GE Oil & Gas
Bently Nevada Asset Condition Monitoring
Electric Motor Condition Monitoring and
Protection Application Guide
4.3.1 MSIM Components..................................................................................14
4.3.2 High Sensitivity Current Transformer (HSCT)............................15
4.4Multilin* Motor Protection Relay......................................................................16
Table of Contents
1Introduction..............................................................................................2
1.1 Purpose...........................................................................................................................2
1.2Acronyms.......................................................................................................................2
1.3 Glossary of Terms......................................................................................................3
2 Electric Motor Overview.........................................................................4
2.1 Mechanical and Electrical Motor Components..........................................4
2.2 Induction Motors........................................................................................................4
2.3 DC Motors......................................................................................................................4
2.4 Synchronous Motors................................................................................................5
2.5 Multiple-speed Motors ...........................................................................................5
2.6 Motor Name Plate Information...........................................................................6
2.7 Three-phase Motor Winding Types..................................................................6
2.8 Variable Frequency Drives....................................................................................7
2.9 Motor Control Center ..............................................................................................8
3 Motor Failure Categories...................................................................... 10
3.1 “Fault Zone” Methodology..................................................................................10
4 Condition Monitoring Solutions......................................................... 12
4.1 Vibration Monitoring Systems.........................................................................12
4.1.1 Journal Bearing Motors.........................................................................12
4.1.2 Rolling Element Bearing Motors........................................................12
4.2 Bently Nevada AnomAlert – Anomaly Detection System.................12
4.2.1 AnomAlert Components......................................................................13
4.3 Motor Stator Insulation Monitor Overview...............................................14
4.4.1 Multilin Relay to System 1 Interface...............................................16
5 Selecting the Proper Condition Monitoring Solution...................... 17
5.1 Condition Based Maintenance Objectives................................................. 17
5.2 Condition Monitoring Challenges Specific to Electric Motors......... 17
5.3 Product Application............................................................................................... 17
5.3.1 Product Capabilities................................................................................18
5.3.2 Product Discussions................................................................................18
6 Complementary Predictive Maintenance
Tests and Technologies ........................................................................ 19
6.1 Offline Testing..........................................................................................................19
6.1.1 Motor Insulation Condition Testing – Offline.............................19
6.1.2 Offline DC Tests.........................................................................................20
6.1.3 Insulation Resistance (IR) to Ground Testing..............................20
6.1.4 Polarization Index, Dielectric Absorption Ratio and
Polarization Index Profile Testing.....................................................20
6.1.5 Step Voltage Test......................................................................................20
6.1.6 DC High-potential Testing and Electrical Surge........................20
6.2 Offline AC Tests.......................................................................................................21
6.2.1 Capacitance to Ground..........................................................................21
6.2.2 Capacitance and Dissipation Factor ) – Tan Delta
Measurements or “Tip-Up” Tests.....................................................21
6.3 Other Motor Condition Testing beyond Insulation –
Offline Testing..........................................................................................................22
6.3.1 Phase-to-Phase Resistance (DC Test)............................................22
6.3.2 Phase-to-Phase Inductance (AC Test)............................................22
6.3.3 Rotor Influence Check ...........................................................................22
6.4 Motor Insulation Condition Testing – Online............................................23
6.4.1 Partial Discharge ......................................................................................23
6.4.2 Motor Current Signature Analysis and
Electrical Signature Analysis............................................................... 24
6.4.3 Infrared Thermography......................................................................... 24
6.4.4 Lubrication Analysis................................................................................25
application note
application note
1Introduction
1.1 Purpose
1.2Acronyms
The following common acronyms are associated with electric
motors:
This guide is intended to help Bently Nevada asset condition
monitoring sales managers and field application engineers
better understand:
AC
Alternating Current
AA
AnomAlert – GE Bently Nevada anomaly
detection system
•
Electric motors and related equipment
CBM
Condition Based Maintenance
•
Methods, technologies and products used for
condition based maintenance of motors
CT
Current Transformer
DAR
Dielectric Absorption Ratio
•
Appropriate selection criteria for protection and/or
condition monitoring solutions for specified electric
motor types
DC
Direct Current
C & DF
Capacitance and Dissipation Factor
ESA
Electrical Signature Analysis
Hi-Pot
High Potential
HSCT
High Sensitivity Current Transformer
IR
Insulation Resistance or Infrared
KVA
Kilovolt-Ampere
KVAR
Kilovolt-Ampere Reactive
KW
Kilowatt
MCSA
Motor Current Signature Analysis
Megger®
Megohm Meter
MSIM
Motor Stator Insulation Monitor
NEMA
National Electrical Manufacturers Association
PD
Partial Discharge
PdM
Predictive Maintenance
PF
Power Factor
PSD
Power Spectral Density
PT
Potential Transformer
RTD
Resistance Thermal Detector
SF
Service Factor
TEFC
Totally-Enclosed, Fan-Cooled
THD
Total Harmonic Distortion
VFD
Variable Frequency Drive
2
application note
1.3 Glossary of Terms
Table 1 –Electric Motor Terminology
Air Gap
The radial gap between the outer circumference of the rotor and the inner circumference of
the stator
Apparent Power
The total amount of electrical power drawn by the motor to produce the electric flux and drive the
load – usually expressed in kilovolt-amperes (KVA); the product of the RMS values of the voltage and
current drawn by the motor
Core
The metal structure of the rotor or stator, typically consisting of sheet metal laminations or plates, stacked
and fastened together, which define the shape of the rotor or stator, and hold the electrical current-carrying
bars or windings
Dissipation Factor
The phase angle relationship of capacitive and resistive components of the current passing through stator
insulation; as the insulation system degrades, the resistive component of the current increases, resulting
in a corresponding increase in dissipation factor
Flux
The magnetic field surrounding a current-carrying conductor or magnet, used when referring to the magnetic
field between the rotor and the stator; Flux lines are used to depict and describe the magnetic field, with the
density of the flux lines corresponding to the strength of the magnetic field
Frame Size
A number corresponding to the numbering system of standard motor size and dimensions established by
the National Electrical Manufacturers Association (NEMA)
Full-load Current
The current drawn by the motor when operating at full-load torque and speed and at the motor’s rated
line voltage and frequency
Full-load Torque
The torque of the motor when it is producing its rated horsepower and running at full-load speed
(also known as “running torque”)
Insulation Class
A series of letters, from A through H, representing temperature increments from 105° C to 195° C,
corresponding to the temperature that the motor insulation is designed to withstand under continuous
operating conditions
Poles
The electromagnet “poles” created in an AC motor by the windings; the number of poles determines the
running speed of the motor; in AC motors it is always an even number (2, 4, 6, and so on) and is inversely
proportional to the speed of the motor
Power Factor
A dimensionless number corresponding to the ratio of real and apparent power drawn by an AC motor, and
indicative of the electrical characteristics of the motor; in other words, real power divided by apparent power
Reactive Power
The portion of the electrical power required for the magnetization of a motor to establish and sustain the
magnetic flux – usually expressed in kilovolt-amperes Reactive (KVAR)
Real Power
The portion of the electrical power drawn by the motor that is converted into mechanical power that drives
the load (driven machine) – usually expressed in kilowatts (KW)
Service Factor
A multiplier, usually listed on the motor nameplate, that when applied to the motor’s rated horsepower,
corresponds to the maximum horsepower loading the motor can be run at without risk of damage
Slip
The rotational speed difference in an induction motor between synchronous speed and the actual rotating
speed of the motor – can be expressed as a speed value, or as a percent of the synchronous speed
Squirrel-cage
The rotating part (rotor) used in the most common form of AC induction motor; consists of a cylinder of steel
with aluminum or copper conductors embedded in its surface1
Starting Current
The current drawn by the motor when it is initially started; typically much higher than running current and
equal to the locked rotor current (sometimes referred to as “inrush current”)
Synchronous Speed
The motor speed corresponding to the rotating magnetic field created at the frequency (f) of the applied
alternating current divided by the number of stator pole windings
A synchronous motor rotates at synchronous speed (RPMsync = (2 x f x 60)/p where p in the number of poles –
for example: f = 60 Hz and p = 2 RPMsync = 3600)
An induction motor rotates at less than synchronous speed (see slip above)
The rotating part (rotor) used in the most common form of AC induction motor; consists of a cylinder of steel with aluminum or copper
conductors embedded in its surface1
1 https://en.wikipedia.org/wiki/Squirrel-­cage_rotor
3
application note
2Electric Motor Overview
2.1 Mechanical and Electrical Motor
Components
Motors are made up of a number of stationary and rotating parts,
typically classified as either mechanical or electrical components.
The vast majority of motors in industrial applications are induction
motors. Figure 1 below provides an example cut-away view of an
induction motor that illustrates the many motor components,
each with its own failure modes.
