b)Contactors with V.F.D.(line)

QUESTION:
Which application criteria shall be considered for use of contactors on line side of variable speed drives?

ANSWER:
Application criteria for use of contactors on line side of variable speed drives:

Variable speed drives incorporate DC voltage link capacitors which cause high making current peaks unless there are suppression measures pre chokes or pre charge resistors provided inside the frequency converter.

If those peaks are higher than the contactor’s making current capacity then even previously closed main contacts will lift off and chatter.
Resulting switching arc melts the contact material and in consequence the contacts will weld together when re-closing.

Since level of occurring current peaks at the converter’s place of installation mainly depends on the prospective short circuit current.


For project planning following options are available:

- under awareness of current peak value:

Select contactor according to its making capacity (10 x Ie AC-3).This making capacity shall not be lower than the current peaks.


- without awareness of current peak value:
Select a capacitor contactor .Capacitor contactors incorporate pre charge resistors. Therewith making current peaks are damped to uncritical values.

Following capacitor contactors Siemens make contactors are recommended for usage on the line side of variable speed drives:

- 3RT1617: up to 5,5 kW / 400 V
- 3RT1627: up to 7,5 kW 11 kW / 400 V
- 3RT1647: up to 22 kW 45 kW / 400 V


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