Woodward MRM4, Manual, Page: 385 / 460

Protective Elements
Value Description Menu path
LoadBear1 Load Bearing 1 [Operation
/Measured values
/URTD]
LoadBear2 LoadBear 2 [Operation
/Measured values
/URTD]
Aux Auxiliary [Operation
/Measured values
/URTD]
Theta - Thermal Model [49M, 49R]
Available Elements:
ThR
General – Principle Use
Thermal Protection and Alarm
This protective device provides a thermal model. The thermal model can work with or without the URTD. The
RTD-based direct temperature trips and alarms are independent of the thermal model. Without the URTD,
meaning the URTD is not connected to the protective device or it is connected but not configured for the thermal
protection trips, the thermal model protection will be solely based on the following settings:
1.Ib Full Load Ampere (FLA);
2.Locked Rotor Current (LRC);
3.Maximum Allowable Stall Time (Tc);
4.UTC (Ultimate Trip Current) or k-Factor;
5.Thermal Model Trip Threshold if enabled;
6.Trip Delay;
7.Thermal Model Alarm Threshold if enabled; and
8.Alarm Delay.
The first four settings (1-4) dictate the maximum allowable thermal limit curve of the protected equipment, and
the last four settings (6-9) define the thermal trip and alarm curves relative to the thermal limit curve.
Mathematically, the thermal limit curve can be expressed as the following:
TripTime
=
I
LR
2
∗
T
LR
I
ef
2
when
I
ef

k
Factor
∗
I
b
If the direct stator temperature measurements are available, the thermal replica model will be modified to include
the heat loss between stator and rotor. As a result, the motor will be able to run longer under overload
conditions. The heat loss serves as a cooling. At some point, the cooling effect will cancel the heat increment
so that the thermal capacity used will reach some steady-state level that may be below the trip or alarm limit.
This equivalently raises the » k-factor« and shifts the trip curve right.
If the thermal capacity used is held at a level that is below the trip threshold, the thermal model will not trip. To
prevent the protected equipment from overheating, the direct temperature trip function must be enabled. Keep
in mind that in order for the stator temperature to be effective in the thermal replica model, the following
conditions must be met:
EN MRM4 01/11 Page 385