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Entry type: FAQ Entry ID: 1017401, Entry date: 07/08/2020
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MICROMASTER, SINAMICS G110/G120 (CU2x0x)/G120C/V20: Using a contactor / MCCB / isolating switch on the inverter output.

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The use of a switching devices between the inverter output and motor should be avoided, if possible. In some circumstance’s operation of a switching device between the inverter output and motor could damage the inverter or the motor, respectively, or unintended faults (trips) could occur.

Question:
Can I use a contactor / MCCB / isolating switch on the inverter output?

Answer:
The use of a switching devices between the inverter output and motor should be avoided, if possible.

In some circumstance’s operation of a switching device between the inverter output and motor could damage the inverter or the motor, respectively, or unintended faults (trips) could occur.

For example, if…

  • …you or the upper control opens (doesn’t matter whether intended or unintended) the switching device during inverter operation or a trip occurs, then the motor inductance might cause overcurrent or overvoltage, and inverter damage

  • …you or the upper control (doesn’t matter whether intended or unintended) closes the switching device during inverter operation, then overcurrent might be caused depending on the state of the motor (e.g. spinning or stalled)

In addition, the integration of a switching device between inverter output and motor can compromise the EMC performance, even when using EMC shielding plates. The end-product/installation should be tested to confirm EMC performance requirements, e.g. radiated emissions, are met.

For advice on EMC compliant installation please refer to 60612658 

Therefore, we do not recommend the use or intended operation of switching devices between inverter output and motor during inverter operation.

If the use of a switching device between the inverter output and motor cannot be avoided, please note the following aspects for the safe operation of switching devices / maintenance switches between the inverter output and motor:

  1. activate "OFF2" or STO

    It is mandatory that switching (meaning opening of the main current carrying contacts of the switching device) is carried out at zero motor current. That means the inverter must be in the status “pulses disabled / pulses inhibited” (see the relevant bits in the status word). This can be achieved by either activating the “OFF2” function or the STO function.

  2. after approx. 100ms open the main contacts

    Particularly during active motor operation at the inverter output (meaning the inverter is applying voltage and current to the motor), the minimum delay time of approx. 100ms between ‘pulses disabled/inhibited’ and opening of the main contacts must be observed. ‘Pulse inhibit’ can be achieved through switching devices with leading auxiliary contacts wired to the STO input, or alternatively wired to a digital input with parameterized function of ‘external fault”. Opening the switching device when being live can damage the inverter’s power circuit or the switching device due to voltage/current induced by the motor.

  3. closing of the switching device must be avoided whilst the motor is spinning, or the inverter is operating

    A reflected voltage from the motor can also damage the inverter’s power circuit. Due to this it is not permitted to use the ‘flying restart’ function of the inverter. A high current can be induced by connecting the motor to an operating inverter, this can also damage the inverter’s power circuit or cause unintended trips, eg. over current.

  4. To switch on the inverter again after the switching device has already been closed, you must remove at first the "ON" and "OFF2" commands before a new “ON” command can be applied (switch-on inhibition).


According to VDE stipulations the "EMERGENCY STOP" function may be destructive. Non-destructive operation of the "EMERGENCY STOP" with isolation of the inverter from the motor can only be ensured when the above described sequence of actions will be observed.

The STO input of the inverter shall be used where possible: this STO input supports faster switching speeds than a standard digital input, the switch ON / switch OFF sequence with reference to ‘pulse inhibit’ (OFF2) and reapplied ON command.

Regarding maintenance switches with leading auxiliary contacts, as it is offered by the SENTRON manual motor disconnect switch 3LD2, a digital input should be parameterized with “external fault” if the inverter does not offer an STO input. This supports the switch ON / switch OFF sequence of the inverter with regard to pulse inhibit (OFF2) followed by a reapplied ON command.

When using a maintenance switch a warning plate “Do not operate under load!” must be placed close to the maintenance switch to inform the user.

Finally it is within the responsibility of the end user/plant operator to ensure the correct switch OFF conditions (no load, motor de-magnetising finished, etc.) and respectively ensure the correct switch ON condition (non-spinning motor, ON/OFF cycle, etc.). The provision of inverter functionalities like pulse inhibit, STO, “external fault” as well as the provision of the leading contact of the maintenance switch, the possibility to lock the maintenance switch in 0- and I-position, can only support the correct system integration.


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