θ
τ
B
r r
t
I
k I
K
I
k I
e p=
×
+ ×
×
× −
( )
×
−
2
2
2
2
1
/
%
(Equation 18)
θ
τ
A
r r
t
I
k I
K
I
k I
e=
×
+ ×
×
× −
( )
×
−
2
2
2
2
1 100
/
%
(Equation 19)
I TRMS value of the measured max of phase currents
I
r
set
Current reference
, FLC or internal FLC
I
2
measured negative sequence current
k set value of
Overload factor
K
2
set value of
Negative Seq factor
p set value of
Weighting factor
t time constant
The equation θ
B
is used when the values of all the phase currents are below the
overload limit, that is, k x I
r
. The equation θ
A
is used when the value of any one of
the phase currents exceeds the overload limit.
During overload condition, the thermal level calculator calculates the value of θ
B
in
background, and when the overload ends the thermal level is brought linearly from θ
A
to θ
B
with a speed of 1.66 percent per second. For the motor at standstill, that is,
when the current is below the value of 0.12 x I
r
, the cooling is expressed as:
(Equation 20)
θ
02
initial thermal level when cooling begins
Figure 181: Thermal behavior
1MRS757644 H
Protection functions
620 series
Technical Manual
365
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