Switched Reluctance motors, an old design is suddenly important now

October 12, 2019
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Reluctance is an engineering term that (as far as I can tell) means that when you create an electromagnetic field (in the shape of a loop)…any steel part that is located near that field is “reluctant” to remain un-aligned with the field that is provided. The electromagnetic field will exert a strong force onto any steel part near it…to align with the invisible magnetic field.

The “switched” part just means that the location for the magnetic field that is performing the work is switched from one location to the next adjacent location [inside the motor] so that the rotor will continuously spin. I will sometimes use the terms “iron or steel” interchangeably when mentioning a magnetically permeable material in a motor. Steel is simply iron that has about half a percent of its mass being some added carbon, which makes it harder, but doesn’t change its magnetic properties.

Note: when you google “reluctance” motors, there are two styles…the “switched” reluctance, and also “synchronous” reluctance. They use very similar principles. A Synchronous Reluctance motor has the same number of magnetic poles in the stationary stator and the rotating rotor. A switched reluctance typically has fewer poles on the rotor than the stator. Neither one is better or worse, they each have their benefits and drawbacks, depending on the application…There are more details about synchronous reluctance farther down.

This is a side-view of the Earths magnetic field, but if you spin it around 360-degrees, it ends up shaped like a doughnut

The event that got this whole mess started was the fact that I read too much, and I stumbled across a wonderful article on the motor that is inside the Tesla Model-3 [written by cleantechnica, click here], which is a switched reluctance style (SR). The SR style isn’t just good, it’s better…and I hope this article will explain why.

Using iron filings to show the shape of the invisible magnetic field around a simple bar magnet

There are several things that are important to notice about the bar magnet picture (posted above) in order to understand Switched Reluctance (SR) motors. Notice that the magnetic field is in the shape of two symmetrical loops (shown in red). Also notice that in most of the field, there are scattered iron filings, indicating that the field is somewhat weak in those places.

The most important part to see is that there is a thick cluster of iron filings right next the north and south poles of the bar magnet, and the size of the cluster is small and tightly packed.

This means that there is a tiny space in two specific locations where the field is very strong, and the rest of the field (just a short distance away) is relatively weak. An SR motor uses this phenomenon to create some movement between a stationary “stator”, and a rotating “rotor” in a motor.

William Sturgeon, and the electromagnet he invented that can be turned on and off

William Sturgeon, the inventor of the electromagnet, which can easily be turned on and off. The two bowls shown hold mercury, and dipping an energized wire (d) into the mercury (Z) forms a simple switch. The copper wire is wrapped around a soft iron bar in the shape of a “U”. When it is energized, an iron object is attached to the two tips, which completes a circular iron path, and forms a magnetic field in the shape of a doughnut (engineers call this shape a toroid).

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Electromagnets, Steel, and Motors

The pic below shows the classic “double loop” of a magnetic field (in blue). A straight wire that passes electricity through it has a magnetic field around it, but that field is very weak. If you configure the wire into a coil, it magnifies and strengthens that field. A coil of wire with nothing in the center is an “air core solenoid”

The basic components of an electromagnet

However, once you place some iron in the center (or steel, which is 99% iron), the iron temporarily becomes magnetic, and it will exert a strong force onto any iron that is near it, to align with it’s magnetic field. Compare the double-loop of the field lines in the graphic above with the graphic just below.

A switched reluctance motor graphic. When the two electromagnets are energized, the rotor will experience a very strong force to align with the magnetic field. There are no permanent magnets in the rotor, it is only made from iron (stacks of thin steel plates).

Remember when I mentioned that the bar magnet had a small but very strong magnetic power concentrated at the two tips? Notice that in this graphic above, when the electromagnets are energized, the corners of the poles on the rotor are almost touching the poles on the stator. The “air gap” between the stator tooth tips and the rotor tooth tips is very narrow to take advantage of the strongest part of the magnetic field.

A 4-phase SR motor with eight poles on the stator, and six poles on the rotor (8/6). This combination is reversable.

A few historical notes on motors, leading up to today

Nikola Tesla’s 3-phase brushless “induction” AC motors from 1888 were an incredible leap, rather than the slow step-by-step of inventions that came before it. However, they work well at a constant speed, and that speed is most efficient near 3,000-RPMs. They do require multiple loops of copper wire to be located in the rotor, and there is currently a trend of converting these common industrial motors to a “Synchronous Reluctance” motor when they wear out, need repair, and also for new installations. By replacing the copper in the rotor with only steel, the price is much cheaper, without any reduction in performance.

