GE Multilin 489

5–86 489 GENERATOR MANAGEMENT RELAY – INSTRUCTION MANUAL
CHAPTER 5: SETPOINTS
generator is assumed to be running if current is measured or the generator is online). A
machine with a stopped rotor normally cools significantly slower than one with a turning
rotor. Machine cooling is calculated using the following formulae:
(EQ 5.34)
(EQ 5.35)
where: TC
used
= thermal capacity used
TC
used_start
= TC
used
value caused by overload condition
TC
used_end
= TC
used
value dictated by the hot/cold safe stall ratio when the
machine is running (= 0 when the machine is stopped)
t = time in minutes
τ = Cool Time Constant (running or stopped)
I
eq
= equivalent heating current
overload_pickup = overload pickup setpoint as a multiple of FLA
hot / cold = hot/cold safe stall ratio
FIGURE 5–23: Thermal Model Cooling
TC
used
TC
used_start
TC
used_end
–()e
t τ⁄–
()TC
used_end
+=
TC
used_end
I
eq
overload_pickup
-----------------------------------------
⎝⎠
⎛⎞
1
hot
cold
----------–
⎝⎠
⎛⎞
100%×=
808705A2.CDR
0
25
50
75
100
0 30 60 90 120 150 180
Time in Minutes
Therm al Capacity Used
Cool Time Constant= 15 min
TCused_start= 85%
Hot/Cold Ratio= 80%
Ieq/Overload Pickup= 100%
0
25
50
75
100
0 30 60 90 120 150 180
Time in Minutes
Thermal Capacity Used
Cool Time Constant= 15 min
TCused_start= 85%
Hot/Cold Ratio= 80%
Ieq/Overload Pickup= 80%
0
25
50
75
100
0 30 60 90 120 150 180
Time in Minutes
Thermal Capacity Used
Cool Time Constant= 30 min
TCused_start= 85%
Hot/Cold Ratio= 80%
Motor Stopped after running Rated Load
TCused_end= 0%
0
25
50
75
100
0 30 60 90 120 150 180
Time in Minutes
Thermal Capacity Used
Cool Time Constant= 30 min
TCused_start= 100%
Hot/Cold Ratio= 80%
Motor Overload
TCused_end= 0%
MOTOR TRIPPED
100% LOAD
MOTOR STOPPED
80% LOAD
Courtesy of NationalSwitchgear.com