GE M60 UR Series

GE Multilin M60 Motor Protection System 5-153
5 SETTINGS 5.6 GROUPED ELEMENTS
5
Figure 5–60: RTD BIAS CURVE
• START INHIBIT TCU MARGIN: This setting prevents starting of a motor if there is insufficient thermal capacity avail-
able for a successful start. The motor start inhibit logic algorithm is defined by MOTOR START INHIBIT MARGIN setpoint. If
MOTOR START INHIBIT MARGIN is set to “0”, the MOTOR START INHIBIT FlexLogic operand is asserted only when motor is
tripped by thermal protection (TCU = 100%). After being asserted, the MOTOR START INHIBIT operand resets and a
new motor start is permitted when TC
used
decays to a level of 15%. The thermal lockout time is calculated as follows.
(EQ 5.20)
where TCU
start
is the thermal capacity level when the MOTOR START INHIBIT operand is asserted, TCU
end
is the ther-
mal capacity level when
MOTOR START INHIBIT operand is reset, and  is the COOL TIME CONSTANT STOPPED setpoint.
To calculate the thermal lockout time for the case when
MOTOR START INHIBIT MARGIN is set to “0”, the following TCU
values are to be applied to the above equation: TCU
start
= 100% and TCU
end
= 15%.
If the motor is tripped or stopped by a means other than thermal protection (TCU < 100%), the
MOTOR
START INHIBIT
operand is not asserted and a new start is permitted. There is a potential risk in this situ-
ation to trip out the motor by thermal protection during the new start.
If MOTOR START INHIBIT MARGIN is greater than “0”, the MOTOR START INHIBIT operand will be asserted either when
motor is tripped by thermal protection (TCU = 100%) or tripped/stopped by any other reason (TCU < 100%).
Each time motor is tripped/stopped by a means other than thermal protection (TCU < 100%) the available thermal
capacity (100% – TCU) is evaluated and compared to the TC required for starting the motor. If the available thermal
capacity is not sufficient to perform new motor start, then the
MOTOR START INHIBIT operand will be asserted. The ther-
mal capacity required for starting the motor (TCU
onStart
) is defined from the following equation:
(EQ 5.21)
where TCU
margin
is the relay setpoint, and TC
maxStart
is the maximum thermal capacity value from the last five suc-
cessful motor starts. The relay monitors motor starting and captures the TCU for each successful start. The largest
value from the last five (5) starts is used in the start inhibit calculation.
In this case, the
MOTOR START INHIBIT operand resets when TCU decays to the level satisfying the following equation:
(EQ 5.22)
To calculate the thermal lockout time for the case when the MOTOR START INHIBIT MARGIN setting is greater than “0”, the
following TCU values are to be applied:
(EQ 5.23)
808721A1.CDR
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
TCU
end
TCU
start
e
T –
=
NOTE
TCU
onStart
TCU
maxStart
TCU
margin
100%+
100%
------------------------------------------------


=
TCU 100% TCU
maxStart
TCU
margin
100%+
100%
------------------------------------------------



–
TCU
start
TC accumulated from the moment the motor was stopped=
TCU
end
100% TCU
maxStart
1
TCU
margin
100%
--------------------------
+





–=