Woodward MRM4, Manual

Page: 387 / 460

Protective Elements
In the Thermal Replica Model Trip Curves with and without RTD, the unmarked lines are the thermal limit curves
and the marked lines are the trip curves. From the curve without RTD, it can be seen that one can change the
thermal current threshold to shift the upper portion of the trip curve right to allow the motor to run at a higher
overload condition than is specified with the service factor. From the curve with RTD, it can be seen that the
stator RTD pushes the effective thermal current threshold to 2.55 * Ib (FLA) on the thermal limit curve
(unmarked line). The marked line is the trip curve with 80% thermal capacity trip threshold, so actual effective
thermal current threshold for the trip curve is about 2.05 * Ib (FLA). Although in this case, the thermal current
threshold is set to 1.50 * Ib (FLA), it is effectively raised to a higher level with the stator RTD. Keep in mind that
thermal limit and trip curves shown are based on the example above. They will vary with other sets of the
settings.
EN MRM4 01/11 Page 387
Thermal Replica and Trip Curves without RTD
0.1 1 10 100
1
10
100
1000
k-Factor=1, Thermal Capacity=100%
t [
s]
multiples of Ib
lth=120%. Tth=80%* Thermal
Capacity