"VanDerWal, Peter MSgt" wrote: > > At any rate it's still pretty damn quick. I don't see where having a second > gear with a ratio higher than 11:1 would help any, especially since, as you > point out, the tires probably couldn't get any more torque to the road. If > you spent a lot of time driving at 5mph it might improve your efficiency > some, I guess. > > >It's my opinion that AC is only rarely flexible enough to do the job. > >And for now I am keeping that opinion. I have never driven an EV-1. I > >live in the northeast, so I probably never will. I sat in the Impact > >once, but that didn't tell me much. > > I agree you are completely entitled to your own opinion. I'm just curious > which AC powered vehicles you have driven that you've developed this opinion > from? > > P.S. I'll grant you that the Siemens motors would need a ratio closer to > 8.5:1 if you wanted a top speed of 80 mph; but I still don't think that an > even higher ratio would help efficiency or torque much, certainly not enough > to make it worth the extra hassle, weight, and complexity. And you could > always solve that by deciding to have a top speed of 65-70mph, going faster > than that is just a waste of energy anyway. > I have some expertise in this area and can chime in with some hard data.
Peter is right that switching gears within normal AC motor range does not impact efficiency or it's very minimal. But the ratios must be lower than 8.5:1 for "normal" RPM range (3000-6000). Take my ACRX: 1st gear ratio is 3.25:1 2nd is 1.65:1 3rd is 1.033:1 Final diff is 2.954:1 So total reduction on the second gear is only 4.87:1 and on the third - 3.05:1. Now, I drive on the second gear all the time. ACRX goes 65 mph at exactly 5000 RPM and consumes about 12 kW to do that. I have a battery power monitor (part of inverter's software) and can watch the power value as I drive. If I switch to the third, the RPM becomes exactly 3000. Acceleration is not as quick anymore because the torque at the wheels is lower but the torque the motor puts out is the same at 3000 and 5000 RPM (the case for my voltage). However, power consumption is identical - still 12 kW because main contributor at that speed is aero drag loss, not motor efficiency. The motor current on the third is proportionally 1.7 times higher than on the second so its losses may be few watts more, but nothing compared to 12 kW overall. Switching to the first gear in my case is not possible for highway use - the motor would need to make 9800 rpm. It can do that, but there will be little useable motor torque there - even multiplied by 1st gear high ratio I suspect I will have less torque at the wheels than on the second gear. Not to mention the motor efficiency at near 10,000 rpm is lower - about 75% (http://www.metricmind.com/line_art/efficiency.gif) and the gear box will be very hot (more watts wasted to that). Victor
