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Old 11-11-2011, 06:53 PM   #1
Car&Driver
Car and Driver
 
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Info We Build the Chevy Spark EV’s AC Permanent-Magnet Motor


Charles Kettering, GM’s first engineering wizard, christened the modern automobile a century ago with the invention of three crucial systems: dependable lighting, reliable ignition, and a means of cranking the engine at the push of a button. Boss Ket’s Dayton Engineering Laboratories became the ACDelco brand still in wide use for GM service parts.
Over time, the company that first electrified cars lost interest in manufacturing motors and generators, instead farming out that task to specialists who could make them better and cheaper. But eight years ago, after the EV1, before the Volt, and during the two-mode hybrid’s gestation, GM rethought that position.* According to Larry Nitz, GM’s executive director of everything electric, “We began taking electric motors to heart.”
Realizing that electric-motor technology is now a core competency, GM invested $270 million in preparing a White Marsh, Maryland, facility for volume electric motor manufacturing. (That plant, a former Allison enterprise, currently builds the two-mode hybrid transmission used in various Cadillac, Chevy, and GMC trucks.) A support facility in Wixom, Michigan, builds prototype electric motors for testing and validates the manufacturing processes that will be used in Maryland beginning in early 2013.
Pete Savagian, GM’s chief engineer for electrical equipment, is of the opinion that the only way to really understand how electric motors work and the best means of manufacturing them is the hands-on method. So GM dropped its guard for a few hours, had a select few journalists don gloves and safety glasses, and turned us loose at Wixom to build motors. Be advised that nothing we touched and no motor we assembled will ever be used to power any GM product.* No magnetic fields were injured during my motor plant incursion.
Electric motors seem simple to the uninformed. A cylindrical housing laced with wires—the stator—supports a rotating drum called, logically, the rotor. On cue, electric currents flow, magnetic fields are created, and the rotor produces a torque potent enough to make the wheels turn. *The magic part is maximizing efficiency while providing suitable durability and avoiding the undesirables: noise, vibration, excess cost, and unnecessary weight.

GM and other hybrid- and electric-car makers have zeroed in on two types of AC motors.* Two-mode-hybrid applications and the Volt both use permanent-magnet motors because they’re best suited to wide-ranging rotational speeds, heavy loads, and sustained use. (The permanent magnets are located in the rotor.) Induction motors are the alternative. Their rotors are magnetized by means of induction (magnetic fields created by the stator windings), and this type of motor is ideal for high speeds, moderate loads, and intermittent use. GM’s cost-effective eAssist mild-hybrid system uses an induction motor in part because the expensive rare-earth materials that comprise a permanent magnet aren’t needed.
My job was to make a few of the parts that go into the 114-hp permanent-magnet motor that will power electrified Chevy Sparks when they go on sale sometime in 2013.* Consider this a preview of coming magnetic attractions.
In essence, the stator is a tube consisting of electrical steel and copper. The steel portion has a high silicon and iron content and consists of many thin plates called laminations. Slots in the laminations are first lined with insulating sleeves made of paper and then filled with approximately 20 pounds of copper.* Instead of using round-section wires, GM’s design for the Spark’s motor fills the stator slots with rectangular-section copper bars. (The roughly 3-by-4-mm bars have a higher surface-to-volume ratio than round wire, which optimizes both their electrical and cooling characteristics.)

GM uses a two-lever press to form the copper bars to the desired shape. After loading a straight chunk of varnish-coated copper bar stock into the die jaws, I gave the first handle a hefty swing followed by a pull of the second lever.* Upon returning those handles to their original positions, the finished product—what looks like a hairpin for Andre the Giant—is ready for further processing.

The next step is carefully loading 120 of those hairpins into the correct stator slots. There are eight types of pins and they fit closely together, so care is needed to get the location and sequence perfect. After the pins are started, they’re shoved home—seated all the way inside the stator—in a hydraulic press exerting more than 1000 pounds of force to overcome the friction between the copper bars and the paper insulators.
To complete the electrical circuits, the protruding ends of the bars must be TIG welded together in a machine that heats each junction to 3000 degrees Fahrenheit in less than a second. Then the dielectric insulation must be replaced at the bare areas of the copper bars where the welding took place. After heating the entire stator to 300 degrees, the portion that needs to be coated is dipped into a container of aerated dry epoxy. To gauge the feel of this step, our GM hosts have me dip a single heated hairpin.

Another automated press pushes and turns the protruding bars into their final shape. During a few minutes in another automated fixture, varnish is carefully dribbled into stator voids to make sure that the bar-to-laminate insulation and attachment is complete. Filling the voids to eliminate relative motion is crucial to make sure the motor runs quietly, dependably, and without thermal issues. After attaching leads and conducting electrical tests, the stator is a finished component. Luckily, the component I worked on passed this acid test.

Now it’s time to construct the rotor. Like the stator, it consists of electrical steel laminations.* In this case they’re supported by a steel hub that turns in permanently sealed ball or roller bearings. A total of 400 permanent magnets—made of the rare-earth elements neodymium and dysprosium with smidgens of iron and boron mixed in for good measure—must be individually loaded into V-shaped slots in the rotor laminations. There are 20 Vs per layer, 2 magnets per V, and 10 total layers. The magnets, which look and feel like black Chiclets, come in two sizes.* The weird thing is that these “permanent magnets” are not actually magnetic! After they’re all glued in place with a drop or two of thermo-set epoxy adhesive, the finished rotor is sent out to a facility that makes them magnetic with a quick shot—some 9000 amps—of electrical current.

The stator and rotor are mounted inside a two-piece cast-aluminum housing. A shower of automatic-transmission fluid pours over the stator to remove excess heat. It’s important to realize that the manufacturing steps I experienced are only used to prove out the quicker, largely automated processes that will be used in Maryland and to manufacture prototype motors for testing and development of the Chevy Spark EV and other GM electric cars. Once production settles down, GM expects to construct 20 percent of the motors it sells.* Sixty percent of its requirements will be met by suppliers Continental, Hitachi, Remy, and possibly others building in accordance with GM designs.* The remaining 20 percent will be motors designed and produced by suppliers.
When the White Marsh facility goes online in 2013, GM will be the first carmaker to have a U.S.-based electric-motor manufacturing facility. I only wish Charles Kettering could enjoy these fruits of his pioneering labors.


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