Electric Motorcycle Armchair Engineers Discussion Thread. Truths, Half-truths, and Myths.

Discussion in 'Electric Motorcycles' started by T.S.Zarathustra, Mar 22, 2019.

  1. MJSfoto1956

    MJSfoto1956 Been here awhile

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    We are running out of CHEAP-TO-EXTRACT DynoJuice. While there is plenty of oil still in the ground, getting to it is increasingly costly and risky. This is why the "tipping point" is right around the corner: adoption of EVs will all be about value not principals.
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  2. Mambo Dave

    Mambo Dave I cannot abide.

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    [​IMG]

    1915 electric car
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  3. MJSfoto1956

    MJSfoto1956 Been here awhile

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    Waaaay ahead of their time...
  4. ctromley

    ctromley Long timer

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    Those ran on the Edison nickel-iron battery. Not a great performer by today's standards of course, but they never wear out. Just replace the pig intestine membrane separators occasionally and refresh the electrolyte (basic, not acid I think) and you're good to go.

    I've read that Edison was not really that great a scientist, but he was certainly a relentless experimenter. If only he had kept tinkering he might have evolved the beginnings of the modern nickel-metal-hydride battery. Imagine how our transportation landscape would look today if he had....

    Our successes are obvious. It's harder to comprehend what we've let slip through our fingers for lack of a bit more vision and effort - but it's still a loss.
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  5. T.S.Zarathustra

    T.S.Zarathustra Been here awhile

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    11. If you put a small windmill, or dynamo, on your electric motorcycle you can go further on a charge.

    If you put a windmill or dynamo on your bike, it will cause extra drag. You'd need to use more throttle to overcome the extra drag, so you can maintain speed. Law of energy conversion, that "a system can never be 100% efficient", dictates that there's more energy used to overcome the drag created by the windmill or dynamo, than they can create. So the net effect is shorter range.
    A possible way to create energy by bike movement is by harvesting energy in the movement of the suspension. The energy harvested will be small, and barely worth it. On the other hand, if the system would be used to create active suspension, handling could improve.
    You can also put solar cells on the bike. Again, barely worth it. Regular bikes have small surface area, and solar cells are not very effective. You get about 15W per square foot from run of the mill solar cells. Put 2 square feet on your bike, leave it in bright sun for 10 hours and you'll get 0.3 kWh. Zero goes about 10 miles on 1 kWh, so 0.3 would give you 3 miles per day. A two hour ride in the sun would give you up to, and maybe a little over, half a mile range.
    Magnussohn's post/link above, regarding efficiency, is IMHO much better solution. I've mentioned that few times before:pope. Increasing the efficiency of the Zero mentioned above, for example by adding a fairing, will give you much better improvement in range than any other method I've mentioned here, or even all of them combined. 10% efficiency increase, by adding a fairing, will give you 10-15 miles extra range on a charge. Being very careful with the throttle, and drive slower, will double that.
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  6. liberpolly

    liberpolly Nu, shoyn, nudnick!

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    I want to know how the fuck did it even come up!
  7. T.S.Zarathustra

    T.S.Zarathustra Been here awhile

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    "12. Electric motors have full power from 0 RPM."

    That is wrong. Sort of. :-) What many think is that Torque equals Power, that is not correct.
    Let's look at the graph in the picture here below. This is pretty typical graph of power characteristics of general purpose electric motor. This is not specific motor, this graph is theoretical, used in textbooks. All motors are different, you'd go through the graphs of available motors and choose motor characteristics to fit your use.
    The X axis is load, the Y axis is speed (RPM)
    The blue line is Power (see also formula below the graph).
    The black line is Torque.
    The red line is Current.
    The green line is Efficiency.
    Where the lines end, on the right, is where the load has become so big that the motor stalls (overheats and destroys itself in few seconds, if not properly protected).
    You can see that max power (the blue dot) is roughly midway between no-load and max-load.
    Max Torque (which many people incorrectly think is equal to power) is highest at high RPM and low load. It steadily decreases with higher load.
    Current draw increases with load.
    The Efficiency curve is interesting. This is the speed/load at which most motors are designed to run at. At max power the motor has lost a lot of its efficiency. To gain range in electric vehicle, it is best to keep the load near max efficiency. In other words, it would be beneficial to gear the motor output so you can keep it close to the sweet spot.

    lettura-curva.jpg


    Formula to calculate power output.

