AC Vs. DC Motors in Electric Cars

by Mike Southern

As gasoline becomes more expensive, the environment more threatened and technological breakthroughs more common, electric cars are finally on the verge of becoming a viable alternative. Because production models, such as the Tesla Roadster and the Nissan Leaf (as well as various hybrid models), feature AC motors, many consumers wonder what the difference is between an AC-powered car and a DC-powered car.

DC Motors

Until recently, most electric cars were built by do-it-yourselfers. DC motors are a natural choice because they are cheap to build. The components are readily available off-the-shelf; for example, a large generator from a jet engine can be quickly adapted for use as a motor. And because DC motors use the battery power directly, the control circuitry is much simpler. From an operating standpoint, DC motors generate more torque (starting power), so it is not unusual for a DC electric car to have no transmission---another cost-cutting aspect. And brushless DC motors generate less heat than an AC induction motor, meaning less energy is wasted.

AC Motors

AC motors are more complicated. Because the DC power of batteries must be converted to AC, the car needs a voltage inverter. The speed control circuitry is more involved, and the entire system is more expensive simply because the electric vehicle industry has not yet developed the necessary infrastructure to drive prices down. But AC motors have important advantages in electric vehicles. As motors get larger and performance becomes a consideration, AC can really show off; for example, the Tesla zips from 0 to 60 MPH in a mere 3.9 seconds. And while the circuitry is more complex, AC systems are actually simpler to install and safer. DC systems can short-circuit the entire battery pack, possibly causing the motor to catch fire, while a blown AC inverter simply fails and the car stops.

Batteries

The real breakthrough technology that is making electric cars a practical alternative to gas engines concerns the batteries. Unlike the lead acid batteries that power the lights and radio on our family autos, cutting-edge electric cars use lithium-ion batteries--the same technology that powers most laptops and consumer electronics. These batteries overcome many of the weight and power limitations of older battery types. Their ability to store a greater charge makes them a near-perfect power source for electric cars.

Production Cars

The Tesla and Nissan Leaf share some basic technology. Each uses an AC motor, and each uses lithium-ion batteries. The batteries can be recharged overnight from a 120-volt outlet using an on-board charger, as well as using special charging stations and chargers. Hybrids typically use the regular gasoline engine to recharge nickel-metal-hydride batteries. Unlike the Tesla and Nissan, which get all of their power from the batteries, these electric motors are only intended to reduce the time the engine runs.

Home-built Cars

Home-built cars will probably increase in popularity as more production electric cars hit the roads since many of the car manufacturers will see the profitability of making retrofit kits available. When this happens, AC conversions will probably become the dominant type of electric car.

About the Author

North Carolina native Mike Southern has been writing since 1979. He is the author of the instructional golf book "Ruthless Putting" and edited a collection of swashbuckling novels. Southern was trained in electronics at Forsyth Technical Community College and is also an occasional woodworker.

Photo Credits

  • photo_camera gas station image by Mat Hayward from Fotolia.com