Electric Vehicle Drivetrains

From Mars to cars by sea

NASA used a simplified version of the Electric Wheel® in the Sojourner Mars rover and has supported its development since 1994 through its Mid-Atlantic Technology Center in Pittsburgh. "The Electric Wheel," said former NASA Administrator Daniel Goldin "has application to everything that moves." NASA approached Solomon Technologies first to offer help after learning about the Electric Wheel through a patent search. After considering the full Electric Wheel for the Sojourner Mars rover, NASA adapted Solomon Technologies' revolutionary technology to the rover's limited speed and light weight requirements. The result was a single motor version with planetary gear set in each wheel to power the rover. In return for the rights to use the patented technology in the rover and other space vehicles, NASA set up an Electric Wheel Working Group with engineers from universities, government research centers and private industry. With the Working Group's help, Solomon Technologies has been able to test and refine the system and establish cost-effective manufacturing procedures.

To further advance marine propulsion for the ever-expanding boating industry, we developed a line of single- and dual rotor/stator drive motors without the integral transmission. In many marine applications, the STI high-torque, low rpm motor does not need the infinitely variable transmission (IVT) required for land vehicles and heavily loaded machinery. Powering the prop, the water acts as a form of minimal torque converter. The propeller can simply slip in the water at startup until the vessel gathers way.

Nevertheless, the Electric Wheel's dual-motor, integral transmission does provide two major advantages in a boat. If one drive motor malfunctions, the other still is able to spin freely to turn the prop and bring the boat home. And at low and medium speeds, only one of the two motors in the unit is needed, which greatly extends the charge in the batteries.

Looking beyond sailboats, the NASA Working Group experts contend that the Electric Wheel design is scalable up to 5,000 horsepower. Larger models could replace the diesel engines and hydraulic winch motors in tugboats on the Mississippi, dinner cruise ships on the Potomac, ocean-going fishing trawlers in the Pacific Northwest and the thousands of barges that ply the network of canals in Europe. It's also ideal for crane winches, farm tractors, conveyor belts, fork lifts, and mining machinery, logging equipment, HVAC systems and steel rolling mills.

And our R&D plans include the development of Electric Wheel motor/transmissions for hybrid-electric cars, trucks, trains and buses. "The list of applications for this technology is mind-boggling," said James Dunn, Executive Director of NASA's Center for Technology Commercialization. "The Electric Wheel," he told a 1995 NASA Technology Transfer Conference "is destined to be the propulsion system for the next millennium."

Far Beyond Conventional Motors

The Electric Wheel is a device that is composed of the least number of parts that can be joined to form motor and variable speed/gear ratio transmission. Unlike other devices though, the transmission is integral instead of external to the motor. The gears are physically attached to the motor rather than in an external housing. This characteristic eliminates external coupling losses and minimizes parts. In most motive applications torque is required at all speeds.

For this reason, every motor and engine needs a transmission. A transmission trades motor RPM and torque for a more appropriate RPM and torque. These two, RPM and torque, are inversely proportional when passing through a transmission.

Revolutionary design

The Electric Wheel device is, in many ways, no different than these examples. Even electric motors have efficiency ranges and require transmission functions to allow efficient operation at slow speeds. These efficiency ranges are an inherent characteristic of all motors and engines. When used in transportation and other applications, they need the aid of a transmission to vary output. The revolutionary difference with the Electric Wheel is that it performs the transmission function with a minimum number of parts and when the transmission function is no longer needed, the frictional losses (heat and wear) are eliminated. This is accomplished by actually rotating the entire transmission inside the motor and with the motor when it is no longer needed. This is all possible because within the Electric Wheel both the sun gear and the ring gear of the planetary gear set are powered.

This method of operation offers a very important benefit of the Electric Wheel; it can operate with one electric motor disabled, or half of its parts. It can also operate with full power with the gear set fused together. These important benefits make the Electric Wheel like no other device - an entirely redundant propulsion system that is totally self-contained. It is for these reasons that NASA has called the Electric Wheel "the propulsion system for the next millennium." The Rocky Mountain Institute has stated: "the most elegant and simplest example of an engine on the planet."

The patent was granted such that "at least one of the inputs being of electrical power". This is to say that the Electric Wheel can derive its second input power from a source other than electric. It could have an electric ring and hydraulic sun, or steam ring and an electric sun or an electric ring and fossil fuel sun, or any combination. It could also have multiple electric technologies combined, for example an AC motor sun and DC motor ring. More importantly, it allows for the motor technology to be employed that is most appropriate for the specific platform needs.

The Electric Transaxle™

The Electric Transaxle™ is a special adaptation utilizing one and a half Electric Wheels. It has all the same features of the Electric Wheel with two independently operated output shafts. This makes it ideal for terrestrial vehicle applications where turning, and even steering, require one wheel to drive while the other slips. The Electric Transaxle consists of a completely self-contained motor/transmission/differential device, containing as few as thirteen moving parts. In the simple application of a 4-wheel vehicle, the Electric Transaxle could be employed in the front axle and/or the rear axle. Redundancy would be achieved and the shock sensitive parts (close tolerance electric motors) would be moved to a safe location securely mounted to the frame.

In the simplest of applications, a rear wheel driven vehicle has the I.C.E., transmission and drive shaft removed, the rear differential replaced with an Electric Transaxle and batteries installed in locations to re-establish the weight balance of the vehicle. This allows for easy integration into existing vehicle lines. The Electric Transaxle patents also allow for any input power devices. However, the real beauty of the Electric Transaxle is its scalability and self-contained aspect. With the electric motors within the Electric Transaxle properly sized there is no need for an I.C.E. to be attached to the drive train. As a stand alone device an Electric Transaxle powered vehicle could employ a conventional, highly efficient diesel generator for auxiliary power when battery power alone is insufficient for range desired.

Remarkable Capabilities

The Electric Wheel consists of a driven ring gear element, a driven sun gear element and planetary gear element that drives an output shaft. The Electric Transaxle consists of two independent planetary gear elements that drive independent output shafts and either joined driven ring gear elements and two independently driven sun gear elements or joined sun gear elements and two independent ring gear elements.

Both devices provide torque multiplying features thru the use of infinitely variable transmissions. Full torque at any shaft output speed - from one revolution per minute to the motor's maximum. In engineering terms, its torque curve is flat. That's a big advantage over an internal combustion engine, which only develops full torque within its narrow peak operating band and consequently requires a continuously engaged transmission to turn a loaded shaft at low RPM.

Instantaneous, precision control - ideal for automated response to rapidly changing external forces, for example, for differential wheel slip control in a four-wheel-drive vehicle. Altering the speed or direction of either motor can change the gear ratio, drive shaft rpm and output torque.