c Contactors with V F D load

Contactors with V.F.D.(load)

Q) Which application criteria shall be considered for use of contactors on load side of variable speed drives?

Ans) For load side of variable speed drives it is appropriate to select contactors for AC-3 utilization category according to rated motor power. Since voltage and frequency of VSD are virtually always directly proportional, load breaking is not critical for the contactor even at low frequencies.

Example:

Referring to a network 400 V 50 Hz, at only 5 Hz a voltage value of 80 V would apply. This value for breaking operation is easily handled by an AC-3 sized contactor.

In theory it may happen that in a failure case DC current flows under full net voltage. In such case the HRC fuses which protect the VSD would blow and disengage the circuit.

ATTENTION!
Vacuum contactors 3RT12 and 3TF6 are not recommended for switching DC and very low frequencies below 16.7 Hz.


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