Motor Cooling - GE MOTOR MANAGEMENT RELAY 469 Instruction Manual

4 SETPOINT PROGRAMMING
The thermal capacity used value decreases exponentially when the motor current is below the OVERLOAD
PICKUP setpoint. This reduction simulates motor cooling. The motor cooling time constants should be entered
for both stopped and running cases. Since the cooling is exponential, the time constants are one-fifth of the
total time to go from 100% thermal capacity used to 0%. A stopped motor normally cools significantly slower
than a running motor. Motor cooling is calculated as follows:
(
TC TC
=
used used_start

TC = ------------------------------------------ -

used_end
overload_pickup
where: TC = thermal capacity used
used
TC = TC
used_start
TC = TC
used_end
(= 0 when the motor is stopped)
t = time in minutes
τ
= Cool Time Constant (running or stopped)
I
= equivalent motor heating current
eq
overload_pickup= overload pickup setpoint as a multiple of FLA
hot / cold = hot/cold curve ratio
100
75
Cool Time Constant= 15 min
TCused_start= 85%
Hot/Cold Ratio= 80%
50
Ieq/Overload Pickup= 80%
25
0
0 30 60
Time in Minutes
Thermal Model Cooling, 80% Load
100
75
Cool Time Constant= 30 min
TCused_start= 85%
Hot/Cold Ratio= 80%
Motor Stopped after running Rated Load
50
TCused_end= 0%
25
0
0 30 60
Time in Minutes
Thermal Model Cooling, Motor Stopped
GE Power Management
t τ ⁄
–
) e ( )
TC TC
– +
used_end
I
 1  
hot
eq ×
– ---------- - 100%
  
cold
value caused by overload condition
used
value dictated by the hot/cold curve ratio when the motor is running
used
90 120 150 180
90 120 150 180
Figure 4–17: THERMAL MODEL COOLING
469 Motor Management Relay
4.6 S5 THERMAL MODEL
used_end
100
75
Cool Time Constant= 15 min
TCused_start= 85%
Hot/Cold Ratio= 80%
50
Ieq/Overload Pickup= 100%
25
0
0 30 60 90
Time in Minutes
Thermal Model Cooling, 100% Load
100
75
Cool Time Constant= 30 min
TCused_start= 100%
Hot/Cold Ratio= 80%
Motor Stopped after Overload Trip
50
TCused_end= 0%
25
0
0 30 60 90
Time in Minutes
Thermal Model Cooling, Motor Tripped

4.6.5 MOTOR COOLING

120 150 180
120 150 180
4-43
4