Three-phase squirrel-cage induction motors are widely used
in industrial drives because they are rugged, reliable, and
economical. Single-phase induction motors are used extensively
for smaller loads, such as household appliances like fans.2
The induction motor does not have any direct electric supply
onto the rotor; instead a secondary current is induced in the
rotor. To achieve this, stator windings are arranged around the
rotor so that when energized with a poly-phase supply they
create a rotating magnetic field pattern that sweeps past the
rotor. This changing magnetic field pattern induces current in
the rotor conductors. These currents interact with the rotating
magnetic field created by the stator and in effect cause a
rotational motion on the rotor.
However, for these currents to be induced, the speed of the
physical rotor must be less than the speed of the rotating
magnetic field in the stator, or the magnetic field will not move
relative to the rotor conductors and no currents will be induced.
If by some chance this happens, the rotor typically slows slightly
until a current is re-induced and then the rotor continues as
before. This difference between the speed of the rotor and
speed of the rotating magnetic field in the stator is called slip.
Due to this, an induction motor is sometimes referred to as an
asynchronous machine.
2.3 DC Motors
Figure 1 – Cut-away view of a 2000HP induction motor
2.2 Induction Motors
The induction or asynchronous motor (shown in Figure 2) is an AC
electric motor in which the electric current in the rotor needed to
produce torque is induced by electromagnetic induction from the
magnetic field of the stator winding. An induction motor therefore
does not require mechanical commutation, separate-excitation, or
self-excitation for all or part of the energy transferred from stator
to rotor, as is required in universal, DC, and large synchronous
motors. An induction motor’s rotor can be either wound type or
squirrel-cage type.
A DC motor (shown in Figure 3) is a mechanically commutated
electric motor powered from direct current (DC). The stator is
stationary in space by definition and therefore the current in the
rotor is switched by the commutator to also remain stationary in
space. This maintains the relative angle between the stator and
rotor magnetic flux near 90 degrees, which generates the
maximum torque.
Figure 3 – Typical DC Motor
Brush DC motor Construction https://www.pinterest.com/edgefx/elprocus/
DC motors have a rotating armature winding (winding in which
a voltage is induced) but a non-rotating armature magnetic
field and a static field winding (winding that produces the main
magnetic flux) or permanent magnet. Different connections
of the field and armature winding provide different inherent
speed/torque regulation characteristics. The speed of a DC
Figure 2 – Typical induction motor
GE Multilin: Motor Protection Principles www.gegridsolutions.com
2https://en.wikipedia.org/wiki/Induction_motor
4
application note
motor can be controlled by changing the voltage applied to the
armature or by changing the field current. The introduction of
variable resistance in the armature circuit or field circuit allows
speed control. Modern DC motors are often controlled by power
electronics systems called DC drives.
The introduction of DC motors to run machinery eliminated
the need for local steam or internal combustion engines, and
line shaft drive systems. DC motors can operate directly from
rechargeable batteries (this system provided the motive power
for the first electric vehicles). Today DC motors are still found in
applications as small as toys and disk drives, or in large sizes to
operate steel rolling mills and paper machines.
2.4 Synchronous Motors
A synchronous electric motor is an AC motor in which, at steady
state, the rotation of the shaft is synchronized with the frequency
of the supply current; the rotation period is exactly equal to an
integral number of AC cycles.3 In other words, it has zero slip
under usual operating conditions. Contrast this with an induction
motor, which must slip in order to produce torque. A synchronous
motor is like an induction motor except the rotor is excited by a
DC field. Slip rings and brushes are used to conduct current to
the rotor. The rotor poles connect to each other and move at the
same speed – hence the name: synchronous motor. The speed at
which synchronous motors rotate depends on the frequency of the
AC power line and the number of poles (p) – RPMsync = 2 x f x 60/p.
Synchronous motors contain electromagnets on the stator of the
motor that create a magnetic field which rotates in time with the
oscillations of the line current. The rotor turns in step with this
field, at the same rate.
Synchronous motors (as shown in Figure 4) can be applied
to driving compressors, grinding mills, metal rolling, mine hoists,
refiners, fans, generators, and many other applications. Small
synchronous motors are also used in timing applications such as
in synchronous clocks, timers in appliances, tape recorders and
precision servomechanisms in which the motor must operate
at a precise speed.
2.5 Multiple-speed Motors
Multispeed motors (as shown in Figure 5) are applied when
operation at two, three or four definite speeds is desired. The
motors are classified based on the relationship of full-load torques
at rated speeds (constant torque, variable torque and constant
horsepower). Different speeds are obtained by switching electrical
connections. The speed of each connection has the constant
speed characteristic typical of single-speed induction motors.
Multispeed motors may have a single reconnectible winding or two
independent windings. It is possible to arrange a single winding
so that it can be reconnected for a different number of poles (and
speed) by suitable reconnection of the leads. However, such an
arrangement permits only two speeds and the speeds must be in
the ratio of two-to-one. An alternative way of securing two speeds
is to have two separate windings, each wound for a different
number of poles and speed.
Figure 5 – Multiple-speed motor
GE Motor Brochure
Such an arrangement means that one winding is not in use
when the other is connected to the line; motor frame sizes usually
are larger in order to accommodate the idle winding. But the use
of two windings permits two speeds that are not in the ratio of
two-to-one. Speeds with a two-to-one ratio can be delivered by
two-winding motors as well as by single-winding motors.
Figure 4 – Typical synchronous motor
GE Motor Brochure
3 https://en.wikipedia.org/wiki/Synchronous_motor
5
application note
2.6 Motor Name Plate Information
2.7 Three-phase Motor Winding Types
Motor rating and identification data are furnished on labels
and nameplates. Packing nameplates provide a permanent
record of motor characteristics, plant identification, and date
of manufacture. Figure 6 shows an example of a label that is
attached to the motor.
There are several different AC motor types, each one with
different operating and mechanical characteristics. The most
common type is the squirrel-cage rotor. It is called squirrel-cage,
because its rotor looks like the exercising wheel found in squirrel
or hamster cages.
A typical three-phase squirrel-cage motor (single voltage) has
six connection leads in the electrical connection box for the
three stator windings. It is important to know how to connect
AC three-phase motors in Star and Delta connection, as
illustrated in Figures 7, 8 and 9.
Figure 6 – Typical motor nameplate
http://www.inverter-china.com/blog/articles/ac-motor/name-plateof-ac-induction-motor.html
Terminology used in typical motor nameplate (see Figure 6)
Volts:
Rated terminal supply voltage
Amps:
Rated full-load supply current
H.P.:
Rated motor output
(Note: In North American, motors are
typically rated in output horsepower (HP),
while internationally the input power is
commonly given in kilowatts (KW)
R.P.M.:
Rated full-load speed of the motor
Hertz:
Rated supply frequency
Frame:
External physical dimension of the motor
based on the NEMA standards
Duty:
Motor load condition, whether it is
continuous load, short time, periodic,
and so on
Date:
Date of manufacturing
Class Insulation:
Insulation class used for the motor
construction; specifies the maximum limit
of the motor winding temperature
NEMA Design:
To which NEMA design class the
motor belongs
Service Factor:
Factor by which the motor can be overloaded
beyond full load
NEMA Nom. Eff.: Motor operating efficiency at full load
6
PH:
Number of stator phases of the motor
Pole:
Number of poles of the motor
Y:
Whether the AC motor windings are
star (Y) connected or delta (Δ) connected
In a star or Y connection, each one of the three phases (R-S-T)
is connected at one end of each coil. The other ends of the coils
are connected together in a common point. A star connection
can be easily accomplished simply by bridging one of the two
horizontal rows in the connection box of the motor. The phases
are then connected on the leads of the other horizontal row.
In a Delta connection, the end of each coil is connected with
the start of another coil. The three coils are then connected
in a circle, thus creating three nodes. The three phases are
then applied on these nodes. A Delta connection is easily
accomplished by vertically bridging the three columns in the
connection box.
Figure 7 – Connection diagram of Delta and Star (WYE)
http://www.pcbheaven.com/userpages/check_the_windings_
of_a_3phase_ac_motor/
application note
Figure 9 – Star (WYE) Terminal Connection
Figure 8 – Delta terminal connection
http://www.pcbheaven.com/userpages/check_the_windings_
of_a_3phase_ac_motor/
2.8 Variable Frequency Drives
A variable-frequency drive (VFD) system (as shown in Figure 10)
is used to control the rotational speed of an induction (AC)
electric motor by controlling the frequency of the electrical
power supplied to the motor. The typical design first converts
AC input power to DC intermediate power using a rectifier
bridge. The DC intermediate power is then converted to
quasi-sinusoidal AC power using an inverter switching circuit.
The standard method used to achieve variable motor voltage
is pulse-width modulation (PWM). With PWM voltage control,
inverter switches are used to construct a quasi-sinusoidal
output waveform by a series of narrow voltage pulses with
sinusoidal varying pulse durations.
