GE 869, Instruction Manual

Page: 255 / 606

CHAPTER 4: SETPOINTS PROTECTION
869 MOTOR PROTECTION SYSTEM – INSTRUCTION MANUAL 4–123
FLEXCURVES
In some applications, the shape of the motor thermal damage curve substantially
deviates from the standard. Furthermore, the characteristics of the starting (locked rotor
and acceleration) and running thermal damage curves may not correspond smoothly.
In these cases, it may be necessary to use a custom curve so the motor can be started
successfully and used to its full potential without compromising protection. For these
conditions, it is recommended that the FlexCurves be used. FlexCurves allows the user to
program selected trip times for pre-determined current levels. As seen below, if the
running (2) and the locked rotor thermal limit curves were smoothed into one standard
overload curve, the motor could not start at 80% line voltage. A custom curve (1) is
required.
For high inertia load applications (when the Voltage Dependent Function is enabled), the
locked rotor thermal limit section of the programmed motor or the FlexCurve overload
curve is modified and becomes dynamically adaptive to system voltage changes. The
detailed explanation of this function is covered later in this section.
Figure 4-40: FlexCurve Example
NOTE
NOTE:
The FlexCurve programming is based on per-unit current values.
The relay incorporates four programmable FlexCurves - FlexCurve A, B, C, and D. The
points for these curves are defined by the user in the EnerVista program. User-defined
curves can be used for Thermal Model protection in the same way as standard Motor
Curves and IEC curves. Each of the four FlexCurves has 120-point settings for entering