Software and Systems Engineering Standards: Verification and Validation of Sy...
Motor Current Signature Analysis R K Gupta
1. Predictive Maintenance of Induction Motors
With MCSA & TSA Technology
R K Gupta
Principal Consulting Electrical Engineer (Power System)
Reliability Engineer
BE Electrical
Protection, Automation, PLC & SCADA, Level 3 MCSA, Colorado
USA
2. What is MCSA & TSA ?
MCSA: stand for Motor Current Signature Analysis. The basis of MCSA is that the
stator current contains current components which is directly linked to rotating
magnetic flux components caused by electrical or mechanicals faults. These
harmonics current components caused by fault can be used for early failure detection.
TORQUE SIGNATURE: The acquisition of torque ripple and the analysis of the torque
spectrum is a relatively new addition in the realm of condition based maintenance
technology. By capturing the real components of the current and deriving the flux
generated from voltage a signal equal to air gap torque is developed. Mechanical
information buried in a complex modulated current signal can now be easily accessed
and interpreted for electrical and mechanical conditions.
4. Dynamic Motor Testing
A motor is only one part of a “Machine System” that includes: Power in, the Motor
and the Load.
Dynamic testing diagnoses issues for all three links.
5. Gives clarity to the Root Cause Analysis
Power Quality, Motor, Load issues
Helps to find
Mechanical issues including: Bearing problems, Rotor Bar issues, Cavitation's
and Eccentricities
Electrical issues including: Oscillations, Transients and Harmonics (Distortion)
6. Benefits of Dynamic testing
Helps separate Mechanical from Electrical issues
Provides “early warning” information
Provides “support” for other technologies
Can be performed more often than static testing
It is safe, fast and non-intrusive
7. Defining Dynamic Electric Motor
Monitoring
WHAT IS IT?: The ability to find Power Condition, Motor and Load Related issues
while the motor is running under normal conditions.
HOW?: We measure, analyze and trend the currents and voltages of the motor-
load system (typically at the MCC).
8. Maintenance Program Types
Corrective – Run to failure
Preventative – Pre-planned periodic motor replacement or recondition regardless
of condition, run time or history.
Predictive – A complete program that includes static and dynamic motor testing,
etc. All diagnosed and trended.
9. Cost Vs. Benefits
Corrective $17 - $18 Per Hp
Preventive $11 - $13 Per Hp
Predictive $ 7 - $ 9 Per Hp
1983 EPRI Study of Maintenance Programs
11. What are we really after?
Reduce unscheduled downtime by predicting imminent motor failures and
identifying problem areas
Determine root-cause of problem
Ultimate goal: Save time & money
12. The Questions:
Is the motor operating properly?
Is the load operating properly?
How is the motor reacting to the load?
What is the condition of the power?
13. To Find the Answers:
Observe voltages & currents supplying motor.
Obtain motor speed.
Obtain motor torque.
Analyze data.
Trend routinely.
16. Here is how this thing works: The MCC tells the motor: ‘I am sending you 50Hz.’
Motor replies ‘Ok. Load, I would like to turn you at this speed ‘(1500rpm, say). Load
says: ‘Ok, you want to do that, then give me this much torque’. Motor responds ‘Fine’.
Turns around and says to the MCC: ‘I need this much power’ (remember, mechanical
output power of the motor is speed times torque. Both of those parameters are set
right now). MCC says ‘Well, I know that I am giving you this particular voltage here’.
Then the motor says ‘All right, granted that you offer me this voltage, then give me this
amount and type of current’.
In short: If you find anything wrong with the torque, it m ust be caused by the load. If
you find anything wrong with the voltage, it m ust be caused by the MCC. If you find
anything wrong with the current, then it is the play of MCC, motor and load together.
Action & Reaction
17. Note
Torque related problems are caused by the load
Power related problems come from “upstream” and effect the whole buss.
If you find anything wrong with the current then it is the play of the MCC, motor and
load together.
21. Motor
Load
CTs
Breaker
Explorer
PTs
No physical connection to motor required.
No transducers required to be installed on the motor
Permanent installation available
All values derived from voltage and current measurements only
Connection for HV/HT Motors
24. Accuracy of Speed Critical:
Consider a 4 pole motor:
Synchronous speed = 1800RPM
Full Load Speed = 1780 RPM
1% Error in speed measurement = 18 RPM!