Figure 10 – VFD circuit and PWM
GE Presentation Monitoring Electric Motor by Petri Nohynek
7
application note
2.9 Motor Control Center
Wherever motors are used, they must be controlled. The
sections above explained how various control products are
used to control motor operation. For example, the most basic
type of AC motor control involves turning the motor on and off.
This is often accomplished using a motor starter made up of a
contactor and an overload relay (see Figure 11).
Figure 12 – Soft starters
Siemens Basic Motor Control Center
Some soft starters also allow the phase control process to be
applied in reverse when the motor is being stopped. This controlled
starting and stopping significantly reduces stress on connected
devices and minimizes line voltage fluctuations.
Typically one motor starter controls one motor. When only a few
geographically dispersed AC motors are used, the circuit protection
and control components may be in an enclosure mounted close to
the motor (as shown in Figure 13).
Figure 11 – Circuit breaker and motor starter circuit
Siemens Basic Motor Control Center
The contactor’s contacts are closed to start the motor and
opened to stop the motor. This is done electromechanically
and often requires using start and stop push buttons and other
devices wired to control the contactor.
The overload relay protects the motor by disconnecting power
to the motor when an overload condition exists. Although the
overload relay provides protection from overloads, it does not
provide short-circuit protection for the wiring that supplies
power to the motor. For this reason, a circuit breaker or fuses
are also used.
Often called soft starters, solid-state reduced-voltage starters
(as shown in Figure 12) limit motor starting current and torque
by ramping up the voltage applied to the motor during the
selectable starting time. They accomplish this by gradually
increasing the portion of the power supply cycle applied to
the motor windings, a process sometimes referred to as phase
control. Once the startup is completed, soft starters use integrated
bypass contacts to bypass power switching devices (thyristors).
This improves efficiency, minimizes heat, and reduces switching
device stress.
Figure 13 – Typical motor starter cabinet
Siemens Basic Motor Control Center
In many commercial and industrial applications, quite a few
electric motors are required, and it is often desirable to control
some or all of the motors from a central location. The apparatus
designed for this function is the motor control center (MCC) (see
Figure 14).
MCCs are simply physical groupings of combination starters
in one assembly. A combination starter is a single enclosure
containing the motor starter, fuses or circuit breaker, and a
device for disconnecting power. Other devices associated with
the motor, such as push buttons and indicator lights, may also
be included.
A motor control center is an assembly of one or more enclosed
sections having a common power bus and principally containing
motor control units. In modern practice, an MCC is a factory
8
application note
assembly of several motor starters. An MCC can include variable
frequency drives, programmable controllers, and metering
and may also be the electrical service entrance for the building.
Motor control centers are usually used for low voltage three-phase
alternating current motors from 208 V to 600 V. Medium-voltage
MCCs are made for large motors running at 2300 V to around
15000 V, using vacuum contactors for switching and with
separate compartments for power switching and control.
Motor control centers have been used since 1950 by the
automobile manufacturing industry, which used large numbers
of electric motors. Today they are used in many industrial and
commercial applications. Where very dusty or corrosive processes
are used, the MCC may be installed in a separate air-conditioned
room, but often they are used on the factory floor adjacent to
the machinery controlled.
An MCC consists of one or more vertical metal cabinet sections
with power bus and provision for plug-in mounting of individual
motor controllers. Very large controllers may be bolted in place,
but smaller controllers can be unplugged from the cabinet for
testing or maintenance. Each motor controller contains a
contactor or a solid-state motor controller, overload relays to
protect the motor, fuses or a circuit breaker to provide short-circuit
protection, and a disconnecting switch to isolate the motor circuit.
Three-phase power enters each controller through separable
connectors. The motor is wired to terminals in the controller.
Motor control centers provide wire ways for field control and
power cables.
A Multilin motor relay from GE is frequently provided for monitoring
and protecting each motor in an MCC. GE also provides a variety
of bus protection products for the bus voltages feeding the MCC.
Knowledge of the Multilin relay and bus protection configuration
installed may be helpful in interfacing with AnomAlert and other
Bently Nevada products from GE.
Each motor controller in an MCC can be specified with a range
of options such as separate control transformers, pilot lamps,
control switches, extra control terminal blocks, various types of
thermal or solid-state overload protection relays, or various classes
of power fuses or types of circuit breakers. A motor control center
can either be supplied ready for the customer to connect all field
wiring, or can be an engineered assembly with internal control and
interlocking wiring to a central control terminal panel board
or programmable controller.
Figure 14 – Motor Control Center overview
Siemens Basic Motor Control Center
9
application note
3Motor Failure Categories
3.1 “Fault Zone” Methodology
Data from an Allianz 2001 survey identified the proportions
of failure modes of general purpose motors constructed with
rolling element bearings (as shown in Figure 15).
Before discussing the testing and technologies used for
condition monitoring, it is helpful to review the failure modes
for electric motors. Figure 17 illustrates damaged motors
from several different causes.
PdMA*, a leading motor predictive maintenance company
(PdM), has developed a methodology called the “Six Electrical
Fault Zone” approach in categorizing motor failure modes. Its
training includes a table that lists the fault zones (or faulty
components) and cross-references tests that its equipment is
capable of making to detect those failures. Unfortunately, its
information is not complete since it only lists the fault zones
and tests that fall under the scope of its equipment’s capabilities.
For example it does not include any mechanical faults and is
somewhat limited in its online testing capabilities.
Figure 15 – Failure modes for typical AC motors
up to 4 kV (REB)
GE Presentation AnomAlert
This study indicated a large number of bearing-related problems,
followed by stator and rotor-related faults. The “Other” category
includes failures such as insulation, air gap, and other non-bearing
mechanical problems.
Therefore, the following table utilizes some PdMA methodology
but modifies and adds information to include other industry
knowledge, terminology, and technologies; plus it includes a
brief explanation of the failure modes. This is done in an effort
to categorize the possible motor fault-components, list the
possible specific failure modes, and provide testing options
to detect and confirm the failures. More detail on the offline
tests (also termed “static tests”, conducted during motor
stoppage) and online tests (also termed “dynamic tests”,
taken during motor operation) is provided in the following
sections of this application guide.
Note: The terminology of static and dynamic motor testing
can easily be confused with Bently Nevada terminology that
has differing definitions of “static” and “dynamic” data.
Figure 16 – Failure modes for typical large AC motors
more than 4 kV
GE Presentation AnomAlert
The 2001 Allianz survey also revealed that large AC motors
(defined as operating at 4 kV and higher and typically rated
from 1,000 to 50,000 HP) with fluid-film journal bearings show
a much larger proportion of stator-related failures, particularly
insulation faults (as shown in Figure 16). This is probably due to
the relative reliability of journal bearings as well as the high
supply voltage levels used in this class of large motors.
10
Figure 17 – Motor failure photos
PdMA Presentation Introducing MCEmax
application note
Table 2 – Motor Failure Categories
Faulty Component
Failure Modes
Offline (Static) Test
Power Quality
N/A
Voltage supply that does not match nameplate
ratings (over/under voltage or fluctuating supply)
Voltage supply imbalance
Online (Dynamic) Test
ESA*, MCSA*
Phase-to-Phase Voltages, THD,
Harmonic Voltage Factor
High supply harmonics
VFD faulty operation (improper filtering or
smoothing parameters, open/shorted diodes)
Power Circuit
(i.e., phasing
problems)
Loose, broken connections
Bad contactor coils
Faulty capacitors
Phase-to-Phase
Resistance*,
Resistive Imbalance
Voltage Imbalance, Current
Imbalance
Thermography*
ESA*, MCSA*
Cabling problems
Vibration
Insulation
Stator
Partial short faults in phase-to-phase or phaseto-ground Insulation:
Resistance to Gnd
Capacitance to Gnd
Bently Nevada MSIM* system
(using HSCT*)
•
•
•
•
•
•
PI*, DAR*, PIP*
Partial Discharge*
Step Voltage*
Stator Temperature
Hi-Pot*, Surge*
RF Detection
C&DF (Tan Delta, Tip-Up)*
Acoustic Measurement
Winding faults
Inductive Imbalance*
ESA*, MCSA*
•
•
•
•
Resistive Imbalance*
Voltage Imbalance, Current
Imbalance
Thermal breakdown (overload or age)
Cracks/fissures
End-winding contamination
Moisture absorption
Uncured resin
Voids
Turn-to-turn
Phase-to-phase
Faulty motor connection
Core loss
Rotor Influence Check*
Vibration*
Thermography*
Temperature sensors
Rotor
Cracked/broken rotor bars or shorting ring or
interconnections
Inductive Imbalance*
ESA*, MCSA*
Rotor Influence Check*
In-Rush/start-up measurements
Vibration*
Magnetic Flux Monitoring
Air Gap
High eccentricity
Inductive Imbalance*
ESA*, MCSA*
Rotor Influence Check*
Vibration*
Thermography*
Bearings
Other
Mechanical
Faults
Fluting, electrical discharge machining (EDM)
Vibration*
Rolling element bearings: spalling, cracked races,
increased clearances
Temperature
Fluid film journal bearings: fluid-induced
instability
Lube/Tribology*
Misalignment, coupling fault
Thermography*
ESA*, MCSA*
Vibration*
Mechanical imbalance, rotor bow
Laser or dial-indicator
measurements
Structural looseness (soft foot/bad foundation)
Modal analysis
Operating Deflection Shape
(ODS) Analysis
MCSA*
* More details follow for several of the techniques listed in this table.
