AC vs DC relay coil

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. :-)

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

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

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.