effect of cable capacitance on contactors

No. Look elsewhere for the problem.

No check your control circuit for bad wiring or defective PB

No, not capacitance.

Look for:

mechanical binding, including a weak spring if you use springs

magnetization of the core/frame

welding pits in the contacts

a malfunction in the control

I would have to agree with TVP45. It appears probable that you are welding your contacts together. This can at the beginning cause the contacts to "stick" for up to several seconds before dropping out. eventually the contacts will most likely weld together permanently. If it is a NEMA style contactor, pull it apart and check the contacts. If it one of the cute new mini contactors made of cheap plastic look for a lot of arcing when it closes. Could also be a mechanical bind, but in my experience it is usually contactors welding.

The capacitance effect of cables does cause problems of contactors not dropping off. This mostly happens with control cable lengths of above 500mtrs. Generally seen when the STOP command is given in the field from a local control station near the motor and the contactor (feeding switchboard) is loacted in a far away loacted substation.

Assuming something like 33pF/m for the cable, that gives about 50nF. Given a contactor coil resistance of, say, 1kΩ, the time constant comes out to 50μs.

Don't see how you could possibly store enough energy is the cables to hold a contactor in.

As the cable capacitance increases with length, a limit is placed on the distance at which the switching device can be located to avoid latching of the contactor
even after Stop / Open command. The limiting distance is impacted by control voltage. Lower the voltage, higher the permissible length of the control cable.

The limiting value of cable length can be calculated as follows, which in turn
indicates the maximum limit on the length of control cable in metres.

L < (BM/(2pi(f)(r)(squareof U)))x (P/(P+1))x10 raised to power 9

where BM is hold on VA

f is frequency

r is capacitance of cable in microfaraday per km

U is rated control circuit voltage

P is U/drop of voltage

As can be seen from the formula, the limit on length of cable can be increased
by using lower control voltage. However, the selection of lower voltage level should
be selected such that it doesn't affect the pick up operation of contactor.

thanks a lot, it really helped!

Refer to page 327 of ABB Switchgear Manual (10^th revised edition)

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Creative thinking is enjoyable, Then think about your surrounding things and other thought products.

It's not in my library - is it available on line? If so, please give us a link.

If the question is written correctly, cable capacitance plays no part. The OP says the supply is turned off at the control end. Off means off. If that is the case, the capacitance will hold the contactor only a few msec at best.

However, cable capacitance may prevent the controller from actually turning off. That is a different matter and a different question.