5–58 469 MOTOR MANAGEMENT RELAY – INSTRUCTION MANUAL
CHAPTER 5: SETTINGS
In simple terms, the RTD bias feature is real feedback of the measured stator temperature.
This feedback acts as correction of the thermal model for unforeseen situations. Since
RTDs are relatively slow to respond, RTD biasing is good for correction and slow motor
heating. The rest of the thermal model is required during starting and heavy overload
conditions when motor heating is relatively fast.
Note that the RTD bias feature alone cannot create a trip. If the RTD bias forces thermal
capacity used to 100%, the motor current must be above the overload pickup before an
overload trip occurs. Presumably, the motor would trip on stator RTD temperature at that
time.
FIGURE 5–16: RTD Bias Curve
Maximum Stator RTD Temperature
0
20
40
60
80
100
–50 0 50 100 150 200 250
RTD Bias Maximum
RTD Bias Center Point
RTD Bias Minimum
Hot/Cold = 0.85
Rated Temperature=130°C
Insulation Rating=155°C
RTD Thermal Capacity Used
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