This document describes a method for detecting broken rotor bars in induction motors using motor current signature analysis (MCSA) and a zero-setting protection element. It discusses simulations showing the characteristic current signatures caused by broken bars. The method analyzes the magnitudes of frequency components in the stator current to determine the number of broken bars. Experimental results are presented using a test motor with intentionally broken bars under varying load conditions. The protection system also includes a Fourier transform function and event history for expert analysis of complex fault cases.
2. CONTENTS
Introduction
Objective
Simulations of a motor with broken bars
Method for detecting broken bars
Fourier transform function
Dependability for motors with light loads
Experimental results with actual motor broken bars
Conclusions
References
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3. INTRODUCTION
Induction motors are an important part of many industrial processes.
5%motor faiures happens because of problem in the rotor cage.
Broken bar condition starts with a facture at the junction between the
rotor bar and the end ring as a result of stresses. Stresses are high for high
inertia loads. Bending of fractured bar cause the bar to break.
When one bar breaks, adjacent bars carry currents greater than their
design values, causing damage in the laminations of the rotor core.
Motor current signature analysis (MCSA) is the most popular method to
detect rotor cage faults. MCSA performs frequency spectrum analysis of
the stator currents.When one or more bars breaks, upper and lower
sidebands appear at (1±2sf˳), where s is motor slip frequency and f˳ is
system frequency
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4. INTRODUCTION (CONTINUE)
Here we describe a protection element that determines the number of
brocken bars using relative magnitudes of the signals at the ±2f˳ and f˳
frequencies.
Loads such as pumps, mills and machines with gearboxes cause
amplitude modulation in stator currents with corresponding sideband
frequencies at f̥ ±kfr where k=1,2,3… This sidebands may affect
magnitude of sideband caused by broken bar conditions. For this reason,
the proposed solution aiso includes a Fourier transform function for
analysis by a motor expert.
For monitoring and protection we have a protective relay featured by
• A broken bar detection element that uses MCSA for detection
• A hystory report that includes the date and time of BBDE operations
• A Fourier transform function that calculates the freaquency
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5. OBJECTIVE
Show how broken bars causes characteristic current signature
Show how to detect broken bars with a zero-setting protection
element, which uses the current signature method
Use cases with different motor operating and fault conditions to
analyze the performance of zero-setting broken bar protection
element
Identify cases when the current signature method is dependable and
when security is a concerns.
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6. SIMULATIONS OF MOTOR WITH BROKEN BARS
SIMULATION RESULTS
As in fig.2 the phase current
exhibits an amplitude modulation
because of broken bars
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Fig.1 Squirrel-cage equivalent circuit for a
motor with one broken bar
Fig.2 Motor A phase current for three
Broken bars
7. SIMULATION RESULTS
• Fig.3 shows frequency spectrum
of A phase stator current when
motor has three broken bars
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• Fig.4 shows relative current
magnitude of phase current
components at sidebands with
respect to the fundamental current
component for one two ana three
broken bars
8. METHOD FOR DETECTING BROKEN BARS
BROKEN BAR DETECTION ELEMENT
WITH ZERO-SETTINGS
The BBDE runs periodically
regardless of the status of the
motor. Every time the BBDE
runs,the BBDE goes through
three phases:initialization, data
collection, and data processing, as
Fig.5 illustrates
Fig.5 Brocken rotor bar detection process.
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Fig .6 Alpha current data collection phase
Fig .7 Data processing phase
9. FOURIER TRANSFORM FUNCTION
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The fourier transform function is designed to provide
protection engineers with detailed data about the behavior of
motor. This fuction can point to developing problems in the
motor or load.
It can help to identify motor, load, and supply problems.
Fig.8 Fourier Transform Function Fig.9 Fourier Transform Report
10. DEPENDABILITY FOR MOTORS WITH LIGHT LOADS
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The BBDE needs to distinguish
between the 0 Hz component and the
broken bar component at twise the slip
frequency.
When the motor is lightly loaded,
the slip frequency is smaller than at
full load and leads to a smaller
difference between the 0 component
and the broken bar component.
The BBDE will not be able to
distinguish the broken bar components
when 2s < D,where D is the minimum
frequency difference at which we can
distinguish between two frequency
components.
Fig.10 slip falls below detection limit
for motors with light load
11. EXPERIMENT WITH ACTUAL MOTOR BROKEN BARS
The motor protected by the BBDE
is coupled to another motor driven
by a torque–controlled variable
speed drive that acts as a variable
load
Components of starting current,
low frequency voltage
oscillations, low frequency load
oscillations cause current
signature similar to broken bars.
This can be separated by
FFTbased steadystate detection
logic,voltage meaurements
closer to source and analysis of
sidebands at higher current
harmonics respectively.
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Fig.11 Laboratory test setup
12. BROKEN BAR TEST CASES AND RESULT
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Fig.12
spectrum shows as the load on
Motor grows, the sideband component
Separate from the system frequency.
Fig.13
spectrum shows as motor bars break,
the magnitude of the sidebands
grow.
13. CONCLUSIONS
The broken bar detection element with zero settings,along with the
event history and the Fourier transform function, allow us to detect
broken rotor bars under a wide variety of motor conditions.
The detection element identifies the most common broken bar cases.
The event history shows when problems start and how they evolve.
Specific cases such as when the motor drives an oscillating load or
when voltage containing low frequency components, still require
expert analysis.For these cases Fourier transform speed up the
measurement process because no extra equipment needs to be
connected to the motor.
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14. REFERENCES
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1. Carlose Pezzani, Pablo Donolo, Guillermo Bossino, Marcos Donolo, Armando
Guzman, Stanly E.Zocholl, ",Detecting Broken Rotor Bars With Zero-Setting
Protection", IEEE trans.
2. A.H. Bonnett and G.C. Soukup, Analysis of rotor failures in squirrel cage induction
motors,IEEE trans.
3. H.Penrose, Electrical motor Diagnosis.
4. G.R.Bossino, C.H. De Angelo, J.A.Solsona, and M.I. Valla, Effects of rotor bar and
end ring faults over the signals of a position estimation stategy for induction
motors.
