A magnetic braking primer: BMW i3 versus Tesla 70D

BMW i | July 10th, 2015 by 13
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The most revolutionary aspect of modern electric vehicles, from a driver’s perspective, is almost certainly single-pedal speed control.  This intuitive feature is shared by the BMW i3 and Tesla’s new 70D, and made possible by the intrinsic flexibility of the EV’s traction motor.
Single-pedal speed control is simply the control of both acceleration and deceleration with a single pedal.  Traditionally this has been done with two pedals, as the slowing system (hydraulic friction brakes) was always completely separate from the motive system (the internal combustion engine).  When electrics first starting appearing (and magnetic forces in the motor could be used to slow the car as well as drive it), EV manufacturers continued to use two separate pedals for go and slow, so that driving an EV would be the same as driving a conventional car.  The emphasis was recovering energy more than exploiting a better way to control the speed of the car.
With this traditional system, as the EV driver depresses the brake pedal, first magnetic braking is engaged, and as the pedal is pushed harder, the friction brakes also come into play.  The goal is for this to happen seamlessly and smoothly, but the complex control system for meshing hydraulic and electronic braking systems has often presented uneven results.  Further, because the driver has to use the conventional brake pedal to engage the magnetic brakes, the friction brakes invariably get used more often, resulting in a less efficient EV.
The electric motor as a braking force
In virtually all street EVs today, the electric motor is directly connected to the drive wheels via a single-speed transmission—and no clutch.  When the motor turns, the wheels turn.  When the car is rolling, the motor is turning.  When the EV is “coasting,” the motor is turning (albeit without using electricity), and acting like a flywheel.  Make the motor speed up—the car speeds up.  And if you decrease the motor speed, the car slows.  A single pedal can control the motor speed—up or down, so it makes no sense to control the motor with two different pedals.  With single-pedal speed control, the emphasis has shifted to using the braking power of the motor to enhance the driving experience.
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It’s technically a straightforward matter to design a single pedal to control the car’s speed, either up or down or constant—it’s just a matter of writing the software that determines the path, frequency, and strength of electricity into and out of the motor, which in turn controls the magnetic fields in the motor, making it go faster or slower.  To be sure, writing that software requires some real talent, but that’s the direction of automotive engineering.
The basic principle involved is the same as with your ordinary refrigerator magnet.  As you bring the magnet closer to the refrigerator, the magnet pulls (accelerates) your hand toward the metal door.  The closer you get, the more powerful the magnetic field, the stronger the force.  When you try to pull the magnet off, it resists (magnetic braking).  If you do the same experiment with two magnets brought together, the forces are much greater—opposite poles (north and south) attract, and like poles repel.
For a motor to work, however, you have to be able to control the magnetism in at least part of the motor, and you can’t do that with just permanent magnets (where the north pole is always north, and the south pole stays south).  You need electromagnets.  As the name suggests, they derive their magnetism from an electric current flowing through a coil of wire.  Increase the current and the magnetic field gets stronger; reverse the flow of current and the polarity of the magnet reverses (north becomes south, south becomes north).
There are different types of motors used in EVs, but they all work on the same basic principles (the diagrams that follow are simplified and generic; the actual motors and their electronic controls are quite complex).  The inner rotating part of the motor is called the rotor.  It contains a series of magnets around its periphery, often permanent magnets, that generally maintain their polarity (that is, the north pole is always north, and south is always south).  The fixed outer part of the motor (the stator) contains a series of electromagnets around its periphery.  The electric current going into the motor powers these electromagnets, but because EV motors use alternating current (electrons reversing direction), the poles of the stator’s electromagnets continually change from north to south to north, etc.  From the rotor’s perspective, the stator’s magnetic fields—its north and south poles—seem to be rotating.
The rotor’s magnets are pulled along by these rotating magnetic fields in the stator.  Speed and torque are changed by varying the frequency of the alternating current and how far the stator magnetic field is placed ahead of the rotor’s magnets (“lead”).
When power to the motor is cut and magnetic braking engaged, the stator’s rotating magnetic fields continue in the same direction as the rotor rotation, but the motor’s electronic controller directs the stator’s magnetic poles to lag behind the rotor’s opposite poles, so the magnetic forces are now pulling against the direction of shaft rotation.
When magnetic braking is activated, an electric current is induced in the wires of the stator’s electromagnets (caused by the wires passing through the rotor’s magnetic fields), and that alternating current flows out of the motor, is converted to direct current, and charges the battery.  (The phenomenon of electromagnetic induction is widely used in the wireless charging of many consumer products.)
 Motor Schematic Regen.pages
Figuring out how the motor slows the car is step one; deciding what to call it, using a term that makes sense and is self-explanatory, could be just as important.
Using magnetic forces in the motor to slow an EV has most often been labeled “regenerative braking,” or “regen” for short.  When electric power from the battery is cut off, and the car is rolling and therefore rotating the motor, the motor can generate electricity to charge the battery.  But then where does the “re” in “regeneration” come from?
