|
Abstract:
This paper will discuss
the root causes of electrical failures in motors and how
high voltage testing can help in early detection. The
paper will review the insulation resistance test,
polarization index test, DC hipot test, and the surge
test along with outline the types of problems they can
and cannot find.
Most
electrical failures are caused by a combination of the
voltage spikes that occur at start up and normal
deterioration. The problem often begins as a
turn-to-turn short that will eventually go to ground.
Without high voltage testing many of these problems will
go undetected.
I. Introduction
Before making a logical
decision about what kinds of testing should be done on
motors to predict electrical failures, it must be
understood what makes these failures occur. It is
important to understand the different insulation groups,
the aging process of the insulation, and review typical
failure scenarios. Only then can decisions be made as
to which tests we should do and if high voltage testing
should be included.
II. The
Insulation Groups
The insulation system of
a motor consists of the groundwall insulation, the
phase-to-phase insulation and the turn-to-turn
insulation. In a typical induction motor the groundwall
insulation is the slot liner paper that protects the
insulated copper to ground. The Phase-to-Phase is often
a sheet of insulation paper that is laid between the
phases. The weakest link in the insulation system is
often the turn-to-turn insulation. This is the enamel
on the copper of a random wound motor or the tape found
on form coils. This insulation’s purpose is to protect
from copper to copper failures.
To properly
test the total insulation system several different tests
must be performed. The groundwall insulation can be
tested with a megohmmeter to determine insulation
resistance values, a polarization index test to evaluate
the elasticity of the insulation, and a DC hipot to test
the dielectric strength of the insulation to some
predetermined level. The phase-to-phase insulation can
also be tested with some of the same tests mentioned
above if the motor is completely disconnected. In most
predictive maintenance scenarios this will not be the
case and the phase-to-phase insulation must to tested in
the same manner as the turn-to-turn insulation. Surge
testing is the only available test of the turn-to-turn
insulation.
III.
The Insulation Aging
Process
The insulation aging
process can be affected by one or more of these five
factors:
Contamination: A chemical
deposit on the windings that causes deterioration of the
insulation.
Mechanical: Vibration or
movement within the windings or the motor which wears
the insulation system.
Normal thermal aging: The
slow deterioration of the insulation over the windings
natural life.
Early thermal aging:
Excessive winding temperatures causing premature
failure.
Overvoltage spikes: High
Voltage surges caused by switching, lighting, and VFD
designs.
All five of these should
be considered when designing a test program. We will
look at the normal thermal aging process and how it is
effected by Mechanical, Early thermal aging, and the
Overvoltage spikes. For this paper we will not deal
with the contamination problems.
IV. What
makes motors fail?
Depending on which study
is referred to; electrical failures are responsible for
35% to 40% of all motor failures. These same studies
often show that many of these winding failures begin as
turn-to-turn shorts caused by steep-fronted surges due
to switching2. These studies date back to
1936. Measurements of these surges began as early as
1960 showing spikes of .5 micro-seconds at up to 5 pu.
When
discussing dielectric strength and voltage spikes in
this paper the measurement of “pu” will be used. One
per unit (pu) is the peak line to ground voltage.
These steep-fronted
surges are caused by a variety of sources. The most
common and main cause of breakdown of the inter-turn
insulation is switching surges.3 These
switching surges can occur both when opening and closing
the contacts. Restriking will create multiple surges.
Studies show
that these surges will range from 1 to 5 pu with rise
times of .1 to 1 micro second. A 4160V motor will see
surges of up to 17,000V.
In normal
operation a typical coil will only see 10 to 100V
turn-to turn. Pashens law states that a difference of
350V is required to initiate an arc. (see figure 1).
With this small of a potential difference a motor should
not fail due to turn-to-turn shorts from normal
operation. It is the combination of weak insulation and
the steep-front surges that eventually lead to motor’s
electrical failures.
Figure 1
Mechanical
abrasion within the winding is another deterioration
mechanism operating on the motor’s insulation. At start
up a squeezing action caused by the magnetic forces will
cause wear between the moving components4.
The magnetic fields changes 120 times a second causing
this squeezing to occur each time. Even though wear
does exist between the winding and the ground
insulation, studies show that less than 17% of the
ground insulation can be worn away due to this
movement. It is the turn-to turn insulation that is
most affected by the abrasion.
