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True AC, Fake AC and Super-Fake AC Solenoid Coils

This post covers the three variations of AC solenoid coils that you will (most likely) encounter in your hydraulic practice: True AC, Fake AC, and Super-Fake AC coils.

I'll discuss some of the practical points that hydraulic technicians should be aware of, and I'll also do a little bit of experimenting and expose a doubt I have no answer to, in hopes that someday an educated reader with an engineering background can provide an explanation for the phenomenon that I encountered in my tests.

Let us begin with the True AC coils - i.e. solenoid coils that are designed to work with alternating current in a given voltage and frequency range:

Such coils have relatively low DC resistance (for their target voltage). Here are the figures for the coils in the image:

• Vickers "D" (220V/50Hz): ~135 ohm
• Parker S1 (220V/50Hz): ~106 ohm
• ZB09 (24V/50-60Hz): ~10 ohm
• ZH14 (115V/50-60Hz): ~157 ohm

Their proper operation (at rated voltage and frequency) relies entirely on their inductive reactance, which ensures that the total impedance limits the current to a safe value. Since these coils are exactly that - coils of wound copper wire - their integrity is easy to check with a digital multimeter in resistance mode, especially if you have a reference figure.

Back in the "old days" (think 70s and 80s), when control systems were based on massive relay logic panels, such coils were the industry standard. Today, however, modern installations rely almost exclusively on 24VDC power. Why the shift? All the reasons boil down to one simple truth: AC coils come bundled with "frustrating pain points that DC coils just don't have." Let me list just a few:

• Inrush Current: AC coils draw massive inrush currents (5 - 10x holding current) until the plunger reaches its designed end of stroke - usually the spot where it hits the pole piece, closing the... I want to say air gap, but I guess in a wet armature it should be called an oil gap. This is distinct from DC coils and is something you typically need to consider when you size the switching and power-providing elements, especially if you plan to energize multiple valves simultaneously.
• Burnouts: I just said that safe operation of these coils relies on their inductive reactance, which, in its turn depends a great deal on what exactly is inside the coil. When it's not what it should be - the reactance, and, consequently, the impedance, drops, the current shoots through the roof, and the coil burns out. The plunger does not go to the end of travel because of a stuck spool - it burns. A tech does not seat the coil all the way - it burns. A tech removes a connected coil from the core "for a quick test" - it burns!
• The Buzz (and the Shading Ring): AC coils require special cores with an integrated shading ring due to their inherent zero-crossing nature. If you somehow still manage to find an AC coil, I urge you to mount it on a normal DC valve core to see what happens - just for the heck of it! What you will hear is essentially a very loud buzzer that sounds like a tattoo gun. A shading ring (a.k.a. shading coil) is the copper ring embedded in the pole face to stop the buzz by providing a secondary magnetic field that is phase-shifted in relation to the main one. In other words, it's another specialized feature that AC solenoid valves require, and DC ones don't.
• The "Hammer" Effect: AC solenoids are snappy. Too snappy. They hit so hard that they often require substantial cushioning to prevent mechanical damage. If, for some unknown reason, you are converting a DC valve to a true AC valve, make sure you double-check the parts manual, and confirm if anything else aside from the AC core (like a dampener) should be replaced or added.
• The Power Factor Headache: AC coils, like all inductors, suffer from the "power factor condition". It's actually pretty bad. Anything with a power factor below 1 is bad, but with AC coils, it's notoriously terrible. If you look, for example, at the data presented in this Solenoid Operated Directional Valves catalog from Vickers, you will see that the full-power AC coils for DG4V-3 have a declared holding VA of 60. (VA for volt-ampere - the unit of measurement for apparent power in AC circuits.) Now, they never specify the power factor for their AC coils, however, according to the same catalog, a standard full-power DC coil (which is relatively equal in size) consumes 30W (W = True Power). Since the True Power (W) is what the coil actually heats up with, we can estimate the power factor: PF = 30W/60VA = 0.5 (which is pretty poor). In other words, even for the lower holding current, the lines, relays, and power supply elements must be over-sized to account for that apparent power - a headache you never have to worry about with DC coils.

Let me show you something cool now. After a (very) extensive search, I managed to source these little babies right here:

These 220VAC NG06 valves come from a very old and dusty project box (I'm told it's at least from the 90s) that was found in the "dark corners" of our warehouse. The valves had definitely been tinkered with. Both are coded as 220VAC@50Hz-capable, but one of them has a DC core tube (the longer one), which - as we know - can't work with AC coils. When I removed the cores, I saw that the one with the shorter AC core had a standard cross-shaped spool washer, while the parts list for this series states that an AC actuator should use a full-bodied round washer (my guess is for increased cushioning). Strangely, the one with the longer DC core did have an AC washer in it... All of this tells me that these have been serving as "part donors." But since the one with the AC core still works, and the coils are True AC (220V@50Hz), I'll use it to demonstrate the said power factor with the help of an oscilloscope - purely for educational purposes.

