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# AC relay theory

Steve
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Posts: n/a

 08-17-2007, 03:01 PM
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.

Straw Man
Guest
Posts: n/a

 08-17-2007, 04:13 PM
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.

Dave Plowman (News)
Guest
Posts: n/a

 08-17-2007, 05:30 PM
In article <(E-Mail Removed) .com>,
Steve <(E-Mail Removed)> 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.

AC coil relays are common. A magnet of any type will attract a magnetic
substance regardless of polarity - it's just if the second substance is -
or gets - magnetized that they can repel. So you use a low permeability
substance for the armature. Something like soft iron.

--
*Gaffer tape - The Force, light and dark sides - holds the universe together*

Dave Plowman (E-Mail Removed) London SW
To e-mail, change noise into sound.

Derek Geldard
Guest
Posts: n/a

 08-17-2007, 09:23 PM
On Fri, 17 Aug 2007 12:13:25 -0400, Straw Man <(E-Mail Removed)> 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

DaveM
Guest
Posts: n/a

 08-17-2007, 11:10 PM
"Dave Plowman (News)" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> In article <(E-Mail Removed) .com>,
> Steve <(E-Mail Removed)> 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.

>
> AC coil relays are common. A magnet of any type will attract a magnetic
> substance regardless of polarity - it's just if the second substance is -
> or gets - magnetized that they can repel. So you use a low permeability
> substance for the armature. Something like soft iron.
>
> --
> *Gaffer tape - The Force, light and dark sides - holds the universe together*
>
> Dave Plowman (E-Mail Removed) London SW
> To e-mail, change noise into sound.

I haven't seen anybody describe the real difference between AC and DC relays.
A relay has a coil and the pole piece, or armature. Voltage applied across the
coil causes current to flow in the coil, creating a magnetic field, which causes
the armature to be pulled into the center of the coil, thus energizing the
relay's contacts. Both types of relays operate on the same principle of
electromagnetism.
The difference between the DC and AC relay is that the AC relay has a shading
pole, or a heavy shorted turn imbedded into one end of the armature. Its
purpose is to maintain a high flux level in the armature when the current in the
main coil goes through zero. This acts to eliminate buzzing or chattering that
is evident when you drive a DC relay with an AC voltage.
There is no diode in an AC relay. A diode is commonly used across the coil of a
DC relay to eliminate the high reverse EMF caused by the collapse of the coil's
magnetic field. If a diode were used on an AC relay, it would create a short
circuit every half cycle, something you want to avoid.

An AC relay can be used in a DC circuit, but not vice versa. If you drive a DC
relay from an AC source, the lack of a shading pole in the relay will cause
buzzing, possibly allowing the contacts to bounce during zero crossings of the
power source.
If you drive an AC relay from a DC power source, the shading pole on the AC
relay will cause the relay to be slow to release when power is removed.

Cheers!!!
--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in the
address)

"In theory, there isn't any difference between theory and practice. In
practice, there is." - Yogi Berra

Sam Goldwasser
Guest
Posts: n/a

 08-17-2007, 11:58 PM
Straw Man <(E-Mail Removed)> writes:

> 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.

You will usually find a copper "shading ring" wrapped around the end of the
pole piece. The current induced in the shading ring delays the decay of the
magnetic field long enough to smooth it out between cycles. There is no
diode or smoothing cap!

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Ross Herbert
Guest
Posts: n/a

 08-18-2007, 03:16 AM
On 17 Aug 2007 19:58:41 -0400, Sam Goldwasser
<(E-Mail Removed)> wrote:

>
>You will usually find a copper "shading ring" wrapped around the end

of the
>pole piece. The current induced in the shading ring delays the decay

of the
>magnetic field long enough to smooth it out between cycles. There is

no
>diode or smoothing cap!
>

In the telephony industry these are known as "slugged" relays. Such
relays have a solid copper slug of a specific length - eg. 1/2" or 1"
depending on delay period required - the same diameter as the coil
itself. The slug could either be at the armature end or the heel end
of the coil depending upon whether a predominantly slow operate or
slow release was required. For ac operation it hardly matters which
end is slugged and if pushed for a part you could use a relay with a
spare winding on it and simply short circuit this winding to produce a
"slugging" effect.

James Sweet
Guest
Posts: n/a

 08-18-2007, 05:25 AM

"Steve" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) oups.com...
> 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.
>

They have a shorting bar like a shaded pole motor.

Peter Dettmann
Guest
Posts: n/a

 08-19-2007, 04:39 AM
On Sat, 18 Aug 2007 03:16:00 GMT, Ross Herbert
<(E-Mail Removed)> wrote:

>On 17 Aug 2007 19:58:41 -0400, Sam Goldwasser
><(E-Mail Removed)> wrote:
>
>>
>>You will usually find a copper "shading ring" wrapped around the end

>of the
>>pole piece. The current induced in the shading ring delays the decay

>of the
>>magnetic field long enough to smooth it out between cycles. There is

>no
>>diode or smoothing cap!
>>

>
>In the telephony industry these are known as "slugged" relays. Such
>relays have a solid copper slug of a specific length - eg. 1/2" or 1"
>depending on delay period required - the same diameter as the coil
>itself. The slug could either be at the armature end or the heel end
>of the coil depending upon whether a predominantly slow operate or
>slow release was required. For ac operation it hardly matters which
>end is slugged and if pushed for a part you could use a relay with a
>spare winding on it and simply short circuit this winding to produce a
>"slugging" effect.

No there is a difference here Ross, in the AC relay, the "slug" does
not cover the whole of the magnetic iron path, it is typically only
applied to about a quarter of the iron circuit. The process is to
delay the decay of flux in that slugged path so that there is a useful
magnetic pull during the time that the un-slugged path has zero flux,
(and therefore zero magnetic pull). Using the DC relay slug is not
really useful for the AC case as it covers the whole magnetic path.

Peter Dettmann

Peter Dettmann
Guest
Posts: n/a

 08-19-2007, 06:27 AM
On Sun, 19 Aug 2007 14:39:50 +1000, Peter Dettmann
<(E-Mail Removed)> wrote:

>>In the telephony industry these are known as "slugged" relays. Such
>>relays have a solid copper slug of a specific length - eg. 1/2" or 1"
>>depending on delay period required - the same diameter as the coil
>>itself. The slug could either be at the armature end or the heel end
>>of the coil depending upon whether a predominantly slow operate or
>>slow release was required. For ac operation it hardly matters which
>>end is slugged and if pushed for a part you could use a relay with a
>>spare winding on it and simply short circuit this winding to produce a
>>"slugging" effect.

>
>No there is a difference here Ross, in the AC relay, the "slug" does
>not cover the whole of the magnetic iron path, it is typically only
>applied to about a quarter of the iron circuit. The process is to
>delay the decay of flux in that slugged path so that there is a useful
>magnetic pull during the time that the un-slugged path has zero flux,
>(and therefore zero magnetic pull). Using the DC relay slug is not
>really useful for the AC case as it covers the whole magnetic path.
>

For completeness I should have added that we did extensively use
relays on AC fed from a full wave bridge rectifier, and without
capacitor for smoothing. This gave a tendency to chattering as there
is still a pulsating current to the relay, however this chattering was
overcome by the use of an armature end slug (as you describe) which
was only about 1/16" long.
A longer slug could be used, but fast operating speed was critical.

Peter Dettmann

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