DN Insight: What Rare Earth Shortages Mean for Engineers, Part 5
SR motors feature salient rotors, which have gaps between the poles (left). Less mass at the periphery of the rotor reduces inertia for a more agile performance compared to the cylindrical rotors of induction motors (center). By eliminating the magnets of a PM motor (right), the design cuts cost. (Source: Nidec SR Drives Ltd.)
Sport: no, not really. Dysprosium raises the Hc, which helps with temperature, but it hurts Br, so energy product drops. If you want a high-flux magnet, leave the Dy out. Neo magnets, even with lots (12%) of Dy, do NOT have great temperature performance. SmCo, Alnico, and ceramic magnets all are capable of performing at higher temperatures than Neo magnets. The advantages of Neo magnets are small size and low mass.
Neodymium Iron Boron magnets offer high energy products, but the higher-energy versions do NOT retain their magnetism well at high temperatures (compared to other magnet types)! Based on this information, the first sentence of this article is incorrect, and is probably based on the writer's lack of experience with Neo magnets.
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.
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.
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.
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.
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.
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.
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