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DN Insight: What Rare Earth Shortages Mean for Engineers, Part 5

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apresher
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Rare Earth Shortages
apresher   5/16/2012 8:25:15 AM
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Kristin, Excellent article.  I know some suppliers are moving to SR technology as a way to improve the performance of induction motor systems.  A key advantage is that SR motors can be paired up with variable speed drives to the eliminate slip losses associated with induction motors while also boosting efficiency and create more compact sizes. Other benefits of the technology are related to higher reliability. The rotor construction itself is robust, plus there is no squirrel cage on the rotor side.  Great series on rare earth shortages.

Jon Titus
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Excellent!
Jon Titus   5/16/2012 12:03:00 PM
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Wow, this has been an informative series of articles. Thank you for the research and deep insight into not only the rare-earth elements, but also motor design. Much appreciated.

TJ McDermott
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Relative cost?
TJ McDermott   5/16/2012 1:39:03 PM
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Kristin, do you have numbers for the cost of an SR motor compared to a conventional induction motor?

Charles Murray
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Re: Excellent!
Charles Murray   5/16/2012 6:00:54 PM
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Fantastic series, Kristin. You've provided in-depth information at a time when design engineers sorely need it.

Jerry dycus
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Re: Excellent!
Jerry dycus   5/16/2012 7:50:04 PM
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            I said this on the first article, we have used non REE motors, generators  for 140 yrs or so and could even be better without them as better control and far more starting torque and peak power, things REE's don't do well.

            Where is any mention of brushed motors like series, sep-ex, compound that ran the industural age?  These in many ways are superior especially in tractive uses like EV's as they can stand far more heat, thus make more peak power.  Add to that the controllers are 1/3 the cost and weight.

             Only in the last yr has AC come anywhere near being as cost effective as these and I still have to pay 100% more for the joy of AC.  That of course would raise the EV price by $2-3k and accepting lower starting power for that joy. 

              Another benefit is in smaller EV's the use of lower voltages lowering cell count, controller costs.

              The cost of this is brushes that need to be changed in 100k miles or so but likely the bearings will needed anyway in DC or more likely in AC,  so little extra cost in real life. 

               Improvements that don't do better at lower costs than past alternatives are not improvements.

                Now in small wind generators REE's are hard to beat. I know as I've tried and building the production prototype now for a home size 3kw cost effective unit.

            Luckily I've cut the needed REE's by 75% and if tests work out, 87.5% compared to the present competition.  It looks like I'll even beat the Chinese WG's  on price/proven  kw/mph and destory them in quality.

ChasChas
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Not out of the woods, yet.
ChasChas   5/17/2012 9:49:48 AM
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Excellent engineering advice in this article, but it sure adds complexity to motor design - and engineering/prototyping/testing isn't free. Seems like many industries will still be forced to pay the higher price of REE. 

bdcst
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Re: Relative cost?
bdcst   5/17/2012 10:46:54 AM
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I wonder how switched reluctance motors compare with other designs regarding cogging?  Where motion has to be fluid, cogging can add vibration and velocity errors, flutter.  This was an issue in the days of analog magnetic tape transport design.  But that era is long gone.

SparkyWatt
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Re: Relative cost?
SparkyWatt   5/17/2012 1:27:52 PM
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Do you mean "cogging" or "torque ripple"?  Torque ripple is an effect where the torque is reduced as the magnetic field moves from pole to pole in an AC motor or stepper motor or as a commutated motor switches windings.  It stems from the fact that the rotor doesn't see a uniform magnetic field as it rotates.  "Cogging" on the other hand is an effect commonly seen in stepper motors, but also appears in AC motors, where a motor under a heavy load slips back a full pole.  "Cogging" is a catastrophic failure, because in most cases onee it happens, it continues to happen.  The motor stops rotating and is subjected to an alternating torque of its full rated torque.  This is usually loud and often destructive.

I suspect that SR motors don't cog, because the switching follows the rotor, similar to brushless DC motors.  An overloaded motor would just slow down and stop switching.

I have no idea what their torque ripple performance is like.

bdcst
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Re: Relative cost?
bdcst   5/17/2012 5:20:10 PM
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Terminology, cogging versus torque ripple.  Well...back in the early days of reel to reel audio tape recording Ampex was the pre-eminent manufacturer of tape decks.  They employed Bodine hysteresis synchronous motors to drive the tape deck capstan.  With a heavy flywheel as a low pass filter, both bearing noise and flutter could be reduced.  That style motor could be made to exhibit a much higher degree of tape playback flutter.  I suppose you could call it torque ripple since the effect was milder than slipping or skipping a pole.

If, while the capstan motor was energized, you changed its speed by switching windings, it would induce a temporary magnetization of the armature that would produce noticeable flutter.  Simply de-energizing the motor and re-energizing it would erase the slight ripple effect.

SparkyWatt
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Re: Relative cost?
SparkyWatt   5/17/2012 6:41:04 PM
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Yes, I've noticed that different groups use terminology differently.  It results in no end of confusion.  That is why I took the trouble to clarify.  It is possible that people in your industry use the term "cogging" differently than the areas I have been around.

What can you do?  Language is organically grown and sprouts from many roots.

Sounds like the effect you are talking about comes from using a "soft" material for the motor's core.  The core becomes magnetized, and therefore "sees" the gaps between the poles.  When I took motor construction in 1980, that was considered a bad idea.  A soft material would usually increase the torque of the motor for a given geometry, but lower its efficiency (it increased the core losses) and causes that form of ripple.

Of course, today that would be a concern also because it increases the motor's electrical emissions.

I am not into SR motors, but my guess would be that (like other styles of motors) it is dependant on the "hardness" of the material used for the rotor.  A "hard" material would show very little effect.

And Ampex did make wonderful tape decks.

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