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Author Topic: when is a shorted turn in an inductor a problem ?  (Read 10321 times)
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w4bfs
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« on: August 29, 2013, 02:51:54 PM »

I know .... this is a basic question but still puzzles me

In a low frequency inductor (power transformer or choke) the presence of a shorted turn will upset the action that is going on.  As the inductor has ac passed thru it the resultant flux excites the the shorted turn which builds a flux in opposition.  This results in a net lower flux and lower inductance.  With a net lower inductance this requires more current to magnetize the core.

In a rf inductor (with or without a core other than air) it is common practice to short out turns for frequency selection.  Do not these shorted turns get activated when rf current flows in the non-shorted turns of the inductor?  Is this not a loss causing situation? Evidentaly not.

Inquiring minds want to know
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« Reply #1 on: August 29, 2013, 05:29:30 PM »

The Q of a solenoid inductor is nearly optimum if the length of the inductor equals its diameter. So if you short windings, as you said, the L drops as does its physical length. Likely so does the Q and taken to extremes I have measured a substantial loss in Q. So, if you want to reduce the inductance your better off changing its form factor in lieu of shorting turns. However, this is in the extreme case. Measurements I have made, shorting the middle turns, or the extreme end turns and therefore reducing the L value, showed a minor drop in Q. If I can find my data I will post. Most of these inductors were either air wound or wound on low perm powered iron cores and ranged in value from 100 nH to 2 uH. Larger plate tank coils, air-wound, say 20-60 uH I would think, would show similar trends. The degree of coupling (pitch) of the coil would also come into play on shorted turns vs. no shorting.

This gentleman cites Wheeler's reference as well as some additional comments that are insightfull.

http://vk1od.net/tx/concept/TappedCoil/

Alan
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« Reply #2 on: August 29, 2013, 05:41:23 PM »

Beefus

There is an important difference between shorting turns of a typical air-wound RF inductor and shorting one or more of the turns of a typical 60Hz AC inductor or transformer with a high permeability magnetic core.

In a typical AC inductor or transformer with a high permeability magnetic core, almost all of the magnetic flux (i.e. the integral of the B-field over the cross sectional area of a turn) is in the core, and passes through every turn. The voltage induced around every turn is the cross sectional area of the core x the rate of change of the B-field with time (i.e. dB/dt). The leakage inductance of any turn is negligible (because almost all of the magnetic flux that passes through the cross sectional area of any turn will also pass through the cross sectional area of all of the other turns).

Imagine that an inductor (or one of the windings of a transformer) is driven by a 60Hz AC current source.  If a single turn of the inductor (or a turn in any one of the windings of the transformer) shorts out, additional current will flow through the closed loop corresponding to that shorted turn because of the voltage that is induced around the turn by the time varying magnetic flux passing through the turn's cross sectional area. The effect of the H-field produced by the additional current flowing around the shorted turn on the total time-varying magnetic flux (v. what the time varying magnetic flux was before the short circuit occurred) will typically be small, because there are typically many turns in each winding. The total magnetic flux is the sum of the magnetic fluxes produced by the current flowing around each turn of each winding (shorted or not). Therefore, to a good approximation, the induced current in the shorted turn will be equal to the voltage around that turn, before the short circuit occurred, divided by the impedance (essentially the ohmic resistance) of that turn. The wire in that turn will get very hot...and probably fuse. You can think of this as a 1-turn separate winding in a transformer, that has a short across its ends.

In an air wound RF coil, the turns are magnetically coupled, but a good deal of the magnetic flux passing through each turn does not pass through all of the adjacent turns (this depends upon the diameter of the coil and the spacing between the turns). Therefore (unlike the high permeability core case) each section of the air wound RF coil can be thought of (i.e. as an equivalent circuit) as an isolated coil (not coupled magnetically to the other sections of the total coil, and with fewer turns than the total coil) + a coupling transformer whose primary winding is in series with that section. Each coupling transformer has an associated magnetizing inductance across its primary. Each of these coupling transformers has an associated secondary winding. The secondary winding of each of these coupling transformers (in the equivalent circuit of the total coil) is connected to the secondary windings of the coupling transformers associated with the adjacent sections of the total coil.

If there are two sections (as would be the case when we are considering a tank coil with some of its turns shorted out), then the equivalent circuit is as shown in the attachments.

If one of the sections is shorted out (as shown in attachment 2) then the primary winding of the associated coupling transformer is not shorted... but has the shorted section's inductor across it... in parallel with the magnetizing inductance of the primary of the shorted section's coupling transformer. Therefore, shorting out some of the turns of an air wound RF transformer does not produce a loop with a very low resistance (and therefore a very high current). Shorting a section of the coil will result in a greater reduction of the inductance of the coil than the reduction that would result if the section were left open or if the section were entirely removed.

Stu

    


* Slide1.JPG (36.83 KB, 960x720 - viewed 563 times.)

* Slide2.JPG (41.78 KB, 960x720 - viewed 567 times.)
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« Reply #3 on: August 29, 2013, 06:43:31 PM »

Nice explanation.
Also At lower Q's, say of a PI net output, some designs progressively short all unused contacts with a rotary wiper, some designs just short with just one contact to the output.
At a loaded Q of 12 to 15, probably not a lot of difference.
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« Reply #4 on: August 29, 2013, 08:03:04 PM »

Nice explanation indeed !   I have not seen anything like this explanation in any technical texts and it took a reduction of field theory to get it into this simpler language ....nice job Stu !
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« Reply #5 on: August 29, 2013, 10:38:19 PM »

very well said and maybe a reason why unused turns in power tank coils are usually always shorted, as a better alternative to tapping and leaving the remains open which makes its own problems.
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« Reply #6 on: August 30, 2013, 07:13:51 PM »

very well said and maybe a reason why unused turns in power tank coilps are usually always shorted, as a better alternative to tapping and leaving the remains open which makes its own problems.

And the problems being extremely high voltage at the end of the open pi-net coil. That can cause a big corona like a small Tesla coil under some circumstances.
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73 Mike 
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