    Pout = τ * ω

    where Pout – output power, measured in watts (W);
    τ – torque, measured in Newton meters (N•m);
    ω – angular speed, measured in radians per second (rad/s).
  8. ctromley

    ctromley Long timer

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    Well, the short answer to the max power at zero rpm myth is that power = torque x rpm. So at zero rpm you have zero power.
    Not sort of, it's flat out false. See the power equation above.
    I'm not the expert here, but my understanding is that different motor types have VERY different characteristics, rendering a 'universal' graph kind of useless. For example, the motor equation for a series motor shows it produces infinite torque and draws infinite current at zero rpm, meaning you can't know what it does without knowing the supply capabilities, meaning the controller in the case of EMs. Maybe show real graphs for some different motor types?
    Uhh, certainly not a universal characteristic regarding rpm (back EMF tends to limit torque as speed climbs), and how can you even measure torque without loading the motor? High torque at low load makes no sense. Please explain?
    I've seen motor curves where the efficiency stays very high over a very broad range, not the peak and drop-off that you show. What types of motors have that characteristic?

    And also, what are the test conditions that the graphs represent? I thought they were all done at constant voltage, with the load varying to run at different speeds. (But for a PM motor speed is directly linked to voltage - is that tested differently?) How do we know how these different motor types perform in the real world, like varying speeds at light load? For example, how does that affect efficiency? (I think I read that PM motors get poor efficiency at low load.) Adding a controller to the system which varies voltage, and the extremely variable loads of real-world use make the theoretical descriptions pretty limited to very basic concepts, many of which go out the window in real-world use. Can you fill in some blanks here? I've been looking for that kind of background for a long time.
  9. woodsrider-boyd

    woodsrider-boyd Wow, these guys are fast

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    These torque discussions are very interesting, I've been wondering about the "max torque at zero rpm" claims since I first heard them. I was thinking it makes literally no sense, as stated, how can you have torque with no rpm? Especially given the torque formula, seems pretty clear when you apply that.

    Having said that, I've been amazed at the torque of the Alta MXR that I own, seems to have tons of torque starting from 1 rpm (as soon as I twist the throttle from zero rpm), all the way through the rev range.
  10. ctromley

    ctromley Long timer

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    Torque is a twisting force. No rotation required. Power is how rapidly you can continue applying that twisting force, by adding the element of time in the form of shaft speed.

    Imagine applying a twisting force to a shaft, by applying a stationary force to a lever welded to the end of the shaft, and the shaft is locked. Force applied, no rotation, but that force is putting a bend in the lever and a twist in the shaft. Now release the shaft and the stored energy in the lever and shaft will make it rotate slightly, but because the force is stationary, it doesn't follow and the shaft stops immediately. The torque has to keep up with the shaft for power to be made, and the faster the shaft turns, the faster the torque needs to chase it to make power. Think of power as torque applied per unit of time.

    If you're still confused, you're not alone. You'd be amazed how many engineering school graduates still don't understand the difference between power and torque.
  11. woodsrider-boyd

    woodsrider-boyd Wow, these guys are fast

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    How can you have twisting but no rotation? Seems like something has to be rotating in order for twisting force to be applied.

    Not trying to dispute what you're saying, I'm no engineer.

    Was more just commenting on the full torque at zero rpm claims. If there's no energy being applied to the motor, no rotation, no forces, no torque.

    Feel free to ignore if I'm completely misunderstanding. Sometimes there's laws of physics I will never understand despite my best efforts to do so.
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  12. SteveAZ

    SteveAZ Long timer

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    I forgot the name of the class of motors where rotor and stator coils are in series but they consume maximum power and provide extremely high torque at 0 RPM

    Clearly if not turning, it's doing no work and so delivering no power as you point out
  13. SteveAZ

    SteveAZ Long timer

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    Torque is a force

    In the linear world (vs. rotational) you can push on something (e.g. a big rock) that doesn't move and you are applying a force to it - since it isn't moving no energy is expelled, no power is delivered, no work is done... until it moves

    It's the same concept except rotational
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  14. ctromley

    ctromley Long timer

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    Yes. When you open a jar with a tight lid, you apply a torque (twisting force) to the lid that exists even before the lid moves and comes off.

    In the linear world, you need to keep applying force to keep your rock or whatever moving, or it will stop. If you want it to move faster, you need to apply your force at a higher rate, meaning your force covers distance faster, i.e. more power. When you lift something, it weighs the same at all times (weight is a force), but it will take twice the power to lift it twice as fast.

