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Extending the
Cooling Time
Constant
Minimum Inhibit
Time
Behaviour in Case
of a Power Supply
Failure
Emergency
Starting
Blocking
Logic
7UM62 Manual
C53000-G1176-C149-3
In order to properly account for the reduced heat exchange when a self-ventilated
motor is stopped, the cooling time constants can be increased relative to the time
constants for a running machine by the factor Kt at STOP (address 6608). A stopped
motor is defined by current below an adjustable current flow monitoring threshold
BkrClosed I MIN, assuming that the motor idle current is greater than this
threshold. The pickup threshold BkrClosed I MIN affects also the thermal overload
protection function (see Section 2.9).
While the motor is running, the heating of the thermal profile is modeled with the time
τ
constant
calculated from the motor ratings, and the cooldown calculated with the
L
τ
⋅ Kt at RUNNING (address 6609). In this way, the protection caters
time constant
L
to the requirements in case of a slow cooldown (slow temperature equilibrium).
Regardless of thermal profiles, some motor manufacturers require a minimum inhibit
time after the maximum number of permissible starting attempts has been exceeded.
The duration of the inhibit signal depends on which of the times, T MIN. INHIBIT
or T
, is longer.
ReInhib time
Depending on the setting of parameter 0274A ATEX100, the value of the thermal
profile is either reset on a failure of the power supply voltage, or cyclically buffered in
a"non-volatile" memory until the power supply voltage returns. In the second case, the
thermal replica uses the last buffered value for calculation and matches it to the
operating conditions.
If, under emergency conditions, motor starting in excess of the maximum allowable
rotor temperature must take place, the motor start blocking signal can be terminated
via a binary input (">Emer. Start QR"), thus allowing a new starting attempt. The
thermal rotor profile continues to function, however, and the maximum allowable rotor
temperature will be exceeded. No motor shutdown will be initiated by the motor restart
inhibit, but the calculated excessive temperature of the rotor can be observed for risk
assessment.
If the motor start blocking function is blocked or switched off, the thermal profile of the
excessive rotor temperature and the equilibrium time T EQUAL as well as the
minimum inhibit time T MIN. INHIBIT are reset, and any existing motor start inhibit
signal is terminated.
The thermal profile can also be reset via a binary input. This may be useful for testing
and commissioning, and after a power supply voltage failure.
Figure 2-105 shows the logic diagram for the restart inhibit function.
Restart Inhibit for Motors (ANSI 66, 49Rotor)
219
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