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PROTECTION
NOTE:
4–120
The curve created when K = 8 is almost identical to the NEMA derating curve.
Figure 4-35: 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.
869 MOTOR PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 4: SETPOINTS
Eq. 15
Eq. 16
Eq. 17
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