Electric ftransaxle drive systems power new aircraft GSE vehicles

NASA Tech Briefs, Aug 2002 by Vogler, Kevin

Motion CONTROL

Tech Briefs

Strenuous efforts are being made by the airline industry to reduce pollution and improve air quality at airports around the globe. Airport ground support equipment (GSE) can be seen dashing back and forth between aircraft, passenger terminals, and other facilities. The internal combustion (IC) engines used in increasing numbers of GSE vehicles are a significant source of airport pollution. To counteract this condition and meet recent state and federal EPA requirements in the U.S., increasing numbers of zero emission electric GSE vehicles are being introduced into the working environment of airports.

The use of an electric traction drive system eliminates nitrous oxide, carbon dioxide, and other internal combustion engine pollutants. Electric vehicle power systems are much more efficient and cleaner than IC power systems, even when the pollution from the electric power plant is considered.

Real World Application Baggage Tractor

Many types of GSE vehicles provide the necessary mobility to move passengers, cargo, and crew into and out of aircraft. Service technicians and mechanics require special mobile lifts to provide critical service and inspection activities. The airport baggage tractor is one of the first GSE applications to begin conversion to all electric drive systems.

Tractor weight is typically 7,000 to 8,000 pounds with battery. The maximum speed can reach 14 miles per hour (mph) with a 25,000-pound load. The draw bar pull required is 3,500 to 4,000 pounds. These tractors are able to accelerate to 15 mph in less than 12 seconds when pulling a 7,000 pound load.

Developed with input from six major airlines, the recently introduced integrated 80 Volt AC electric drive system is designed for GSE vehicles rated up to a 5,000 pound static drawbar pull and a top speed of 20 mph. Ballard Power Systems' Ecostar powertrain has been installed in aircraft ground support vehicles manufactured by Stewart and Stevenson TUG, Charlatte, Eagle Industrial Truck, Fleet Body Equipment, and others. Today these vehicles are being used at many airports, including Chicago's Midway and O'Hare, Tulsa, Oklahoma City, San Francisco, Atlanta, Houston (IAH), Denver, and Toronto.

The Ecostar system, which meets the EPA's zero emission program, currently provides six major components, all integrated to function as a single vehicle drive and control system for this application:

1. Transaxle power transmission assembly.

2. 80V AC system controller, using a

torque-based vector control algorithm, with MOSFET power devices; system includes contactors and fusing.

3. Hydraulic brake line pressure sensor for dynamic regenerative brake control.

4. Electronic accelerator pedal assembly (first used in the Ford Electric Ranger).

5. DC/DC Converter, provides 400 watts of 12V DC power for vehicle systems.

6. Battery state-of-charge display gauge with a seven-character LCD readout.

The system electronics are passively cooled via a panel mounted directly to the vehicle's chassis. The AC induction motor and axle assembly are mounted directly to the vehicle chassis for mechanical rigidity. The transaxle attaches to the vehicle using two metal-elastic pivot bushings and uses coil springs and shock absorbers. Any DC power source such as 80V flooded lead acid batteries of today or fuel cells of tomorrow can power the electric drive system.

Transaxle Assembly

The drive axle (tee) assembly is composed of a drive axle, AC motor, planetary gear reduction, and wet disc brakes all integrated into a single co-axial assembly. The overall transmission gear reduction is approximately 25:1. Low gear noise is achieved by the use of helical gears in the planetary gear train assembly The transaxle drive system transmission can pull a GSE vehicle up an 18% grade while pulling 7,800 pounds of baggage or cargo.

AC Induction Motor Operation

The AC induction motor is a brushless motor that has been used in general appliance and factory applications for decades. The 4-pole, 3-phase AC motor operates up to 6,000 rpm and 225 footpounds of torque. The AC motor reaches a peak power level of 40 horsepower. Power is rated at 35 horsepower of continuous operation up to 60 minutes. The motor's operating ambient temperature covers the range of -31 deg F to 123 deg F. The AC motor includes a temperature sensor located in the stator windings for protection from extreme conditions.

Invertor Power Stage Topology

The electronic drive system utilizes an inverter power stage and inverter control board that interfaces to the AC induction motor. The inverter power stage uses a 6-switch MOSFET based power bridge drive configuration to convert DC battery power into AC inverter power. The inverter power stage sequentially energizes the 3-phase AC motor at a pulse-width modulation (PWM) frequency of 8 kHz over a wide output frequency range of 0 to 250 Hz. Peak inverter power stage efficiencies reach 95%. The compact 7.87" (L) x 11.02" (H) x 3.15" (NW) overall package (weighing less than 32.3 pounds) develops a peak power of 47 horsepower for up to 2 minutes. A current level of up to 500 amps rms (for 2 minutes) at 122 deg F ambient drives the AC motor at desired torque levels. This level of current is compatible with, but always lower than the drive current levels from traditional DC series-wound traction motor drive systems.