θ
τ
A
r r
t
I
k I
K
I
k I
e=
×
+ ×
×
× −
( )
×
−
2
2
2
2
1 100
/
%
(Equation 39)
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 40)
θ
02
initial thermal level when cooling begins
Figure 239: Thermal behavior
The required overload factor and negative sequence current heating effect factor
are set by the values of the
Overload factor
and
Negative Seq factor
settings.
In order to accurately calculate the motor 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.
1MRS759142 F
Protection functions
REX640
Technical Manual
413
To Next Page
Loading...
