Table 486: Time constant and the respective phase current values
Time constant in
use
Phase current
tStart
Any current is over 2.5 ∙
I
IFLC
tNormal
Any current is over 0.12 ∙
I
IFLC
and all currents are below 2.5 ∙
I
IFLC
tStop
All currents are below 0.12 ∙
I
IFLC
Setting
Weighting factor p determines the ratio of the thermal increase of the two
curves θA and θB.
The thermal level of the rotor at power up of the protection relay is defined by the
setting
Initial thermal Val.
The temperature calculation is initiated from the value defined in the
Initial thermal
Val setting. This is done if the protection relay is powered up or the function is
turned off and back on or reset through the Clear menu.
Calculated thermal level TEMP_RL is available in monitored data view.
Thermal levels at the beginning and the end of the latest startup are
available through the monitored data view at THERMLEV_ST and THERMLEV_END
respectively.
Internal FLC calculator
Full load current (FLC) of the motor is defined by the manufacturer at an
ambient temperature of 40°C. The FLC of the motor is set by
Current reference
setting. Special considerations are required with an application where the ambient
temperature of a motor exceeds or remains below 40°C. A motor operating in a
higher ambient temperature, even at or below rated load, can subject the motor
windings to excessive temperature. The motor rating has to be appropriately
reduced for operation in such high ambient temperatures. Similarly, when the
ambient temperature is considerably lower than the nominal 40°C, the motor can
be slightly overloaded. For calculating thermal level, it is better that the FLC values
are scaled for different temperatures. The scaled currents are known as internal FLC
(I
IFLC
). An
I
IFLC
is calculated based on the ambient temperature shown in the table.
The
Env temperature mode setting defines whether the thermal level calculations
are based on FLC or
I
IFLC
.
When the value of the
Env temperature mode setting is set to the "FLC Only" mode,
no I
IFLC
is calculated. Instead, the FLC given in the data sheet of the manufacturer is
used. When the value of the
Env temperature mode setting is set to "Set Amb Temp"
mode, the
I
IFLC
is calculated based on the ambient temperature taken as an input
through the Env temperature Set setting. When the
Env temperature mode setting
is on "Use input" mode, the
I
IFLC
is calculated from temperature data available
through resistance temperature detectors (RTDs) using the
AMB_TEMP
input.
Table 487: Temperature dependency of I
IFLC
Ambient Temperature (T
amb
)
I
IFLC
<20°C
FLC x 1.09
20 to <40°C
FLC x (1.18 - T
amb
x 0.09/20)
40°C
FLC
>40 to 65°C
FLC x (1 –[(T
amb
-40)/100])
>65°C
FLC x 0.75
1MRS759142 F
Protection functions
REX640
Technical Manual
427