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14.7.3.2.1 Measurement mode
The thermal overload protection function evaluates the square average of phase currents at the
fundamental frequency. The instantaneous temperature estimation is based on the average of the
measured phase currents and on the environment temperature set in the protection dialogue
window (Tenv).
14.7.3.2.2. Operation Criteria
The Thermal Overload protection function estimates the instantaneous value of motor temperatu-
re.
If the estimated instantaneous temperature exceeds the first setting threshold value (Twarn), then
the protection function enters the START status and generates a WARNING signal.
If the estimated instantaneous temperature exceeds the second setting threshold value (Ttrip),
then the protection function generates a TRIP signal.
The protection function will exit START status, return to passive status and the start signal will be
cleared if the estimated temperature falls below the setting threshold value Twarn.
The protection function will exit the TRIP status and the trip signal will be cleared when the estimated
temperature falls below the setting threshold value Ttrip.
The protection function also prevents reconnection after a trip of overheated machines until the
estimated motor temperature has fallen below the warning temperature Twarn (according to
calculated motor cooling process, based on Time Constant OFF).
In the same way, an overheated motor, for example for a number of starts, cannot be reconnected
until its estimated temperature has fallen below the warning temperature (Twarn), according to
the motor cooling-down time estimation.
The Configuration Tool command "Reset Temperature" in the monitoring window forces the estimated
motor temperature value to the configured value Trst (Reset Temperature).
14.7.3.2.3 Thermal model
t
It is assumed the heating (or cooling) process works according to the following equation
where:
T
is the final (asymptotical) temperature
f
T
is the initial motor temperature
ini
τ
is the thermal constant of the heating (or cooling) process
is the actual time, counted from t=0 starting at
T
.
ini
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