On Fri, 17 Aug 2007 12:13:25 -0400, Straw Man wrote:

On Fri, 17 Aug 2007 08:01:28 -0700, Steve wrote:

How does an AC relay work? I understand that the magnetic field of a
DC relay coil attracts the contact arm and I assumed an AC relay just
had a diode to convert coil current to DC - however when I tried to
find an AC relay fault there was no diode. I thought 50 or 60 Hz
alternating magnetic field cannot produce such a corresponding
movement in a mechanical contact arm so I would have thought the net
magnetic effect would be zero (no overall attraction or repulsion).
Given the relay obviously operated before, I just can't see how. I
did think the 'AC' rating just meant the contacts but I don't see why
this wouldn't just be a current rating.

Enough latent magnetism in the core to somewhat negate the alternating
field? Same goes for an AC solenoid as in a doorbell plunger I suppose.

No.

An AC relay is just an electromagnet operating a set of contacts.

An electromagnet attracts ferromagnetic material during both halves of
the AC cycle. You could try it at home with a home wound electromagnet
and a battery, the electromagnet will attract iron no matter which way
round the battery is connected, or for that matter which way round the
coil is wound, (same difference !) .

The only difference is that the wound coil of a solenoid (esp. with an
iron core) forms an inductor which has an impedance which acts to
reduce the AC current through the coil for any particular AC voltage
and frequency, it does this without causing electrical energy to be
wasted (For the pedants I'm not saying it's perfect).

This figures significantly in the design of AC relays and
elecromagnets.

HTH

DG