Load Jam Protection; Mode Of Operation - Siemens SIPROTEC 7SJ61 Manual

2.6.3

Load Jam Protection

The load jam protection serves to protect the motor during sudden rotor blocking. Damage to drives, bearings
and other mechanic motor components can be avoided or reduced by means of quick motor shutdown.
The blocking results in a current jump in the phases. This is detected by the function as a recognition criteria.
The thermal overload protection would of course also pickup as soon as the configured threshold values of the
thermal replicas are exceeded. The load jam protection, however, is able to detect a locked rotor quicker, thus
reducing possible damage to the motor and powered equipment.

2.6.3.1 Mode of Operation

Principle of Operation
Figure 2-36 illustrates the feature of an asynchronous cage motor. Nominal current is flowing at normal load. If
the load is increased, the current flow also increases and the speed decreases. Above a certain load, however,
the motor is no longer able to adjust the speed by increasing the torque. The motor comes to standstill in spite
of an increase in current to a multiple of its nominal value (see Figure 2-37). Other types of induction motors
have similar characteristics. Apart from the thermal heating of the motor, a locked rotor causes substantial me-
chanical strain on coils and bearings.
Figure 2-36
SIPROTEC, 7SJ61, Manual
C53000-G1140-C210-1, Release date 02.2008
Typical characteristic of an asynchronous cage motor
Functions
2.6 Motor Protection
117