As usual, Nice ASCII art, Lee. I would note two things, one is that with a motor and controller pair that can flux weaken, you can get something like constant power past base speed so the HP curve stays relatively flat after the climb, so you could have another graph for the second to last ASCII curve. (Power loss past base speed is somewhat exaggerated)
| ----____ | / ----- | / HP | / |/ --------------- rpm Also, depending on how "cost efficient" the controller design, is, you can get a torque sag at very low shaft apeeds. If one phase is doing the brunt of the work, and you don't have excess silicon or near zero thermal resistance, then you have to fold back torque at stall. Which makes additional mechaincal torque multiplication nice. Or motor reconnection from delta to wye and back. So the same current gets you more torque. But this takes some not inexpensive switchgear to do. The perfect contorller is sized so that it has the silicon to not worry about cooking at stall, and the ability to be an electric gearbox. But we rarely get perfect designs, as they cost so much. Seth Lee Hart wrote: > > Sigh... we aren't thinking very clearly about the one-speed vs. > multi-speed situation. It doesn't matter if it is an AC vs. DC drive. > What matters is that the motor+controller+transmission system produce > *enough* torque over a wide enough speed range. > > The torque-speed curve for *BOTH* an AC drive and a DC drive looks like > this: > > torque > |__________ > | \ > | \ > | \ > | \ > |______________\____speed > > The flat portion is when the controller is in current limit; fixed > current = fixed torque. The falling portion is when the controller comes > out of current limit, and battery voltage is limiting torque. > > If the peak torque is high enough to spin the wheels, and the top speed > is high enough to reach your maximum desired road speed, then the > motor+controller is "big" enough to work without a transmission. A > single gear ratio can satisfy both your max torque and max speed needs. > > If you pick a smaller motor+controller, its max torque and/or max speed > capability is less. But, you can boost the torque at low speeds with a > transmission. > > torque > |____low gear > | \ > |_____\______high gear > | \ \ > | \ \ > |________\_____\____speed > > Obviously, the peak horsepower isn't as high; but you have the same > torque (hill-climbing and accelleration capability) at low speeds, and > the same top speed as the more expensive motor+controller without a > transmission. > > Or, you can look at it from a horsepower point of view. Horsepower is > torque times speed, so the horsepower curve for the large single-speed > motor+controller looks like this: > > horsepower > | _ > | / \ > | / \ > | / \ > | / \ > |/_____________\____speed > > And a 2-speed transmission and smaller motor+controller horsepower curve > looks like this: > > horsepower > | > | low gear > | _ _high gear > | / \ / \ > | / \/ \ > |/___/_\_______\____speed > > The smaller motor+controller+transmission delivers less peak horsepower, > but delivers its smaller horsepower over a wider range. In fact, an > ideal infinitely variable transmission would deliver constant horsepower > over a wide range, so a smaller motor+controller+transmission would > outperform a larger single-speed motor+controller. > > The devil is in the details. Exactly how lossy is your transmission? How > fast can you shift? How wide a speed range can you get per gear ratio? > -- > Lee A. Hart Ring the bells that still can ring > 814 8th Ave. N. Forget your perfect offering > Sartell, MN 56377 USA There is a crack in everything > leeahart_at_earthlink.net That's how the light gets in - Leonard Cohen -- vze3v25q@verizondotnet
