I TRMS value of the measured max of phase currents
I
r
set
Rated current
, 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:
GUID-2C640EA9-DF69-42A9-A6A8-3CD20AEC76BD V2 EN (Equation 12)
θ
02
initial thermal level when cooling begins
GUID-A19F9DF2-2F04-401F-AE7A-6CE55F88EB1D V2 EN
Figure 126: 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,
Section 4 1YHT530004D05 D
Protection functions
250 615 series
Technical Manual
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