drawing excessive current and overheating, which causes the premature insulation
failures of the windings and, in worst cases, burning out of the motors.
4.1.9.4 Operation principle
The function can be enabled and disabled with the Operation setting. The
corresponding parameter values are "On" and "Off".
The operation of the motor thermal overload protection function can be described
using a module diagram. All the modules in the diagram are explained in the next
sections.
ALARM
BLK_RESTART
Max
current
selector
I_A
Thermal
level
calculator
Internal
FLC
calculator
Alarm and
tripping
logic
START_EMERG
BLOCK
OPERATE
I_B
I_C
I
2
AMB_TEMP
GUID-1E5F2337-DA4E-4F5B-8BEB-27353A6734DC V2 EN
Figure 125: Functional module diagram
Max current selector
Max current selector selects the highest measured TRMS phase current and reports
it to Thermal level calculator.
Internal FLC calculator
Full load current (FLC) of the motor is defined by the manufacturer at an ambient
temperature of 40°C. Special considerations are required with an application where
the ambient temperature of a motor exceeds or remains below 40°C. A motor
operating at a higher temperature, even if at or below rated load, can subject the
motor windings to excessive temperature similar to that resulting from overload
operation at normal ambient 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. An internal FLC 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 internal FLC.
When the value of the Env temperature mode setting is set to the "FLC Only"
mode, no internal FLC 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 internal FLC is calculated based on the ambient
temperature taken as an input through the Env temperature Set setting. When the
Section 4 1YHT530004D05 D
Protection functions
248 615 series
Technical Manual
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