4-44
469 Motor Management Relay
GE Power Management
4.6 S5 THERMAL MODEL
4 SETPOINT PROGRAMMING
4
4.6.6 HOT/COLD CURVE RATIO
The motor manufacturer may provide thermal limit information for a hot/cold motor. The 469 thermal model
adapts for these conditions if the
HOT/COLD CURVE RATIO
setpoint is programmed. This setpoint value dictates
the level of thermal capacity used the relay will settle at for current levels below the
OVERLOAD PICKUP LEVEL
.
When the motor is running at a level that is below the
OVERLOAD PICKUP LEVEL
, the
THERMAL CAPACITY USED
register will rise or fall to a value based on the average phase current and the
HOT/COLD CURVE RATIO
setpoint.
The
THERMAL CAPACITY USED
will either rise at a fixed rate of 5% per minute or fall as dictated by the running
cool time constant.
where: TC
used_end
=
THERMAL CAPACITY USED
if
I
per_unit
remains steady state
I
eq
= equivalent motor heating current
hot / cold =
HOT/COLD CURVE RATIO
setpoint
The
HOT/COLD CURVE RATIO
may be determined from the thermal limit curves if provided or the hot and cold
safe stall times. Simply divide the hot safe stall time by the cold safe stall time. If hot and cold times are not pro-
vided, there can be no differentiation and the
HOT/COLD CURVE RATIO
should be entered as
"1.00"
.
4.6.7 RTD BIAS
The 469 thermal replica operates as a complete and independent model. The thermal overload curves how-
ever, are based solely on measured current, assuming a normal 40
°
C ambient and normal motor cooling. If the
ambient temperature is unusually high, or if motor cooling is blocked, the motor temperature will increase. If the
motor stator has embedded RTDs, the 469 RTD bias feature should be used to correct the thermal model.
The RTD bias feature is a two-part curve, constructed using 3 points. If the maximum stator RTD temperature
is below the
RTD BIAS MINIMUM
setpoint (typically 40°C), no biasing occurs. If the maximum stator RTD temper-
ature is above the
RTD BIAS MAXIMUM
setpoint (typically at the stator insulation rating or slightly higher), then
the thermal memory is fully biased and
THERMAL CAPACITY USED
is forced to
"100%"
. At values between the
maximum and minimum, the
THERMAL CAPACITY USED
created by the overload curve and the thermal model is
compared to the RTD Bias thermal capacity used from the RTD Bias curve. If the RTD Bias thermal capacity
used value is higher, then that value is used from that point onward. The
RTD BIAS CENTER POINT
should be set
at the rated motor running temperature. The 469 automatically determines the
THERMAL CAPACITY USED
value
for the center point using the
HOT/COLD SAFE STALL RATIO
setpoint.
At < RTD_Bias_Center temperature,
At > RTD_Bias_Center temperature,
where: RTD_Bias_TCused = TC used due to hottest stator RTD
Temp
acutal
= current temperature of the hottest stator RTD
Temp
min
= RTD Bias minimum setpoint
Temp
center
= RTD Bias center setpoint
Temp
max
= RTD Bias maximum setpoint
TCused @ RTD_Bias_Center = TC used defined by the
HOT/COLD SAFE STALL RATIO
setpoint
TC
used_end
I
eq
1
hot
cold
-----------
–
100%
×
×
=
TC
used
@ RTD_Bias_Center
1
hot
cold
-----------
–
100%
×
=
RTD_Bias_TC
used
Temp
actual
Temp
min
–
Temp
center
Temp
min
–
--------------------------------------------------------
100
TC
used
@ RTD_Bias_Center
–
(
)
×
TC
used
@ RTD_Bias_Center
+
=
RTD_Bias_TC
used
Temp
actual
Temp
center
–
Temp
max
Temp
center
–
-------------------------------------------------------------
100
TC
used
@ RTD_Bias_Center
–
(
)
×
TC
used
@ RTD_Bias_Center
+
=
