Siemens SIPROTEC 7SJ62 Manual page 167

Restart Threshold
If the rotor temperature has exceeded the restart threshold, the motor cannot be restarted. The blocking signal
is not lifted unless the rotor temperature has fallen below the restarting limit, that is, when exactly one start
becomes possible without exceeding the excessive rotor temperature limit. Based on the specified motor pa-
rameters the device calculates the normalized restarting limit Θ
Where:
Θ
Restart
k
R
I
STARTUP
I
MOTNom
T
start max
τ
R
n
cold
The restarting limit Θ
Rotor Overload Detection
If the rotor temperature exceeds 100% of the maximum temperature calculated from the thermal rotor profile,
there is a risk of motor damage. If this threshold value is exceeded, either tripping occurs or an overload
message is issued. The desired reaction can be determined via parameter 4311 ROTOR OVERLOAD. If param-
eter is set to OFF, rotor overload will not be detected.
Restart Time
The motor manufacturer allows a maximum number of cold (n
after, another startup is not permitted. A certain time must have passed — restarting time T
that the rotor has cooled off (operational measured value 661).
Equilibrium Time
This thermal behavior is provided for in the protection as follows: Each time the motor is shut down, the timer
starts (address 4304 T Equal). It takes into account the different thermal conditions of the motor parts at the
moment of shutdown. During the equilibrium time, the thermal replica of the rotor is not updated. It is maintained
constant to replicate the equilization process in the rotor. Then, the thermal replica with the corresponding time
constant (rotor time constant x extension factor) cools down. During the equilibrium time the motor cannot be
restarted. As soon as the temperature sinks below the restarting limit, the next restart attempt can be made.
Minimum Inhibit Time
Regardless of thermal replicas, some motor manufacturers require a minimum inhibit time after the maximum
number of permissible startup attempts has been exceeded.
The total duration of the inhibit signal depends on which of the times T
SIPROTEC, 7SJ62/64, Manual
C53000-G1140-C207-2, Release date 01.2008
= Temperature threshold below which restarting is possible
= k-factor for the rotor, calculated internally
= Startup current
= Nominal motor current
= Maximum startup time
= Thermal time constant of the rotor, calculated internally
= Permissible number of startups in cold condition
is displayed as operational measured value in the "thermal measured values".
Restart
:
Restart
) and warm (n ) startup attempts. There-
cold warm
or T is longer.
Min Inhibit Restart
Functions
2.8 Motor Protection
— to ensure
Restart
167