Application planning
5.6 Use in motor protection
3VL IEC molded case circuit breakers
66 System Manual, 11/2013, 110 0110 - 02 DS 03
All 3VL molded case circuit breakers with electronic trip unit have a "thermal image" 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 unloaded (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 3VL molded case circuit breakers with fixed thermal image to provide
maximum protection for the motor.
Functional principle of the thermal image
During operation, a thermal model of the motor is simulated in the ETU. This reduces the
response time of the molded case circuit breaker with thermal image such that further
overloads cannot damage the motor windings. The motor is switched off within a time limit
that is specified by the pre-loading.
An overload may also be the switch-on current of the motor.
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. This prevents the motor from being excessively thermally loaded by a current
immediately after an overload release occurs.
Figure 5-4 Response time of the trip unit after overload release
Phase failure sensitivity
The "phase failure sensitivity" function is also integrated into the 3VL molded case circuit
breaker for motor protection. This ensures that the motor is reliably protected against
overheating if a phase interruption or a large fluctuation occurs.
The specified operational current I
R
is automatically reduced to 80% of the set value if the
RMS values of the operational currents in the three phases differ by 5 to 50% (depending on
release type).
Deviations of more than 50% mean the value of the current in the least loaded phase drops
to a level below 50% of the maximum loaded phase.
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