They are similar in principle to the “Switched Reluctance” (SR) motors that this article is about, but with distinct differences. Both types of motors use a rotor that is only made from stacks of thin laminated steel, with no copper or expensive neodymium magnets. The “Synchronous Reluctance” style (SynR) uses a stack of laminations in the shape of a round disk, with curved air-slots to act as a flux “fence” that guides the flux loops that are created. They are shaped this way to interact with the existing induction motor stators, which use overlapping “distributed” windings around the stator teeth.

An SR rotor is on the left. The middle example, plus the one on the right are two styles of SynR rotor

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SynR, Synchronous Reluctance

So, if this article is about “Switched Reluctance” motors, why am I still continuing to talk about SynR motors? I’ll tie it all together at the end, so trust me, there’s a reason.

A SynR motor graphic. The stator is gray, and the rotor is blue. The copper wire coils in the stator-teeth are not shown, but the loops of generated magnetic flux are shown in red. When those five magnetic loops are all shifted over the distance of one tooth as a group, the rotor will be pulled over one tooth to re-align itself.

The pic below shows a common industrial induction motor, and the SynR rotor that will be replacing the standard induction rotor.

One of the basic principles of magnets is that the identical poles push away from each other (N/N…S/S), and opposite poles are attracted to each other (N/S…S/N). If you reverse the current (negative/positive) that is flowing through an electromagnet, you also reverse its magnetic field. Using these basic principles, it is not difficult to design a simple and powerful electric motor that can be scaled up to just about any size. It works.

In 1982, General Motors and Sumitomo each independently developed a powerful permanent magnet, which turned out to be the Nd2Fe14B “neodymium” magnet (which replaced expensive AlNiCo and SmCo magnets being used at the time). Although the price and strength of Neo magnets was better than the previous options, they use “rare earth” elements that are not common, and can easily have their access restricted by international trade disputes (I’m looking at you, China). Despite this concern, their use expanded rapidly.

The use of neo’s in mass-produced computer hard drives in the 1980’s dropped their prices to more affordable levels, and their use expanded even more. This included the motors in electric bicycles, which are very sensitive to the size of a motor, requiring them to be both small in size and still have high power. Which brings us back to the modern era.

The T-Zero. This was the Tesla prototype before the company even had a name.

In 2003, Martin Eberhard and Marc Tarpenning test-drove a new EV car prototype called the T-Zero, which had absolutely stunning performance, and was nothing like the failed General Motors EV-1. They commissioned a battery pack made from the most recent lithium-based cells, which made the car lighter and increased it’s range. It could now accelerate to 60-MPH in 3.6 seconds.

When T-Zero decided to not convert their original production battery pack to lithium, Eberhard and Tarpenning saw this as an opportunity, and immediately formed the Tesla car company. Whatever anyone might have previously complained about when it came to electric cars, they could no longer claim that they were all ugly and slow.

The Tesla Model-3. It uses an SR motor, and the AWD P100D uses an SR motor on the front.

An early wealthy American investor in Tesla was named Elon Musk (born in South Africa), and it wasn’t long before he became CEO of the company. Under his direction, the engineering staff has improved their motors and batteries with each version. However, if the company was to grow and become profitable, there were two bottlenecks that loomed on the horizon.

The first concern was being able to get enough cells to make the battery packs, and the 18650 format cells that they had designed the pack around were made in Asia. Musk decided to form a partnership with Panasonic, with the requirement that an entirely new factory for the Tesla cells would be built in the USA (the “Gigafactory” was built in Nevada).

The second bottleneck was the the early Tesla models were expensive and somewhat exclusive. All of the parts of the Tesla cars are fairly conventional, except for the battery and the motor. The Gigafactory ensured that Tesla would now have a steady supply of batteries, but the second problem is that…other dramatic changes were needed to lower the costs of the 2017 Model-3, so that they could generate many more sales than the earlier Roadster and Model-S. The Model-3 motor design was targeted for a major reduction in costs, but…it could not sacrifice the performance that the Teslas had become known for.

And that leads us back to this article in Cleantechnica (click here), which just blew me away. The motor engineers started with a clean slate, and explored every possible option. For the past 100 years, the existing types of motors worked well enough that nobody needed to reinvent the wheel, and the Switched Reluctance (SR) principle was not impressive enough in the early days of Tesla to warrant spending money and effort in trying to improve it.

To be fair, the controllers of the day were simply not sophisticated enough to make up for its shortcomings, and SR motors were known for having torque-ripple that was bad enough to create significant vibration and noise.

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Switched Reluctance, and Shared Poles

Remember earlier when I talked about how industrial inductance motors were being converted to Synchronous Reluctance (SynR) motors? That started around 2011, when a German firm began advertising a more sophisticated controller, which allowed existing Induction stators and their cases to simply substitute a SynR rotor for a significant savings in cost for new motor installations.

