GE M60 Instruction Manual page 296

GROUPED ELEMENTS
where
TCU =
THERMAL CAPACITY USED
end
I = I motor heating current
eq equivalent
hot / cold =
HOT/COLD SAFE STALL RATIO
— This setting enables or disables the RTD bias mechanism. The RTD bias feature is active only if the optional RTD
RTD BIAS
Input module (module 5C or 5E) has been installed. See the
setting descriptions that follow for details on using the RTD bias mechanism.
RTD BIAS MINIMUM / CENTER / MAXIMUM
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
increases. If the motor stator has embedded RTDs, use the RTD bias feature to augment the thermal model calculation of
Thermal Capacity Used.
The RTD bias feature is a two-part curve (RTD Bias Thermal Capacity Used) constructed from three points: minimum, center
and maximum. If the maximum stator RTD temperature is below the
occurs. If the maximum stator RTD temperature is above the
rating or slightly higher), then the thermal memory is fully biased and RTD bias thermal capacity used is forced to 100%. At
values in between, the present RTD bias thermal capacity used created by other features of the thermal model is
compared to the RTD bias thermal capacity used. If the value of the RTD bias thermal capacity used is higher, then this
value is used from that point onward. Set the
relay automatically determines the RTD bias thermal capacity used value for the center point using the
setting.
STALL RATIO
5
At < RTD_Bias_Center temperature,
At > RTD_Bias_Center temperature,
RTD_Bias_TCU
where
RTD_Bias_TCU = thermal capacity used due to hottest stator RTD
Temp = current temperature of the hottest stator RTD
acutal
Temp = RTD Bias minimum setting
min
Temp = RTD Bias center setting
center
Temp = RTD Bias maximum setting
max
TCU at RTD_Bias_Center = thermal capacity used defined by the
In simple terms, the RTD bias feature is feedback of measured stator temperature. This feedback acts to correct the
assumed thermal model. Since RTDs have a relatively slow response, RTD biasing is useful for slow motor heating. Other
portions of the thermal model are required during starting and heavy overload conditions when motor heating is relatively
fast.
The RTD bias feature alone cannot create a trip. Even if the RTD bias feature forces the RTD bias thermal capacity used to
100%, the load current must be above the overload pickup setting to set the output.
Do not program
RTD BIAS MAXIMUM
temperatures up to 200°C.
5-168
if I remains steady state
equivalent
setting
— The relay thermal replica operates as a complete and independent model. The
RTD BIAS CENTER
TCU at RTD_Bias_Center
Temp Temp
–
actual
------------------------------------------------- -
RTD_Bias_TCU =
Temp Temp
–
center
Temp Temp
–
actual center
= ----------------------------------------------------- -   100 TCU at RTD_Bias_Center
–
Temp Temp
–
max center
setpoint greater than 200°C if using a remote RTD unit, since the RRTD only measures
,
RTD BIAS MINIMUM RTD BIAS CENTER
setting (typically 40°C), no biasing
RTD BIAS MINIMUM
setting (typically at the stator insulation
RTD BIAS MAXIMUM
setting to the rated running temperature of the motor. The
hot
 
--------- 
= 1 100%
–
 
cold
min

TCU at RTD_Bias_Center
min
 + TCU at RTD_Bias_Center
HOT/COLD SAFE STALL RATIO
M60 MOTOR PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 5: SETTINGS
, and
RTD BIAS MAXIMUM
HOT/COLD SAFE
Eq. 5-17
Eq. 5-18
Eq. 5-19
setting