CHAPTER 5: SETTINGS GROUPED ELEMENTS
M60 MOTOR PROTECTION SYSTEM – INSTRUCTION MANUAL 5-205
5
Figure 5-105: RTD bias curve
START INHIBIT TCU MARGIN — This setting prevents starting of a motor if there is insufficient thermal capacity available 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 thermal capacity level when the MOTOR START INHIBIT operand is reset
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 previous equation: TCU
start
= 100% and TCU
end
= 15%.
If
MOTOR START INHIBIT MARGIN is greater than “0,” the MOTOR START INHIBIT operand is asserted either when the motor is
tripped by thermal protection (TCU = 100%) or tripped/stopped for any other reason (TCU < 100%).
Each time the 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 a new motor start, then the MOTOR START INHIBIT operand is asserted. The thermal
capacity required for starting the motor (TCU
onStart
) is defined from the following equation:
Eq. 5-21
where
TCU
margin
is the relay setpoint
TCU
maxStart
is the maximum thermal capacity value from the last five successful motor starts
The relay monitors motor starting and captures the TCU for each successful start. The largest value from the last five starts
is used in the start inhibit calculation.
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 situation to trip out the motor
by thermal protection during the new start.
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