Presumably because part of the energy expended accelerating the car, or moving it up a hill, is re-couped when decelerating or driving down the hill.  But the motor is simply generating electricity; it’s not “regenerating” it.  Why create a new meaning for a term that is sure to baffle ordinary people?
Dr. Who regenerates.  A lizard regenerates a new tail when the old one has been snatched by a predator.
BMW uses the term “brake energy recuperation,” which also begs an explanation.  Recuperation is what you do after you get sick, but yes, it also means recovering or regaining something.  But then, is the meaning of “brake energy” apparent?
What is apparent when you lift up on the speed pedal, is the car slows.
Neither “regenerative braking” nor “brake energy recuperation” explains what slows the car (as a parallel to friction braking).  With conventional brakes, it is friction that slows the car (creating waste heat, a coating of black dust on your wheels, and regular income for auto service departments).  With your EV, it is magnetic force that slows your car (creating electrical energy to charge your battery as a side-effect).  Hence, friction braking and magnetic braking.
Whatever you choose to call it, it should be implemented in a way that appeals to drivers.
The psychology of magnetic braking
Magnetic braking is new.  It’s much more technically sophisticated than the mechanics of hydraulic pistons pressing brake pads against a brake disc.  And unlike friction brakes, magnetic brakes are invisible.
It’s no wonder marketing and sales people don’t want to explain them.  They are eager to talk horsepower, torque, 0-60 mph/0-100 kph times—all familiar.  How often do they highlight stopping distances?  Braking is arguably much more important to a driver than how fast a car will accelerate, but braking performance seems to be off the radar.
And yet, a year-long study of magnetic braking from a user’s perspective revealed that drivers found it very appealing.  That 2014 German study (a PhD psychology dissertation by Peter Cocron, Chemnitz University of Technology), also considered the results from similar studies in several other countries.  Forty MINI Es were involved in each of the two six-month study periods.  The MINI E uses single-pedal speed control and has strong front-wheel magnetic braking.
The study looked at single-pedal speed control of magnetic braking (versus using the separate brake pedal), strong versus mild magnetic braking, and the importance of the driver being able to select the maximum level of magnetic braking.
Single-pedal speed control
Cochron found that study participants greatly preferred magnetic brake activation by accelerator (versus brake pedal), and “quickly adapted to driving with one pedal.”  Many of the drivers acclimated to the single-pedal speed control by the end of their first test drive.
Both the i3 and the Tesla 70D incorporate single-pedal speed control; their brake pedals only control the friction brakes.  The (not-to-scale) chart below shows that the forward part of the pedal travel sends power to the motor, and the rearward part of the pedal travel slows the car.  But don’t overthink this control mechanism—just drive the car intuitively.  The pedal controls the speed—push down some to go faster, let up some to go slower, or put your foot at a spot that maintains a constant speed.  It’s seamless; there’s no shifting your foot from one pedal to another to inform you when you go from power to braking.  Because the motor is always directly connected to the wheels, you won’t readily be able to tell where the transition is.


Remember, just as pushing the pedal to the floor will give you maximum acceleration, releasing the pedal completely will give you maximum magnetic braking.  For everything in between, your foot needs to stay on the pedal.
Strong versus mild
The MINI E’s magnetic braking has consistently been described as strong—stronger than in the i3.  A great majority of the MINI E drivers in the study “liked the strong regenerative braking after they became accustomed to it, and tried to use the friction brakes as little as possible.”   In a 2012 study, test drivers found EVs with stronger magnetic braking to be “more directly controllable than EV concepts with [weaker regen].”
A few drivers in the later study mentioned that in the beginning they “often stopped too early, for instance, at a traffic light as they underestimated the deceleration of the system. However, they reported that after the initial adaptation phase they managed to decelerate quite accurately as they used the pedal more sensitively.”  In other words, they discovered they could modulate the braking force by not fully lifting off the pedal.
Magnetic braking in the Tesla Model S is not quite as strong as in the i3, but strong enough so that use of the friction brakes in normal traffic can be avoided.  But while the i3 can come to a complete stop with just magnetic brakes, the Tesla needs the application of friction brakes for the last few miles per hour before a stop.  Tesla did not offer clarification on this, but I suspect it’s because the i3 uses a hybrid synchronous motor with permanent magnets embedded in the rotor, while the Tesla uses an AC induction motor without any permanent magnets, and its induced magnetic fields may collapse as motor speed approaches zero.
Driver-selected level of magnetic braking
Both the BMW i3 and Tesla 70D employ single-pedal speed control, so drivers can control the level of magnetic braking from zero to maximum.  In this section, we’re talking about users being able to adjust the maximum strength of the car’s magnetic braking.
