For a long time, induction motors could only offer fixed-speed operation. With the advent of reliable and cost-effective power electronic switches, motor designers developed a method to achieve variable-speed operation from AC induction motors. A DC power supply converts AC wall plug power to DC. Leveraging sophisticated power electronics enabled by insulated-gate bipolar transistors (IGBTs), an AC inverter drive uses pulse-width modulation (PWM) to chop the DC output into something that simulates a variable-frequency sine wave.
AC inverters can provide useful solutions for applications like fans and pumps where energy saving can be achieved by using variable speed control to replace inefficient bypass or throttling control. More sophisticated versions can be used for simple point-to-point motion from one or two axes in machines that don’t require high-speed or highly synchronized motion.
SR systems comprise a motor and a drive and offer an alternative to inverter-fed induction or PM machines, particularly in applications that are required to operate with high efficiency across a wide speed and load range. SR motors don’t include brushes, rotor windings, commutators, or magnets. Instead, their rotors consist purely of a stack of electrical steel laminations mounted about a shaft. Torque is produced by the magnetic attraction of the steel rotor poles to stator electromagnets. The rotor position relative to the stator is detected using a simple hardware sensor or by electronic "sensorless" means. The controller then energizes each stator winding only when it can produce useful torque. By suitable timing of the stator excitation, the machine can operate as a motor or generator, with good efficiency over a wide range of speed and torque.
The SR motor concept originated more than a hundred years ago, but only with the advent of cheap, powerful microprocessors did the approach become practical. Thirty years later, the technology has graduated from a laboratory curiosity to a viable solution for applications ranging from crushers and weaving machines to automotive and white goods, particularly in light of the high cost of rare earth magnets.
SR rotors feature a so-called "salient" design, which refers to gaps between the rotor poles. This design significantly reduces the rotor weight. “The designs remove mass from the periphery of the rotor, and not only does that make the rotor lighter, but it makes it substantially lower in inertia, typically less than half that of the equivalent induction machine,” says Cummins.
That makes them effective for applications that require fast dynamic performance such as high-speed traction drives. Lower rotor inertia also benefits gear-driven applications, which experience large transient loads, such as rotors, in which the lower rotor inertia of the SR motor reduces shock loading on the primary of the gearbox.
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.
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.
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.
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 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.
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.
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.
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.
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