Rockwell Automation 1398-DDM-xxx USE MNL/ULTRA 200 DIG.SERVO.DR - Manual (Page 277)

Publication 1398-5.0 – October 1998
Creating Custom Motor Files D-13
The thermal time constant of the motor is measured by stabilizing the
motor temperature at its rated condition, disabling the drive, and
measuring the time for the hottest part of the motor winding to drop
63% of the difference from ambient. Allen-Bradley, as well as many
other motor manufacturers, specifies this parameter for motors,
although it may not be published in catalogs or data sheets.
If the thermal time constant of a motor is unknown or unavailable, an
estimated value is preferable to disabling the motor thermal protection
software. A reasonable substitute is to find an Allen-Bradley motor
with similar capability, and use its thermal time constant value for the
custom motor.
The motor thermal protection algorithm filters the square of the torque
current (using the motor thermal time constant value) and generates a
motor thermal protection fault if the output of the filter exceeds the
square of the motor’s continuous torque current rating. The square of
the current is used because the power dissipated in the motor is
approximated as I2R losses. Figure D.6 shows the method to be used
for protection, with t defined as the motor thermal time constant.
Integral Thermostat
The integral thermostat check box should be checked if the motor has
a built-in thermostat. If this box is not checked, the thermostat inputs
to the drive are ignored.
When the integral thermostat check box is selected, a motor
overtemperature fault is displayed if the thermostat inputs to the drive
are in an open state.
Maximum Speed
This value specifies the maximum speed in RPM that the motor can
safely maintain on a continuous basis. The maximum speed value can
be in the range from 0.00002 to 32767.99998 RPM.
Figure D.6 Motor Thermal Protection Software Method
Intro
1
1+s
1
1+s
τ +
-
I
TORQUE
2
I
RATED
2
Motor
Overcurrent
Fault