Functions
2.8 Motor Protection
SIPROTEC, 7SJ62/64, Manual
C53000-G1140-C207-2, Release date 01.2008
165
2.8.2 Motor Restart Inhibit 66
The motor restart inhibit prevents restarting of the motor when this restart may cause the permissible thermal
limits of the rotor to be exceeded.
Additionally, the function can trip directly if the rotor temperature exceeds the maximum admissible temperature
(100%) (rotor overload).
2.8.2.1 Description
General
The rotor temperature of a motor generally remains well below its maximum admissible temperature during
normal operation and also under increased load conditions. However, high startup currents required during
motor startup increase the risk of the rotor being thermally damaged rather the stator, due to the short thermal
constant of the rotor. To avoid that multiple starting attempts provoke tripping, a restart of the motor must be
inhibited if it is apparent that the thermal limit of the rotor will be exceeded during this startup attempt. Therefore,
the 7SJ62/64 relays feature the motor restart inhibit which outputs a blocking command until a new motor
startup is permitted for the deactivated motor (restarting limit). The blocking signal must be configured to a
binary output relay of the device whose contact is inserted in the motor starting circuit.
Determining the Rotor Overtemperature
Since the rotor current cannot be measured directly, the stator current must be used to generate a thermal
replica of the rotor. The r.m.s. values of the currents are used for this. The rotor overtemperature Θ
R
is calcu-
lated using the largest of these three phase currents. It is assumed that the thermal limit values for the rotor
winding are based on the manufacturer's data regarding the nominal starting current, maximum permissible
starting time, and the number of starts permitted from cold (n
cold
) and warm (n
warm
) conditions. From this data,
the device performs the necessary calculations to establish the thermal replica of the rotor and issues a block-
ing signal until the thermal replica of the rotor decreases below the restarting limit at which startup is permitted
again.
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