2.25 Restart Inhibit for Motors (ANSI 66, 49Rotor)
151
7UM61 Manual
C53000-G1176-C127-3
τ
R
– rotor time constant, calculated internally:
τ
R
= t
Start
· (n
cold
– n
warm
) · I
Start
2
where:
t
Start
= Startup time in s
I
Start
= Startup current in pu
Θ
pre
– thermal replica at the instant the motor is switched off (depends on
operational condition)
The operational measured value T
ReInhib time
(to be found in the overload measured
values) shows the time remaining until the next restart is allowed.
Extension of
Cooldown Time
Constant
In order to properly account for the reduced heat removal when a self-ventilated motor
is stopped, the cooldown time constant can be increased relative to the time constants
for a running machine with the factor
K
τ
at STOP
(address
6608
). A stopped motor
is defined by current below an adjustable current flow monitoring threshold
BkrClosed I MIN
. The assumes that the idle current of the motor is greater than
this threshold. The pickup threshold
BkrClosed I MIN
also effects the thermal over-
load protection function (see Section 2.9).
While the motor is running, heating of the thermal profile is modeled with the time con-
stant
τ
R
calculated from the motor ratings, and the cooldown is calculated with the time
constant
τ
R
·
K
τ
at RUNNING
(address
6609
). In this way the requirements for a slow
cooldown (slow temperature leveling) are met.
Minimum Inhibit
Time
Regardless of thermal profiles, some motor manufacturers require a minimum inhibit
time after the maximum number of permissible startup attempts has been exceeded.
The duration of the inhibit signal depends on which of the times, T
MIN INHIBIT
or T
ReInhib
time
is longer.
Behaviour on
Power Supply
Failure
Depending on the setting of parameter
274
ATEX100
, the value of the thermal profile
is either reset to zero on failure of the power supply voltage, or cyclically buffered in a
non-volatile memory until the power supply voltage returns. In the latter case when
power supply is restored, the thermal profile uses the stored value for calculation and
matches it to the operating conditions.
Emergency Startup
If, for emergency reasons, 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
Θ
R”
), thus allowing a new starting attempt. The
thermal rotor profile continues to function, however, and the maximum admissible
rotor temperature can be exceeded. No motor shutdown is initiated by the motor
restart inhibit, but the calculated overtemperature of the rotor can be observed for risk
assessment.
Blocking
If the motor restart blocking function is blocked or switched off, the thermal profile of
the rotor overtemperature and the leveling time
T EQUAL
as well as the minimum
inhibit time
T MIN. INHIBIT
are reset, and any existing motor start inhibit signal is
retracted.
Logic
The thermal profile can also be reset via a binary input. This may be useful for testing
and commissioning, and after power supply voltage restoration.
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