# AC vs DC relay coil

Hi,
I would like to know what is the internal difference between AC and DC relay? Why the AC coil works on DC power and not DC coil on AC power?
Sorry for my english, I'm french!
Steve A.
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Steve & Julie wrote:

A relay coil made to work on AC has the magnetic core split, and a copper "shading coil" of one turn placed on one side. This causes the magnetic field to hold during the reversals of the AC current. This is not needed for DC, but does not affect the relay's operation.

I would respond in french, but I've forgotten most of my high school french. :-)
--
Virg Wall, P.E.

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With an AC source, the 'pull' pulses 120 times a second (100 times a second on 50hz). This uneven magnetic pull causes the relay to chatter/buzz. AC coils deal with this by putting a short-circuited shading coil around 1/2 of the pole face. The eddy currents induced in this coil build up a magnetic field. Because of the inductance, the current in this coil is out of phase (time shifted) from the main current. So although the magnetic field in this section of the pole face alternates at 120 hz (or 100hz), it crosses zero at a different time than the rest of the pole face's magnetic field. Result is a steadier 'pull' and no chatter.
Keep in mind that many AC coils can *not* be used on DC of the same voltage. AC coils often rely on the inductive reactance of the coil to limit current. Connect them to DC of the same voltage and the current will be excessive, and the coil can burn up.
daestrom
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All that matters is amp turns, there was an interesting article in one of the "free" engineering magazines a few years ago about converting AC coils to DC. It involved adding resistance in series with the AC coil to make it operable on DC. Handy for cases when the proper DC coil is not commecially available.
On Fri, 04 Mar 2005 14:55:26 GMT, "daestrom"

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And that was my point. A coil designed for 120VAC will draw too much current if connected directly to 120VDC. You must either use a lower voltage, or install a series resistor. Just hooking 120VAC coil to 120VDC and it will 'pick up' just fine, but then it may burn out from excessive current.
Using a coil rated for 120VDC in an AC application and two things can go wrong. The reactance will be such that it won't be able to stay 'picked up', or if it does pick up, it may chatter and buzz excessively because it doesn't have a shading coil.
daestrom
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Using an AC coil on DC can have another problem. It may not drop out. You need a brass or other non-magnetic "residual" between the core and the armature so it won't seal.
On Sat, 05 Mar 2005 20:56:27 GMT, "daestrom"

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Another problem of AC relay on DC - the solid iron core on a DC relay will act as a shorted turn when the coil is fed AC. This will result in overheating. An AC relay core is laminated - made of thin layers of iron to reduce these eddy currents. The main problem is, as previously stated, an AC relay has a significant inductance in addition to the wire resistance to limit the current; when fed the same DC voltage the current will be much too high. DC relays would have a higher resistance.
snipped-for-privacy@aol.com wrote:

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Why would one bother to not use a DC coil relay to replace the one thats there? Hey you smart guys!....Please dont "convert any coils" to run on ac from dc and such. In real world applications this can make for much machine down time, while some idiot like me grabs the relay, reads the label and puts another one in its place...I may not see the clever addition of a non metalic reidual doo dad or a resistor, especially if inside the case of the relay. Thank you all, however for the vast insight I have gained by reading this group.
-
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Not all coil types are available in all voltages, sometimes it can't be helped. To your point, off the shelf is far preferred to custom any day.
On Sun, 13 Mar 2005 05:38:54 GMT, snipped-for-privacy@adelphia.net wrote: