How an Electric Car Motor Works

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Electric car technology hasn't really changed in the last 100 years, it's just gotten smaller.
Electric car technology hasn't really changed in the last 100 years, it's just gotten smaller.

Electric vehicles may be far simpler in design than engine-powered vehicles, but building one is far from simple. Think of the electric car like a natural-stone building; its straightforward construction belies the amount of engineering involved beforehand. Understanding how an EV's motor system works is the first and most important step toward building one that you can be happy with or getting the most out of the one that you have.

  1. The Energy Source

    • Electric cars almost always get their power from some sort of battery back. Older hybrid technologies used sealed lead-acid batteries, nickel-cadmium (NiCad) or nickel metal-hydride (NiMH). The push in recent years had been toward lighter and more powerful lithium-based batteries, including lithium-polymer (LiPoly), lithium-ion (Li-ion, often used in laptops) and the still-underdevelopment lithium-air design. Hydrogen fuel-cell cars use an on-board power station to constantly recharge the batteries, and a series electric (which uses an engine-driven generator to power the drive motors) may use no battery at all. Almost all electric cars take advantage of some sort of regenerative braking. ReGen involves switching the motor's output so that it turns into a generator instead of a motor.

    AC vs DC Power

    • Electric current comes in two varieties: alternating and direct current. Direct current electrical power, which batteries produce, moves in only one direction. Power exits one of the battery's terminals, goes through the motor and loops back into the battery. Generators and alternators produce alternating current; in this configuration, the "positive" and "negative" wires constantly switch in polarity. In effect, alternating current transfers energy by "vibrating" the electricity as opposed to sending it through the wires. Alternating current is more efficient because the AC configuration's electrons don't actually move through the wires in the way that DC configuration's do.

    Field Switching

    • An electric motor works by constant shifts in the fields in an electromagnet to change the magnet's north/south field polarity. Electromagnets work by using spiral-wound copper wire to induce electron movement around a metal conductor; if the electrons more one way, the magnet is "north," and the other way makes it "south." If you connect an electromagnet coil to an AC power source, it'll automatically switch field polarity because of the electron's "vibration." If you connect the electromagnet to a DC source, you'll have to manually switch the current from positive to negative and back to obtain the same effect.

    How the Motor Works

    • Imagine lining up a bunch of bar magnets end-to-end and bolting them together so that the north pole of one butts up against the north pole of the next. Now the you've got a "track" of magnets, imagine what would happen if you placed another bar magnet on top of it. That bar magnet would immediately stop where its south pole sat over a north on your track. Now quickly change the track magnet's north pole to a south; what happens? The loose bar magnet shoots either forward or backward and stops. Keep switching the track's polarities north-south-north-south-north, and so on, and the loose magnet will travel along the "track." This is exactly how a monorail train works. Now, take your track, wrap it around in a loop and put the center of the loose magnet on a spindle. As the track polarity changes, the center bar magnet spins around and you've got an electric motor.

    AC and DC Motor Function

    • An AC motor works just in the way described above. The constantly switching AC current changes the polarity on a set of electromagnets mounted on the inside of the motor's case, and a permanent motor mounted on the shaft inside spins to keep up. The DC motor is a little backward because it needs a way to switch the field polarity. The DC motor uses permanent magnets on the outer case and electromagnets on the shaft. The electromagnets get their power from a commutator on the shaft. As the shaft spins, the alternating commutator pads come in contact with the motor's positive or negative input "brushes," thus switching the power as the shaft spins.

    AC vs DC Strategy

    • Electric cars may use either an AC or DC motor, depending upon what the builder or manufacturer is looking for. The DC motor's commutator does a fine job of switching polarity, but it also increases drag, heat and electrical resistance inside the motor. This makes the DC motor inherently less powerful and less efficient than the AC motor. But AC setups are far more expensive to manufacture. The AC motor may cost no more than the DC, but it requires a power inverter to switch the battery's DC signal to AC. In an electric car, the power inverter is the AC motor's electronic equivalent to the DC motor's commutator. This isn't so much a problem for large manufacturers with access to wholesale, specially built, high-current-capacity inverters, but it can create a significant stumbling block for do-it-yourself EV builders. Often, an inverter capable of delivering the required 50,000-plus watts of power to the motor will cost more than the entire car.

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References

  • "Build Your Own Electric Vehicle"; Seth Leitman; 2008
  • "The Electric Car: Development and Future of Battery, Hybrid and Fuel-Cell Cars"; 2001
  • "Foundations of Electrical Engineering, Second Edition"; John R. Cogdell; 1995
  • Photo Credit Medioimages/Photodisc/Photodisc/Getty Images

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