Motor circuit breaker

Motor circuit breakers are used for protecting a motor and its supply cable against over-heating by overload. Depending on the overload level, they disconnect the motor from power supply immediately in the case of a short-circuit or they disconnect the motor if an overload has occurred for some time.

Conventional motor circuit breakers are commercially available for various applications with different trigger characteristics (L, G/U, R and K), as shown in the diagram below. As variable frequency drives in most cases are used for supplying motors which are classified as operating equipment with very high starting currents, only the K-characteristic was realized in this function.

Unlike the operation of a conventional motor circuit breaker which disconnects the equipment to be protected immediately if the trigger threshold is reached, this function provides the possibility of issuing a warning instead of disconnecting the equipment immediately.

The rated values of the VFD are to be considered accordingly when it comes to dimensioning the application. The function of the motor circuit breaker can be linked to different data sets. In this way, it is possible
to operate different motors via one variable frequency drive. Thus, each motor can be equipped with its own motor circuit breaker.

In case a motor is operated via the VFD for which some setting values, e.g. minimum and maximum frequency, are changed via the data set switch-over, only one motor circuit breaker may be installed. This functionality can be set for single or multi-motor operation via variable frequency drive parameter.

Multiple motor operation
In multiple motor operation, it is assumed that each data set is assigned to a corresponding motor. For this, one motor and one motor circuit breaker are assigned to each data set. In this operation mode, the rated values of the active data set are monitored. The current output current of the VFD is only taken into account in the motor circuit breaker activated by the data set. In the motor circuit breakers of the other data sets, zero current is expected, with the result that the thermal decay functions are taken into account. In combination with the data set changeover, the function of the motor circuit breakers is similar to that of motors connected alternately to the mains with their own circuit breakers.

Single motor operation
In single motor operation, only one motor circuit breaker, which monitors the output current of the VFD, is active. In the case of a data set changeover, only the switch-off limits derived from the rated machine parameters are changed over. Accumulated thermal values are used after the changeover as well. In the case of the data set changeover, ensure that the machine data are stated identically for all data sets. In combination with the data set changeover, the function of the motor circuit breaker is similar to that of motors connected alternately to the mains with one common circuit breaker.