Let me show you the test rig. The 12V transformer from this old table lamp will provide the voltage waveform (I found it in the garage, and if I'm not mistaken, my wife told me to get rid of it some fifteen years ago!):

This random clamp-on current transformer connected to a 220-ohm resistor will provide the current waveform:

Finally, I prepared a purely resistive load, in the form of a 60W incandescent lamp, and also a wire with a simple DIN plug and another wire with a DIN plug with a full-wave rectifier:

Before anything else, let's test this arrangement with the incandescent light bulb, which is a purely resistive load with a Power Factor of 1. (The yellow line is the voltage, and the blue line is the current):

The system is definitely noisy, but you can clearly see that the current and voltage waveforms align perfectly, peaking and crossing zero at the exact same moments. But when we connect the solenoid coil, the picture is very different:

Now you can see that the current waveform is lagging some 3.2 - 3.3 milliseconds behind the voltage. Given the period of 20 ms (for 50 Hz), the lag is almost 60º, which confirms our Power Factor estimation: PF = cos(60º) = 0.5. Almost exactly our "guesstimate!" Although I am not sure if this particular valve is operating properly, because its current waveform does have a slightly higher amplitude than the 60W bulb did. According to the 60VA figure stated in the catalog, it should have been the same. In any case, the current lag clearly seen here is a great illustration of the "power factor condition."

Now let's power this 220V RAC coil (Parker S1-205000) with the rectifying plug and check out the waveforms:

As you can see, the coil inductance "irons out" the current waveform. With it being essentially a DC coil, there's virtually no current lag and also no "zero current" zones, which means that this coil is perfectly stable and "hum-free", and it does not require any "shading coil hassles."

There's one last thing I want to say about the True AC coils: like any other coil of wire, they will, mostly, work with DC, as long as you provide enough voltage to reach the nominal current (think: target ampere-turns). Usually, this voltage will be about a half of the nominal RMS voltage. Occasionally, you may even come about so called "dual voltage coils" (e.g., 48 VAC/24 VDC), often in smaller form factors. But to summarize - True AC coils are to be avoided in all but the simplest and the cheapest installations. I never used them in any of my designs, and I don't think that I ever will.

Now let us move on to the Fake AC coils, like this little guy right here:

I call them "Fake AC" because, while they are marked as AC coils, they are actually DC coils, or, even more correctly (as we discussed last week), RAC coils, because they have an integrated full-wave rectifier. This automatically removes all of the true AC headaches from them, but gives them a catch that you should know of: checking their integrity with a DMM in the resistance mode will often show an abnormally high resistance, which can make a tech think that they are damaged. This happens because the voltage that a DMM uses to probe resistances is not enough to open the diodes of the integrated rectifier:

The coil measures 7 Megaohms - and if you think it's a normal coil, you may think that it's busted, but I assure you that it's not. Now let us check it again in the diode mode:

What we are reading is the 1.4 volt drop of the two diodes connected in series. If you check both polarities, you can (at least) confirm if the rectifying bridge is OK or not, but you can't directly check or verify the resistance of such a coil with a multimeter - that is - without an external power source - and this is definitely something you should understand.

Finally, let us move on to the last type - the Super-Fake AC coils:

I've encountered these on cheap aftermarket NG10 valves, made in China. They are essentially coils with a built-in rectifier, but... there's an even bigger catch! Just like the Fake AC coils that we just saw, these ones can fool a tech by reading an abnormally high resistance when measured in the resistance mode:

But then, if you check them in the diode mode, you see something... peculiar:

Yes, my friends, everything indicates that these coils, instead of a full-wave rectifying diode bridge, have a single series-connected diode! (That is, a half-wave rectifier.) I actually tried this coil with the rectifying plug just to see if I was seeing this right, and indeed - it only works when the voltage is applied in the correct direction. This is crazy. Definitely the cheap solution nobody asked for! Also, when you connect this coil to the mains and insert something metallic in the core, you can actually feel the half-wave pull vibration. I don't like this solution in the slightest, and I am surprised something like this exists. But it does, and so you should be aware of it, too.

But now, here's the strangest thing. When I connect this coil with a rectifying plug, I get the expected ironed-out constant current waveform, similar to the one I saw with the Parker's RAC coil:

It heats up like crazy, of course, because it consumes double the rated power, but the current waveform makes sense. And now - here's the crazy waveform when it's connected to the mains with a normal plug (relying only on its internal, single diode):

I can see how the evened-out current goes through the coil in the first half wave, but then, when the voltage reverses and the series-connected diode supposedly closes, the current apparently flows back into the mains for about 2 ms. Inductive kickback during the reverse recovery time of the diode, maybe? I can buy that. But then - there's the current wave that lasts for another 8 ms - and I can't see this happening with the closed (not conducting) diode. It may be the diode's parasitic capacitance in play? The coil's parasitic capacitance? It is supposed to be negligible, should it not? I am not sure what is going on here, to be honest, but in any case, this means that this coil design is... let us be honest - pretty crappy.

I am hoping that some day an electrical engineer can explain what's going on here.

I suppose the main takeaway of today's post is - Go DC of Go Home! For industrial applications, one hundred percent - ditch AC coils and stick with the good stuff. That "good stuff" is pure DC (24 volts, if possible, or 12V for mobile, when needed). Make sure you do that, and you will be "golden."