AC vs DC relay coil

[Steve & Julie]

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.

 

[daestrom]

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!

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

 

[daestrom]

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.

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

 

[[email protected]]

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.

 

[Bud]

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.