Must be able to resolve shaft speed to the 1RPM level or better: 0.05% (0.05% =
1/1800*100%)
Special signal processing techniques required: DFLL (Digital Frequency Locked
Loop) – basically a form of correlation.
FFT will not work due to very long observation times (60 seconds or more) to get
enough frequency resolution to resolve 1 RPM.
25. Current Signature: FFT vs. DFLL
Amplitude: 20dB
Amplitude: 60dB
Resolution: 0.13Hz Resolution: 0.005Hz
FFT DFLL
28. Motor Speed
Air gap changes as rotor rotates within the stator core because of excentricities,
misalignments of rotor and stator, out of round rotors, etc.
Current in stator changes (slightly) as a result of the changing gap distance.
Change in current shows up as a peak in the current spectrum.
29. Torque Derivation
Use “dq0” theory – also called two axis theory.
Theory exists since 1929
R.H. Park, “Two-Reaction Theory of Synchronous Machines – Generalized Method of
Analysis. Part 1”, AIEEE Transactions, Vol. 48, July 1929, pp. 717-717.
Used in VFD’s since the 1980s
E.C. Lee, “Review of Variable Speed Drive Technology”, www.powertecmotors.com/avsde4.pdf.
T.A. Lipo, A.B. Plunkett, “A Novel Approach to Induction Motor Transfer Functions,” IEEE
Transactions on Power Apparatus and Systems. Vol. PAS 93 pp. 1420-1419, 1979.
A. B. Plunkett, “A Current Controlled PWN Transistor Inverter Drive,” IEEE/IAS 1979
Annual Meeting, pp 785-792.
Well documented in motor control texts.
P.C. Krause, O. Wasynczuk, S.D Sudhoff, “Analysis of Electric Machinery,” IEEE Press
NY, ISBN 0-7803-1101-9, 1995.
30. Torque Shows What the Load is
Doing
Variations in torque indicates a problem
with the smooth operation of the motor
and or load.
Intermittent load variations -> T(t)
Repetitive load variations -> T(freq)
To the right: Fan with a flapping belt
causing excessive bearing wear on both
motor and fan pillow blocks.
31. Power Quality
Voltage level, voltage unbalance, harmonics distortion, total distortion, power, harmonics
Machine Performance
Payback period, effective service factor, load, operating condition, efficiency
Current
Current level, current unbalance
Spectrum
Rotor bar, V/I spectra, demodulated spectra
Connections
Waveforms, ABC/SYM components, phasors
VFD details
Torque and speed vs. time, frequency and voltage vs. time.
Overall Check
35. Low Voltage
High Voltage
Over Currents (Over Heat)
Low Power Factor
Iron Saturation
Ultimately Higher Losses
Incoming Power - Voltage Issues
36. Over/Under Voltage
Motors are designed to operate with +/- 10% of rated voltage
Ideally, voltage supply deviation should be less than +/- 2%
When operating over/under voltage a motors performance, efficiency, and power
factor change
37. Over/Under Voltage
Voltage deviations usually caused by
Poorly performing or improperly adjusted transformers
Undersized conductors
Poor connections
Low power factor sources in the distribution system
38. Acceptable System Voltage Ranges
Nominal System
Voltage
Allowable Limits % Allowable Voltage
Range
120V(L-N) +/- 5% 114V- 126V
240V(L-L) +/- 5% 228V- 252V
480V(L-L) +/- 5% 456V- 504V
39. Comparison of voltage level to average
winding temp & motor efficiency*
Voltage -10%
(414V)
Normal
(460V)
+10%
(506V)
HPFull
Load Temp Eff Temp Eff Temp Eff
10 66 90.0 56 91.4 55 91.5
20 84 90.4 70 91.8 67 92.1
50 84 91.9 69 93.1 62 93.6
100 82 94.2 72 94.8 69 94.9
200 90 94.9 77 95.5 74 95.7
41. 6.6kV 1000 hp Pump Motor at a
Nuclear Power Plant
Motor being overdriven. Operating at 7.7kV
Plant engineering thought there should be no problem.
Currents are below nameplate – so its ok, right?
