Rockwell Automation Publication 2198-UM004D-EN-P - December 2022 147
Chapter 7 Configure the Drive with KNX5100C Software
You can use the MotorOverloadLimit ID629 (PN.001) attribute (default of
100%, max of 200%) to increase the motor overload trip-time by artificially
increasing the motor rated current (for thermal protection only). Increase
MotorOverloadLimit above 100% only if cooling options are applied.
Increasing MotorOverloadLimit causes MotorCapacity ID656 (PN.038) to
increase more slowly. The generic motor thermal model also derates the motor
rated current (for thermal protection only) when operating at low speeds. The
derating factor is 30% at 0 Hz and 0% at 20 Hz, with linear interpolation
between. Operating at output frequencies less than 20 Hz causes
MotorCapacity to increase more quickly. When the generic motor thermal-
model is active, the MotorCapacity attribute increases only if the motor output
current is greater than the effective motor rated current (taking into account
the MotorOverloadLimit and low speed derating factor). The default
MotorThermalOverloadFactoryLimit and MotorThermalOverloadUserLimit
values for this thermal model are both 100%.
Thermally Characterized Motors
If the ID635 (PN.007) MotorWindingToAmbientResistance and
MotorWindingToAmbientCapacitance ID636 (PN.008) attribute values are
both non-zero, the motor is considered thermally characterized and an
alternate motor thermal model is run. The purpose of this algorithm is to limit
the time a motor is operating with excessive levels of current. This thermal
model uses the firstorder time constant determined from the
MotorWindingToAmbientResistance and
MotorWindingToAmbientCapacitance values to estimate the motor thermal
capacity based on the motor output current. The MotorOverloadLimit ID629
(PN.001) attribute (default of 100%, max of 200%) can be used to increase the
motor overload trip-time by increasing the
MotorThermalOverloadFactoryLimit value. The MotorOverloadLimit should
be increased above 100% only if cooling options are applied. Increasing
MotorOverloadLimit does not change the behavior of MotorCapacity ID656
(PN.038). This thermal model supports setting the MotorOverloadAction
attribute as Current Foldback. Selecting the Current Foldback action results in
a reduction in the current reference via the MotorThermalCurrentLimit
attribute value that is reduced in proportion the percentage difference
between the MotorCapacity and the MotorOverloadLimit values. When this
thermal model is active, the MotorCapacity attribute is non-zero if the motor
output current is non-zero. The default MotorThermalOverloadFactoryLimit
and MotorThermalOverloadUserLimit values for this thermal model are both
110%.
IMPORTANT The generic motor-thermal model does not support Current Foldback as
a Motor Overload Action.
IMPORTANT This thermal model does not derate the motor-rated current when
operating at low speeds. Operating at low output frequencies does not
cause the MotorCapacity behavior to change.
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