EMR-4000
IM02602009E
In addition, at least one of these winding temperatures must be valid.
Knowing the maximum steady stator temperature qS (°C), the thermal capacity used can be estimated by the following
formula.
TC
Used
%
=
0
S
240
I
ef
2
∗
50
I
LR
2
∗
T
LR
when
I
ef
ITH
∗
FLA
Take for example, ILR = 6FLA, TLR = 15, and thermal trip level of 100%. The relationship between the effective current
threshold and the stator temperature can be seen in the Stator Temperature Effect on Current Threshold Curve.
Stator Temperature Effect on Current Threshold Curve
From the graph, it is seen that the lower the stator temperature, the higher the effective current threshold. Without
stator temperature, given the current threshold of 1.0 FLA and 2.0 FLA of the stator phase current, the thermal model
will use the full thermal capacity in 139.54 seconds. However, if the stator temperature is known as 100°C (212°F), the
effective ultimate trip current threshold is raised to 2.55 FLA and the thermal capacity used will reach a steady state of
77.5%. As a result, the thermal model will never trip under this condition. From this example, it can be seen that the
stator RTD could keep the motor running under overload condition. In this case, the appropriate direct stator
temperature trip function must be enabled.
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