    Rate of motion in a straight line (speed) is the equivalent of shaft rpm in the rotational world - applying torque at a higher rate (rpm) requires more power.
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  15. ctromley

    ctromley Long timer

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    That's a series motor, the darling of forklift designers and electric drag racers for its monstrous bottom-end torque. That's the one with a theoretical motor equation that has torque and current draw going to infinity as speed goes to zero (inversely proportional).

    A clarification - they consume maximum electrical input power (volts x amps) while producing zero power at zero rpm because they want to draw infinite current. That's why you don't want to stall a series motor. What current it can get will be turned into Big Torque. Something big and expensive is likely to happen, and you don't want to be there. I've heard of e-dragster competitors snapping driveline parts like twigs - parts that were designed for Top Fuel rails.
  16. SteveAZ

    SteveAZ Long timer

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    I think that's what I posted :D

    They'll draw as much power at stall as the supply working into the windings' resistances allow - pure resistive load until turning (big heater)

    Yup, low/zero speed torque monsters
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  17. ctromley

    ctromley Long timer

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    It is, but this stuff is pretty mysterious to a lot of people (including me), and I just wanted to distinguish input from output power, which to a noob might not be clear.
  18. liberpolly

    liberpolly Nu, shoyn, nudnick!

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    [​IMG]
    https://www.engineeringtoolbox.com/electrical-motors-hp-torque-rpm-d_1503.html
  19. ctromley

    ctromley Long timer

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    Doesn't tell quite the whole story, but offers a good opportunity to fill in some useful details.

    I don't know of any motor type that has a straight, constant torque curve, and since power is the product of torque and speed, a straight-line power curve. (The most oddball is the series motor mentioned before, where torque starts at infinity, drops as speed picks up and then drops more slowly.) But the motor isn't the only component in play. In an EV you need precise and widely variable speed and power control, so the motor gets its power through the controller. The controller has a physical limit to the current it can provide, and in most you can program the current limit to pretty much anything you want below that physical limit. All the motor types I know of have a direct relationship between current and torque. So the curves for the motor/controller pair under load actually are as pictured above, because the current limit forces current (and therefore torque) to remain truly constant. Perfectly flat torque curve, perfectly linear power curve, exactly what every ICE vehicle test editor represents as the perfect set of curves for road vehicles, but which an ICE can never really achieve.

    However.

    Since the motor spins as a result of the current you feed it, and motors are also generators of you spin them, increasing speed generates something called back EMF - essentially a voltage in the field windings that opposes the voltage you're feeding it. This suppresses the motor's ability to draw current as speed climbs, and the current it draws typically drops below the controller's current limit somewhere in the operating range of the motor. This 'dropping out of current limit' means that at some point those perfect straight-line curves start tapering off. Torque declines and power levels off. Where it happens and how rapidly it tapers depends on the particular motor.

    Caveat: this is what I know as it pertains to DC-fed, wound-field motors. Not sure, but I think my description above also describes PMAC (permanent magnet motors fed by AC synthesized from a DC input), and brushed PM motors running on DC. AC induction is a different animal, controlled in a different way. AC induction controls speed through variable AC frequency, which is a different ball game (the others control speed with voltage), and I think the torque/power tapering off can be mitigated to a great extent.

    I'm out beyond my knowledge limits here; can any experts correct any errors and fill in my inevitable gaps?

    This is important because I believe AC induction is a much better way to implement EV drivetrains. Wound-field DC motors tend to be heavy and expensive, PM tend to be cheaper but less flexible or durable. This is one reason I had high hopes for the Lightning Strike with its tiny and powerful AC induction motor. I hope they can sort themselves out, or someone else goes to AC induction.

    Experts, what are your thoughts?

    Oh, and while we're pinging the experts, Tesla set the world of motor science on its ear a couple years ago by finally taming the Switched Reluctance motor (used in the Model III), thanks to the latest advances in electronics and computing speed, and some clever tweaks to the magnetic fields (adding some judiciously-placed permanent magnets). Industrial motor manufacturers had been working on that for decades, because switched reluctance motors are dirt-simple, dirt-cheap and powerful. Has any of this new tech resulted in EM-size motors and controllers yet?
  20. liberpolly

    liberpolly Nu, shoyn, nudnick!

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    I am sorry, but I cannot offer any corrections to a stream of non-sequiturs. Where are you getting all this stuff? This is not how it works. This is not how any of this works.
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