Once in a while at work (paper mill) I come across a situation where someone has inadvertently replaced a device with a dc coil (relay, contactor, solenoid, etc) with an ac-coil device or vice versa. Nothing bad happens except the machine just does not run, usually easy to find and repair. I started wondering though, what the difference is between the two types of coils, after all both are just coils of wire around a core. I know about the shading ring on (some) ac contactors which keep the coil held in during the zero-crossing of the voltage but that’s about it; I took a few old ones apart and don’t see what the difference is. Can anyone enlighten me?
I can’t find a good reference to explain this, so this might not be the whole picture, but since nobody else is trying…
The current drawn by a coil in an AC circuit is limited by its impedance, while its DC resistance can be relatively low. The current drawn by a coil in a DC circuit is limited only briefly by its impedance, so its DC resistance might need to be relatively high.
So I would expect an AC-designed coil in a DC circuit to draw too much current and possibly burn out. A DC-designed coil in an AC circuit might not draw enough current to actuate.
The physical difference in design could be something as subtle as the permeability of the electromagnet core material, or the gauge and number of windings. All the same components are there in either design, so it makes sense that you can’t see a difference.
There’s another possible difference in that the AC relay’s switching current actually switches on and off rapidly. I’ve heard a DC relay in the same circuit might draw enough current to actuate, but its contacts might buzz and wear out rapidly. So the two designs might have differences in details like spring tension and actuator weight, as well as coil design.
All that said, I think it’s possible that some AC relays might work in some DC circuits, and vice-versa. But in those cases you wouldn’t be called in, so you wouldn’t necessarily know the error occurred.
DC powered relays often have a diode in series with the coil. This supresses voltage transients when the coil is deenergized. It likely also halves the peak to peak voltage of any AC coming into the coil.
Don’t you mean in parallel? Oriented opposite the circuit polarity?
Not that that’s any better, when hooked up to an AC circuit. In fact, I fancy it’d be worse!
Imagine a path around the outer surface of the core that goes all the way around it just under the wires. That path is clearly the same thing as a wire wrapped around it and shorted together, which would be a short circuit when the coil is operated with AC. For this reason the core in an AC solenoid is generally made with thin laminations that are electrically insulated from one another (though the insulation may be as little as the oxidation on the surface as it came from the mill). There are no longer conductive paths that look like a shorted winding. DC cores don’t have to deal with this and the much cheaper solid core can be used here. This would make DC devices used in AC overheat. A laminated AC core can be used in DC just fine, though it’s excessively expensive to do so.
I don’t get it. The coil’s windings are electrically isolated from the core anyway, and the field in the core is magnetic, not electric. Where does this “short circuit” come from, and where does it go? Why doesn’t an RF coil wound on a solid ferrite core short out?
Ferrite isn’t electrically conductive–it’s a ceramic with a high magnetic permeability (a characteristic called mu). Silicon steel (a common alloy used in power transformers and solenoid/relay cores), on the other hand, is both electrically conductive and high mu. There are still currents induced in it by the B field produced by the windings, and those currents can be large in a solid steel core. Laminations electrically insulated from one another reduce the magnitude of these circulating currents; higher frequencies require thinner lams–this is why RF cores are generally ferrites, since lams at these frequencies would need to be impractically thin.
Appreciate all the great answers. This especially, clears up a lot for me. Thanks.
OK, so, is the inefficiency caused by eddy currents the whole picture? There’s no difference in the winding of an AC coil versus a DC coil, even though the AC current is limited by reactive impedance and the DC current is limited only by resistance (after the magnetic field charges)?
I thought hard to work out my explanation. Was I entirely off base? Or does it just so happen that there’s not much difference between AC and DC current through a given inductor? (I guess 60 Hz is practically DC, in some circles.)
I think your explanation was very good, when I started thinking about the difference between the two types of coils I hadn’t given much thought to the permeability of the core or anything like that. I knew I was overlooking something but couldn’t figure out what. I responded to Q.E.D.'s post mainly because it cleared up some stuff about ferrite cores that I didn’t understand at all (and was wondering about).
As a bit of background:
Part of the reason this came up is a new tech is going to be hired soon. The training for the new guys is a group effort, they train with the day shift for a while and then follow the guys on night shift (me) for a while. Everyone else does a good job of covering safety, equipment, etc. I like to cover basic theory and troubleshooting so I plan to make up a test board with relays, etc to practice with. I plan to have a relay fed from 24Vdc but substitute a 24Vac relay and let the trainee figure it and a few other things out.
>I don’t get it. The coil’s windings are electrically isolated from the core anyway, and the field in the core is magnetic, not electric. Where does this “short circuit” come from, and where does it go?
The coil’s windings have no direct electrical connection to the core, but they are inductively coupled, because of the relationship between electricity and magnetism. A conductor in a varying magnetic field will have voltages and currents induced in it according to the variation in the magnetic field and according to whatever else is connected to that conductor.
In a transformer, there are typically two coils that are electrically isolated but coupled by the magnetic field. Most of the magnetic flux circulates around through the core and through the space both coils are wound around. This means changing current in one coil will cause changing currents and voltages in the other coil.
The point I was making about cores and AC was that the surface of the core also functions as a winding. That is, even though it isn’t made of insulated copper wire, it is still a path around most of the core, and most of the changing magnetic flux passes through the space circumscribed by the surface of the core. So, electrically, it is also a winding, whether you wanted it to be or not.
And, typically, you don’t want it to be. A shorted winding has the effect of trying to prevent the magnetic flux passing through its circumscribed area from changing, by having a big induced current flowing around inside it and generating nothing but heat.
So, the strategy typically is to design the core in some way that acknowledges that there are paths through it that encircle most of the magnetic flux, and breaking those paths with insulation of some sort. By far the commonest method is using laminations, which have poor electrical connectivity with each other. This problem gets worse and worse with higher frequency, so heavy laminations work OK for 20 Hz power and thinner laminations for 60 Hz. Audio transformers (I believe) use much thinner laminations, at higher cost. Then the powdered iron and ferrites come into play.