CHAPTER 4: SETPOINTS PROTECTION
869 MOTOR PROTECTION SYSTEM – INSTRUCTION MANUAL 4–115
CURVE EFFECT
Range: Cutoff, Shift
Default: Cutoff
This setting affects the trip time thermal curves when the Overload Curve is selected as
“Motor”. This setting takes into account the design of the machine with respect to
overload capability as determined by the overload (service) factor. For motor designs
where temperature rise above ambient is based on full load current, this setting should
be selected as “Cutoff”. The trip time is then calculated using the following equation:
Eq. 10
For specialized motor designs where temperature rise above ambient is based on the
product of the overload factor and full load current (OL × FLA), this setting must be
selected as “Shifted”. The trip time is then calculated using the following equation:
Eq. 11
In both the above equation, the motor stator current (I
motor
) and motor rated current
(FLA) are expressed in Amps. In case of uncertainty, the more conservative “Cutoff” value
should be used. The following figure illustrates the impact of this setting on the time to
trip thermal curves.
Figure 4-32: Impact of Thermal Model Curve Effect Setting
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