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I am currently studying the effects of current on the coil of an electromagnetic relay and came across this graph:

Measured and simulated coil current during the 24 V experiment

Image source: Ramirez-Laboreo, E., Sagues, C., & Llorente, S. (2015). A new model of electromechanical relays for predicting the motion and electromagnetic dynamics. https://doi.org/10.1109/ias.2015.7356818 (author's website download link)

Why is there a dip in the current around 11 ms and then a crest again around 35 ms? Is there a way of improving this?

The relay armature was closed for the first 25 ms (when the control circuit was on i.e relay coil had current) then open (when the control circuit was off i.e relay coil had no current.)

SamGibson
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Syed Muhammad Ghufran
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    Would be much easier to answer with more context: what's the circuit? What's controlling coil current or is it a constant-voltage setup? – pjc50 Jul 05 '23 at 10:37
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    What needs to be improved about the current? And aren't the dips and crests due to the armature moving in the magnetic field? – Justme Jul 05 '23 at 11:14
  • @pjc50 Since current is continuous, we can assume constant-voltage drive. Evidently a square wave between 24V (high) and 0V (low). I have measured similar waveforms with hard switched and PWM driven solenoids. – Tim Williams Jul 05 '23 at 20:56
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    Syed Muhammad Ghufran - Hi, Please remember the [site rule](/help/referencing) which requires that when a post includes content (e.g. text, image, photo etc.) copied or adapted from elsewhere, that copied content must be correctly referenced. As a minimum, the source webpage or PDF etc. should be linked. In order to help you, I found what I believe to be a source PDF link & added it for you. For the future, please remember it's your responsibility to do that :) To remind yourself of the main site rules, please see the [tour] & [help]. Thanks. (Please [edit] the link I added if it's wrong.) – SamGibson Jul 06 '23 at 21:08

3 Answers3

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A relay coil is inductive, and the mechanical parts form part of the magnetic circuit. When the clapper moves after current is passed through the coil the magnetic circuit closes, changing the inductance significantly, so the current does not simply increase with the kind of exponential relationship you'd expect with a R+L circuit, rather you get the kind of behavior shown. It's not just inductance change, there is also energy change related to the mechanical motion and changing force.

I believe the graph you have has the relay being de-energized at t=25ms. The details of that will depend on the circuit (especially if there is a flyback diode, zener network or snubber) as well as the relay design.

Wherever you copied that graph from should have context.

It is not clear what you think "improving" the behavior would constitute. Usually we're mostly concerned with the contact behavior and coil current vs. time is not a great concern for DC relays. The force ends up being much higher at the end of travel since the magnetic circuit is closed- that's a good thing, and why it's often you see relays with a higher current rating for the N.O. contact than for the N.C. contact.

Spehro Pefhany
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  • as @GrahamNye suggested theres some charging effect due to variations in inductance. what i am currently searching for is a solution to mitigate this effect so that the graph shows a proper L+R exponential rise. if not that than something close to it. – Syed Muhammad Ghufran Jul 06 '23 at 09:53
  • You could open the relay case and (for example) remove the clapper. It won't work as a relay any more, obviously. – Spehro Pefhany Jul 06 '23 at 11:12
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    @SyedMuhammadGhufran what *problem* does it cause? Does something not operate properly, or does looking at the graph just make you sad? If it's a technical problem, maybe we can give you advice on fixing it. If it's a spiritual one, maybe not. – hobbs Jul 06 '23 at 14:37
  • well i am currently doing my thesis on this. the company i am doing it for needs a solution to this problem that they are facing (i still dont understand how its causing a problem, maybe in the next meeting theyll inform me more on it) but till then i thought id get opinions. – Syed Muhammad Ghufran Jul 06 '23 at 20:24
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Welcome to EE.SE.

The relay armature was closed for the first 25 ms (when the control circuit was on...

The power is on for the first 25 ms but the armature won't close instantaneously. As the armature is pulled in it will change the reluctance of the magnetic circuit and hence the inductance of the coil giving rise to the effects seen. It looks like the armature is starting to pull in from around 5 ms and completes closure by 11 ms. When the relay opens the same effect happens in reverse as the armature opens giving you the blip on the opening cycle from 33 ms.

Graham Nye
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  • is there a way of reducing this effect so that the new relay that im modelling has less of this effect – Syed Muhammad Ghufran Jul 06 '23 at 09:42
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    @SyedMuhammadGhufran Is this effect giving a problem in practice or is this only a concern for modelling? The document you took the picture from (unknown to Google image search) seems to have a reasonable approach to modelling this; can you follow their approach? As Spehro noted, you can't remove this effect without stopping the relay from working. You could, I suppose, drive the relay with a constant current source at the cost of having a corresponding blip in the voltage vs time curve across the relay. A storage scope will let you plot your own VI curve for your relay. – Graham Nye Jul 06 '23 at 15:40
  • the picture is from a research paper i am studying. they didn´t really tackle this issue of pull-in phenomenon. i was wondering if i a snubber circuit on the coil or maybe use a lower voltage before applying the actual voltage could help reduce this effect or make the graph more smoother?? – Syed Muhammad Ghufran Jul 06 '23 at 20:29
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    @SyedMuhammadGhufran The principal author of the paper Sam found (nice find!) has a [webpage](https://webdiis.unizar.es/~ramirlab/publications.html) with papers addressing the same area. You may find those useful. – Graham Nye Jul 06 '23 at 23:00
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As the anchor comes in, the magnetic resistance of the circuit is reduced drastically. If the current through the coil would remain the same, the magnetisation would stay the same but the magnetic flux would increase drastically. But an increase in flux induces a countervoltage that will reduce the current. Conversely, while the relais falls off, the magnetic resistance of the circuit increases. This reduces the flux without a corresponding change in magnetisation and thus causes a positive bump in the switch-off current.

A constant inductivity would exhibit the exponential rise/falloff curves without the bumps. The closing anchor causes an additional dip (with a sharp transition as the anchor closes with a jolt), the opening anchor causes a blip (with a soft start as the anchor takes off smoothly, and a comparatively soft end as the anchor stops moving in a far-off position).

user107063
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