GE 869 Instruction Manual page 258

PROTECTION
4–126
• hot / cold = hot/cold curve ratio
For the case when the motor is running cyclic or reciprocating load of small load cycle, it
is recommended to calculate the value of Cooling Time Constant Running using the
below equation.
where
TDM is the TD Multiplier.
However, the Cool Time Constant Running can be only selected using the above
equation when Overload Curve is selected "Motor".
COOL TIME CONSTANT STOPPED
Range: 1 to 1000 minutes in steps of 1 minute
Default: 30 minutes
The Thermal Capacity Used value is reduced in an exponential manner when the motor
current is stopped after running rated load or tripped due to overload. The motor Cool
Time Constant Stopped must be entered for the stopped cases. A stopped motor
normally cools significantly slower than a running motor.
For the motor stopped case, motor cooling is also calculated using the same equation
above, however; the value of the cool time constant (τ) is now Cool Time Constant
Stopped and TCU is now 0.
end
Figure 4-42: Thermal Model Cooling
869 MOTOR PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 4: SETPOINTS
Eq. 18