11
application note
4Condition Monitoring Solutions
•
3500/42M Monitor – This monitor provides continuous
monitoring and machine protection when used with a relay
card output, but has some diagnostic limitations. Vibration
channels can be configured for acceleration or velocity
sensors, but separate channels are required to provide
both measurement units. Also, as previously mentioned,
the 3500 monitor is limited to 800 line spectrums. The most
severe limitation is not having acceleration enveloping (AE)
capability for early bearing fault detection.
•
1900/65A or 2300 Monitor – These monitors can also
be used for a lower criticality motor with roller element
bearings. These units are designed to continuously monitor
and protect equipment that is used in a variety of applications
and industries. Their low cost makes them an ideal solution
for a general purpose machine and a process that can benefit
from continuous monitoring and protection.
•
Trendmaster Dynamic Sampler Module (DSM) – This
rack-based data acquisition system is fully integrated with
GE’s System 1 software. It is an online scanning system and
does not provide shutdown capability.
•
Essential Insight.mesh – This wireless data acquisition
system is fully integrated with System 1 software. A typical
system requires a manager gateway, wSIM devices – wireless
sensor interface module, and repeaters that create a robust,
auto-forming mesh network. Each wSIM device has four
channels that can be individually configured to support
vibration and temperature measurements.
•
Scout Portable Data Collector – This portable analyzer
offers the power and convenience of two- or four-channel
measurement and dual plane balancing. If a portable
data collector is used without permanently mounted
accelerometers, special care must be taken in the data
collection routine in order to avoid significant variability in
trending the data.
4.1 Vibration Monitoring Systems
There are a number of GE’s Bently Nevada monitoring and
transducers systems that can be appropriate for monitoring
motors. The ideal hardware suite depends on bearing types and
motor criticality.
4.1.1 Journal Bearing Motors
Orthogonal proximity probe (X-Y pair) sensors are highly
recommended for monitoring motor journal bearings. The proximity
measurements can directly assess the motion of the rotor within
the journal bearing clearances. Seismic or case-mounted sensors
are sometimes used on motor journal bearings, but they are less
effective. The vibration energy is dampened by the oil film in the
journal bearing and the seismic sensor may not sense the vibration
until the shaft comes in contact with the bearing journal.
GE’s Bently Nevada 3500 monitoring system is highly recommended
for monitoring large, critical assets with fluid-film journal bearings
(typically for motors rated over 500 HP). The 3500 system provides
continuous, online monitoring suitable for machinery protection
applications, and is designed to fully meet the requirements of the
American Petroleum Institute’s API 670 standard for such systems.
The 1900/65A monitors can also be used for motors with lower
criticality. They are designed to continuously monitor and protect
equipment that is used in a variety of applications and industries.
The monitors’ low cost makes them an ideal solution for general
purpose machines and processes that can benefit from continuous
monitoring and protection.
4.1.2 Rolling Element Bearing Motors
Smaller motors with rolling element bearings (REB) are very
common. These smaller motors may still be critical components
of plant operations and justify condition monitoring equipment.
Accelerometer or velocity sensors are recommended for motors
with roller element bearings for several reasons:
•
•
•
•
Tight bearing clearances effectively transmit vibration
out to the motor casing
Lower installation and sensor cost
Acceleration enveloping provides good early detection
of REB bearing faults
Casing measurements allow easy axial measurements
Within GE’s Bently Nevada product line, there are several options
for making acceleration and velocity measurements, appropriate
for monitoring motors with rolling element bearings. The
customer’s preferred option is a function of hardware costs,
cost of installation, the frequency of data collection samples,
permanently installed versus route-based portable data collection,
the ability to collect velocity and acceleration information on the
same channel, the ability for acceleration enveloping, and the
criticality of the motor asset and process. The following list
provides a comparison of applicable Bently Nevada products
along with the associated capabilities and benefits listed in
approximate order of higher to lower hardware costs:
12
4.2 Bently Nevada AnomAlert –
Anomaly Detection System
The AnomAlert motor anomaly detector is a system of software
and networked hardware (see Figure 18) that continuously identifies
faults on electric motors and their driven equipment. The AnomAlert
system applies an intelligent, model-based approach to provide
anomaly detection by measuring the current and voltage signals
from the electrical supply to the motor. It is permanently mounted,
generally in the motor control center, and is applicable to three-phase
AC, induction or synchronous, fixed or variable speed motors. The
AnomAlert diagnostic solution can be used with a complementary
vibration monitoring system for detecting electrical faults.
Alternatively, it can be used where dedicated vibration monitoring
is not practical, economical, or comprehensive enough. It can detect
changes in the load the motor is experiencing due to anomalies in
the driven equipment or process such as cavitation or plugged filters
and screens. Because it doesn’t require any sensor installation on
the motor itself or associated load, the AnomAlert detection system
is especially attractive for inaccessible driven equipment and is
applicable to most types of pumps, compressors, and similar loads.
It is also well suited to the monitoring of canned pumps.
application note
The AnomAlert system uses a combination of voltage and
current dynamic waveforms, together with learned models,
to detect motor or driven equipment faults. Active learning is
backed up by an additional fleet model in case the system has
been installed on an already defective motor. The AnomAlert
system detects differences between observed current
characteristics and learned characteristics and relates these
differences to faults.
Motor fault detection is based on a learned, physics-based
motor model, in which constants in the model are calculated
from real-time data and compared to previously learned values.
Figure 18 – AnomAlert - front view (left) and rear terminals (right)
GE Presentation AnomAlert
4.2.1 AnomAlert Components
•
One AnomAlert specific to voltage, current, and motor speed
(fixed or VFD)
•
Three current transformers (fixed speed) or sensors (VFD),
rated for the motor nameplate rating
•
Two or three potential transformers if the motor supply
voltage is greater than 480 Vac
AnomAlert Modeling, Analysis
The AnomAlert detection system samples motor supply conditions
every 90 seconds. When it first begins collecting data for a motor,
it checks the input connections to determine if there are any
installation errors. Upon a successful first sample, the system
enters a learning period of approximately 10 days. This length of
time is long enough to allow for the AnomAlert system to encounter
all normal conditions typically experienced during a weekly cycle.
During the learning period, the AnomAlert system learns and builds
a separate internal motor model for each operating mode that is
encountered. Afterwards, by comparing the motor’s operation with
the learned model within the same operating mode, the system
can easily detect small changes in motor condition. Note that in
order to detect existing motor faults during the learning phase, the
AnomAlert unit also features a comparison between the learned
values and an average fleet-based level. A typical AnomAlert
installation diagram is shown in Figure 19.
Figure 19 – AnomAlert typical sensor connections
GE Presentation AnomAlert
13
application note
The AnomAlert data (the equivalent of “static” data in Bently
Nevada terminology) that can be viewed, trended, and alarmed
upon includes:
•
Directly Measured Values – Current, voltage for
each phase
•
Calculated Values – Active power, apparent power,
power factor, voltage and current balances, THD for
odd harmonic line frequencies
•
•
Power Spectral Density (PSD) Frequency-based
Bands – Loose foundation, unbalance/misalignment/
coupling/bearing, rotor, stator/loose windings/short circuit,
transmission element, bearing
4.2.1.1 AnomAlert Diagnostics and Validation
The strength of the AnomAlert detection system lies in its
ability to detect small but critical changes in motor operation
that could indicate potential faults in the motor or its driven
load. After alarms are generated, the user should complete the
following steps to verify the alarms and look for supporting
evidence of faults:
1.
Trend the motor data to get an overall picture of how the
motor has been operating.
2.
Try to cross-correlate the alarming variables:
– For mechanical-based alarms, view the PSD to correlate
the alarming variable with the actual spectrum peaks.
Model-based Values – Internal and external electrical
fault 1,2,3 and 4
Motor fault detection is based on a number of abnormal
conditions.
Unhealthy Voltage supply levels or unbalances will be
flagged with a watch line alarm. The motor operators should
verify the proper motor power quality conditions or look for
faults in the power circuit.
Motor current changes will cause a watch load alarm
indicating that the user should investigate one of two
possible scenarios:
–
3.
For electrical alarms, view the PSD for correlations
and look for changes in the internal or external electrical
fault variables for additional corroboration.
Perform external testing to examine the validity of the alarms
(see Table 2 for corroborating evidence for motor faults).
