Magnetic shunt on a relay coil.

[kinarfi]

https://www.electro-tech-online.com/custompdfs/2013/07/C0_v4bg_5.pdf
bottom of page 82, it's called a "shader ring"
also of interest, at least to me, is https://www.electro-tech-online.com/custompdfs/2013/07/app-note1.pdf page 14 & 15
but all this has very little to do with building a small dynamo, how are you going do that?

 

[MrAl]

Hi,


Yes nice link there. They state outright that it is and i quote, "a shading or short circuit ring", that is used to delay the fall of the flux when used with AC.

The basic theory for this would come from the simple inductor:
V=L*di/dt

and solving for the time delay dt:
dt=L*di/V

With a normalized L*di we have:
dt=1/V

so with the voltage V equal to 10 volts it takes time:
dt=1/10

or 0.1 second to decrease the current i (and thus the flux) by some amount di. But if the voltage is lower, like 1v, we get:
dt=1/1=1 second

which is a full second now. Decreasing the voltage to 0.1 volts we get:
dt=1/0.1=10 seconds

which is much longer now. So as the voltage is decreased the time delay gets longer and longer until we reach zero volts:
dt=1/0 => +infinity

and then the relay would never open. But of course in a real relay the coupling of the field to the ring is going to be minimal (no where near a perfect 1) so the time delay will be much less.

It's also of side interest that it should cause more current to flow in the relay wires when it is energized. How much more would depend on the degree of coupling and the resistance of the winding vs the resistance of the ring.

However, it would still be interesting to see this run with and without the ring to see just how much better this particular relay works with the ring. A video with and without the ring would be nice

 

[ericgibbs]

hi,
In the good old days we called them 'slugged relays' ,ie: delayed release.

**broken link removed**

E.

 

[MrAl]

Hi Eric,

Now that i think way back it has been extremely rare that i ever used a AC coil relay for anything. Mostly DC coil relays for switching either AC, DC, or signals. That's because most of the stuff i did used control circuits that had to turn the relay on and off. Even large contactors used to switch heavy AC currents used the DC coils so that we could turn them on and off with a control signal from another circuit that had to measure some quantities and make the decision to turn on or off.

Would be interesting to see some applications that require a relay with an AC coil.

 

[kinarfi]

Hi Eric,

Now that i think way back it has been extremely rare that i ever used a AC coil relay for anything. Mostly DC coil relays for switching either AC, DC, or signals. That's because most of the stuff i did used control circuits that had to turn the relay on and off. Even large contactors used to switch heavy AC currents used the DC coils so that we could turn them on and off with a control signal from another circuit that had to measure some quantities and make the decision to turn on or off.

Would be interesting to see some applications that require a relay with an AC coil.

I worked as an electrician most of my career in industrial and oil field settings and every thing was ac coil, 3 phase motor starters mostly, lighting controls, solenoids and and electrical valves, control valves, control relays, etc. with voltages of 24 and 120 vac most common for the smaller relays and up to 480 for the motor starters.
It was during this time that I found the explanation for what the shading ring does as a change to the hysteresis loop (from trapazoidal to more of square pattern) of the magnetic field as the voltage passes through 0 vs that the of the return spring. On a few occasions, I ran into coil armatures with broken shading rings and it did cause the contactor to buzz, on one occasion, I tried to replace the ring with a piece of bare copper wire with the ends twisted together, but it didn't work.

 

[dr pepper]

I seem to have created a popular thread.
The pdf was interesting, the breakdown of the relay with the shading ring is almost identical.
We have a lot of control relays, all the early machines tend to be ac, and later ones with plc's are 24v dc.
Relay dropout has been a problem for us on ocaision.
I did try ripping out a shunt and trying the relay, wouldnt pull in, with assistance the armature did pull in but as expected loud buzz, in fact 50hz was probably available at the contacts, with the coil be 230 I didnt was to mess anymore.
My application is rather mundane, pushbike lights, with a ring of neo magnets glued inside the rimspace of a alloy wheel the output is surprising, one coil being 12v at nearly 100mA, 1 watt, 3 coils would light a 3w led, nice for cycling home on back lanes.

 

[kinarfi]

not so mundane, pretty neat actually,
thanks

 

[dr pepper]

yes I spose not every project needs a 16bit processor and a dsp to be usefull.

 

[MrAl]

Hi,

What? We need at least an 8 bit microcontroller on every project from now on, or else we dont get much functionality
I like analog myself though.

But a twisted wire on the core may not be the same as the actual piece made to fit on the core nicely. It may be a matter of machining a piece to the same shape as the original copper piece. In magnetic circuits usually the parts have to be machined to decent tolerances so that they fit together nearly perfect. There's very little room for error or else the coupling factor goes up tremendously, and reducing the coupling to a winding means it does not perform the same way and can actually be drastically different.

 

[dr pepper]

I'm conversant with pic micro, and I have a little machine shop so either isnt a problem, however I dont think I'll be needing a micro, unless I decide to make the lights flash, which will invoke a 10f200.

I was thinking of milling the core down flush with the coil to improve or should I say not ruin the coupling, I might do a test with this as the little protrusion generated by pulling off the shunt gives me a bit of metal to poke through a plastic housing, getting the core closer to the rotating magnets.

 

[kinarfi]

can you post a photo or two, please and thanks.

 

[dr pepper]

OK, but I'd like to get a little further than masking tape and tie wraps first.