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How to Reach 200 MPH on Hydrogen Fuel Cells
the_manatee writes "Ford's 999 hydrogen-powered speedster is making waves for its upcoming speed record attempt in the Bonneville Salt Flats, but details on what's actually going on under the hood have been scarce. As it turns out, there are NASCAR-style brakes, steering, and suspension components, along with 16 Ballard Mk902 fuel cells that produce 350 kW of electricity. All that juice spins up a 770-hp motor and the rest is (hopefully) history. One final ingredient: 400 lbs of ice for cooling, which will melt in seconds once the car gets up to speed."
Installing a motor in every wheel is intuitively a nice idea. Unfortunately, electric motors have a great deal of inertia. At high speeds, the effects of this rotational inertia dramatically affect the stability of the vehicle when it hits a bump.
At lower speeds, vehicle performance is maximized when the motors torque/speed curve is matched to the maximum speed of the vehicle while simultaneously matching motor diameter to wheel diameter. Unfortunately, the wheel diameter, tire diameter, motor diameter, and peak motor RPM rarely agree. Thus mechanical gearing often helps.
An AC induction motor has the highest power/weight density of all electric motors. Brushless DC motors are only competitive for very small motors.
Er, no. A "brushless DC" and a "variable-frequency synchronous AC" motor are the same thing. Smaller motors tend to be called "brushless DC" and are driven by "motor controllers", while larger motors are called "variable-frequency AC" and are driven by "drives" or "inverters". The threshold is around 1KW. The difference in terminology comes from different industries.
All motors are AC at the windings, or they'd reach a steady state position and stop. "Commutation" refers to the means provided to switch power to the windings so the motor continues to chase the minimum position for the magnetic field. Commutation can be performed with brushes and a commutator (which is just a drum of contacts), with external electronics, or simply borrowed from the power line frequency. "Brushless DC" and "variable AC" motors are driven by external electronics. They're usually at least 3 phase devices; this allows starting from a stationary position without the possibility of being stuck at a neutral point.
This concept scales up just fine. Here's the General Electric AC6000, the most powerful locomotive in the world, driven by 3-phase AC variable-frequency motors. The software, written in C++, locks all the wheels together as if they were geared together, even though there's a separate motor for each axle. This allows more tractive effort without wheel slip than any previous locomotive. There are thousands of these locomotives (mostly the smaller AC4400, but a few hundred of the big AC6000) in use today.