4.3 Motor Stator Insulation Monitor
Overview
All the PSD- and Model-based variables listed above are
trended based on the amount of change from the initial learned
period. If the AnomAlert unit detects a consistent anomaly in
these values, it will generate an Examine 1 or Examine 2 alarm,
depending on the amount of change that corresponds to
the severity.
The motor stator insulation monitor (MSIM) system is designed to
measure and monitor the motor’s stator insulation condition. It
uses the advanced technology, high sensitivity current transformer
(HSCT) to measure the leakage current of the motor online and
process the measured data in real time to determine the condition
of the motor’s stator insulation. Processed data can be displayed
in the 3500 monitoring system and System 1 software. The HSCT
is a specialized variant of a differential current transformer that
incorporates high-sensitivity and noise reduction technology, thus
providing a very low-amplitude leakage current measurement in the
presence of large load currents. This contrasts with conventional
differential protection current transformers that are limited in their
ability to detect very small leakage currents. With this development
it is possible to provide an online capacitance and dissipation factor
(Tan Delta) measurement, enabling a series of diagnostics that
previously were only available with offline testing.
Additional details related to AnomAlert operation can be
found in the document: “AnomAlert Under the Hood” published
in the April 2012 edition of the Orbit magazine.
The MSIM system is applicable to medium and high voltage motors
(4 KV or higher), and the motor connection must be externally WYE
connected. It is not applicable to variable speed motors.
1.
The motor current level has changed due to a faulty
process in the machine (for example a clogged filter
for a fan)
2.
The motor current level is due to a normal operating
condition change. The user can initiate an Update
command that causes the AnomAlert system to add
the present operating condition to its learned model.
4.3.1 MSIM Components
The MSIM system (as shown in Figure 20), includes:
14
•
Three High Sensitivity Current Transducers (HSCTs)
•
Three HSCT interface modules
•
Two or three High Voltage Sensors (HVSs)
•
Two or three HVS interface modules
•
One to three Resistance Thermal Detector (RTD) temperature
sensor interface modules
•
One MSIM 3500 I/O module
•
One 3500/82 MSIM monitor
application note
Figure 20 – MSIM system layout
4.3.2 High Sensitivity Current Transformer (HSCT)
The line and neutral lead for each motor phase is routed through
the HSCT (Figure 21) as shown schematically below in Figure 22.
The HSCT has been designed to measure the resistive current
losses. Losses through insulation faults cannot be measured
directly, but by routing both the outgoing current and the return
current for each phase through an HSCT, the sensor is able to
measure the difference between the currents. This current
differential is equal to the leakage current through the insulation
faults. The HSCT is designed to detect very low leakage levels,
typically as low as 10 mA.
Figure 22 – HSCT measurement scheme
Figure 21 – High sensitivity current transformer (HSCT) and
high voltage sensor (HVS)
In an ideal motor, the amount of electrical current that flows in
and out of each motor phase should be exactly the same. As a part
of the motor manufacturing process, the motor stator windings
are coated with insulation. The insulation in its pure state acts
as a capacitor. Even in a brand new motor, there is always some
leakage current; input does not equal output. The leakage current
15
application note
has two components: resistive and capacitive. As the insulation
deteriorates, the resistive component of the leakage current
becomes dominant and the phase angle, delta (δ), between the
current and voltage is less than 90 degrees. The capacitance and
dissipation factor (also termed the “Tan Delta” and “Loss Angle”
test) can be calculated using the phase angle (see Figure 23);
these measurements are used to detect deterioration in the
motor winding insulation. The Tan Delta test works on the
principle that any insulation in its pure state acts as a capacitor.
Therefore, the current phase should lead the voltage phase
by 90 degrees. As the insulation system degrades, insulation
resistance decreases which increases leakage current and
changes the phase angle between the current and voltage. In
standard industrial practices, the measurement of the dissipation
factor has always been made during offline motor testing. But, by
measuring the value of current losses related to resistive defects,
the dissipation can be calculated during motor operation.
Figure 23 – Circuit equivalent diagram and vector diagram
4.4Multilin* Motor Protection Relay
GE’s Multilin relay is a digital motor protection system designed to
protect and manage medium- to large-sized AC motors and their
driven equipment. It contains a full range of selectively enabled,
self-contained protection and control elements. There are several
models of the Multilin relay, and it is important to determine either
which one is installed, or what information you want to collect,
to determine which relay to choose. Multilin relays can provide
some or all of the following information: humidity, A, B and C phase
voltages (RMS), number of starts, ambient temperature, ground
current, A, B and C line currents as well as differential currents,
motor speed, motor load, running time, stopped time, cause of last
trip, three-phase real power, three-phase apparent power, threephase reactive power, three-phase power factor, three-phase
power demand, average current total harmonic distortion (THD),
and average voltage THD.
4.4.1 Multilin Relay to System 1 Interface
GE’s Multilin relay provides data (see Figure 24) to System 1
software via Modbus over Ethernet. Three-phase motor current and
voltage waveforms are available within System 1. Multiple relays
can be daisy-chained together and one serial
to Ethernet converter is used for a link to
System 1. The following Multilin relay models
are compatible with System 1 software:
269 Plus (communication: RS232, RS485)
369 (communication: RS232, RS485)
469 (communication: RS232, RS485)
M60 (communication: Ethernet, RS232, RS485)
16
Figure 24 – Typical data available through System 1
(Multilin relay model 469 shown)
application note
5Selecting the Proper Condition
Monitoring Solution
5.2 Condition Monitoring Challenges
Specific to Electric Motors
5.1 Condition Based
Maintenance Objectives
Electric motors are made up of a large number of mechanical
and electrical components with many failure modes. They are
also very dependent on external systems. The following common
considerations relate to monitoring motors:
Operators at industrial facilities typically have a vast array of
motors with differing horsepower ratings, operating at various
voltage levels, and used for a variety of purposes. Depending on
how critical a motor is to the process, and its failure modes and
consequences of a failure, it may be beneficial to have a condition
monitoring (CM) program for the motor that provides protection
and/or early warning of impending mechanical or electrical failures.
Operators and maintenance personnel may want to identify failing
motor components as well as determine severity, and manage the
operations and stress on the motor until it can be shut down for
repairs. This may also provide time for the facility to procure any
necessary parts and services.
An essential benefit of condition based maintenance (CBM)
or predictive maintenance (PdM) is the ability to detect initial
faulty motor components before they result in more costly
subsequent damage. This is harder to justify on smaller, less
expensive, and less critical machinery. Therefore, it is generally
easier to justify CM equipment on larger, more critical motors.
However, there are many factors besides asset cost and process
criticality that can influence a customer’s CM needs. Many of
these technologies are used in parallel so that the strengths of
one system can overcome weaknesses in others. PdM and CM
technologies are also used in series so that, for example, one
technology flags a problem and then another system is used to
isolate and troubleshoot the exact fault.
Another important piece of PdM information for a customer
is not only detecting a failing component, but also identifying
any faulty operating conditions that may be the root cause for the
failed motor component. For example, a spalled bearing race may
be the result of a misaligned or unbalanced drive train. Similarly,
insulation degradation is a detectable failure mode, but it is
often caused by excessive winding temperatures, power quality
problems, or contamination. Furthermore, it is easy to see the
advantage of preventing any damage to motors by detecting
any potentially damaging motor conditions.
•
The CM solution must cover the wide variation of motor sizes,
power ratings, electrical voltages, and monetary value.
•
Motor construction and features determine the operation
and diagnostic analysis. The typical variables include:
– Asynchronous, synchronous, and DC motors
– Rolling element or journal bearings
– Varying insulation types and ratings
– Lap wound or concentric wound stators
•
The low cost of smaller motors makes it difficult to validate
the return on investment of condition monitoring.
•
For induction and synchronous motors, the CM solution must
be able to detect rotor electrical faults even though
the rotors are not directly connected in the circuit.
•
The low cost of variable frequency drive (VFD) technology
makes variable speed motor applications much more
common, but these applications often complicate the
diagnostic requirements.
•
Motors are very dependent on external conditions:
– Power quality (voltage values and imbalances)
– Poor conductors, contactors, and so on
– Ambient and internal temperatures
5.3 Product Application
Proper application of GE’s Bently Nevada solution involves an
understanding of the customer’s CBM objectives, the criticality
of the machine, the type and characteristics of the motor,
and the motor’s typical or anticipated failure modes and their
consequences. Because these factors are not readily distilled down
to a “cookbook” for product selection, the following two sections
provide guidance and advice to aid in proper CM solution selection.
In preparation for using these sections, the sales manager or
field application engineer should discuss the following items
with the customer or CM system specifier.
•
•
•
•
•
•
•
•
Motor voltage levels
Motor horsepower levels
Motor bearing types (journal or REB)
Motor criticality (based on the customer’s
motor-driven processes)
Availability of spare or backup motors
The existence or implementation of customer’s other
PdM program(s)
The customer’s desire for automatic shutdown options
The customer’s preference for certain technologies
versus others
17
application note
5.3.1 Product Capabilities
The information represented in this table should be combined with the comments and clarifications in the next section.
