PSNet - basic training
Here is a short description of some fundamental effects in degradation
of
insulation from a recent article.
Failure Mechanism of the Interturn Insulation of Low Voltage Electric Machines
Fed by
Pulse-Controlled Inverters.
IEEE Electrical Insulation Magazine Sept/Oct 1996 - Vol 12, No 5
by M Kaufhold, G Borner, M Everhardt and J Speck - Institute for High Voltage
and High
Current Engineering, Dresden University of Technology.
Key words: converter-fed motors, very fast transients, voltage distribution,
partial
discharge probability, surface charge accumulation, breakdown of interturn
insulation.
This article discusses the basis for breakdown of interturn insulation
in
low voltage
induction motors in motors driven by pulse-width modulated adjustable speed
drives.
"This paper describes the failure mechanism of low voltage interturn
insulation as a
consequence of partial discharges (PDs) and shows why and how it is influenced
by the
insulation design, temperature and the applied voltage. Understanding the
failure
mechanism leads to better founded maximal permissible stresses and proper design
of the
interturn insulation to avoid premature drive failures."
Importantly, the paper claims "PDs occur in the air-filled gaps of
enameled wires
that are touching. They erode the insulation and consequently lead to an
interturn
breakdown."
The article discusses issues based on typical constructions where the
high
frequency capacitive currents with steep voltage pulses (short rise times) are
unevenly
distributed because of the random wound nature of the motor coils. The ensuing
voltage
stress can be an order of magnitude greater than in coils operated directly from
line
frequency. It is quite clear from the paper presentation that the resulting
breakdown of
the interturn insulation is a result of partial discharge in the space between
the windings.
These PDs "occur when the voltage stressing of an electrical insulation does not
reach the
breakdown voltage, yet the field intensity locally exceeds the breakdown field
strength.
These discharges transform part of the capacitively stored energy in the
insulation into
heat and radiation as well as mechanical and chemical energies, which can
degrade
insulation materials. This aging process in the winding insulation
progressively reduces its
breakdown voltage, until the breakdown voltage reaches the applied voltage
level. At the
point, the breakdown and failure of the whole drive occurs."
The article concludes: "The premature failures of low voltage motors
with
ASDs
(Adj Speed Drives) are due to breakdowns of the interturn insulation. ...
- The PDs occur in the air-filled gaps of enameled wires that are
touching. the
PDs erode the insulation and consequently lead to an interturn breakdown.
- High frequencies, short rise times and fast oscillating pulses shorten
the lifetime.
However, of no PDs occurred, even after two years under considerable high
electrical and
thermal stress, no breakdown was observed.
- Satisfactory lifetime of inverter-fed low voltage motors can be
achieved if PDs in
the winding insulation are avoided. This can be done by an appropriate
limitation of the
risetime and amplitude of the terminal voltage ... Great care should be taken
with proper
insulation design to avoid a low PS inception voltage.
- PD inception voltage decreases with the increase of temperature and
the
decrease
of insulation thickness. To improve the electrical endurance properties, the
impregnation
of the winding should permanently fill up the gaps between the wires. ... "
It would be very interesting to see the results of a similar study for
SMPS.
:>) br, Pete Perkins
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Peter E Perkins
Principal Product Safety Consultant
Tigard, ORe 97281-3427
+1/503/452-1201 phone/fax
[email protected] email
visit our web site:
http://ourworld.compuserve.com/homepages/peperkins
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