Motor Restart Inhibit 66; Description - Siemens siprotec SJ62 User Manual

2 Functions
2.8.2

Motor Restart Inhibit 66

2.8.2.1

Description

General
Determining the
Rotor
Overtemperature
159
The restart inhibit prevents restarting of the motor when this restart may cause the per-
missible thermal limits of the rotor to be exceeded.
The rotor temperature of a motor generally remains well below its maximum admissi-
ble 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 ex-
ceeded during this startup attempt. Therefore, the 7SJ62/63/64 relays feature the
motor start inhibit which outputs a blocking command until a new motor startup is per-
mitted for the deactivated motor (restarting limit). The blocking signal must be config-
ured to a binary output relay of the device whose contact is inserted in the motor start-
ing circuit.
Since the rotor current cannot be measured directly, the stator current must be used
to generate a thermal profile of the rotor. The r.m.s. values of the currents are utilized
for this. The rotor overtemperature Θ
currents. Therefore, 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
warm (n ) conditions. From this data, the device performs the necessary calcula-
warm
tions to establish the thermal profile of rotor and issues a blocking signal until the
thermal profile of rotor decreases below the restarting limit at which point starting is
permitted anew.
is calculated using the largest of these three
R
SIPROTEC 4, 7SJ62/63/64 Handbuch
C53000-G1140-C147-A, Edition 07.2015
) and
cold