Wrong! Saturating the iron in the stator caused overheating
Motor has burned up 3 times - $150,000 to repair.
Thermal output of plant reduced to 50% during 3-4 weeks per repair.
Lost $10,000,000 in electrical sales during each repair.
43. Voltage Unbalances
When a voltage unbalance reaches 5 %, the phase currents can differ by as much
as 40 %.
Unbal = 100 Vmaxdev-V
V
Where:
Unbalance = Voltage unbalance in %
Vmaxdev = Line to line phase voltage deviating most
from mean of 3 phases
V = RMS voltage, mean line to line of 3 phases
48. Harmonics Can Damage Motors
Shotton Paper (Flintshire, UK, Shaw Bowden, shaw.bowden@upm-kymmene.com)
Large inverter fed motor on the same bus as a smaller induction motor.
Harmonics from the VFD destroying smaller 660V, 550hp motor.
Voltage THD = 11%, 9%, 8% (three phases)
Voltage Imbalance = 3.6%
NEMA de-rating = 60%
Running at Effective Service Factor = 1.29
End result, motor running very hot.
Time to get out the Baker Advanced Winding Analyzer to check for insulation
damage.
60. Operating Condition
Goal: To compare machine’s operation with the past.
Method:
If past data exists at present torque & frequency level, then
Is the voltage level similar to previous?
Is the current level similar to previous?
Is the phase angle similar to previous?
Successes
Flagged very early rotor bar deterioration
Flagged high resistance connection on terminal box
Issue
Undiagnosed flags DO happen
64. Fundamental Relationships on
Efficiency:
Example: 100hp motor at 90% efficiency,
with perfect voltage condition.
a) What is the Input Power and the Output Power?
Efficiency = 100% - Losses[%]
Pout = 100hp
Pin = 100hp / 0.9 = 111hp
Efficiency = Output Power / Input Power
Ploss = 111hp – 100hp
Ploss = 11hp = rated losses
b) What are the losses?
65. Fundamental Relationships on
Efficiency:
Example: 100hp motor at 90% efficiency,
with 2% Voltage unbalance.
c) What is the Input Power and the Output
Power?
Pout = 100hp
Ploss = 11hp * 1.1 = 12.1hp
Pin = Pout + Ploss = 112.1hp
67. Variability Study of Identical
Motors
Motor HP No. of Test Efficiency
Max. Min. Ave.
Average
Losses
Variation
Losses (1)
200 10 96.6 96.0 96.3 5.722 +7.9%
75 6 93.9 92.8 93.2 4.06 +6.5%
40 7 91.0 90.0 90.6 3.085 +7.9%
(1) MaximumPercent increase in Total Loss fromthe Average Loss
73. Current Issues
Over Current
Increased heat causing faster degradation in the insulation system
Current Unbalance
Can be caused by voltage unbalance
Can indicate internal motor problems – or
Connection problems
76. Symptoms
Effect of Broken Bars
Requires constant torque level or it will
pulsate
Next one breaks sooner
Current increases
Temperature increases
Insulation life shortens
Typically non-immediate death
Symptoms of Broken/Degradation
Rotor Bars
High Rotor Impedance
Increase in Rotor Current
Heat/Temperature
80. Rotor Bar
MCSA detects current components induced in the stator winding due to rotating flux
components caused by problems such as broken rotor bars or high air gap eccentricity. It
is true that broken rotor bars will result in a change to the vibration spectrum, but vibration
is traditionally sensed at the bearings and for each motor there is a different mechanical
stiffness between the electromagnetic forces caused by broken bars or air gap eccentricity
and the position where the vibration is sensed. Consequently the vibration from
electromagnetic forces is a second order effect compared to current components directly
induced from the specific rotating flux waves. This is not the case with MCSA. With
respect to detecting air gap eccentricity problems a similar reasoning applies. It has also
been shown that the current spectrum changes due to mechanical misalignment and due
to abnormal rotor drive dynamics.
81. DETECTION OF BROKEN BARS
Primary Causes:
Direct on-line (DOL)
The starting duty cycles which the rotor cage winding was not designed to withstand.
This causes high thermal and mechanical stresses since the starting currents are
typically of the order of five times the full-load operating current.
