What’s the difference between AC and DC electric car motors, and which one will work best for your needs? This article introduces the basics of electric vehicle (EV) motors and compares AC with DC and DIY (do it yourself) with premade, in terms of cost and popularity.
Direct current (DC) motors are the most popular do-it-yourself choice for EV conversions, for a few good reasons. First, they are the least expensive and most readily available; The NetGain WarP 9 is a popular DC choice which retails for about $1,800, and has enough power for a small or medium-sized car.
Second, the DC Series electric motors are known for their high torque capacity from standstill. While combustion-style engines lack power when getting started, DC motors shine! Applications such as diesel locomotive traction motors and drill motors (both of which are usually DC electric) help illustrate this feature. While torque is considered a benefit, though, continuous hill-climbing high-load driving with a DC motor is not ideal; AC motors do this much better. This is because DC motors have permanent magnets mounted on the inner surface of the motor housing. These magnets are able to provide high torque, but they don’t transmit heat well, leading to heat buildup in the windings. DC continuous operating horse power is substantially less than peak horse power (hp).
It’s worth noting, though, that DC motors have a reputation for being inexpensive, reliable, and accessible with extremely high peak power. Some DC motors for EVs are able to generate over 1,000 hp long enough to race a quarter mile drag strip, making DC the preferred choice for drag racers. The image below is a WarP 9 DC motor (similar to the one discussed above) that was adapted to fit directly onto the driveshaft of an existing manual transmission car for “direct drive”. For people who want to keep their manual gearbox and extend their speed range, ready-made adaptor plates can be ordered and bolted on between the electric motor and the transmission. It doesn’t get any easier than that, particularly if you’re a DIY type and making an EV yourself.
While alternating current (AC) motors aren’t used in DIY electric cars nearly as often as DC motors, it isn’t because they don’t perform well. On the contrary, AC motors shine in many ways — including continuous power for hill climbing, higher RPM, regenerative braking, wide range, light weight and overall power. AC motors just tend to be a lot more expensive than DC.
Why so expensive? A few reasons; one of which is the fancy converter system that changes the direct current coming out of EV batteries into an alternating current. Another is the sophisticated exchange system that allows for regenerative braking. As mentioned previously, AC motors are more efficient, and when combined with regenerative braking they are a clear winner for distance applications.
Don’t make up your mind just yet, though… For many smaller lighter cars (such as a converted Honda Civic) regenerative braking isn’t going to add value for the money. Plus the perceived value of a Honda Civic is fairly low (and much lower than the cost of installing an AC system, which is about $50K with labor). When you do the math, you realize why this is not a popular choice for DIY electric cars.
If you shell out the bucks for an AC system (and can convince a producer like AC Propulsion to sell you one) you’ll also want top-of-the-line batteries to go with it; the price tier for these systems is higher across the board. Keep in mind that the sooner you convert, the more gas you’ll save and the cooler you’ll look in your converted EV car. You’ll also be wasting fewer resources.
For those who aren’t DIY types and can afford it, a whole slew of branded pre-made AC electric cars are already on the market, including the $109K Tesla Roadster and the $45K Phoenix EV Truck, with the Chevy Volt and Dodge EV in the works. One final note: AC motors spin from zero to 13,000 RPM without needing to change gears. This means that the electric cars using them don’t require a transmission or manual shifting interface (something you’ll need in your DC EV car), which can save a lot of weight. AC motors are easier on the drive train, too, and will wear cars out a bit slower. They also require little to no maintenance, since they don’t have brushes like DC motors. On the other hand, DC motors use their brushes to create an oscillating AC current from the DC input current, so DC motors are, in fact, partly AC technology!
Many of the same trade-offs discussed here regarding motor performance vs. cost and availability also apply to motor battery systems. There are many factors that go into building or modifying a car, so do your research and use a supplier that offers support or has a number you can call before placing your order. You can also check back here on a regular basis, for more detailed articles on EV conversions and electric car parts.
