ABB RELION REX640 Technical Manual page 426

Protection functions
k
ρ
t
The equation θB is used whenever all phase current is below overload limit i.e. 2.5 ∙
I
IFLC
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 ∙
=
θ
02
The thermal behavior can be understood from
Figure 246: Thermal behavior
The required
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
settings. Only one time constant is valid at a time. Different running conditions are
defined by the comparison of the
Table 486
426
is the set value of
is the set value of
is the time constant.
, whereas equation θA is used when any of the phase current exceeds overload
I
, the cooling can be expressed as:
IFLC
−
02
= initial thermal level when cooling begins
% Thermal capacity
100

A
80
60
Thermal level
For e.g. at Startup
Overload factor is used to define the highest permissible continuous
Time constant start , Time constant normal , and Time constant stop
.
Overload factor ,
Weighting factor and
Figure 246

B
Thermal level
For e.g. at Standstill
I
and the phase currents, which can be seen in
IFLC
1MRS759142 F
(Equation 47)
.
REX640
Technical Manual