GE 469 Instruction Manual page 91

4 SETPOINTS
g) RTD BIAS
The 469 thermal replica 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 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
setpoint (typically 40°C), no biasing occurs. If the maximum stator RTD temperature is above the
RTD BIAS MINIMUM
setpoint (typically at the stator insulation rating or slightly higher), then the thermal memory is fully biased
BIAS MAXIMUM
and
THERMAL CAPACITY USED
created by the overload curve and the thermal model is compared to the RTD Bias thermal capacity used from the
USED
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
value for the center point using the
MAL CAPACITY USED
At < RTD_Bias_Center temperature,
RTD_Bias_TC
used
At > RTD_Bias_Center temperature,
RTD_Bias_TC =
used
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
In simple terms, the RTD bias feature is real feedback of the measured stator temperature. This feedback acts as correc-
tion of the thermal model for unforeseen situations. Since RTDs are relatively slow to respond, RTD biasing is good for cor-
rection and slow motor heating. The rest of the thermal model is required during starting and heavy overload conditions
when motor heating is relatively fast.
It should be noted that the RTD bias feature alone cannot create a trip. If the RTD bias feature forces the 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.
GE Multilin
Courtesy of NationalSwitchgear.com
is forced to "100%". At values between the maximum and minimum, the
should be set at the rated motor running temperature. The 469 automatically determines the
TC @ RTD_Bias_Center
used
Temp – Temp
actual min × (
= -------------------------------------------------------- 100 TC –
Temp – Temp
center min
Temp – Temp
actual center × (
------------------------------------------------------------ - 100 TC –
Temp – Temp
max center
100
Hot/Cold = 0.85
80
Rated Temperature=130°C
Insulation Rating=155°C
60
40
20
RTD Bias Minimum
0
–50 0
Maximum Stator RTD Temperature
Figure 4–14: RTD BIAS CURVE
469 Motor Management Relay
setpoint.
HOT/COLD SAFE STALL RATIO
 
hot
×
1 100%
= – ---------- -
 
cold
)
@ RTD_Bias_Center + TC
used used
)
@ RTD_Bias_Center + TC
used
HOT/COLD SAFE STALL RATIO
RTD Bias Maximum
RTD Bias Center Point
50 100 150 200
808721A1.CDR
4.6 S5 THERMAL MODEL
THERMAL CAPACITY
@ RTD_Bias_Center
@ RTD_Bias_Center (EQ 4.10)
used
setpoint
250
RTD
THER-
(EQ 4.8)
4
(EQ 4.9)
4-41