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Rockwell Automation Publication 2198-UM002E-EN-P - February 2018
Appendix D
Motor Control Feature Support
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 MotorWindingToAmbientResistance and
MotorWindingToAmbientCapacitance 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 first-
order time constant determined from the
MotorWindingToAmbientResistance and
MotorWindingToAmbientCapacitance values to estimate the motor thermal
capacity based on the motor output current.
The MotorOverloadLimit 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.
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.