k
is the set value of
Overload factor
,
ρ is the set value of
Weighting factor
and
t is the time constant.
The equation θB is used whenever all phase current is below overload limit i.e. 2.5 ∙
I
IFLC
, whereas equation θA is used when any of the phase current exceeds overload
limit.
During overload condition, the function calculated the θB in background, when
overload ends the thermal level is brought linearly from θA to θB, with speed of
1.66% per second. For the motor at standstill i.e. when the current is below the value
0.12 ∙
I
IFLC
, the cooling can be expressed as:
(Equation 47)
θ
02
= initial thermal level when cooling begins
The thermal behavior can be understood from
Figure 246
.
% Thermal capacity
100
80
60
Thermal level
For e.g. at Startup
Thermal level
For e.g. at Standstill
Figure 246: Thermal behavior
The required
Overload factor
is used to define the highest permissible continuous
load. The recommended value is at minimum 1.05.
In order to accurately calculate the rotor thermal condition, different time constants
are used in the above equations. These time constants are employed based on
different motor running conditions, for example starting, normal or stop, and are
set through the
Time constant start
,
Time constant normal
, and
Time constant stop
settings. Only one time constant is valid at a time. Different running conditions are
defined by the comparison of the
I
IFLC
and the phase currents, which can be seen in
Table 486
.
Protection functions
1MRS759142 F
426 REX640
Technical Manual
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