GE Multilin 469, Instruction Manual

Page: 199 / 338

CHAPTER 5: SETTINGS
469 MOTOR MANAGEMENT RELAY – INSTRUCTION MANUAL 5–57
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 SAFE
STALL RATIO
settings. 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.
(EQ 5.7)
where: TC
used_end
=
THERMAL CAPACITY USED
if I
per_unit
remains steady state
I
eq
= equivalent motor heating current
hot / cold =
HOT/COLD SAFE STALL RATIO
settings
The
HOT/COLD SAFE STALL 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 SAFE STALL RATIO
should be entered as “1.00”.
RTD Bias
The 469 thermal replica operates as a complete and independent model. However, the
thermal overload curves are based solely on measured current, assuming normal 40
° C
ambient and normal motor cooling. If the ambient temperature is unusually high, or motor
cooling is blocked, the motor temperature will increase. If the motor stator has embedded
RTDs, the 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
settings (typically 40°C), no biasing
occurs. If the maximum stator RTD temperature is above the
RTD BIAS MAXIMUM
(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
settings.
(EQ 5.8)
At < RTD_Bias_Center temperature,
(EQ 5.9)
At > RTD_Bias_Center temperature,
(EQ 5.10)
where: RTD_Bias_TC
used
= TC used due to hottest stator RTD
T
actual
= current temperature of the hottest stator RTD
T
min
= RTD Bias minimum settings, T
center
= RTD Bias center settings
T
max
= RTD Bias maximum settings
TC
used at RBC
= TC used defined by the
HOT/COLD SAFE STALL RATIO
settings
TC
used_end
I
eq
1
hot
cold
----------
–
⎝ ⎠
⎛ ⎞
100% × ×
=
TC
used at RBC
1 hot cold ⁄
–
( ) 100% ×
=
RTD_Bias_TC
used
T
actual
T
min
–
T
center
T
min
–
------------------------------- TC
used at RBC
×
=
RTD_Bias_TC
used
T
actual
T
center
–
T
max
T
center
–
------------------------------------ 100 TC
used at RBC
–
( ) × TC
used at RBC
+ =