GE 469 Instruction Manual page 84

4.6 S5 THERMAL MODEL
b) CUSTOM OVERLOAD CURVE
If the motor starting current begins to infringe on the thermal damage curves, it may become necessary to use a custom
curve to tailor the motor protection so that successful starting may occur without compromising protection. Furthermore, the
characteristics of the starting thermal damage curve (locked rotor and acceleration) and the running thermal damage
curves may not fit together very smoothly. In this instance, a custom curve may be necessary to tailor motor protection to
the motor thermal limits so it may be started successfully and be utilized to its full potential without compromising protec-
tion. The distinct parts of the thermal limit curves now become more critical. For these conditions, it is recommended that
the 469 custom curve thermal model be used. The custom overload curve feature allows the user to program their own
curve by entering trip times for 30 pre-determined current levels.
As seen in the figure below, if the running overload thermal limit curve were smoothed into one curve with the locked rotor
overload curve, the motor could not start at 80% line voltage. A custom curve is required.
4
During the interval of discontinuity, the longer of the two trip times is used to reduce the chance of nui-
sance tripping during motor starts.
NOTE
4-34
Courtesy of NationalSwitchgear.com
GE Multilin
TYPICAL CUSTOM CURVE
6500 HP, 13800 VOLT INDUCED DRAFT FAN MOTOR
10000
1
PROGRAMMED 469 CUSTOM CURVE
2
RUNNING SAFETIME (STATOR LIMIT)
3
ACCELERATION SAFETIME (ROTOR LIMIT)
4
MOTOR CURRENT at 100% VOLTAGE
1000 5
MOTOR CURRENT at 80% VOLTAGE
100
10
1.0
0.1
MULTIPLE OF FULL LOAD CURRENT SETPOINT
Figure 4–7: CUSTOM CURVE EXAMPLE
469 Motor Management Relay
1
2
3
4
5
806803A6.CDR
4 SETPOINTS
GE Multilin