacity? The capacity for lifting and moving equipment
should be sufficient to handle your motors.
•
___ Is the shop clean? The repair facility should be clean and orderly, with tools and
equipment in good repair.
•
___ Does the repair center have the proper testing equipment? The facility should have a
calibration system in place and test equipment to handle all of the mandatory motor testing.
•
___ Is the motor shop willing to provide a documented pricing structure?
•
___ Is the warranty clearly understood and in writing?
Repair Specification A-2
•
___ What type of winding removal equipment do they have? _____________
•
___ What type of insulation systems do they use? ______________________
•
___ What type of winding equipment is used? _________________________
•
___ Does the repair center use its own machine shop and balancing equipment?
•
___ Is there a recognized quality program in place? (ie: ISO 9002)
•
___ Is the shop capable of testing at full voltage of your motors?
•
___ Does the repair center have in-house engineering staff?
•
___ Does the repair center have field testing and repair capabilities?
Communication
You should have a clear understanding with the shop about the following, possibly in the form of a
written contract:
•
___ Final Report requirements
•
___ Repair vs. Replace - Requirements should be outlined to provide for early repair vs.
replace decisions in order to avoid unnecessary evaluation charges.
•
___Points of Contact - Preferably more than one person who can make decisions concerning
repair and repair priority.
•
___ Turnaround time requirements - Should also be reasonable, as a faster than normal repair
can result in short cuts that, while the motor may be put back into service a few hours faster, will
fail more frequently than if the motor is repaired correctly.
Repair Specification B-1
Attachment 2
Motor Repair Job Ticket
Job Number:
Date Received:
Bill To:
Ship To:
Urgency:
Emergency
Shipping Instructions:
Rush O/T
Our Truck
ASAP
Date Due:
Pick Up
Service Call
Other:
Manufacturer:
Volts:
Type:
Amperes:
Serial Number:
Type of Application:
Power:
(hp/kW)
Speed:
Frame:
Ins. Class:
Temp. Rise:
Other:
Reason Sent for Repair:
Past Repair Types:
Special Instructions / Requests:
Missing Parts:
Customer Service Representative:
Contact:
Phone Number:
Job Number:
Customer Number:
Repair Specification B-2
Quotation Information:
Price:
Repair
Replace
Repair Specification C-1
Attachment 3
Motor Repair Test Sheet
Job Number:
Customer Service Rep:
Customer:
Customer Contact:
Manufacturer:
Model Number:
Power:
(hp / kW)
Serial Number:
Insulation Class:
Speed:
Frame Size:
Volts:
Amps:
Temperature Rise:
Other:
Visual Inspection:
Motor Connected For:
Megger:
No Load at:
Coupling: Y / N
Continuity:
_________ Volts
Type:
Test Run: Y / N
Amperes:
A _____ B _____ C _____
Observations:
Shaft Straightness:
Rotor Bar Test: Pass / Fail
DE Bearing Journal: ___________ ODE Bearing Journal: ____________
DE Bearing Housing: ___________ ODE Bearing Housing: ___________
Repair Specification C-2
Bearing Inspection (ie: Bearing Races Pitted from Shaft Currents?):
Megger Test: _________ Polarization Index: ______ MCA: Pass / Fail
Hi Potential: AC: Y / N Pass / Fail
DC: Y / N Pass / Fail
Voltage:
Impedance Balance: A ________ B ________ C ________
Spin Test: Y / N
Rotor:
Voltage Applied:
Pass / Fail
Stator:
Tested By:
Amps: A____ B____ C ____
Pass / Rewind
Witness:
Date:
Stripping Information
Length: Conn End _____mm Opposite Conn End _____mm Leads(in.)______
Number of Leads:
Slots:
Coil Turns:
Numbering:
Span:
Stripping Temp:
Connection:
Wires in Parallel:
Wire Sizes:
Notes:
Data Taken By:
Witness:
Date:
Rewind Information
Winding Machine:
Winding Head:
Setting:
Changes from Original Data:
Megger:
Hi Pot: AC: Y / N Pass / Fail
DC: Y / N Leakage:
Repair Specification C-3
MCA: Pass / Fail
Impedance Balance: A _____ B _____ C _____
Spin Test: Voltage:
Amperes: A _____ B _____ C _____
Rewound By:
Tested By:
Witness:
Date:
Machining Information
Journals:
Final:
DE: ______ ODE: ______ How Machined:
Housings:
Final:
DE: ______ ODE: ______ How Machined:
Other Machining:
Tested By:
Witness:
Date:
Final Tests
Megger:
Continuity:
Test Voltages:
Test Run: Y / N
Test Speeds:
Current at all Voltages and Speeds:
Final Test Voltage:
Final Test Speed:
Amps: A____B____C____
Observations and Additional Tests:
Pass / Fail
Tested By:
Witness:
Date: