Page 6 - A O Smith 2012
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Speed Engineered® Motors
Why Specify Speed
Engineered
®
Inverter
Duty Motors?
Variable frequency drives (VFDs),
while offering advantages of greater
control and energy savings to commer-
cial and industrial motor users, can also
cause premature winding failure
in motors not designed specifically
for inverter duty. Now A. O. Smith
engineers have developed a solid
solution...Speed Engineered
®
Inverter
Duty Motor.
Speed Engineered Inverter Duty
Motors are specially designed and
constructed to eliminate the destructive
forces that can occur when motors
are applied with drives. The Speed
Engineered “Corona-Free” solution
eliminates the causes of premature
winding failure.
All Speed Engineered motors meet
or exceed NEMA MG1-31 performance
standards, in addition to carrying A. O.
Smith’s Speed Engineered warranty for
inverter duty applications.
The Causes of Premature
Motor Failure
Research we conducted identified
why motors can fail when used with
variable frequency drives under cer-
tain operating conditions. The results
were published in a white paper,
The
Simple Truth About Motor/Drive
Compatibility,
which is available from
A. O. Smith. Our findings revealed
that “corona” as well as other poten-
tial hazards, can materialize and even-
tually damage motors applied with a
drive.
What is Corona?
VFDs create high voltage pulses at
the motor, especially when the motor
and drive are separated by long power
leads. Those high voltage pulses (or
voltage spikes) develop voltage poten-
tial between adjacent conductors in
the motor winding.
When the voltage generated in the
air between the conductors is high
enough, the air breaks down.
This breakdown is known as
“corona.” The discharge that is created
forms ozone, which causes the
motor’s magnet wire insulation to
disintegrate, causing premature failure.
This phenomenon has been around
for a long time and affects a limited
number of earlier vintage motor/drive
applications. But with drives becoming
more sophisticated, inverter switching
rates increasing and the percentage of
motors operating with drives growing
rapidly, incidents of downtime are also
growing, and corona is now getting a
lot of attention in the motor/drive
industry.
There are several techniques
employed in the market to increase
motor tolerance to corona. Although
simpler and less costly, these prac-
tices are not always effective since
corona is not cured...only bandaged.
The only way to be sure the destructive
efforts of corona will not compromise
your motor/drive application is to
eliminate corona altogether. This is
easily accomplished by specifying
A. O. Smith Speed Engineered motors
on your next project.
What Makes Corona-Free
Speed Engineered
®
Motors
Best For Motor-Drive
Compatibility?
There are several solutions to the
problem of motor insulation stress
caused by inverters. Rather than just
squelching the voltage overshoot which
leads to corona, as mentioned earlier,
the preferred method and the approach
used by A. O. Smith is to design the
motor to be corona free at expected
peak voltage. We begin with a design
premise of understanding the magnet
wire corona inception voltage (CIV)
and distribution of voltage in the motor.
From that, our design approach
becomes simply to:
Choose a winding layout
that minimizes the proximity
voltage differences and reliably
positions insulation materials
to improve dielectrics above the
threshold of corona…
You may recognize this as the
design approach for any motor,
regardless if it is line operated or
driven by an inverter. The difference is
that with an inverter you must anticipate
a much higher peak voltage and the
rapid rise times of these potentially
harmful pulses.
E-Plus
®
Speed Engineered
®
Inverter Duty Motor
Motors specially designed, tested and
warranted to be
Corona-Free
for
compatible inverter duty are marked
in this catalog with a
J
.
