Texas Instruments | Fluxgate for Motor Diagnostics | Application notes | Texas Instruments Fluxgate for Motor Diagnostics Application notes

Texas Instruments Fluxgate for Motor Diagnostics Application notes
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Fluxgate for Motor Diagnostics
Scott Vestal, Magnetic Sensing Products
Permanent magnetic synchronous motors are
commonly used in industrial applications. These
motors are attractive because of their high efficiency,
increased torque-current ratio and high power density
yet compact and lightweight. These performance and
size attributes make them ideal in industrial
automation, Hybrid Electric Vehicles (HEV) traction
motors and robotics. However, faults are unavoidable
in these designs as a result of manufacturing defects,
material lifetime, and intermittent use. Motor
monitoring and fault detection are important in
providing lifetime and performance, while minimizing
safety concerns and potential costly shutdowns.
Fault monitoring techniques can include electrical
(electrical signature analysis, motor current analysis,
motor circuit analysis), mechanical (vibration, torque)
thermal (infrared, ultrasonic) and magnetic (magnetic
field, flux density) analysis. The most prevalent faultdetection methods for permanent magnetic
synchronous motors are vibration signal analysis and
phase current information. However, there are issues
associated with each of these techniques. Vibration
fault monitoring is typically effective under stationary
conditions. When used in nonstationary applications,
there is a need for extensive signal processing. For
phase current analysis, fault signatures are dependent
on a variety of design specific configurations including
stator winding, winding type, load levels and fault
location. Thus, operating point dependency is needed
for setting detection thresholds. As a result of these
issues with both vibration and phase current, fault
detection techniques in the final solutions can be
costly, bulky, and impractical in some applications.
Figure 1. Time Domain Signature
Figure 2 shows the same radial axis leakage flux
component comparison in the frequency domain.
Magnetic Fault Detection
An alternative is using the leakage flux spectra to
detect magnetic field related faults. Magnetic field
faults include defects, such as magnets that are
broken, flawed or deformed and demagnetization due
to external magnetic fields and temperature. One of
the advantages of using the leakage flux density is it
allows for detection of magnetic defects in both the
time and frequency domain. Figure 1 shows the radial
axis leakage flux component comparison of a healthy
motor versus 20% broken magnet motor versus a 50%
broken magnet motor in the time domain.
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Figure 2. Frequency Domain Signature
Both figures above show distinct measurable
differences between healthy motors and damaged
motors. Another advantage of using the leakage flux
density is that the fault signatures are independent of
speed. Where other detection techniques, such as
current-based analysis, require different threshold
settings for motor speed, the flux density has similar
signatures at different motor speeds, as shown in
Figure 3 and Figure 4.
Fluxgate for Motor Diagnostics Scott Vestal, Magnetic Sensing Products
Copyright © 2017, Texas Instruments Incorporated
1
Related Documentation
www.ti.com
gain drift, and very low nonlinearity provided by the
internal compensation coil, results in unrivaled
magnetic field measurement precision. The DRV425
comes in a 4-mm x 4-mm QFN package allowing for
easier installation on or around the motor frame. The
internal fluxgate sensor in the DRV425 is a single-axis
sensor. The DRV425 will measure magnetic fields only
in its axis of sensitivity allowing for the installation of
the sensor to detect radial, tangential or z-axis
component of the magnetic field. By using three
DRV425 devices, all three axis components of the
magnetic field can be detected in a small form factor.
5.38
-6.01
-12.16
Figure 3. Frequency Signature of 600 RPM
5.31
-6.76
There are advantages to using fluxgate sensors to
detect magnet defect faults in permanent magnetic
synchronous motors, such as easy mount on the
motor frame, both frequency- and time-domain
analysis remote sensing, and speed-independent fault
detection. The high sensitivity, low noise, low
temperature drift, and small size enable the DRV425
device to be an excellent choice for use in motormonitoring and fault-detection applications.
Acknowledgment
The author would like to acknowledge Dr. Bilal Akin at
The University of Texas at Dallas, Electrical
Engineering Department for his research using the
DRV425 device. All diagrams are created from the
research of Dr. Akin.
-13.12
Table 1. Alternative Device Recommendations
Device
Optimized
Parameters
Performance
Trade-Off
DRV5055-Q1
Automotive, Ratiometric,
Linear, Hall Effect Sensor,
SOT-23 or TO-92 package
Magnetic field
measurement range
µT to mT
1
Figure 4. Frequency Signature 3000 RPM
DRV425 Integrated Fluxgate Sensor
The DRV425 is a precision integrated fluxgate
magnetic field sensor capable of measuring magnetic
field strength from the nT to mT with a bandwidth of 47
kHz. The low offset, offset drift, and noise of the
fluxgate sensor, combined with the precise gain, low
2
Related Documentation
T. Goktas, M. Zafarani, K. W. Lee, B. Akin and
T. Sculley, "Comprehensive Analysis of Magnet
Defect Fault Monitoring Through Leakage Flux,"
in IEEE Transactions on Magnetics, vol. 53, no.
4, pp. 1-10, April 2017. doi:
10.1109/TMAG.2016.2617318
Fluxgate for Motor Diagnostics Scott Vestal, Magnetic Sensing Products
Copyright © 2017, Texas Instruments Incorporated
SLYA035 – December 2017
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