Pulsating Mechanical Load
Pulsating Load arise when driving reciprocating compressors, ESP, Agitating
Pumps etc which can subject the rotor cage to high mechanical stresses.
82. DETECTION OF BROKEN BARS
Broken rotor bars result in current components being induced in the stator winding at
frequencies given by Equation
fsb = f1(1 ± 2s) Hz
83. CURRENT SIGNATURE DUE TO
BROKEN BARS
LOWE SIDEBAND
f1 =SUPPLY FREQUENCY (50 Hz)
Hz
dB
UPPER SIDEBAND
84. SIGNATURE PATTERN DUE TO BROKEN BARS
TWICE SLIP FREQUENCY SIDEBANDS
fsb = f1±2sf1
-2sf1 +2sf1
f1
dB
Hz
IDEALISED Current Spectrum – Due to Broken Rotor Bars
85. Case Study
Vedanta Aluminum Extraction Plant,
India
Motor Details
ID Fan
V=6600
I=40 A
HP=475.87
Speed=992
86. Air Gap Eccentricity
Air Gap Eccentricity consists of two types :
Static
Dynamic
Both air gap exists simultaneously in a real SCIM due to manufacturing
tolerances & installation procedure ( in large motors).
87. Static eccentricity where the position of minimum radial air gap length is fixed in space.
Causes of this include:
Static Eccentricity
Manufacturing tolerances on each component part.
Stator core ovality.
Out of tolerance spigots in the end frames that house the bearings.
Incorrect installation of large HV motors.
88. Dynamic eccentricity where the minimum air gap revolves with the rotor and is a
function of space and time.
Causes include –
Non-concentric rotor and shaft.
Thermal bow of the rotor.
Combination of high air gap eccentricity, consequential unbalanced
magnetic pull (UMP), and a flexible rotor.
Severe bearing wear.
Rotor core lamination movement independent of the shaft.
Dynamic Eccentricity
89. Dynamic Air Gap Eccentricity
Minimum air gap revolves with the rotor and it is the function
of space & time.
90. Diagnosis of Air Gap
Motor Current Signature Analysis (MCSA) is used to identify the characteristic current
signature pattern which is indicative of abnormal levels of air gap eccentricity. The
current signature pattern is identified via the following equation
fec = f1(R(1-s)/p ± nws) ± f1(1-s)/p
Consists of two parts:
frs = normal rotor slot passing frequencies = f1(R(1-s)/p ±
nws)
fr = rotor speed frequency = f1(1-s)/p = Nr (rpm)/60
91. High Air Gap Eccentricity Normal Air Gap Eccentricity
Air Gap Eccentricity Graph
96. Demodulated Spectrum
Analyzes the dynamic behaviour of Motor Load System
It is the demodulated spectrum of Torque Signature
With Channel Control, the demodulated spectrum can be changed from torque to
current or voltage of any one phase
98. Torque Domain
Torque Ripple Displays the instantaneous torque requirements of the load
Torque Spectra
Two tests are available in torque domain:
104. Case Study Bearing Failure
5hp, 4 pole, Baldor Motor
24” Axial Fan
Instrumented with
Cognitive Systems CV395B Spectrum Analyzer
Bentley Nevada ADRE 208P (Automated Diagnostics
for Rotating Equipment)
SWANTECH stress wave analysis system
Air flow meters
Accelerometers
Laser tachometers
Current meters
Thermocouples
Baker Explorer
105. SKF 6503 Deep Groove Bearing
.062 deep, .062 wide grove in outer race.
Vibration should show at the ball pass
frequency in formula below:(n=#balls,
f=rot freq, Bd=ball dia, Pd= path dia,
beta=contact angle) BPFO=107Hz:
)1(
2
)( βCos
Pd
Bd
f
n
BPFORaceOuter −=
108. Known Good Bearing Known Outer Race Defect
Torque Spectrum
Electrical Frequencies Removed Electrical Frequencies RemovedMarking 1 * BPFOAdding Electrical Harmonic Sidebands
BPFO Controlled Lab Test
… how it works
117. Voltage vs. time Torque vs. time
Current vs. time
Transient Analysis - Normal start-up
118. Transient Analysis-Case Study
Using start-up transient analysis to adjust soft-starts and locate problem areas.