Table 3 – Product Capabilities Matrix
Product
Dynamic
Rotor
Protection Plots
Bearing Mech
Rotor
Elect
Stator
Mech
Stator
Elect
Line
Fault
Load
Fault
Foundation
(S1)
3500/40
3500/42
X
S1 =
System 1
Software
X
X
X
X
1900/65A
X
(S1)
X
X
X
X
(S1)
X
X
X
X
See Note
Below
X
X
X
X
Essential
Insight.mesh
(S1)
X
X
X
X
SCOUT100/140
(S1)
X
X
X
X
X
X
Trendmaster
Pro / DSM
Bently Nevada
2300 Vibration
Monitor Series
(S1)
X
AnomAlert
X
X
MSIM/HSCT
GE Multilin
relay
X
X
X
X
X
X
X
X
X
(S1)
X
Note: 2300/20 connects to S1 Evolution while 2300/25 connects to S1 Classic through DSM.
5.3.2 Product Discussions
To select the appropriate product for a given motor application,
it is important to consider the following information when
interpreting Table 3 in the previous section.
5.3.2.1 Bently Nevada 3500 Series Machinery
Monitoring System
GE’s Bently Nevada 3500 Series machinery monitoring system’s
price point and features make it a good choice for critical and highly
critical motors with journal bearings. Note that the 3500/40M and
3500/42M systems do not have an enveloping capability, which is
useful for rolling-element bearings. Velomitor* transducers can be
installed on the stator core or motor frame to detect mechanical
looseness of electrical components. The 3500/42M monitoring
system does not have on-board automated diagnostics capabilities.
However, use with a Keyphasor transducer enables synchronous
sampling and additional vibration vector parameters that can be
interpreted by knowledgeable and trained personnel to provide
initial fault classification. The 3500/40M and 3500/42M systems
have gap alarms that are especially useful in detecting sleeve
bearing wear that has known to go undetected using bearing
housing or casing vibration measurements. The 3500 Series
includes temperature monitors that can be used to measure
and alarm on stator temperature sensors.
18
5.3.2.2 Bently Nevada 1900 Series Machinery
Asset Protection Systems
The 1900/65A general purpose equipment monitor is often a
more cost-effective choice for critical to low criticality motors.
It includes enveloping for rolling element bearing monitoring,
but lacks the Keyphasor (synchronous sampling) capabilities
of the 3500 Series system. It can also be used for stator
temperature measurement.
5.3.2.3 Bently Nevada 2300 Vibration
Monitoring System
The recently introduced 2300 Series vibration monitor provides
many of the features of the 1900/65A monitor and may be a
consideration for appropriate motor applications. The 2300 Series
does not currently support Velomitor, proximity, or temperature
sensors; however, these enhancements are planned. Check with
your field application engineer or product line manager if you
believe you have a motor application for which the 2300 Series
may be suitable.
5.2.3.4 Bently Nevada Trendmaster Pro Online
Condition Monitoring System
This scanning system is intended for use with large numbers
of medium to low criticality machinery. The dynamic scanning
module (DSM) can perform enveloping (see the data sheet for
application note
which cards include the enveloping feature), and can accommodate
Keyphasor transducer inputs for synchronous sampling, as well as
dynamically changing filter corners to correspond with machine
speed (this is especially useful for variable speed motors). System 1
software provides dynamic data plot formats for use in diagnosing
problems such as soft foot.
5.3.2.5 Essential Insight.mesh
Essential Insight.mesh is targeted for applications similar to
Trendmaster Pro, although the lower scanning rate and inability
to take in proximity probes, Keyphasor transducers, or temperature
sensors positions it for lower-criticality motors or motors with
slow failure rates. Enveloping and dynamic plots are available in
System 1 software.
5.3.2.6 Bently Nevada SCOUT 100/140
This portable solution is typically used on medium to low criticality
machines, but can be used to supplement other technologies
on large electric motors. The Ascent software package includes
configurations for detecting vibrations associated with rotor
electrical problems.
5.3.2.7.Bently Nevada AnomAlert
Although not a protection system, AnomAlert has a very
comprehensive monitoring capability. Unlike other products, it
can also provide an automated diagnostic analysis that identifies
a category of faults that should be investigated, usually using
other technologies or products. It is limited to use on three-phase
AC motors, and can be used with VFD and soft start if certain
conditions are met.
5.3.2.8 Bently Nevada Motor Stator Insulation Monitor
The motor stator insulation monitor (MSIM) and associated high
sensitivity current transformer (HSCT) are targeted to motors up
to 7.7 kVA. The MSIM has several other requirements that narrow
its applicability (constant speed AC motors, Y-wound with external
neutral connection). Although it focuses on stator insulation
condition, it is the only product on the market that directly
measures dissipation factor. Customers with large critical or
highly critical motors will be interested in it if they are concerned
about or have had failures with stator insulation. It also has the
advantage of being integrated into the 3500 Series platform.
5.3.2.9 Multilin Relay
GE’s Multilin motor protection relays are commonly found at
many of our customer sites. They protect the motor from internal
as well as line faults. If the customer already has System 1 software,
or is interested in a network to bring not only static values and
statuses, but also dynamic waveforms, into a central location
for analysis, then they will be interested in the System 1 Multilin
relay interface.
6Complementary Predictive
Maintenance Tests and Technologies
Besides regular maintenance practices and procedures, there
are numerous predictive maintenance (PdM) technologies that
operators and diagnosticians can use to verify that motors are
operating as designed. Many of these technologies are used
in parallel to allow the strengths of one system to overcome
weaknesses in others. PdM and CM technologies are also used
in series to allow, for example, one technology to flag a problem
and then use another system to isolate and troubleshoot the
exact fault.
Note: Many of the motor monitoring and diagnostic technologies
mentioned below are applicable to generators and can often be
used interchangeably on motors and generators.
6.1 Offline Testing
At this time, GE’s Bently Nevada product technologies and
methodologies provide only online testing capabilities, and this
document does not consider or discuss adding any ability to
provide offline motor testing capabilities. It only discusses
current online monitoring products and possible improvements
or enhancements that might be made.
Offline testing (static) is used to evaluate the stationary
components of the insulation system; for example, to detect
the condition of the rotor bars or windings. Offline testing can
also detect broken conductors, loose connections, and ground
wall insulation weakness. Windings are the basic indicators of
degradation and failure in an electric motor. Bearing failure can be
prevented by keeping the electric motor in a balanced, grounded,
properly mounted, and lubricated condition.
6.1.1 Motor Insulation Condition Testing – Offline
Many different types of tests for assessing the health of motors
have been developed over the last century. Most of these tests
require that the motor be disconnected from the grid to allow
“offline” tests to be performed. These tests are typically performed
during initial commissioning to provide baseline information and
then repeated during regular maintenance outages so that the
results can be trended and compared with the baseline readings.
Motor insulation is one of the most critical aspects of proper motor
condition. Healthy motor insulation is an important requirement
for properly functioning motors as well as a critical safety concern.
Numerous factors can cause insulation failure, including excessive
heat or cold, moisture, dirt, corrosive vapors, vibration, aging,
and oil or other chemicals. There are many tests in use today for
assessing the health of motor insulation. Unfortunately no single
test is sensitive to all insulation deterioration problems.
The progression of insulation failure often begins in stator
windings shorting to each other at the point of an insulation void
or breakdown. This partial phase-to-phase short causes uneven
current distribution within the stator windings. While the motor
continues to run, the resulting increased current causes localized
overheating and further premature insulation breakdown in the
groundwall insulation until there is ultimately a catastrophic and
dangerous short from high voltage to ground.
19
application note
6.1.2 Offline DC Tests
Direct current offline tests used to quantify insulation health
are very common and easy to perform with low-cost equipment.
These tests are sensitive to contamination, moisture absorption,
and major flaws such as cracks, cuts, or pinholes in the insulation.
Unfortunately, they are insensitive to internal problems in the
insulation such as voids.
6.1.3 Insulation Resistance (IR) to Ground Testing
IR is the most widely used insulation test. It is relatively easy to
perform, requiring the use of a mega-ohmmeter with a timed test
function and a temperature indicator. “Megger” (MEGaohm metER)
is a common name for this test because the Megger Company
originated the test equipment and procedure around 1900.
The insulation of each phase can be tested separately with a
high-voltage DC source applied between each conductor and
ground. The resulting mega-ohm readings are time dependent
and measured at the start, then after one minute, and then at 10
minutes. Due to the popularity of the test, the following standards
have been developed to define the recommended test voltage level
and resistance acceptance levels for different voltage motors:
•
IEEE Std 43-2000
•
IEC Std 60364-6 [1] Table 6A
•
ANSI NETA ATS-2009 [2]
Determining and documenting the testing temperature is
critical because electrical resistance has an inverse exponential
relationship with temperature. The resistance approximately halves
for every 10o C temperature increase. Therefore, readings must be
corrected to a base temperature (typically 20o C or 40o C).
