GE 869 Instruction Manual page 257

CHAPTER 4: SETPOINTS
NOTE:
869 MOTOR PROTECTION SYSTEM – INSTRUCTION MANUAL
The curve created when K = 8 is almost identical to the NEMA derating curve.
Figure 4-41: Medium Motor Derating Factor Due to Unbalanced Voltage
If a value of K = 0 is entered, unbalance biasing is defeated and the overload curve times
out against the measured per unit motor positive sequence current. The following
equations can be used to calculate k.
where I is the per unit locked rotor current.
LR
COOL TIME CONSTANT RUNNING
Range: 1 to 1000 minutes in steps of 1 minute
Default: 15 minutes
The thermal capacity used value is reduced in an exponential manner when the motor
current is below the full load amps times overload factor (OL x FLA) settings to simulate
motor cooling. The motor Cool Time Constant Running should be entered for running
case. Motor cooling is calculated as follows:
where:
• TCU = thermal capacity used.
• TCU = TCU value caused by overload condition.
start
• TCU = TCU value dictated by the hot/cold curve ratio when the motor is running.
end
This value is 0 when the motor is stopped).
• t = time in minutes.
• τ = Cool Time Constant Running.
cool
• I = Equivalent motor heating current (also defined as Thermal Model Biased Motor
eq
Load Current).
• OL = Overload factor specified by the Motor Overload Factor setting.
• FLA = Motor rated current specified by the Motor Full Load Amps setting.
PROTECTION
Eq. 15
Eq. 16
Eq. 17
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