DC vs. AC Electric Car Motors
What's the difference between AC and DC electric car motors and which one will work best for your application? This article introduces the basics of EV motors and compares DC vs. AC in terms of cost, popularity, and DIY vs. premade.
Direct Current DC motors are the most popular do it yourself choice for EV conversions for a few reasons. Firstly, they are the least expensive and most readily available EV motors. The NetGain WarP 9 is a popular choice which retails for ~$1,800 and has enough power for a small or medium sized car.
DC Series wound electric motors are also known for their high torque 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 to illustrate this concept. While torque is considered a benefit, continuous hill climbing high load situations are not ideal due to their construction. For this type of application (hill climbing for example) AC motors can do 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 a high torque, but they don't transmit heat well, leading to heat buildup in the windings. This means that the continuous operating hp of DC motors is substantially less than the peak hp.
Before we skip on to AC it's worth noting that DC motors are known for being inexpensive, reliable, available to the public and do-it-yourselfers, with an extremely high peak power. Some DC motors for electric vehicles are able to make over 1000 hp long enough to run down a quarter mile drag strip, making direct current motors the preferred choice for drag racing. The image below is a WarP 9 DC motor (similar to the one discussed above) that has been adapted to fit directly onto the driveshaft of an existing manual transmission car for "direct drive", and for people who want to keep their manual gearbox to extend their speed range ready-made adaptor plates can be ordered that simply bolt on between the electric motor and the transmission. It just doesn't get much easier than that for DIY!
While 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 capabilities, range and weight, and overall power.
So what makes AC motors so much more expensive than DC, besides performance? A couple of factors, including fancy converter systems that change the direct current coming out of EV batteries into alternating current (AC) and sophistocated exchange systems that allow for regenerative bracking. As mentioned previously, AC motors are more efficient, and when combined with regenerative braking they are a clear winner for distance applications, but don't jump to conclusions just yet... For many smaller lighter cars (such as a converted Honda Civic) regenerative braking just isn't going to add much value for your buck. Add to this that the perceived value of a Honda Civic's as a car is fairly low (and much lower than the cost of an AC system with labor ~$50K) and you start to realize why this type of setup just doesn't float for DIY electric cars. Additionally, if you shell out for an AC system (and are actually able to convince a producer like AC Propulsion to sell you one) you'll probably want top of the line batteries to go with it; the price tier for these systems is higher across the board. On the flip side, 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. If you can afford one of these and are willing to wait (in line), or want to wait for a Chevy Volt or Dodge EV, power to you! Keep in mind, the sooner you convert the more gas you'll save and the cooler (more custom) car you'll have as a conversion vs. buying something new; you'll also be wasting fewer resources and saving a lot of money.
One final note, AC motors spin up from zero all the way to 13,000 RPM without needing to change gears and this means that the electric car's using them don't require a transmission or manual shifting interface (something you'll need in your DC EV car) and that can save a lot of weight. An added benefit here is that AC motors are easier on the drive train and wear cars out a bit slower, and also require zero maintenance since they don't have brushes like DC motors, but in general the differences aren't that extreme.
At present time there are many reasons to be excited about AC systems but still opt for going with DC. And if it makes you feel better, DC motors use their brushes to create an oscillating AC current from the DC input current, so they are actually partially AC machines! Many of the same trade-offs discussed here regarding motor performance vs. cost and availability also apply to battery systems and there are many factors that go into building or modifying a car, so do your research and use a supplier that offers some support or a number you can call before placing your order. Check back often here for more detailed articles on EV conversions and electric car parts.