Surface contamination (as shown in Figure 25) leads to increased
leakage current on the surface of the winding and results in low
Insulation resistance readings and a possible low polarization index.
6.1.4 Polarization Index, Dielectric Absorption
Ratio and Polarization Index Profile Testing
The polarization index (PI) test takes the ratio of IR data measured
at 10 minutes and one minute. Similarly, the dielectric absorption
ratio (DAR) test is the ratio of IR measured at three minutes and
0.5 minutes.
PI =
IR10 min
,
IR1 min
DAR =
IR3 min
IR0.5 min
Equation 1A, B – Calculating Polarization Index and
Dielectric Absorption Ratio
Due to the use of ratios of IR values, the PI and DAR values are
less sensitive to temperature variations because the temperature
compensation factors cancel when division is performed. This
makes trending PI and DAR values easier over time compared to
just using IR values. Similar to IR measurements, PI and DAR values
are compared between phases and trended over time to find
indications of insulation failures due to contamination, moisture
absorption, and cracking in the ground wall insulation.
Another related term is Polarization Index Profile (PIP) testing
which trends the insulation resistance over the full 10 minute
testing time spanned in five second intervals. This test is usually
accomplished using specialized motor testing equipment like the
PdMA MCEMAX*. The PIP plot provides a more comprehensive
look at the resistive properties compared to just looking at the
ratio of two discreet points in time. It starts with a low Mohm
value (near zero Mohms) and smoothly rises to several thousand
Mohms. Contaminated or moisture-ingressed motor insulation
demonstrates inconsistent and low Mohm values (less than
100 Mohms).
6.1.5 Step Voltage Test
Similar to the above tests, in Step Voltage tests two or more
voltage levels are applied across the motor winding insulation.
Then the resistance levels measured at the different test voltages
are compared. Healthy insulation should have consistent resistance
levels. If the resistance values decrease substantially at higher
test voltages, this can indicate insulation deterioration due to dirt,
moisture, cracking, or aging.
6.1.6 DC High-potential Testing and Electrical Surge
DC high-potential (Hi-pot) test is a pass/fail test that is typically
only performed after IR and PI tests indicate a potential fault.
The DC Hi-pot test involves applying an over-potential DC voltage
to the ground wall dielectric insulation for one minute. For the
duration of the one-minute long Hi-pot test, the applied test
voltage can be as high as twice the motor nameplate voltage.
If current flows during the Hi-Pot test, this is an indication that
the winding has cracks or fissures, endwinding contamination,
moisture absorption, or uncured resin.
Figure 25 – Winding condition; oily, dirty on the inside,
but looks pretty good from the outside (top), after steam cleaning,
drying, and varnish retreatment (bottom)
20
Testing determines if the electrical insulation between two
electrically isolated components is adequate to face any overload
voltage conditions. A high voltage is applied across the two
components being tested, and current is measured to detect
the amount of leakage in the insulation.
application note
Hi-pot testing is considered by many to be a critical test because of
the safety aspect of ensuring effective ground wall insulation. But
there is some amount of criticism for utilizing such high voltages,
pointing out that testing with voltage surges beyond the motor
insulation ratings can itself cause overheating and premature
failure of the motor components. So while the IR, PI, and DAR tests
are non-destructive tests, Hi-pot testing is considered destructive.
Step voltage tests are also considered destructive, if the applied
voltages exceed the insulation rated capability.
Electrical surge testing applies only a very brief high voltage
pulse across the winding to momentarily stress the insulation.
This high rise time impulse induces a voltage difference between
adjacent loops of wire within the winding. If the insulation between
the two loops of wire is damaged or somehow weakened, and if
the voltage difference between the wires is high enough, there
will be an arc between the wires. This arc shows up as a change
in the surge waveform.
The surge test is performed with an impulse generator and
a display to observe the “surge waveform” in progress. The surge
waveform is the voltage present across the test leads during the
test. The indication of a turn-to-turn fault is a shift to the left,
and/or a decrease in amplitude of the waveform when the arc
between loops of wire occurs. The wave pattern observed
during a surge test is directly related to the coils inductance.
Figure 26 – Tan Delta measurement related to voltage
and current phases
Dissipation factor measurements are taken during initial factory
motor testing and can be repeated later during maintenance
outages. The measurements setup is fairly elaborate and made
with expensive high-precision measuring equipment (as shown
in Figure 27). The dissipation factor test requires an outage for at
least half a day.
The test can be repeated at various voltage levels, and the
voltage at which the surge test failure occurs can be correlated
to the remaining life of the motor. Some testing companies claim
that the brief electrical pulses do not harm the insulation.
6.2 Offline AC Tests
6.2.1 Capacitance to Ground
DC measurements do not give visibility of insulation contamination
or internal voids. Measuring and trending phase-to-ground
capacitance levels can provide an indication of insulation
contamination and internal voids.
6.2.2 Capacitance and Dissipation Factor –
Tan Delta Measurements or “Tip-Up” Tests
Capacitance and dissipation factor measurement helps detect
deterioration in the motor winding insulation. Also termed
“Tan Delta” (δ) or “Loss Angle” testing (see Figure 26), this test
works on the principle that any insulation in its pure state acts
as a capacitor. Therefore, the current phase should lead the
voltage phase by 90 degrees. As the insulation system degrades,
it takes on resistive properties that change the phase angle
between the current and voltage.
Figure 27 – Typical DF (Tan Delta) measuring bridge
Himalayal Tan Delta Bridge www.himalayale.com
21
application note
6.3 Other Motor Condition Testing
beyond Insulation – Offline Testing
While insulation is a primary concern for motor operators, other
testing may be done to provide condition monitoring for the many
other possible motor failure modes. Traditionally this was done
with offline testing performed during maintenance intervals.
6.3.1 Phase-to-Phase Resistance (DC Test)
Measuring an increase in phase-to-phase resistance values can
indicate a number of faults:
•
Corroded terminals, contactors, or connections
•
Loose cable terminations or bus bar connections
•
Poor crimps or solder joints
To perform this test, the resistance is measured between each
motor phase pair, resulting in three values. For example, for a
motor with phases designated A, B, and C, the three phase-tophase measurements would be: RA-B, RB-C, and RA-C (See Figure 28).
This measurement can also be repeated at different locations
(directly at the motor leads or in the motor control center, etc.)
to try to isolate the location of the faulty component that is
contributing to the added resistance.
6.3.2 Phase-to-Phase Inductance (AC Test)
Phase-to-phase inductance measurements detect changes in the
relationship between the motor stator and rotor. The inductance
measurements are made between each pair of motor leads, which
provides three trendable values: LA-B, LB-C, and LA-C. Inductance
values change as leakage paths develop in the windings. For
example, a turn-to-turn short in the stator would cause a leakage
path and decrease the inductance. Rotor faults typically increase
the measured inductance.
6.3.3 Rotor Influence Check
The rotor influence check (RIC) provides a graphical representation
of the relationship between the rotor and stator (see Figure 29). The
rotor’s residual magnetism affects the phase-to-phase inductance
readings. All three phase-to-phase inductance measurements are
taken at small increments of shaft position (approximately five
degree increments, depending on the number of motor poles).
The result of the measurements is a graph of inductance versus
phase angle. The user must interpret the shape of the graph and
look for how sinusoidal the curves are, minimum and maximum
values, and repeating distortions in the plot that can be interpreted
to signify the presence of rotor or stator faults. The data can also
be used to show air gap (eccentricity) problems if the inductance
values trend up or down through a shaft rotation cycle.
Figure 28 – Example resistive load in motor
power circuit (WYE or “Star” wired motor)
Phase-to-phase resistive measurements can be used to isolate
the location of a faulty terminal causing an additional resistance
“R” (as shown in Figure 28 above). Because RA’-B’ is less than RA’-C’
and RB’-C’ and RB-C, the additional resistive load can be isolated
between the C’ and C measurement points.
Resistive imbalances between the phase-to-phase measurements
can indicate serious motor problems that can result in voltage
imbalances. Voltage imbalances will in turn cause current
imbalances and increased winding temperatures. During motor
operation a 1 percent voltage imbalance can result in a 6 to 7
percent current imbalance. A 3.5 percent voltage imbalance
can raise winding temperatures by 25 percent and lead to
premature insulation failures.
22
Figure 29 – RIC graph for rotor bar fault (top) and
eccentric rotor (bottom)
http://www.pdma.com/pdfs/Articles/Influence_of_Residual_
Flux_on_the_Measurement_of_Inductance.pdf
application note
6.4 Motor Insulation Condition Testing
– Online
While offline testing was historically the only available method
for motor testing (and is still considered critical for the safety
aspects involved), growing in popularity are testing methods
that are performed during the operation of the motor. Online
testing does not require that the normal motor operation be
interrupted or be physically disconnected from its power supply.
