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Introduction
C440/XTOE Electronic Overload Relay, DeviceNet
MN04210002E—March 2011 www.eaton.com
3
Electronic Overload Education
Description
Definition
Cause
Effect if Not Protected
C440/
XT
Protection
Motor Protection
Thermal overload
Overload is a condition in
which current draw exceeds
115% of the full load
amperage rating for
an inductive motor.
•
An increase in the load or
torque that is being driven
by the motor
•
A low voltage supply to the
motor causes the current
to go high to maintain the
power needed
•
A poor power factor
causing above normal
current draw
•
Increase in current draw
leads to heat and
insulation breakdown,
which can cause system
failure
•
Increase in current can
increase
power consumption and
waste valuable energy
•
Thermal trip behavior is
defined by UL, CSA, and
IEC standards
•
Trip class is settable from
10A, 10, 20, 30
Ground fault
A line to ground fault.
A current leakage path to ground.
An undetected ground fault can
burn through multiple insulation
windings, ultimately leading to
motor failure, not to mention risk
to equipment or personnel
Fixed protective setting that
takes the starter offline if ground
fault current exceeds 50% of the
FLA dial setting, for example, if the
FLA dial is set to 12A, the overload
relay will trip if the ground current
exceeds 6A.
Unbalanced phases
(voltage and
current)
Uneven voltage or current
between phases in a
three-phase system.
When a three-phase load is
powered with a poor quality line,
the voltage per phase may be
unbalanced.
Unbalanced voltage causes large
unbalanced currents and as a
result this can lead to motor
stator windings being overloaded,
causing excessive heating,
reduced motor efficiency and
reduced insulation life.
Fixed protective setting that takes
the starter offline if a phase drops
below 50% of the other two
phases.
Phase loss—
current
(single-phasing)
One of the three-phase
voltages is not present.
Multiple causes, loose wire,
improper wiring, grounded
phase, open fuse, and so on.
Single-phasing can lead to
unwanted motor vibrations
in addition to the results of
unbalanced phases as
listed above.
Fixed protective setting that takes
the starter offline if a phase drops
below 50% of the other two
phases.
C440 DeviceNet.book Page 3 Friday, March 25, 2011 10:41 AM
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