What is that supposed to mean??
A shading ring should be in the same plane as the main coil.
In a relay for e.g. it is usually a single ring on top of the main coil.
Never at right angles to the main coil.

Relay e.g. (red arrow indicates it)
View attachment 311472

OK, Max, go back and look at the photo in post #18. Along side of the coil are three legs of steel,with a thick copper sleeve brazed around the center leg. The read my comments following about the massive shading coil. it does not look at all like the shading coils we are used to seeing, does it??

 

How can that be a shading coil if the single turn does not follow the same plane as the coil itself?

 

Hello again,

Just for reference, I did a calculation of the RMS value of a triangle with rounded peaks, but I rounded the peaks a little more than shown in post #18.
The calculation came out to be about 84 percent of a perfect triangle wave. So if a certain triangle RMS value came out to 1.00, the rounded peak RMS value would be 0.84 which is sort of significant but not super super significant. With the rounded a little less, it could easily get up to 90 percent or even 95 percent with just a small amount of rounding.
The slightly curved sides will not change anything that much because they are positioned at a lower level in the wave than the areas near the peaks.

BTW, (in post #18) that's actually not that far off from a triangle, usually the sides are curved more and the peak sharper.

 

How can that be?

OK I see how it is configured now, bit crude but I suppose it works!

 

How can that be a shading coil if the single turn does not follow the same plane as the coil itself?

The electromagnet is a fairly deep "U" shape, with the outside pole slit into three legs and the shading coil wrapped around the middle leg. It is more massive than I am used to seeing but still obviously a shading coil. But not much of the copper shows through the coating on it. Probably I have seen more shading coils over the years.
I disassembled a whole lot of control valves to learn where they would fail. Quite an education, allowed me to provide some good advice.

 

As I said, I got it. !

 

This should be simple, but...

I have a 24V 50Hz AC solenoid.
The DC resistance of the coil measures 70 Ohms at ambient temperature.
The solenoid has a U shaped iron core with a moveable armature.
When energised, the core and armature form a closed magnetic circuit.
The core includes a copper shading ring.

I measured the inductance with the armature closed using a Marconi bridge.
If I measure using the internal 1kHz oscillator I get 180mH.
If I use an external low frequency oscillator in the region of 50Hz, I get 440mH.
I assume the difference is due to the permeability of the iron core changing with frequency?

I want to calculate the expected rms current when the solenoid is connected to a 24V 50Hz AC supply.

Using the 440mH figure, the reactance of the coil at 50Hz is 138 Ohms.
That gives a total impedance of 155 Ohms, phase angle 63deg.
Hence I would expect the current to be 24 / 155 = 155mA rms.

When I hook up a test circuit I actually measure around 220mA rms (using an AVO which approximates rms).
The current drops to about 200mA as the coil warms up.

I don't understand why the measured current differs from the calculated result.
What have I overlooked?
Is it because the shading ring distorts the current waveform?
If it's no longer a true sine wave then the current as measured on an analogue moving coil ammeter won't be accurate.

Did you measure you 24VAC ? Was it exactly 24VAC ?

 

A wise practice is to use DC solenoids where ever possible.
The only time the AC version has the edge, is at turn on time, after that the DC wins.

Another reason for using an AC solenoid is in wet or salty environments. All irrigation systems that I have knowledge or have worked on use AC solenoids. Also they are easy to activate with a Triac using zero crossing.

 

One additional motivation for AC solenoids is that the AC supply, either reduced voltage or mains voltage, costs less than an equally rated AC supply.. And polarity issues are avoided.

 

Another reason for using an AC solenoid is in wet or salty environments. All irrigation systems that I have knowledge or have worked on use AC solenoids. Also they are easy to activate with a Triac using zero crossing.

My biggest beef with AC solenoids is that if they do not shift over completely for some reasons, or a maintenance personnel etc, forces the armature over by some object to test, in both cases often results in coil burn out.
They tend to run hotter than their DC counter part.
See post#25 et-al.
https://forum.allaboutcircuits.com/...uy-the-basic-parts.160535/page-2#post-1404732