5–54 469 MOTOR MANAGEMENT RELAY – INSTRUCTION MANUAL
CHAPTER 5: SETTINGS
FIGURE 5–13: Voltage Dependent Overload Protection at 100% Voltage
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.
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123 4567 8
2
3
4
5
6
7
8
9
10
20
30
40
50
60
70
80
90
100
HIGH INERTIA LOAD OVERLOAD CURVES
8800 HP, 13.2 kV, REACTOR COOLANT PUMP
TIME TO TRIP (SECONDS)
MULTIPLES OF FULL LOAD AMPS
200
300
400
500
600
700
800
900
1000
GE Multilin
806826A4.CDR
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