Hot/Cold Curve Ratio; Rtd Bias - GE MOTOR MANAGEMENT RELAY 469 Instruction Manual

4.6 S5 THERMAL MODEL
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.
TC
used_end
= THERMAL CAPACITY USED if I
where: TC
used_end
I = equivalent motor heating current
eq
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
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.
TC @ RTD_Bias_Center
used
At < RTD_Bias_Center temperature,
RTD_Bias_TC
At > RTD_Bias_Center temperature,
RTD_Bias_TC
where: RTD_Bias_TCused = TC used due to hottest stator RTD
Temp = current temperature of the hottest stator RTD
acutal
Temp = RTD Bias minimum setpoint
min
Temp = RTD Bias center setpoint
center
Temp = RTD Bias maximum setpoint
max
TCused @ RTD_Bias_Center = TC used defined by the HOT/COLD SAFE STALL RATIO setpoint
4-44
 
hot
× ×
I 1 100%
= – ---------- -
 
eq
cold

hot
1
= – ---------- -

cold
Temp – Temp
actual min
= --------------------------------------------------------
used
Temp Temp
–
center min
Temp – Temp
actual center
= ------------------------------------------------------------ -
used
Temp – Temp
max center
469 Motor Management Relay
remains steady state
per_unit

×
100%

× (
100 TC @ RTD_Bias_Center
–
used
× (
100 TC @ RTD_Bias_Center
–
used
4 SETPOINT PROGRAMMING

4.6.6 HOT/COLD CURVE RATIO

4.6.7 RTD BIAS

)
TC @ RTD_Bias_Center
+
used
)
TC @ RTD_Bias_Center
+
used
GE Power Management