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Entry type: FAQ Entry ID: 1017401, Entry date: 05/08/2014
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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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It is forbidden to isolate the device from the supply on the motor side; isolation from the supply must always be carried out on the supply side of the inverter.

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

No, in some cases this could damage the converter or the motor. For example, if:

  • You open the switch during operation, self induction might cause overcurrent or overvoltage.
  • You close the switch during operation, overcurrent might be caused depending on the state of the motor.

For the reasons mentioned we recommend generally not to use the switches whilst the converter is in operation.

Note the following for the occasional use of safety switches:
The "EMERGENCY STOP" function can be destructive according to VDE. You can only guarantee non-destructive handling of the "EMERGENCY STOP" when isolating the converter from the motor if you keep to the following order of actions:

  1. Activate "OFF2"
  2. Approx. 100ms open the motor contactor (opening the contactor when live can lead to destruction of the power circuit due to return induction).
  3. You must avoid closing the motor contact again as long as the motor is turning. (Remanent voltage from the motor could also lead to destruction of the power circuit). It is thus also forbidden to use a flying restart circuit at this point.
  4. To switch the converter back on again, you must first remove the "ON" and "OFF2" commands before you can resubmit an "ON" command (switch-on inhibition).

For maintenance switches with leading contacts you can also use the "OFF2" = inverter pulse inhibition (freely parameterizable). The converter output voltage-free within 100ms. Generally, you should note here that all relays and contacts with RC elements must be switched, or the converter might be damaged.

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