Automobiles and Electric motors in the US!

Electric Motors, Made to Order

By LINDSAY BROOKE

THE electric motors that drive today’s hybrids and electric vehicles are not so different from those pioneered by Nikola Tesla and George Westinghouse a century ago.

Like their predecessors, modern electric machines, to use the engineers’ preferred term, are composed of two elements: a fixed housing that contains copper wire wound around an iron core, called the stator, and a rotor that spins within the stator’s open center.

The interaction of electric current and magnetic fields between the stator and rotor create rotational torque to spin the motor’s shaft — and turn the wheels.

Tailoring electric motors for duty in vehicles has necessitated the development of new materials, sophisticated electronic controls and some clever design variations, said Heath Hofmann, an associate professor of electrical engineering and computer science at the University of Michigan in Ann Arbor.

“The auto companies are focusing on machines capable of operating over a much wider speed range than typical fixed-speed industrial motors,” he said. Two primary designs for electric machines, A.C. induction and permanent-magnet, prevail in today’s hybrids and E.V.’s. They differ mainly in the construction and operation of their rotors.

The magnetic field of the rotor in an induction motor is generated by an electric current flowing through its copper windings.

In the rotor of a permanent magnet design — the type of motor the Chevrolet Volt will use — the field is generated entirely by strong magnets, without the need for current. More powerful motors can be built by making the magnets of rare-earth metals like neodymium, which increases the rotor’s magnetic flux (its total amount of magnetic field) and enables it to make more power.

Each motor type has its benefits and drawbacks. Permanent-magnet motors generate less rotor heat than inductive types, which aids efficiency. But as the motor’s size grows, magnetic losses increase proportionately, reducing efficiency.

Because permanent magnets tend to be brittle, Professor Hofmann said, General Motors, Toyota and others embed the magnets in the rotor, and the magnets are fairly expensive. There are strategic concerns: China has a near-monopoly on known rare-earth metal sources.

Induction motors do not suffer proportionate losses as size increases, and their design makes them capable of generating high power by operating at high speeds; the motor of the Tesla Roadster spins up to 14,000 r.p.m. They are generally less expensive to produce than permanent magnet types.

Specialist motor companies began improving automotive e-motors long before hybrid cars became popular. Remy, for example, invented a new stator-winding design that uses rectangular wire, rather than round wire, with windings that are arranged in multiple layers. The company says its design reduces heat.

For automakers, choosing a design boils down to a horses-for-courses decision. Induction motors appear to be the choice for battery E.V.’s where high performance is a main requirement, Professor Hofmann said. For economy-focused hybrids, permanent magnets may be better.

“I think it will be application-specific,” he said.