Well…those new controllers mean that if anyone wanted to dust off the 1888 SR motor design, it just might have a chance of working well now…and it does (that patent is the header pic of this article).

I’ve found references to the Tesla Model-3 SR motor being designed by an engineer who was hired in 2012, so I imagine that 2012 was about the time that the motor engineers settled on SR as a motor type, and then began experimenting to see how much better they could improve it. The entire reason I decided that this new Tesla SR motor deserved it’s own article was because…the Tesla motor engineers made a motor that not only had the same performance as before (*which was excellent), but…they did it in a motor that was actually smaller and lighter, and ALSO cheaper. This means that we are all going to be seeing more products with SR motors in them, so…let’s take a look at what I’ve been able to dig up so far.

A rotor for a Switched Reluctance / SR motor. There are no copper wires or neodymium magnets in this rotor, only a stack of thin steel sheets that form this shape.

As soon as I started researching SR motors, I came across several terms I hadn’t heard before. One of these terms is “Salient” Poles. It means that the focus of the magnetic pole is formed as a protrusion, with air around it. The stator poles in the PM motors (that we are all familiar with already) have always had salient poles, but they didn’t need a special name to distinguish them during discussions over the years. By also making the poles of the rotor salient, these types of motors are now sometimes referred to as “doubly salient” motors by some design engineers.

If a rotor has less mass to accelerate and decelerate, having that feature can be a desirable. There have been 2500-RPM SR motors that are able to be fully reversed, and back up to full speed in less than one second. The 8-pole rotor configuration shown above is symmetrical and easily reversible.

Also, I mentioned earlier that industrial “induction” motors have a distinct efficiency profile that has them running at a near-constant 3,000-RPMs. SR motors are able to run at unusually high RPM’s if desired. The Dyson company has developed a home-appliance vacuum where it’s SR motor spins up to over 100,000-RPMs.

Another characteristic is that these SR motors run cooler than a Permanent Magnet / PM motor for several reasons. first, there are no flux reversals, like the kind that are typically found on PM motors used in EV’s. As the SR energized coil-pairs are activated in sequence around the motor, they all create magnetic fields that flow the same direction. There are still some eddy-currents created, but nowhere as much eddy current heat compared to constantly reversing the fields in the stator back and forth.

The best way that I can explain eddy current heat is to say that all elements have loose electrons that can be pried out of one atom and shoved into the next atom by a moving magnetic field. Each Iron atom might theoretically have 26 electrons, but in the presence of a moving magnetic field, one atom might have 25, and another might have 27. The farther these loose electrons can be dragged, the more heat is generated (this is a horrible explanation of eddy currents, but its the best I can do right now). This is why the iron inside an electromagnet is not a solid rod, but it is made of a stack of thin plates called “laminations”.

Eddy currents moving (in red) steel laminations on the left, and a solid bar on the right

In the pic above, the green arrows indicate the direction of the magnetic flux. The red arrows on the circular shapes roughly show the paths of loose electrons. The individual laminations are electrically separated by a thin and clear varnish that insulates them. Laminations that are 0.50mm thick are common, but the thinner the lamination, the less eddy current heat you will experience. Although, the thinner laminations are more expensive, since it takes more of them to fill the stack size. I have seen 0.35mm thick lams as an upgrade, and you can even find laminations as thin as 0.20mm

The Tesla car company, Zero Motorcycles, and Alta Motors all use the inrunner configuration of motor, and this is where the rotor is located in the center, and the stator is connected to the outer shell. This makes it easy to use some type of external active-cooling (such as an air-fan or a liquid cooling pump) to remove the heat from the stator. If you have the stator at the core, and make the outer shell the spinning rotor, that style of motor is called an “outrunner”.

Once you have settled on the largest size of motor that can fit on your design, the way to get more power is to give the stator a temporary burst of amps. This causes lots of heat, and…if the magnets on the rotor get too hot (in a PM motor design) then the magnets will get demagnetized and ruined. Of course these motors have temp sensors, but it just means that if you are running hard very frequently, then you may find that the controller suddenly starts limiting the power you can use until the motor cools down.

SR motors can take much more amp-heat and they can do that much more often because they don’t have PM’s on the rotor, which could be damaged by heat.

Putting magnets back in

I mentioned earlier that in an SR design, it’s possible to have the direction of the electromagnetic fields all cycle in the same direction, with the benefit of producing less heat due to having no flux reversals.

There are some permanent magnet (PM) motors that alternate the direction of current through the electromagnets, which alternates the direction of the flow in their magnetic fields. Rapid flux reversals cause additional eddy current heat, but it can be one way to make a compact PM motor very powerful.

