Applications
3.3 Motor protection applications of 3VA molded case circuit breakers
3VA molded case circuit breakers with IEC certificate
Manual, 03/2019, A5E03603177010-03
151
All 3VA2 molded case circuit breakers for motor applications have a "thermal memory" which
takes the pre-loading of the three-phase asynchronous motor into account. The tripping
times of the inverse-time delayed overload releases are only valid for the unloaded (cold)
state.
The pre-loading of the 3-phase motor must be taken into consideration in order to prevent
damage to the motor, e.g. from being frequently switched on without sufficient cooling time.
All 3VA2 molded case circuit breakers for motor protection have a thermal model of the
motor integrated in the electronic trip unit. This reduces the response time of the molded
case circuit breaker with a thermal memory such that further overloads cannot damage the
motor windings. The motor is switched off within a time limit determined by the thermal pre-
loading, see curve
② in the figure below (response time of the trip unit after overload trip).
An overload may also be the next inrush current of the motor.
After an overcurrent trip, the trip times are reduced according to the tripping characteristic.
A cooling time determined by the size of the motor is required before the motor can be
switched on again. This prevents the motor from being thermally overloaded by a current
after an overload trip.
Response time of the trip unit after overload trip:
without thermal memory
with thermal memory
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