As stated above, the
potential difference turn-to-turn during normal
operation is not enough to cause a failure of the
turn-to-turn insulation in a motor. Only the spikes
will have a voltage level high enough to cause this kind
of problem. Adding to this turn-to turn stress is the
non-linear distribution of voltage across the phase. In
a study conducted by Christiansen and Pedersen,5
it was concluded that the rise time of the spikes will
determine how the voltage propagates over the windings.
As shown in figure 2, the faster the rise time the less
linear the voltage divides over the coil. As shown a
typical .2 mico second rise time will drop 50% over the
first coil. Other studies have shown close to 100% drop
over the early turns. Large motors with form wound
coils are much more likely to see this non-linearity
than random wound motors.
Figure 2
V. Typical
Scenario of an Electrical Failure
When a motor is new the
dielectric strength of the insulation system is very
high. On a typical 4160V motor the strength may be over
100KV to ground with turn-to-turn strength of over
50KV. Over time the insulation will deteriorate due to
the normal thermal aging process. To accelerate this
process contamination and mechanical stress will cause a
more rapid deterioration. This will continue until
finally the insulation has deteriorated to a level that
is affected by the surges. (see figure 3)
Figure 3
At this time
each surge will result in an arc causing more
deterioration of the insulation. When the turn-to-turn
insulation erodes to a level close to the operating
voltage the conductors will weld together causing rapid
failure due to the high induced current. (see figure
4).
Figure 4
VI. High
Voltage Testing?
Of the four tests
reviewed in this paper, only two are considered to be
“high voltage”. It is important to understand what each
of these tests can and cannot do. It is the combination
of the right tests that will help meet the goal.
A. Insulation
Resistance Test
Developed
early in the 20th century, the insulation
resistance (IR) test is the oldest and most widely used
test for assessing the quality of insulation to ground.
In this test, the motor frame is grounded, and the test
instrument (megohmmeter) imposes a dc voltage on the
motor windings. Instrument readout is provided in
megohms.
A sound
winding yields a readout in hundreds, or thousands, of
megohms. ANSI/IEEE Std 43 IEEE Recommended Practice
for Testing Insulation Resistance of Rotating Machinery
prescribes as a minimum acceptable reading 1 megohm
plus 1 megohm per KV of the motors rated voltage.
Minimum acceptable resistance for a 460V motor, for
example, is 1.46 megohms. Prudence, however, dictates
that the motor be removed from service for winding
refurbishment while winding-to-ground resistance is
still well above the minimum acceptable value.
IR
test readings are highly sensitive to temperature and
moisture. For accurate, meaningful readings, testing
should be done when the motor has been out of service
for a long enough time for it to have reached
atmospheric temperature. To preclude condensation, the
temperature should be above the dew point. IR readings
obtained must then be corrected to a standard
temperature in accordance with tables provided by the
test instrument manufacturers. This test is a test of
the ground insulation only and has no value in
determining the quality of the turn-to-turn insulation.
B.
Polarization Index Test
This
ten-minute DC test is performed at one third of the
total test voltage as prescribed by IEEE Std 95. A
megohm reading is taken at one minute and again at ten
minutes to determine the elasticity of the ground
insulation. When placed in an electric field, molecules
of the ground insulation should align with that field.
(see figure 5) If the insulation is aged, hard, and
brittle, no polarization can occur.
Figure 5
The Polarization index is the ratio of the ten-minute
insulation resistance reading divided by the one-minute
reading. Over the ten minute period this reading should
increase by a factor of two or more giving a “PI” of two
or more. If the insulation is very brittle the
polarization index will be one or very slightly more
than one, indicating no polarization took place (see
figure 6). This test also looks at only the ground
insulation and will not see the problems in the
turn-to-turn insulation.
Figure 6
C. DC
High-Potential (HiPot) Test
The first of
the two “high voltage” tests, the DC HiPot test can
uncover insulation weaknesses that might not necessarily
be detected in an IR or PI procedure. In addition to
measuring overall insulation resistance to ground, it
provides information on insulation dielectric strength.
In this sense, it can detect insulation weaknesses that
are likely to fault to ground if subjected to the high
transient voltage surges that commonly occur on
industrial power systems.
