Simple Hybrid Auxiliary Motor Package

2/19/06 - Paul E. Schoen


Saturn Hybrid Conversion Project

This describes details of my proposed hybrid conversion project for my Saturn SW1 wagon.

I will probably use two 5 to 20 HP motors, one on each rear wheel. For most driving, and especially for inching along in a trafffic jam, that is plenty, and you can turn off the gas engine. When going down hills, you can use the motors as generators to charge the traction batteries, and save on brakes. If you are stuck on ice or snow, the rear wheels add four wheel drive capability. If the gasoline engine system fails, you may have enough reserve power in the batteries to make it home, or to a safe location.

The most difficult part of this project at this time is designing the most easily implemented mechanical connection from the motors to the drive wheels. I will probably attach large sprockets to the inside (or possible outside) of the wheels or brake drums, and use a roller chain to a chassis mounted motor. The outer diameter of a tire on a 14" wheel is approximately 24", so the circumference is about 72", or 6 feet. I figure the maximum sustained speed for this vehicle will be 75 MPH, which is about 1100 RPM. The highest safe speed for a motor is probably about 4400 RPM, so a 4:1 reduction drive would be ideal. The force required to propel a 4000 lb vehicle up a 10% grade is 400 lb, or 200 lb per wheel. If the radius of the wheel is 1 ft, this is 200 ft-lb. Moving the vehicle at 1/6 the maximum speed, or 12 MPH, which is about 180 RPM. The horsepower required is 180 * 200 / 5500, or about 6.5. The motors would need to deliver 50 ft-lb at 720 RPM.

New technology in variable speed motor drives (variable frequency, variable voltage) has achieved precise speed and torque control with PWM techniques to regenerate a three phase power source from a DC bus supply. However, most large motors are 208, 240, and 480 VAC, which require DC bus voltage of 750 VDC or more. IGBTs are readily available at 1200 VDC or more, and currents of 200 amps or more, which are used for motor controls up to several hundred horsepower. However, even a 350 VDC bus for a 208 VAC motor would require 30 12 VDC batteries, which can be done, but is rather difficult to deal with. Also, the high voltage is a safety hazard.

I plan to use something like four to six batteries for each motor. My unique idea is to wind the motor so it will run on 48-72 VDC, which can produce about 30-45 VAC. I expect to drive the motor at up to three to six times the nominal 60 Hz, which means I could get 15-30 HP out of a 5 HP 60 Hz frame, or use a 2 HP frame to get 6-12 HP. I will experiment to see if I can get even higher HP (or smaller size) at up to 10x frequency.

I have proven the feasibility of winding a three phase induction motor for nominal 8 VAC at 60 Hz. I have a working 12 pole motor I made from a 3/4 HP blower motor that was actually a single phase 120 VAC permanent capacitor type. I have driven this motor with a VF drive to 1800 RPM at 180 Hz, and it should work up to 3600 RPM with 360 Hz. For regeneration, I need to direct any higher voltages from the motor through steering diodes into an energy storage system that charges the batteries and provides a dynamic braking action. For my application, I will need to design a special high current VF drive, as 5 HP corresponds to about 3500 watts, which is about 70 amps at 48 VDC. The technology for this is well established, and the newer PICs introduced over the last 2-3 years have made it much easier to generate the required PWM waveforms, as well as monitoring drive current, motor speed, and other factors (such as temperature, battery condition, etc.).

I am also looking into hub motors, which are generally being designed with BLDC techniques, using permanent magnets in the rotating rim, and a stationary stator on the inside. This is essentially an inside-out design. The problems with BLDC motors are higher cost, due to exotic magnets and position sensing mechanisms, and they are not quite as rugged. They may be more efficient, however.


Interesting Facts about Electric Vehicles (01/23/2006)

Assume an electric vehicle runs with an average of 20 HP at 60 MPH. A four hour trip would cover 240 miles, which is a reasonable distance before a refueling stop is needed. This is about 106 kW-Hours. At 10 cents per kWH, the fill-up would cost about $10.60. At gasoline price of $2.50/gallon, a car would need to get about 60 MPG for the same cost.

Now, assume the electric car has ten 12V batteries in series, for 120 VDC. 106 kWH translates to 889 A-H. The drain on the batteries would average about 225 amperes, which is not too bad. Of course, that was over a four hour period. At the refueling station, you might be satisfied with a six minute fill-up, or 1/10 hour. That means you would need to charge the batteries at 8890 amperes! Also, at 120 VDC, this is a whopping 1.07 MW of power. Assuming you have three phase 480 VAC commercial power, that requires 740 amperes per phase.

If you use the car only for commuting, or a total daily use of 240 miles, however, you could recharge it overnight, in 10 hours. For this, you would need only 10.7 kW of power, which is about 45 amperes on a 240 VAC line. This is about as much as a large hot water heater or a small electric furnace or central air conditioner.

Now, remember, this was for a vehicle rated at only 20 HP! A small, lightweight, aerodynamic vehicle traveling on fairly flat terrain may need only 5 or 10 HP on average, with conservative driving habits, and if regeneration is effective. However, a large, heavy SUV or truck, driven aggressively, might average consumption of 60 HP or more, so the above scenarios would be much worse.