We use contactors rated at up to 1000A that use the same principal.
(i.e. separate pull-in & hold-in windings). There can be problems with
a PWM drive on this type as during pull-in both windings are
energised. This causes transformer action on the AC component of the
PWM with a "shorted turn" effect on the hold-in winding. The overall
result is a high pull-in current which has to be accomodated as well
as protected against in the case somthing jams.

We have to use PWM as we need to use the same contactors over the
range 19-65Volts.

Otherwise they are very effective units.
Richard P

On 16/07/05, Tony Smith <ajsmith@rivernet.com.au> wrote:
> >
> >   But expensive and huge as dimension. There is a much simpler methode
> > which was first used in the 80' on magnetic tape write-readers
> > (commercial stereos).
> > The problem with the electromagnets is just initialing the current.
> > Then required only a very small current to keep them connected.
> > So, the electromagnet has two coils, one for ignition and another for sustain.
> > A built in switch change the coil after turning on. Simple, without
> > capacitors and using the same supply voltage.
> >
> > best wishes,
> > Vasile
> 
> 
> I've got some of those 3-wire solenoids from old hard drives.  They were used to lock the stepper motor that moved the heads into
> place.  I've since been informed that pinball machines use them on the flippers.
> 
> These days you just use PWM to get the holding current.
> 
> Tony
> 
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