01-10-2012, 04:42 PM
ONLINE FAULT ANALYSIS OF DC MOTORS
[attachment=34954]
Abstract
Over the last 20 years, Current Signature
Analysis (CSA) has become an established tool for online
fault analysis of AC Induction motors. Presently, very little
research has been performed using current signature
analysis on DC motors. This paper is a brief introduction to
online fault diagnosis of DC motors using current signature
analysis.
INTRODUCTION
This research initiative was undertaken to further
develop online fault detection of DC motors using
current signature analysis in both the time and frequency
domains. These faults include differential current,
shorted armature windings, shorted field windings, and
off magnetic neutral plane brush positions.
To detect the various faults in DC motors, we must
develop a methodology to properly differentiate normal
operating conditions from those of fault operating
conditions. The first step is to establish a baseline of
normal operating conditions. Once a baseline of normal
operating conditions is established, a method of
differentiating fault operating characteristics from
baseline characteristics must be developed.
Brush Position
Detecting when the brushes are off the magnetic
neutral axis can be difficult, especially if the motor is
inaccessible during operation. Using voltage analysis in
the time domain makes the job of properly setting the
brushes for the desired load much easier. The voltage
waveforms in Figure 9 appear to be clean (i.e., without
noise). When the brushes are off the magnetic neutral
axis, the voltage waveforms in the time domain have a
lot of hash as shown in Figure 10.
Differential Current
Differential current may be analyzed by comparing
two current waveforms in the time domain. There are
two predominant situations where differential current
analysis may provide insight to fault conditions that may
otherwise be overlooked. One of these is when two or
more cables feed a single terminal such as those found in
semi-high current situations. Another situation
differential current analysis may be used is in comparing
the A1 to A2 currents. Situations may occur in which
one of the main power cables may have bypass current.
Bypass current may occur from the high frequency
switching found in DC drives. Differential current may
also occur when alternate return paths offer a lower
impedance than the primary feed cables.
Numerical analysis is the primary methodology used
when analyzing differential current. The deterministic
differential will vary according to the application. For
our study, we used a 2% differential (1% equipment +
1% minimum differential) to compare two cables feeding
a single terminal. One of these cables had a resistance
inserted into the line to represent a high resistance
connection of one of the cables in a multi-cable situation.
SUMMARY
Our research has shown the use of current and voltage
signature analysis in both the time and frequency
domains may provide useful insights to online fault
analysis of DC motors. Many common faults such as
shorted turns or commutator bars, grounded windings,
and off magnetic neutral axis faults may be detected
using online current and voltage signature analysis.
Trending these over time may provide an indication
of a developing fault in the motor. As with all tests, cross
correlation between technologies is imperative in the
decision making process.
[attachment=34954]
Abstract
Over the last 20 years, Current Signature
Analysis (CSA) has become an established tool for online
fault analysis of AC Induction motors. Presently, very little
research has been performed using current signature
analysis on DC motors. This paper is a brief introduction to
online fault diagnosis of DC motors using current signature
analysis.
INTRODUCTION
This research initiative was undertaken to further
develop online fault detection of DC motors using
current signature analysis in both the time and frequency
domains. These faults include differential current,
shorted armature windings, shorted field windings, and
off magnetic neutral plane brush positions.
To detect the various faults in DC motors, we must
develop a methodology to properly differentiate normal
operating conditions from those of fault operating
conditions. The first step is to establish a baseline of
normal operating conditions. Once a baseline of normal
operating conditions is established, a method of
differentiating fault operating characteristics from
baseline characteristics must be developed.
Brush Position
Detecting when the brushes are off the magnetic
neutral axis can be difficult, especially if the motor is
inaccessible during operation. Using voltage analysis in
the time domain makes the job of properly setting the
brushes for the desired load much easier. The voltage
waveforms in Figure 9 appear to be clean (i.e., without
noise). When the brushes are off the magnetic neutral
axis, the voltage waveforms in the time domain have a
lot of hash as shown in Figure 10.
Differential Current
Differential current may be analyzed by comparing
two current waveforms in the time domain. There are
two predominant situations where differential current
analysis may provide insight to fault conditions that may
otherwise be overlooked. One of these is when two or
more cables feed a single terminal such as those found in
semi-high current situations. Another situation
differential current analysis may be used is in comparing
the A1 to A2 currents. Situations may occur in which
one of the main power cables may have bypass current.
Bypass current may occur from the high frequency
switching found in DC drives. Differential current may
also occur when alternate return paths offer a lower
impedance than the primary feed cables.
Numerical analysis is the primary methodology used
when analyzing differential current. The deterministic
differential will vary according to the application. For
our study, we used a 2% differential (1% equipment +
1% minimum differential) to compare two cables feeding
a single terminal. One of these cables had a resistance
inserted into the line to represent a high resistance
connection of one of the cables in a multi-cable situation.
SUMMARY
Our research has shown the use of current and voltage
signature analysis in both the time and frequency
domains may provide useful insights to online fault
analysis of DC motors. Many common faults such as
shorted turns or commutator bars, grounded windings,
and off magnetic neutral axis faults may be detected
using online current and voltage signature analysis.
Trending these over time may provide an indication
of a developing fault in the motor. As with all tests, cross
correlation between technologies is imperative in the
decision making process.