Design Article
Myth or fact? You can increase the force of your solenoid by removing turns (Part 3 of 3)
Paul H. Schimpf, Eastern Washington University
1/20/2012 4:03 PM EST
The solenoid: what's there to learn further about a passive component that is nearly 200 years old, dating back to the 1820s? After all, it's a simple passive component, isn’t it? And there is not that much to understand, right?
Well, yes and no. This article examines the common claim that the strength of a solenoid can be increased by removing some turns from the coil. In the process we look at formulations for inductance, power, energy, and force. The end result clarifies the roles of several solenoid parameters, and the effect of the number of turns may be surprising to many readers. Those who would prefer to skip the mathematical details can find a qualitative explanation in the conclusion.
We are presenting this article—which includes equations, models, and analysis—in three parts, as pdf files:
Part 1: Introduction; Equivalent Circuit; to read it, click here.
Part 2: From Energy to Force; Additional Discussion; to read it, click here.
[Editor's note: some readers have reported that Part 2 equations are not rendering properly in the pdf. If that's the case for you, please try this alternate version here, where Part 2 starts at the bottom of the first pdf page "From Energy to Force", and ends with the beginning of Part 3 (Conclusion).]
Part 3: Conclusion; References; and Appendix; to read it, click here.
About the author
Paul H. Schimpf holds the B.S., M.S., and Ph.D. degrees in Electrical Engineering from the University of Washington, with respective degree specialties in digital electronics, embedded signal processing, and modeling electromagnetic fields in complex domains such as the human body.
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GutenTag
1/25/2012 9:28 AM EST
One remark:
When removing turns the resistance decreases and the current increases, thus the power dissipated in the solenoid. This increases the temperature of the (copper) wire thus increasing the specific resistance of the wire material. Thus the current does increase less than assumend. So removing turns lower the force ...
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pschimpf
1/25/2012 3:23 PM EST
That is a good point, thank you. I assume you meant "So removing turns increases the force less than would be expected without accounting for the rise in temperature."
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cookiejar
1/26/2012 7:45 AM EST
My rules of thumb for solenoid or motor/generator coils:
1. completely fill the available space with your wire turns to achieve maximum efficiency.
2. adjust the gauge of wire to your driving voltage.
3. keep in consideration that the number of turns affects the value of the inductance and inversely the speed of response.
4. don't forget to suppress the inductive kickback voltage which can easily be in the KV area and besides being potentially destructive can couple into your circuitry messing up your circuit's performance. A diode across a DC coil is the simplest and most effective suppression, but will slow the release time. You can also use Zeners, Transzorbs, MOVs and capacitors to suppress the transients with tradeoffs in speed and EMI.
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pschimpf
1/31/2012 3:12 PM EST
Thanks for the comments cookiejar. It is always great to have this kind of advice from experienced designers.
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JmmWill
1/27/2012 1:29 PM EST
This was a top-notch paper, one of the few I've read in full detail from start to finish.
The corrected formulae in Part 2 really should be included in a new version of the whole article.
This is a truly scholarly work, not just a teaser for commercial advertisements, and EET readers, especially EE students, should be given access to it in toto.
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pschimpf
1/31/2012 3:20 PM EST
Thank you for the praise. I did, in fact, end up writing this as a result of trying to prepare some notes to explain what I was thinking would be basic information about cylindrical solenoid design and behavior for an embedded systems class. In the process I discovered that not even coils are covered in a practically useful way in the usual undergraduate textbooks on electromagnetics. Hence the paper. I hope some students find their way to it.
BTW, I happen to agree about how the rendering corrections for part 2 should be posted. The first version could probably simply be taken down. But that is, of course, in control of the editor. :-)
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