Motor Protection; Motor Starting Protection 48; Description - Siemens SIPROTEC 7SJ61 Manual

Functions

2.6 Motor Protection

2.6 Motor Protection
For the protection of motors, devices 7SJ61 are provided with a motor starting protection function, a restart
inhibit and a load jam protection. The starting protection function protects the motor from prolonged startup pro-
cedures thus supplementing the overload protection (see Section 2.7). The restart inhibit prevents restarting of
the motor when this restart may cause the permissible thermal limits of the rotor to be exceeded. The load jam
protection serves to protect the motor during sudden rotor blocking.
2.6.1

Motor Starting Protection 48

When devices 7SJ61 are used to protect a motor, the starting protection feature supplements the overload pro-
tection and protects the motor against prolonged starting operations (see Section 2.7).

2.6.1.1 Description

General
In particular, rotor-critical high-voltage motors can quickly be heated above their thermal limits when multiple
starting attempts occur in a short period of time. If the durations of these starting attempts are lengthened e.g.
by excessive voltage surges during motor starting, by excessive load moments, or by blocked rotor conditions,
a trip signal will be initiated by the protective relay.
Motor starting is detected when a settable current threshold I MOTOR START is exceeded. Calculation of the
tripping time is then initiated. It should be noted that this timer starts every time the motor is started. This is
therefore a normal operating condition that is neither indicated in the fault log nor causes the creation of a fault
record. Only when the locked rotor time has elapsed is the trip command issued.
The protection function consists of one definite time and one inverse time tripping element.
Inverse Time Overcurrent Element
The inverse time overcurrent element is designed to operate only when the rotor is not blocked. With a de-
creased startup current resulting from voltage dips when starting the motor, prolonged startup times are eval-
uated correctly and tripping with an appropriate time is enabled. The characteristic (see formula below) can be
ideally adjusted to the condition of the motor by using different startup times depending on the cold or warm
condition of the motor (see Figure 2-30).
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SIPROTEC, 7SJ61, Manual
C53000-G1140-C210-1, Release date 02.2008