Online tests can be performed during normal motor operation
as long as permanent instrumentation is already installed or
temporary sensors can be installed in compliance with proper
LOTO procedures.
For almost all online diagnostic tests described below, it is
recommended that motors operate above 70 percent of full load
to provide consistent, trend-able readings. The following sections
list several hardware systems and tests used for online motor
monitoring and protection.
6.4.1 Partial Discharge
Partial discharge (PD) monitoring (as shown in Figure 30) can
provide a continuous, online assessment of the motor or generator
insulation condition. Partial discharges are small electrical sparks
that occur within the high-voltage electrical insulation in stator
windings (as well as dry type transformers and switchgear). PD
occurs whenever there are small air gaps or voids in or on the
surface of the insulation. Normally, well-made windings that are
still in good condition display little PD activity. But, the amount of
activity can increase by a factor of ten or more with deteriorated
insulation. Iris Power states that their PD monitoring can generally
provide two or more years of warning for increased risk of failure.
can distinguish PD activity from the motor being monitored versus
other “noise” on the supply bus.
Connection
to high
voltage
Capacitive
coupler and
mounting bracket
Signal
output
Figure 31 – Partial discharge capacitive couplers
http://smsystems.co.in/PDTracII_v6%20(2).pdf
The PD monitoring device keeps track of the size, polarity, and
where in the line frequency cycle the discharges occur. There
is much proprietary knowledge required in determining the
insulation damage severity based on the data characteristics
with respect to the type (manufacturer, model, and vintage) of
insulation being monitored. Also, it is common for PD monitors
to use temperature compensation in the assessment. Iris Power
offers their own software package that trends and displays data
from their PD Trac monitors. The software uses 2-D and 3-D plots
for displaying PD data.
Figure 30 – Partial, or incomplete, electrical discharge that occurs
between insulation and either insulation or metallic electrode
http://smsystems.co.in/PDTracII_v6%20(2).pdf
Partial discharge measurements are applicable for motors
operating at or above 3.3 KV.
To measure PD activity, capacitive couplers (as shown in Figure 31),
are installed at each of the three phases. Due to their capacitive
properties, these couplers pass high-frequency fluctuations
(discharges) through the couplers and out to the signal wires on
the low voltage sides of the capacitive couplers. In general, one
set of three couplers is installed near the generator or motor, but
the monitor may use special filtering techniques to filter out PD
activity from other sources on the power bus. Additional sets of
couplers can be used at a location further from the motor, and,
by viewing the timing difference from the pulses, the PD monitor
Figure 32 – 2D and 3D plots of partial discharge activity
IRIS Power PDTracII http://smsystems.co.in/PDTracII_v6%20(2).pdf
23
application note
In hydro-turbine-generator applications, GE works together with
Iris Power, offering their PD monitors as an option with GE’s Bently
Nevada hydro monitoring products. A special plot option was
created in System 1 software to display partial discharge data
from an Iris Power PD monitor (see Figure 32). Additionally, the
System 1 “Hydro-X” rulepak was created, which provides options
to include PD data in its assessments.
The Iris Power hardware can also integrate PD with magnetic flux
measurements (to detect rotor shorted turns) and end-winding
vibration monitoring.
6.4.2 Motor Current Signature Analysis and
Electrical Signature Analysis
6.4.3 Infrared Thermography
Infrared thermography (shown in Figure 33) uses a thermal
imager to detect radiated heat, not only from the motor itself,
but the complete associated electrical system. Thermography
data can be taken without taking motors offline. Motor-related
faults that can be detected with thermography, include:
•
Stator hot spots
•
Insulation faults
•
Faulty electrical connections
•
Bad motor contactors
•
Wiring problems
A growing trend in motor condition monitoring is evaluating the
motor power lines during operation. Motor current signature
analysis (MCSA) uses current transformers or current sensors
to measure the current flowing through the phases of the motor.
The most common faults detected (not all distinguishable from
each other) by MCSA are:
•
Broken or cracked rotor bars
•
High resistance joints in rotor bars or wound
rotor conductors
•
Broken or cracked end rings in squirrel cage rotors
•
Casting porosity affecting current flow in die cast rotors
•
Static and dynamic eccentricity conditions between
rotor and stator
•
Mechanical defects associated with the rotating element
(e.g., bearing degradation)
Electrical signature analysis (ESA) is a term usually used more
broadly and can also utilize motor voltages in conjunction with
the currents when doing analysis.
Advantages of this type of motor analysis, include:
–
Sensors do not have to be installed on the machine
train itself. Sensors can be installed anywhere along
the power supply lines, but are typically installed in
motor control cabinets in switchgear rooms.
•
Electrical properties of the motor can be detected.
•
Drivetrain mechanical faults cause features in the motor
current that can be analyzed with ESA. The motor line
frequency acts as a signal carrier, and torsional mechanical
disturbances cause amplitude and phase modulation of the
line frequency and harmonics.
ESA may be performed with permanent or walk-around portable
equipment. Condition monitoring and diagnostics with ESA can
be challenging because data taken at various motor operating
conditions results in wide variations in current readings.
24
Figure 33 – Pump motor infrared thermography image showing
localized heating (Thermal image was used to confirm AnomAlert’s
electrical fault warning)
AnomAlert Presentation
When implementing an IR monitoring program, it is recommended
that initial baseline thermal image data be captured. Then a
schedule of regular collection routes should be established for
detecting developing problems. Additional data should also be
collected after maintenance work is performed, or repeated after
repairs have been made to confirm the work was performed
correctly. A significant selling point for using IR monitoring for
motors is the ability to detect motor winding overheating “hot
spots,” take corrective actions to prevent further damage, and thus
extend the life of the motor insulation.
It has been common for end users to rely on specialized training
and consulting companies (such as Snell Group) to perform
data collection routes and provide analysis, but thermography
equipment technology has improved even as prices have decreased,
making it practical for many plants’ predictive maintenance teams
to perform the thermography inspections in-house. A significant
advantage of this technology is that the same thermography
application note
hardware used for motor internals (motor windings, electrical
terminations, and bearings) can also be used in other stationary
electrical equipment like switchgear, transformers, and circuit
breakers. This hardware can also be used for assessing the
condition of a plant’s other rotating or non-rotating equipment
like belts, rollers, piping, heat exchangers, boilers, steam turbines,
and pumps.
The majority of small, general purpose motors use greased rolling
element bearings (REBs). Since all REBs have finite lifetimes and
the mechanical components naturally wear and eventually spall,
liberating some of the bearing material, it is obvious that used
grease wear debris analysis could provide important insight on
the condition of motor bearings. This analysis, though, is not a
common practice in the industry.
Challenges associated with thermography, include:
REB wear particles generally are much larger than particles
analyzed with typical oil analysis procedures such as atomic
emission spectrometry. Particle size from normal bearing wear
ranges from 5 um to 15 um and advanced damage is indicative
with particles greater than 25 um.
•
Need for careful image setup to get images with consistent
internal and external factors (motor load, ambient
temperatures, camera angle, and so on)
•
Difficulty in interpreting data plots to assess what is
healthy or not healthy
While GE’s Bently Nevada products do not utilize infrared
thermography, our System 1 DocuView has the ability to link
images and other files to its enterprise assets. System 1 software,
however, does not have any ability to analyze, compare, or trend
thermal images.
6.4.4 Lubrication Analysis
Large motors typically use fluid-film bearings and many motors
have rolling element bearings lubricated with oil spray-mist
systems. In these cases, it is relatively easy for maintenance
teams to use tribology laboratory services that can analyze oil
samples (see Figure 34) for the following features:
• Oil quality (viscosity, acidity, specific gravity)
• Unwanted oil contamination (water, silicon)
• Machine wear particles
Figure 34 – Oil analysis –an exceptionally powerful tool for
monitoring sleeve bearing wear
25
application note
26
application note
27
© 2015 General Electric Company. All rights reserved. Information provided is subject to change
without notice. Best practices and recommendations herein are applicable to most industrial
electrical motors. This guideline is not intended to replace or supersede any manufacturer or OEM
guidelines concerning proper installation and operation of their equipment.
GE Oil & Gas
1631 Bently Parkway South
Minden, NV 89423
*Denotes a trademark of Bently Nevada, Inc., a wholly owned subsidiary of General Electric
Company. The GE brand, GE logo, Bently Nevada, System 1, Keyphasor, Proximitor, Velomitor,
RulePaks, Bently PERFORMANCE SE, ADRE, SPEEDTRONIC, GE Multilin, Multilin, Mark, SmartSignal
are trademarks of the General Electric company.
24/7 customer support: +1 281 449 2000
www.gemeasurement.com
PdMA, MCEMAX, BusTracII, BusTracII, Megger are trademarks of their respective companies
GEA32339 (2/2016)