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Entry type: FAQ Entry ID: 109758696, Entry date: 07/04/2018
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Hybrid motor starter with electromechanical switching devices

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This entry contains information about voltage peaks at switching off the running motor.

QUESTION:

Which aspects have to be considered when using hybrid motor starters in a row with electromechnical switching devices, for example a mains-contactor?

 

 

ANSWER:

High voltage peaks caused by the interruption of a small motor (high inductive load) must be avoided by an appropriate switch-off-sequence or an RC element.

 

 

Technical explanation

 

Initiation/Base

A advantage of hybrid motor starters is switching off at zero crossing. Therefore no high voltage peaks and electric arcs happen in the main circuit. Nor are there EMC interferences because of the switching.

 

By switching an inductive load, particularly of motors < 1kW with high inductance, with a electromechanical switching device, e. g. a contactor, an induction voltage of several 1000V with steep edges can occur depending on the switching moment.

Due to bouncing contacts of the contactor with electric arcs this can happen several times. This leads to increased contact erosion(wear) and to high EMC interferences for other devices.

 

 

Problematic situation
If a mains-contactor is now additionally connected in front of or behind of an electronic switching hybrid motor starter the switch-off of the mains contactor while the motor is running and the motor starter is not switched off will generate a self-induction voltage as described before, which can reach several 1000V with a very steep voltage rise depending on the switching time.

 

The hybrid motor starter is protected against overvoltage by internal varistors. If the voltage peaks are significantly higher and/or these occur more frequently in succession, e. g. due to contactor bounce, the varistors are overloaded and can no longer limit the overvoltage. The electronic switching elements of the motor starter(Triacs) are no longer protected and this can lead finally to a short circuit of the Triacs. The motor starter is “defect”.

 

 

Technical recommendation
The voltage peaks can be avoided by:

 

  • The switching sequence of the mechanical switching device (mains contactor) and the hybrid motor starter must be realized with a time delay (default value >100 ms). The motor must be switched off by the hybrid motor starter first and then disconnected from the mains by the mains contactor. This increases also the life-cycle of the contactor, because it switches load-free.

 

  • The very high voltage transients and steep voltage peaks in the power circuit must be reduced by adding an RC-element to the three motor phases.

 

Following RC elements are suitable: 3RK1911-6EA00, 3RK1911-6EB00 or RC elements for direct connection to the contactor (3RT2916-1P*)



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