Functions
2.8 Motor Protection
SIPROTEC, 7SJ62/64, Manual
C53000-G1140-C207-2, Release date 01.2008
168
Total Time T
Reclose
The total waiting time T
Reclose
before the motor can be restarted is therefore composed of the equilibrium time,
the time T
Restart
calculated from the thermal replica and the value that is needed to drop below the limit for re-
starting. If the calculated temperature rise of the rotor is above the restarting limit when the motor is shut down,
the minimum inhibit time will be started together with the equilibrium time.
Thus the total inhibit time T
Reclose
can become equal to the minimum inhibit time if it is longer than the sum of
the two first mentioned times:
T
Reclose
= T
Equal
+ T
Restart
for T
Min Inhibit
< T
Equal
+ T
Restart
T
Reclose
= T
Min Inhibit
for T
Min Inhibit
≥
T
Equal
+ T
Restart
if the calculated excessive temperature > re-
starting limit
The operational measured value 809 T
Reclose
(visible in the thermal measured values) is the remaining time until
the next restart is permissible. When the rotor excessive temperature is below the restarting limit and thus the
next restarting attempt is permitted, the operational measured value for the waiting time has reached zero.
Extension of Cool Down Time Constants
In order to properly account for the reduced heat exchange when a self-ventilated motor is stopped, the cool
down time constants can be increased relative to the time constants for a running machine with the factor
K
τ
at STOP
(address
4308
). The criterion for the motor stop is the undershooting of a set current threshold
BkrClosed I MIN
. This understands that the motor idle current is greater than this threshold. The pickup
threshold
BkrClosed I MIN
affects also the thermal overload protective function (see Section 2.10).
While the motor is running, the heating of the thermal replica is modeled with the time constant
τ
R
calculated
from the motor ratings, and the cool down calculated with the time constant
τ
R
·
K
τ
at RUNNING
(address
4309
). In this way, the protection caters to the requirements in case of a slow cool down (slow temperature
equilibrium).
For calculation of the restarting time T
Restart
the following applies:
with
k
τ
at STOP
Extension factor for the time constant =
K
τ
at STOP
, address
4308
k
τ
at RUNNING
Extension factor for the time constant =
K
τ
at RUNNING
, address
4309
Θ
pre
thermal replica at the moment of motor shutdown (depending on operational condition)
τ
R
Rotor time constant, calculated internally
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