With this
test, the motor frame is grounded, and a dc voltage
gradually applied in step increments up to the maximum
recommended test voltage. IEEE Std 95 Recommended
Practice for Insulation Testing of Large AC Rotating
Machinery With High Direct Voltage recommends
maximum test voltage at double the motor rated voltage
plus 1000V. At each step up to this voltage, leakage
current in microamperes in read and plotted against the
corresponding dc test voltage.
The
resulting plot should be a straight line. Magnitude of
leakage current and resulting slope of the line is not
the only consideration. The criterion of importance is
that the plot be, in fact, a straight line. An abrupt
upswing in the slope of the plot indicates an insulation
flaw. The test should be immediately aborted to prevent
the winding from failing under test. The motor can be
returned to service, but winding reconditioning or
replacement should be scheduled for the earliest
convenient opportunity.
The number
of discrete steps in which the test is performed is
optional. However, taking more steps in smaller voltage
increments yields better results and minimize the
possibility of test voltage overshoot. Most dc high
potential test sets incorporate overcurrent trips to
protect the winding if a weakness is detected. The most
sensitive of these overcurrent protective circuits can
operate when leakage current is as low as one microamp.
The DC HiPot is also a test that only looks at the
ground wall and is of no value for the turn-to-turn
insulation.
D. Surge
Test
Although
surge comparison testing was developed more than
40-years ago, it is the newest of the classic tests
performed to determine winding insulation condition.
This test detects turn-to-turn, coil-to-coil, and
phase-to-phase insulation defects that cannot be
discovered by other methods.
Surge
comparison testing is premised on the principle that in
a stator with no winding defects, all three-phase
windings are identical. Each phase is tested against
the others – A-B, B-C, and A-C. The test instrument
imposes a brief voltage pulses on the phase undergoing
the test and reflected ringing pulses are displayed on
the instrument’s oscilloscope screen. If the two
windings are identical (as they should be), reflected
images are identical and appear as a single trace.
This
comparison method has been used in the motor shops
repairing motors for over 40 years. When using a surge
tester as a predictive maintenance tool, the test does
not require the comparison of two wave forms. A simpler
test is performed that looks for a shift to the left by
the waveform of the phase being tested. This shift
indicates that the dielectric strength of the
turn-to-turn insulation has deteriorated to a level
below that of the switching surges. Once the insulation
has weakened to this point, decisions need to be made
concerning the future of the motor. With today’s
digital technology it is possible to acquire data of the
phase under test at several voltage levels and nest them
together. This technique is valuable in detecting and
documenting this shift to the left.
Figure 7
VII. Conclusion
When testing a motor’s
insulation system, it is important that the right tests
are performed. Understanding that the motor sees
voltages of up to 5pu, it is important that the
insulation system be capable of handling stress higher
than its normal operating voltage. As shown in this
paper, if high voltage testing is not performed it is
nearly impossible to detect the weak insulation in
advance of it’s failing.
Of the four tests
discussed in this paper three concern themselves with
the groundwall insulation with no regard for the
turn-to-turn. The Surge test is the only test that looks
at the turn-to-turn insulation. The turn-to-turn
insulation is the root cause of a high number of the
electrical failures. This test simulates characteristics
of a surge at start up, making it an appropriate test
for early detection of weak insulation.
References:
1.
Schump, David E., “Predict Motor Failure
With Insulation Testing, “Plant Engineering Magazine,
September, 1996.
2.
Zotos,
Peter A., Member IEEE, Motor Failures Due to Steep Front
Switching Surges: The Need for Surge Protection User’s
Experience, “ IEEE Transactions on Industrial
Applications, Volume 30, Number 6, Nov/Dec 1994. It an
appropriate test for early detection of weak insulation.
3.
Kema,
N.V., Arnhem The Netherlands, H.G. Tempelaar,
“Determining of Transient Over Voltages Caused by
Switching of High Voltage Motors.”
4.
Crawford, D.E., General
Electric company, “Mechanisms of Motor Failures.”
5.
Christiansen, K.A. and
Pederson, A., “An Experimental Study of Impulse Voltage
phenomena In A Large AC Motor.”
For
more information visit
Baker Instrument
online |