However, if you settle on using an SR design and you also specify that you want to have all the fields cycle in the same direction (unipolar), you have an additional option you can incorporate. In order to achieve the “double loop” magnetic field similar to the bar magnet pic (back at the top of this article), the aligned poles on either side of an energized coil in the stator do not “need” to also have coils, and those two poles can be made of steel laminations with no copper, just like the rotor.

A 2-phase SR motor using 10 salient poles on the rotor, and the stator uses two coil-pairs working along with four shared poles as flux-return paths. Depending on how you want to count the shared poles (as 4 or 8), this would be an 8/10, or a 12/10

The pic above is a graphic of a full motor using 2-phases and four shared poles. The pic below shows only half a motor, but that pic shows the flux lines through the rotor and stator when one of the phases is energized.

In the pic above, the magnetic flux lines are shown from a Finite Element Method Magnetics  analysis graphic (FEMM). The “half-motor” example above is the same configuration as the motor just above it. In this graphic, you can see that the red arrow is pointing to an energized pole (copper wire coil not shown), and the blue arrow is pointing towards the unpowered pole.

Since this is a symmetrical configuration using a 2-phase operation, the other mirrored half would be identical.The thin green lines represent permanent magnets that form a skin on the faces of the shared poles (which have no copper wire coils). The thin green PM’s in this location reduce torque ripple, and also smooth-out cogging.

A 2-phase shared-pole SR motor.

Here is a pic of a prototype above, just so you will know that this isn’t just an engineers theoretical idea. If one of the major benefits of an SR motor is the elimination of PM’s, it seems to be a step backwards to add them back again, but…take notice that two loose PM’s are shown in this pic. One is set on its edge at 11:00 O’Clock (on the stator), and another is laid flat at around the 8:00 O’Clock position.

Not only are the number of magnets significantly fewer in this type of motor, they are also very thin. It was found that a magnet of a one-millimeter thickness provided the best results (but expensive), and ordering magnets to be made at a 3mm thickness was the thinnest that provided the most cost-effective option. That is only 1/10th of an inch thick. The PM in this style of motor don’t need to be strong, so cheap ferrite magnets are a definite option.

Elon Musk has tweeted that the Tesla Model-3 motor is a switched reluctance, and it uses six poles (in the stator?). That struck me as odd, but if three of the poles are shared, it would perform like a 9-pole stator. If that motor has three large electromagnets in the stator (and three shared poles), then what would the rotor look like? And by what sequence pattern would the stator coils be energized? I don’t know.

I know that this may be a let-down after reading all the way through this long-ass article, but I am on the lookout for pics and a verification of the Tesla Model-3 motor configuration. I will post them here when I find them.

I also found out that the Range Rover Defender also has an EV variant that uses an SR motor (click here).

The Range Rover “Kahn” special edition EV

Speaking of clever configurations, remember I stated earlier that the shorter the path that the eddy currents have to travel, the cooler the motor will run? And remember how the double loops of the flux normally travel around the entire perimeter of the steel laminations in the stator? I found a brilliant design proposal (click here)  that uses a 3-phase operation, but instead of six poles, the designer split the poles into six pairs, for a total of 12 poles on the stator, and 10 poles on the rotor. You can see below how short the powerful flux loops are. This style is shown in the pic below.

An unusual 3-phase SR design proposal. One of the “paired” coils pushes, and the other one next to it pulls. The smaller flux loops would be very powerful and would run cooler. The pair opposite to them is doing the same thing.

Why are SR motors hot right NOW?

Once controller electronics became more sophisticated in 2011, and then this advanced style recently became more affordable, it was only a matter of time before someone realized you could make a less expensive motor using the SR type from 1888.

This year, China announced that their production of permanent magnet motors of all sizes and types was growing so fast that they would not be able to export as many of the strong neodymium magnets that these motors needed in order to work.

There is always a pressure to make a better performing product that also costs less, but…an announcement that your EV assembly line will no longer be able to get as many motors is a seismic event. The SR design changes all of that. So now, we are all going to be hearing a lot about switched reluctance motors in the near future…

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Written by Ron/spinningmagnets, November 2019

Grew up in Los Angeles California, US Navy submarine mechanic from 1977-81/SanDiego. Hydraulic mechanic in the 1980's/Los Angeles. Heavy equipment operator in the 1990's/traveled to various locations. Dump truck driver in the 2000's/SW Utah. Currently a water plant operator since 2010/NW Kansas


1 Comments

  1. Interesting article with effective use of diagrams.

    The more the better.

    Here is a link to a new class of small motors that claim 2 to 5 times the torque density and require no rare earths.

    Hunstable Electric Turbine by Linear Labs

    https://spectrum.ieee.org/cars-that-think/transportation/advanced-cars/new-electric-motor-could-boost-efficiency-of-evs-scooters-and-wind-turbines

    ct

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