GE 469 Instruction Manual page 151

S5 Thermal Model
GE Multilin
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FIGURE 5–13: Voltage Dependent Overload Protection at 100% Voltage
e) Unbalance Bias
Unbalanced phase currents also cause additional rotor heating not accounted for by
electromechanical relays and also not accounted for in some electronic protective
relays. When the motor is running, the rotor rotates in the direction of the positive-
sequence current at near synchronous speed. Negative-sequence current, with a
phase rotation opposite to positive-sequence current (and hence, opposite to the
rotor rotation), generates a rotor voltage that produces a substantial rotor current.
This induced current has a frequency approximately 2 times the line frequency:
100 Hz for a 50 Hz system or 120 Hz for a 60 Hz system. The skin effect in the rotor
bars at this frequency causes a significant increase in rotor resistance and therefore
a significant increase in rotor heating. This extra heating is not accounted for in the
thermal limit curves supplied by the motor manufacturer, as these curves assume
only positive-sequence currents from a perfectly balanced supply and motor design.
The 469 measures the ratio of negative to positive-sequence current. The thermal
model may be biased to reflect the additional heating that is caused by negative
sequence current when the motor is running. This biasing is accomplished by
http://www.GEindustrial.com/multilin
GE Multilin
HIGH INERTIA LOAD OVERLOAD CURVES
8800 HP, 13.2 kV, REACTOR COOLANT PUMP
3 4 5
MULTIPLES OF FULL LOAD AMPS
469
Motor Management Relay
6 7 8
806826A4.CDR
5–45