Thermal Overload Protection - ABB REM 610 Technical Reference Manual

REM 610 REM 610
5.1.4.2.
30
Motor Protection Relay
Technical Reference Manual - ANSI Version

Thermal overload protection

The thermal overload protection detects short- and long-term overloads under
varying load conditions. The heating up of the motor follows an exponential curve,
the levelled-out value of which is determined by the squared value of the load
current.
The full load current of the motor is defined by means of the FLA secondary scaling
factor and determines the thermal trip level of element 49. The set safe stall time, t
determines the operate time of the element for a load current of 6 x FLA without
prior load.
If the RTD module has been installed, RTD6 can be selected to measure the ambient
temperature. The selection is made in SGF4. However, if RTD6 is not used for
measuring the ambient temperature or if the RTD module has not been installed, the
thermal protection will use the set ambient temperature, T
The ambient temperature is used to determine the internal FLA. The table below
shows how the internal FLA is modified.
Table 5.1.4.2.-1 Modification of internal FLA
Ambient temperature
<+20°C
20...<40°C
40°C
>40...65°C
>+65°C
There are two thermal curves, one which characterizes short- and long-time
overloads and which is used for tripping and another which is used for monitoring
the thermal condition of the motor. Weighting factor p determines the ratio of the
thermal increase of the two curves. For direct-on-line started motors with hot spot
tendencies, the weighting factor is typically set to 50 per cent. When protecting
objects without hot spot tendencies, e.g. motors started with soft starters, and cables,
the weighting factor is set to 100 per cent.
When one or several phase currents exceed the internal FLA by more than five per
cent, the whole thermal capacity of the motor will be used after a time determined
by the internal FLA, the set safe stall time and the prior load of the motor. When the
thermal level (influenced by the thermal history of the motor) exceeds the set prior
alarm level, 49 ALARM, the element will generate an alarm signal, and when the
thermal level exceeds the set thermal restart disable level, 49 RESTDIS, the element
will generate a restart disable signal. The time to the next possible motor start up can
be read with SPA parameter V52 or via the HMI. When the thermal level exceeds
the trip level, the element will generate a trip signal. For operate times, see
Fig. 5.1.4.2.-1... Fig. 5.1.4.2.-4.
The thermal protection operates differently depending on the value of weighting
factor p. For instance, if p is set to 50 per cent, the thermal protection will consider
the hot spot tendencies of the motor and distinguish between short-time thermal
.
amb
Internal FLA
FLA x 1.09
FLA x (1.18 - T x 0.09/20)
amb
FLA
FLA x (1 - [(T - 40)/100])
amb
FLA x 0.75
1MRS755537
,
6x