In this case the customer had six new soft-starts installed; five were performing as
expected but one was not.
Result: New soft-start had to be replaced.
122. 3 Identical Vertical Pumps Feeding a Manifold
4160V, 1250hp, 254rpm
Pumps designed to pull 32,000Nm from motor.
Motor designed to provide 35,000Nm for full load.
One Pump pulling only 28,000Nm from motor.
Cavitations
Case Study
123. Findings
Problem found in the pump – not the motor:
First pump would only run at 28,000Nm or 75% of capacity
Pump showed fluctuating torque
The utility wanted higher cooling capacity for summer months - pump
pulled for repair
Repair cost $180,000
125. The Saga Continues
The pump was repaired & put back on line.
Power output of the entire 730MW plant was available…..until
One of the other pumps sheared its shaft.
126. Second Pump Failed
It has been speculated that the first pump’s turbulent water flow in the manifold overstressed
the shaft of the second pump. The pump may have have been destined for failure regardless
of the first pump’s performance.
Torque ripple was higher on 2nd and 3rd pump when 1st pump was running at 75% with
loose end bell. Didn’t think too much of this at the time.
Plant now down to 2/3rd of it’s thermal output because 1/3rd of cooling capacity was lost.
Repair of the second pump took 5 weeks
During the 5 weeks, the first pump was able to provide 100% of its rated pumping capacity.
Had it still been at 75% capacity, $4,000,000 in power sales would have been lost due to low
cooling capacity. (E. Wiedenbrug, “Instantaneous Torque as Predictive Maintenance Tool
for Variable Frequency Drives and Line Operated Motors,”)
127. Induced Imbalance
7.6 grams introduced at 2” from center of fan – 0.54ounce-inches of
imbalance.
128. Imbalance Results
Torque spectra showed a significant difference between the balanced and
unbalanced signal at the 1 x shaft speed. Torque signal very usable in finding
imbalances.
Accelerometers mounted on the outside of the duct showed a 6.7% increase in
amplitude at 1x shaft speed. Vibration does change, but it would be tough to be
definitive.
130. Using “Torque Ripple” to locate mechanical issues.
In this case, an air handler was having bearing failures and alignment problems.
The torque ripple identified the very cyclic beat of a loose belt.
131. Air Handling Fan
125hp, 480V, 1780RPM motor driving an air
handling fan – 6 ft between pulleys.
Torque varying 25% +
Can’t keep motor and fan aligned.
Replace fan pillow block bearings every 35-
45 days.
Replace motor bearings every 6 months.
Use torque deviation to determine when to
realign.
No longer necessary to replace bearings so
often.
Editor's Notes
IMP NOTES: Since the torque spectrum is not electrically modulated like the current spectrum it is not necessary to perform sideband analysis around the fundamental frequency. Instead, the spectrum is analyzed in the same way as a vibration spectrum.
Who of you thinks that you get primarily paid to avoid motors from breaking?
Who of you thinks that you get paid to avoid downtime? Key difference: with MPM you can do much more than only avoid motors from burning, and finding the root cause for it.
VFD: Who of you knows exactly what the VFD is doing? Do you know the speed at which it is running when; the rate of acceleration and the transient torque that it is requesting from the motor? If you don’t know what it is doing to your motors, how can you expect to be able to diagnose and repair conditions caused by the VFD that are tearing up your motor or the load it is driving?
We will talk about all these issues during this presentations.
Chain of events. Why is the chain of events important? You need to know as a second nature the order of cause and response. That is what gives you the power of root cause analysis. How many of you know about motors that blow up every year or two? How many times are you planning on replacing them after they blow, and when do you want to fix what is happening? Right now you don’t have the tool that gives you the knowledge so that you can repair what you are responsible for.
Here is how this thing works: The MCC tells the motor: ‘I am sending you 60Hz.’ Motor replies ‘Ok. Load, I would like to turn you at this speed ‘(1800rpm, say). Load says: ‘Ok, you want to do that, then give me this much torque’. Motor responds ‘Fine’. Turns around and says to the MCC: ‘I need this much power’ (remember, mechanical output power of the motor is speed times torque. Both of those parameters are set right now). MCC says ‘Well, I know that I am giving you this particular voltage here’. Then the motor says ‘All right, granted that you offer me this voltage, then give me this amount and type of current’.
