Description - Siemens SIPROTEC 4 7SJ61 Manual

2.6.2.1

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 7SJ61 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
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
data, the device performs the necessary calculations to establish the thermal replica of the rotor and issues a
blocking signal until the thermal replica of the rotor decreases below the restarting limit at which startup is
permitted again.
[temperaturverlauf-bei-mehrfachanlaeufen-260602-kn, 1, en_US]
Figure 2-34
SIPROTEC 4, 7SJ61, Manual
C53000-G1140-C210-6, Edition 05.2016
Temperature curve at the rotor and in the thermal replica during repeated start-up attempts
) and warm (n ) conditions. From this
cold warm
Functions
2.6 Motor Protection
is calcu-
R
99