Rockwell Automation 1398-DDM-xxx ULTRA 100 Series Drives Installation Manual - Manual (Page 268)

Publication 1398-5.2 – PDF 1997
D-14 Creating Custom Motor Files
Occasionally a manufacturer specifies only the maximum
instantaneous torque, and does not include the maximum current
specification. In such a case, the peak current can be computed using
the peak torque and the torque constant. A factor of 1.1 is included to
allow for degradation of the torque constant at high temperatures, etc.
The formula, assuming the torque constant has already been converted
to N-m/A(peak), is given by:
Thermal Time Constant
The thermal time constant check box indicates if a valid thermal time
constant exists for the motor. If this check box is not selected, the
motor thermal protection software is disabled.
Enabling of the motor thermal protection software is recommended,
even if the thermal time constant is not known. This software feature
significantly reduces the chance of damage to the motor, even when
the motor has an integral thermostat.
The thermal time constant value, also known as the cool down time
constant, identifies how fast the motor winding temperature dissipates
heat. The value is entered in seconds. The thermal time constant value
can be in the range from 1 to 65535 seconds.
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although it may not be published in catalogs or data sheets.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.
Intro
I 1.1
maximum torque in N m
K PEAK
T
= ⋅
⋅