In short: If you find anything wrong with the torque, it must be caused by the load. If you find anything wrong with the voltage, it must be caused by the MCC. If you find anything wrong with the current, then it is the play of MCC, motor and load together.
Connection for medium and high voltage motors.
Points to get across:
a) This type of connection is for any voltage larger than 600V
b) More than 9 out of 10 times you will be facing a 2PT and 3CT scenario, which is shown here. We could even do a 2CT and 2PT scenario.
c) Typically, the voltage and the current cabinets are physically different cabinets
d) There is no problem with an open delta, or floating delta configuration (shown here). It is ok to have 2 hot leads and one ground.
Concept: You clip our current clamp ons the following way: one CT around each of the 3 secondaries of their CTs. Typically their CTs are 5 A rating. Use the 10A CT for this.
You clip our voltage clips the following way: First, our ground clip to their cabinet ground. Then one of our clips to the first ‘hot’ of their cabinet. Their ‘hot’ is the secondary of their PT. It will be typically 120V. Then our second clip to their second hot. Then our third clip to ground. Don’t worry, it works!
Think about it: Steady state, three phase system. There can only be 3 things that are wrong with your voltage condition: Level, balance or distortion.
Typically you will have a combo of these, with different grades of severity.
Very important to have engrained: Pretty much all voltage issues come from up stream. And whatever problem you have with the voltage bus, it will affect the whole voltage bus. It is not only this one motor that you are testing, that is in trouble; it is all the motors connected to that bus (think about all the row of MCC at which you are standing, at a minimum. Walk upstream and find what causes the problem and fix it! If you can’t, make sure that all of your critical motors are derated properly, so that they are not running too hot.)
Exception: VFDs create their own voltage condition downstream. Whatever happens downstream of the VFD is not caused by what is happening upstream of it. Think of the VFD as creating 100% of the voltage condition downstream.
Think about it: Steady state, three phase system. There can only be 3 things that are wrong with your voltage condition: Level, balance or distortion.
Typically you will have a combo of these, with different grades of severity.
Very important to have engrained: Pretty much all voltage issues come from up stream. And whatever problem you have with the voltage bus, it will affect the whole voltage bus. It is not only this one motor that you are testing, that is in trouble; it is all the motors connected to that bus (think about all the row of MCC at which you are standing, at a minimum. Walk upstream and find what causes the problem and fix it! If you can’t, make sure that all of your critical motors are derated properly, so that they are not running too hot.)
Exception: VFDs create their own voltage condition downstream. Whatever happens downstream of the VFD is not caused by what is happening upstream of it. Think of the VFD as creating 100% of the voltage condition downstream.
Think about it: Steady state, three phase system. There can only be 3 things that are wrong with your voltage condition: Level, balance or distortion.
Typically you will have a combo of these, with different grades of severity.
Very important to have engrained: Pretty much all voltage issues come from up stream. And whatever problem you have with the voltage bus, it will affect the whole voltage bus. It is not only this one motor that you are testing, that is in trouble; it is all the motors connected to that bus (think about all the row of MCC at which you are standing, at a minimum. Walk upstream and find what causes the problem and fix it! If you can’t, make sure that all of your critical motors are derated properly, so that they are not running too hot.)
Exception: VFDs create their own voltage condition downstream. Whatever happens downstream of the VFD is not caused by what is happening upstream of it. Think of the VFD as creating 100% of the voltage condition downstream.
Think about it: Steady state, three phase system. There can only be 3 things that are wrong with your voltage condition: Level, balance or distortion.
Typically you will have a combo of these, with different grades of severity.
Very important to have engrained: Pretty much all voltage issues come from up stream. And whatever problem you have with the voltage bus, it will affect the whole voltage bus. It is not only this one motor that you are testing, that is in trouble; it is all the motors connected to that bus (think about all the row of MCC at which you are standing, at a minimum. Walk upstream and find what causes the problem and fix it! If you can’t, make sure that all of your critical motors are derated properly, so that they are not running too hot.)
Exception: VFDs create their own voltage condition downstream. Whatever happens downstream of the VFD is not caused by what is happening upstream of it. Think of the VFD as creating 100% of the voltage condition downstream.
