Hello,

I wonder how AC type SOV and relay can produce their uni-directional force in varying electro-magnetic field. Can anyone tell me the theory of them.

 

The plunger in AC actuated devices is soft iron
therefore an alternating magnetic field generated by the coil will attract that piece of iron, like in transformer. On big size devices like big Hydro tranformer even tiny ones the iron core is made of sheets electricaly insulated.
Why? This reduces the Foulcault current which opposes the magnetic field and reduces effectiveness.

Now, that the magnetic field changes polarity
(North/South) at frequency of the AC supply, has no influence on the attraction created except.
The core is always attracted in the center because the magnetic field wants to go through it
absolutely and within the shortest path because soft iron is better conductor to magnetism than
air or else.

Again, the core is constantly driven towards that
direction, but either by the hairs or by the foot.

 

Both AC and normal DC relays and solenoid valves operate on the same principle. They are not "attracting" a permanent magnet, rather they use magnetic reluctance so the polarity of the electromagnetic field doesn't matter. This is similar to the operation of a reluctance motor.

When the coil is energised, the armature presents a lower reluctance path than the air does, so the flux flows through the armature. The armature is then pulled into position as the flux attempts to create a shorter path for itself (the preceeding makes more sense if you look at the actual construction of these devices). A relay has a different mechanical design than a solenoid valve, but the operating principle is the same in each case.

AC coils (valves or relays) are not normally interchangable with DC coils of the same voltage because the inductive reactance of an AC system adds to the total impendance which must be allowed for by lowering the resistance. This is why a 24VAC relay may burn out in a 24VDC circuit.
Also, the AC relay may include a design feature called a "shading coil" to keep it from dropping out or chattering every half cycle as the
current (and therefore the flux) drops to zero. A shaded pole motor also uses a shading coil but for a different purpose.
A shading coil is generally a single turn of conductor (often stamped from a sheet) about the coil which is inductively coupled to it. As
the magnetic field collapses each half cycle, it induces a current in the shading coil, which in turn generates a magnetic field which tends to oppose the collapse of the main field. This combines with the inertia of the armature to prevent it from dropping out before the field generated from the next half cycle can take effect.

Certain latching relays do use permanent magnets to hold themselves in when the power is off. However, the normal operating method is as
outlined above.


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Michael Griffin
London, Ont. Canada
[email protected]
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The shading 'coil' Mike is talking about is called the "Frager ring".
Been a short circuit, it generates a 90 deg out of phase magnetic field and so on...

 

Been a short circuit, it generates a 90 deg out of phase magnetic field and so on...