Siemens SENTRON 3VL System Manual page 75

Trip classes
Trip class 10A is used for motors that have very simple start-up characteristics (those with a
short start-up time and a small moment of inertia). The class 30 releases are used to protect
motors that have to withstand difficult start-up characteristics (long start-up time and large
moment of inertia). The motor must be suitable for difficult start-ups.
The trip class must be selected so that it corresponds to the overload factor of the motor
under operating conditions. See figure: Current-time curve before and after overload, with
thermal memory.
Definition of the trip class
The trip class specifies the release time for balanced 3-pole loads, starting from the cold
state, with 7.2 times the set current I
10 are generally used.
Applications that require a longer start-up time, such as fans with large blades, require a
higher trip class.
Thermal memory
All SENTRON VL circuit breakers possess a "thermal memory" which takes the pre-loading
of the AC motor into consideration. The tripping times of the current-dependent delayed
overload releases are only valid for the uncharged (cold) state.
The pre-loading of the 3-phase AC motor must be taken into consideration in order to
prevent damage to the motor, e.g. after being frequently switched on without sufficient
cooling time.
Siemens offers the SENTRON VL circuit breakers with fixed thermal memory to provide
maximum protection for motors.
Functional principle of the thermal memory
During operation, a thermal image of the motor is simulated in the ETU. This reduces the
response time of the circuit breaker with thermal memory to such an extent that further
overloads cannot damage the motor windings. The motor is switched off within a time limit
that is specified by the pre-loading.
The current required to switch the motor on again could also be considered to be an
overload.
After an overcurrent tripping, the tripping times are reduced in accordance with the tripping
characteristic curves.
A cooling time defined by the size of the motor is required before the motor can be switched
on again. The circuit breaker prevents the motor from being turned on again during this time
interval. This prevents the motor from being excessively thermally loaded by a current
immediately after an overload release occurs.
3VL molded-case circuit breakers
System Manual, 03/2009, 110 0110 - 02 DS 01
6.6 Use in the motor protection area
according to IEC 60947-4-1. Combinations with class
r
Application planning
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