Electromagnetic clutch? How to get a slow engage?

[Logic]

I am looking for/at electromagnetic clutches with an eye on engaging the drive to the alternator and aircon on deceleration only.
(with an override for when the battery is too flat)

The clutches on car aircons and ride on mowers all seem to take instantaneously?
How could one make them 'take' more gradually so as not to shock load the belt etc?

Maybe a capacitor in series with the live?
But think it's a little more involved than that?