Think about it: Steady state, three phase system. There can only be 3 things that are wrong with your voltage condition: Level, balance or distortion.
Typically you will have a combo of these, with different grades of severity.
Very important to have engrained: Pretty much all voltage issues come from up stream. And whatever problem you have with the voltage bus, it will affect the whole voltage bus. It is not only this one motor that you are testing, that is in trouble; it is all the motors connected to that bus (think about all the row of MCC at which you are standing, at a minimum. Walk upstream and find what causes the problem and fix it! If you can’t, make sure that all of your critical motors are derated properly, so that they are not running too hot.)
Exception: VFDs create their own voltage condition downstream. Whatever happens downstream of the VFD is not caused by what is happening upstream of it. Think of the VFD as creating 100% of the voltage condition downstream.
Big question: I have a 100hp motor, that says that it is a 1.25 sf on the nameplate. How much power may I expect to run that motor with in steady state, without causing premature deterioration?
Answer: 100hp only! The number of 125hp is a typical myth. Here is what the sf number tells you: You may run up to 125hp for short times only. This table tells you the story by looking at the temperatures that motors will run at, when running under a) rated conditions; b) 1.15sf and c) 1.25sf.
The most drastic case is the 50hp motor. Lets say that you would wish that motor to last for 20 years, before you have to replace it.
Now, the chart indicates that this motor will be running 50 degrees C hotter at 1.25sf than what it would be running at full load. This means that the first 10 degrees will take it from 20 years down to 10. The second 10 degrees take it from 10 years down to 5. 3rd goes from 5 down to 2.5. 4rth goes from 2.5 down to 1.75. And the last 10 degrees half that value into 0.875 years, (10 months).
Symptoms of broken bars depends on several factor like types of bar, no of rotor bars etc
In CSA, the pole pass frequency (FP) appears as sidebands surrounding the line frequency (FL) after performing a FFT on a captured signal. The synchronous magnetic pattern of the stator rotates faster than the rotor cage. This implies that any given rotor bar is passed by all of the magnetic poles in one rotation of the slip frequency. The rate at which this occurs is termed the FP. Often in vibration analysis the term for this is called the motor pole passing frequency (PPF). (PPF = motor slip X number of poles). The difference in amplitude between the FL and the FP is an indication of rotor health. Empirical research has shown that a difference of over 54 dB indicates a healthy rotor while less than 45 dB indicates a degraded (i.e., high resistance joints, cracked or broken bars) condition exists in the rotor.
UMP = Unbalanced magnetic Pull. Maximum limit is 10% in induction motor.
Acts on rotor but it rotates at rotor speed. Excessive stress on Motor and can cause bearing wear.
Radial magnetic force wave is caused by an eccentricity act on stator and rotor cage resulting winding damage.
Where fec = frequency components (Hz)
which are a function of air gap eccentricity,
f1 = supply frequency (Hz),
R = number of rotor slots,
nws = eccentricity integer (1, 3, 5......)
s = per unit slip,
p = pole-pairs,
Nr = actual rotor speed (rpm).
This sunken pump application is of a larger motor. 1250hp very slow turning application. The torque level of this pump in particular was lower than the sister pumps, which were same design, running in the same pit.
Additionally, the torque band (ripple surrounding the average torque) was very large, and not even constant. The scenario is very clear. Torque is requested by the load, and the motor has to comply. That is why the torque signature is the very best method to stop the mechanical people from pointing at the electrical and vice versa. If you find something wrong with the torque signature, you know that one way or another it is mechanically induced and can typically be found easily.
In this case, they stopped the pump, and pulled it. Pulling the pump costed 60,000U$ (just below 3 times the cost of the MPM Explorer). It was found that this pump had lost its endbell. The endbell is the part that is depicted to the right. Its function is to funnel the water into the pump itself, avoiding for the water to be circulating, and ensuring that it goes up into the pump.
Having lost the endbell, and the endbell having fallen into the pit, there was a lot of water that was circulating in the pump, yet not being sucked in. That explains the lower torque level. Additionally, the amount of torque band was caused by the much higher cavitation that that circulating water caused in the pump. The fixed torque signature to the right again displays how obvious of a finding this is, once one has the right tools in hand.