Load Jam Protection; Mode Of Operation - Siemens SIPROTEC 7SK80 Manual
Motor protection
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2.7.3
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.
2.7.3.1 Mode of Operation
Principle of Operation
Figure 2-39 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 2-40). 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 2-39
Figure 2-40 illustrates an example of a locked rotor caused by mechanical overload. It should be noted that the
current flow increases substantially as soon as the mechanical load reaches the stability limit.
SIPROTEC, 7SK80, Manual
E50417-G1140-C344-A4, Release date 08.2010
2.7 Motor Protection (Motor Starting Protection 48, Motor Restart Inhibit 66, Load Jam Protection)
Typical characteristic of an asynchronous cage motor
Functions
135
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