Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – Of individual circuit component or element
Reexamination Certificate
2001-10-15
2002-11-19
Sherry, Michael (Department: 2829)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
Of individual circuit component or element
C324S701000
Reexamination Certificate
active
06483319
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to detecting stator winding insulation damage in electric motors and more specifically to a method of detecting the onset of insulation damage, identifying the failure mechanism, determining the winding's susceptibility to further damage, and predicting failure using broadband impedance measurements.
2. Description of the Related Art
Electric motors are used to convert electrical energy into mechanical energy. Permanent magnet field motors have low horsepower (hp) ratings and are used commercially for servo applications such as machine tools, robotics, and high-performance computer peripherals. Of particular interest are higher hp levels required in heavy industry and commercial production lines. For example, polyphase ac induction motors, primarily three-phase, are used to drive fans, pumps, and material handling devices. These motors operate at low frequencies, less than 2 kHz and typically at the 60 Hz frequency provided by the local power company.
FIGS. 1
a
and
1
b
illustrate a 3-phase induction motor
10
that converts electrical energy into mechanical energy. As an aside, an induction motor can be operated as a generator which converts mechanical energy into electrical energy. The motor includes a 3-phase stator winding
12
having three separate coils
14
a
,
14
b
, and
14
c
that are physically wound around a stator form
16
at 120 degree intervals and encapsulated in an insulator
18
. Each coil includes a large number of wire strands
20
that are individually provided with an insulative sheath. The stator winding
12
is mounted inside a motor housing
22
and does not move. A rotor
24
is wound or provides means for a closed coil
26
and mounted on a set of bearings
28
inside stator winding
12
. A drive shaft
30
extends axially from rotor
24
through motor housing
22
.
Power delivered by the electric company to industrial plants for large motors is three-phase and is connected to drive coils
14
a
,
14
b
, and
14
c
. At low frequencies, the current flows through the motor's inductance coils such that electrical energy is converted into mechanical energy. Specifically, the induced current flowing through the inductive coils produces a rotating magnetic field that cuts across rotor coil
26
inducing a circulating current in coil
26
that develops its own magnetic field in the rotor. Field intensifiers
32
are positioned around the interior of stator winding
12
to amplify its rotating magnetic field. The interaction between the rotating magnetic field and the rotor field produces the motor action that rotates shaft
30
. Sinewave type motors drive the shaft at the line frequency, typically 60 Hz. Inverter type motors convert the fixed frequency line voltage into a controllable-frequency drive voltage so that the shaft can be rotated at different frequencies, typically less than 2 kHz.
In heavy industry and commercial production lines, unexpected motor failure disrupts the line, which wastes time and money, oftentimes results in discarded products, and may damage other online systems. The cost of reconditioning or replacing a motor is negligible compared to the expense associated with an unscheduled shut down. Consequently, commercial production lines are routinely shut down on a fixed maintenance schedule to test the motors and determine whether a component failure has occurred.
A 1985 Electric Power Research Institute (EPRI) study of failure modes in three phase induction motors revealed that 41% of the motors failed because of the rotor bearings, 37% failed due to problems associated with the stator winding insulation, 10% failed from rotor problems and 12% failed for a variety of other reasons. Although the dominant failure mode is bearing failure, stator winding insulation failure is the most significant from a user's perspective because its the most unpredictable. A stator winding failure may cause the motor's performance to degrade or to fail entirely.
A 1985 brochure “Failure in Three-Phase Stator Windings” from Electrical Apparatus Service Association, Inc. (EASA) illustrated the typical winding failures in three-phase stators when exposed to unfavorable operating conditions—electrical, mechanical or environmental. Typical winding failures include a single-phase failure in which one phase of the winding is opened, phase-to-phase shorts, turn-to-turn shorts, winding grounded to the slot (intensifier) and thermal deterioration of insulation. Single-phase failures are usually caused by a blown fuse, an open contactor, broken power line or bad connections. The phase-to-phase, turn-to-turn and grounded winding failures result from contaminants, abrasion, vibration or voltage surges. Thermal deterioration is caused by imbalanced voltages, load demands exceeding the motor's rating, locked rotor condition and power surges.
Current test procedures are conducted off line and coarsely measure degradation in the motor's performance in its low frequency operating range, below 2 kHz, to detect the existence or non-existence of one of these failure modes in the stator winding. Based upon his or her experience, a technician decides whether a failure has occurred and what action to take. The risk is that the motor will fail or severely degrade before it is pulled off the line or that perfectly good motors will be mistakenly rejected. Known test procedures neither detect the onset of the damage that eventually causes one of these stator winding failures, identify the failure mechanism responsible for the damage, determine the winding's susceptibility to further damage, nor predict when failure will occur.
Typically, motors are subjected to partial discharge and surge tests to detect a stator winding failure. In the partial discharge test, a technician discharges a capacitor across the stator winding and observes the time domain voltage response to detect spikes on the 60 Hz envelope. The magnitude of the spikes is a rough indicator of stator winding damage. In the surge test, a technician applies a large voltage pulse to each phase of the winding and compares their time domain current responses to detect asymmetry that is indicative of damage. At best these tests detect whether a failure has occurred, neither is sensitive enough to detect the onset and progression of damage to the winding prior to an actual failure.
SUMMARY OF THE INVENTION
In view of the above problems, the present invention provides a broadband test procedure for detecting the onset of stator winding insulation damage, identifying the failure mechanism, determining the winding's susceptibility to further damage, and predicting stator winding failure.
This is accomplished by realizing that changes in the stator winding insulation and/or geometry are reflected as changes in the capacitance between the individual windings and, hence, as changes in the stator winding's broadband impedance response. These changes may indicate different types of damage to the winding that can cause failure. Furthermore, the broadband impedance response reveals the stator winding's vulnerability to further damage from high frequency signals far above the motor's operating frequency that are typically associated either with line surges or inverter driven motors.
To detect this damage and predict failure, the stator winding is probed at frequencies substantially above its normal operating range where the winding current has a substantial capacitive component that flows between the individual windings and is dissipated in the insulation. The winding may be probed online by either injecting high frequency test signals into the stator winding or using existing high frequency signals common to inverter motors.
Changes in the broadband impedance response indicate the onset of insulation damage and may identify a particular failure mechanism. In the currently preferred approach, the impedance response includes the frequency, magnitude, width, and phase of the resonant impedance. The stato
Kendig Martin W.
Rogovin Daniel N.
Amin Himanshu S.
Gerasimow Alexander M.
Kobert Russell M.
Reliance Electric Technologies LLC
Sherry Michael
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