GE 869, Instruction Manual

Page: 257 / 606

CHAPTER 4: SETPOINTS PROTECTION
869 MOTOR PROTECTION SYSTEM – INSTRUCTION MANUAL 4–125
NOTE
NOTE:
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.
Eq. 15
where I
LR
is the per unit locked rotor current.
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:
Eq. 16
Eq. 17
where:
• TCU = thermal capacity used.
• TCU
start
= TCU value caused by overload condition.
• TCU
end
= TCU value dictated by the hot/cold curve ratio when the motor is running.
This value is 0 when the motor is stopped).
• t = time in minutes.
• τ
cool
= Cool Time Constant Running.
• I
eq
= Equivalent motor heating current (also defined as Thermal Model Biased Motor
Load Current).
• OL = Overload factor specified by the Motor Overload Factor setting.
• FLA = Motor rated current specified by the Motor Full Load Amps setting.