Why aren’t there a bunch of electric street vehicles for sale at your local car/truck dealers? Are they really hard to build or super expensive? Are they unreliable or worse yet dangerous? These are things that bothered me when I started thinking about making an electric vehicle.
To learn about electric vehicle design options I read “Build Your Own Electric Vehicle” by Bob Brant (McGraw Hill-1994). This laid out the basic considerations and options. Many of the key decisions were discussed with pros and cons listed. After reading this book and finding web sites for electric vehicle conversions, such as those listed in the “Links” tab on this site, and talking with builders at electric vehicle meets, the decisions started coming into focus one after another.
I learned that this was not a start from scratch effort, but that much work had been done and parts were readily available.
Here’s what I found:
Motor: AC versus DC – Alternating current motors are available and offer the advantage of allowing regenerative braking, i.e.: turning the motor into a generator to charge the batteries when braking. For DC motors, this seems to still be in the doctoral thesis realm. However AC motors don’t run directly off batteries and require an inverter to change the battery voltage into an alternating voltage to drive the motor. Some web sites state that AC motors are cheaper but when I priced similar systems, the AC motors were more. I believe in some cases the AC motor’s price includes the controller. After reading, it seemed that most take the DC motor route, so I decided to as well.
While there is some discussion that permanent magnet DC motors are more reliable, the preference for available motors seems to be series wound DC motors. A number of companies make these and I have read that they are maintenance free for 100,000 miles and then only requiring replacement of brushes.
The sizes and ratings for a DC motor needed seem to be in the 20 hp (equivalent) range for cruise and 100 peak horsepower for acceleration for a moderate sized vehicle with reasonable acceleration, and range. Many of the web sites have good explanations on the rating of motors and how they differ from internal combustion engines. Engines have high horsepower ratings, but the horsepower is achieved only at high rpm. So a 200 horsepower engine may not reach 20 hp until 4000 rpm. The dead-stop takeoff horsepower is much less. An electric motor has high torque and power at 1 rpm, so a high horsepower rating is not needed. When a car is cruising at 50 mph, it needs only around 20 hp. I am choosing a motor that is around 32 hp continuous and 100 hp peak.
The DC motor requires a controller. The decision points on the controller are the continuous current, peak current and cooling requirements. The motor curves seem to show that a continuous current in the 200 amp range would be needed for cruising and above 500 amps for acceleration. I am picking a 750 amp controller for this project.
Another decision point is whether to use the transmission or select a direct drive option. After talking with the experts, a motor is available which can hook up directly to the drive shaft. This would eliminate the transmission but require an electrical reversing solenoid, which is available from several sources. The experts tell me that the problem is starting out. A direct drive system has no mechanical advantage provided by the transmission gear reduction and requires huge currents to start off. This means sizing the controller, fuse, contactors, batteries, etc. for the huge startup currents which drives the price up. Also the clutch is an additional way of disconnecting the motor from the wheels for certain failure modes. I picked using the transmission option. There are several sources for kits to adapt the transmission to the electric motor for a variety of vehicles. There are also instructions available for making your own. I selected the buy option, since the spacing is critical between motor and transmission and I worried about the balance of homemade collars and adapter rings.
Another decision point involved selecting a vehicle. My thoughts were there would need to be lots of room for batteries so a pickup would solve that problem. I would have the bed and possibly room under the bed for battery space, if needed. A smaller truck would reduce weight and I preferred an American brand, thinking replacement parts costs would be lower. I found that the Chevy S-10 was a good candidate. It seems to be well liked among owners, uses many standard GM parts and has a strong frame. I found a 1997 truck in good shape for a decent price. Photos are on the site photo page. The truck has 121 K miles but the engine still runs and the transmission seems sound.