S5 THERMAL MODEL 4. SETPOINT PROGRAMMING
4-38
4.6.6 HOT/COLD CURVE RATIO
The motor manufacturer will sometimes provide thermal limit information for a hot/cold motor. The SR469 thermal model will adapt for
these conditions if the Hot/Cold Curve Ratio is programmed. The value entered for this setpoint dictates the level of thermal capacity
used that the relay will settle at for levels of current that are below the Overload Pickup Level. When the motor is running at a level that
is below the Overload Pickup Level, the thermal capacity used will rise or fall to a value based on the average phase current and the
entered Hot/Cold Curve Ratio. 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 I 1 Hot / Cold 100%
used_end eq
=×− ×
Where: TCused_end= Thermal Capacity Used if Iper_unit remains steady state
Ieq = 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 provided, there can be no differentiation and the hot/cold
curve ratio should be entered as 1.00.
4.6.7
RTD BIAS
The SR469 thermal replica created by the features described in the sections above operates as a complete and independent model. The
thermal overload curves however, are based solely on measured current, assuming a normal 40 °C ambient and normal motor cooling.
If there is an unusually high ambient temperature, or if motor cooling is blocked, motor temperature will increase. If the motor stator has
embedded RTDs, the SR469 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
o
C), no biasing occurs. If the maximum stator RTD temperature 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 is forced to 100%
used. At values in between, the present thermal capacity used created by the overload curve and other elements of 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 running temperature of the motor.
The SR469 will automatically determine the thermal capacity used value for the center point using the Hot/Cold Safe stall ratio setpoint.
()
TC RTD Bias Center Hot Cold
used
@__ /
=− ×
1 100%
At < RTD_Bias_Center temperature,
RTD_Bias_ TC
Temp Temp
Temp Temp
TC @RTD_Bias_ Center
used
actual min
center min
used
=
−
−
×
At > RTD_Bias_Center temperature,
RTD_Bias_ TC
Temp Temp
Temp Temp
(100 TC @RTD_Bias_ Center)+ TC @RTD_Bias_ Center
used
actual center
max center
used used
=
−
−
×−
Where RTD_Bias_TC
USED
= TC used due to hottest stator RTD
Temp
ACTUAL
= Current temperature of hottest stator RTD
Temp
MIN
= RTD Bias minimum setpoint
Temp
CENTER
= RTD Bias center setpoint
Temp
MAX
= RTD Bias max setpoint
TC
USED
@RTD_Bias_Center = TC used defined by HOT/COLD SAFE STALL RATIO setpoint
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