The maximum level of magnetic braking strength in the i3 is not user-adjustable.  It’s strong enough to satisfy most of its drivers, at least at lower speeds, but many of those i3 drivers would still like the option of making it stronger.  At highway speeds, BMW reduced the maximum braking strength in response to MINI E and ActiveE driver feedback, presumably so someone inadvertently taking his foot completely off the speed pedal would not slow abruptly in heavy highway traffic.  Of note:  if you disengage the i3’s cruise control with your foot off the speed pedal, magnetic braking will immediately engage at its maximum level (they recommend applying foot to pedal before disengaging).
The Tesla’s maximum magnetic braking strength “is only reached under certain [unspecified] conditions and when the driver completely releases the accelerator pedal.”      But two levels can be user selected, toggled on the touchscreen between “standard” and “low.”  The lower setting simulates a conventional car’s mild engine drag when the driver lifts off the accelerator.
The psychology study made a few observations about driver selection of magnetic braking level.  With respect to the MINI E, it noted that drivers were forced to adapt to the strong braking because they could not change the level (even though they grew to like it a lot).  But some of the drivers resisted, and were slower to adapt.  “As some drivers wished to modify the system individually, and there exists a negative relation between acceptance and the reported learning duration, it may be reasonable to integrate different levels of regeneration in future EVs.  As a consequence, drivers could . . . gradually increase the deceleration during the early days of usage.”
Or drivers could just keep the “low” button pressed and stay in their comfort zone.  Which approach is best:  force new drivers to adjust to strong magnetic braking, humor their old habits, or give them a gradual learning progression?   The issue is illustrated by the following (youtube) exchange between a first-time EV driver testing a Tesla 70D, and the Tesla minder in the passenger seat.
At the end of the first block, the driver took her foot off the speed pedal.
Driver:  “Oh, yeah, that really does stop,” she exclaimed.  “If you back off like I’m used to gas with coasting.”
Minder:  “You can make it do that.”  (meaning he could make the Tesla slow like a conventional ICE car)
Driver: “Oh, you can?”
Minder:  “Yeah, there’s a setting here; I’ll change it.”  He then switched the magnetic braking to the “LOW” setting, but he didn’t explain that she could get less braking simply by letting up just a little bit on the speed pedal.
Driver (later):  “Wow, it does take a few seconds to get used to the driving, as far as letting your foot off, and stuff like that.”
Minder:  “If you leave that regen on, that you felt, it takes just a little while to get used to it.   Then it’s actually kinda nice. cause [unintelligible]…  you can almost drive the car without even using the brake.”
At that point the driver must have thought the two options for (“regen”) when she let up on the accelerator were either mild conventional engine drag, or maximum magnetic braking.  Clearly a little education would have been in order, including what he meant by the word “regen.”  Otherwise drivers will learn how it works either by accident, or running into someone who can explain it.
So, Tesla’s two settings (low and standard) are a good thing (as long as magnetic braking is explained to the new driver), because some people will need more time to adjust to the EV driving experience.  Other drivers will want more aggressive magnetic braking than standard.  The choice between low and standard may be too big a jump.  If there were five steps, for example (with the top step giving stronger braking than what either the i3 or the 70D now have), the progression from old habits to new could be more inviting.
Four-wheel versus two-wheel magnetic braking
Magnetic braking only works through the wheels connected to the motor.  The i3 is rear-wheel drive, and therefore the magnetic braking is through the rear wheels only.  Since the front wheels are the most effective at braking (due to weight shift on deceleration), the effectiveness of the i3’s magnetic braking could be better.  The MINI E, with its front wheel drive, could support stronger magnetic braking, and did (but the RWD ActiveE also had magnetic braking levels higher than the i3).  Both trial cars experienced some magnetic braking anomalies in certain conditions, however.
The all-wheel drive Tesla Model S 70D provides balanced magnetic braking (and more even tire wear).  As implemented, its magnetic brakes are independently controlled front and rear; resistance is automatically adjusted at each axle to match conditions.  It should be less prone than the i3 to traction loss during difficult braking conditions.
The 70D therefore has the potential to have stronger magnetic braking than either the i3 or the MINI E.  But does it?
Unfortunately, a Tesla representative confirmed that the maximum level of magnetic braking in the 70D is the same as that in the RWD Model S 60.  Tesla, however, says that with its “free over-the-air software updates, features such as regenerative braking have the potential to be optimized with time to provide the best driving experience Model S can offer.”
The future  
Magnetic braking is in its infancy.  There is still a lot of room for improvement (or “optimization”).  It’s certainly not going away.  After using single-pedal speed control, drivers are not going to accept anything less.
The German study found that, “Drivers expected that the usage of conventional braking would in most cases no longer be necessary, if the vehicle was equipped with regenerative braking.”  This expectation was often accompanied by the notion that “conventional friction brakes were only used for emergency braking.”
Single-pedal speed control is, in a word, addictive.  Cocron reported that, “After using regenerative braking for an extended period of time, the need to conventionally brake could be challenging . . .”