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0 pointssgtnoodle3y ago0 comments

This is the first I've heard of it. The physics of it seems fine. The engineering is going to be very hard for them to get right.

Just skimming their web site, range is qualified as "WLTP drive cycle". I assume that means software enforced torque and speed limits, and no one is going to want to drive with that enabled.

Using 4 independent motors implies they're using in-hub motors. They don't seem to mention a top speed anywhere. Unless their motors have dynamically adjustable air gaps (which I am guessing they don't), the motors will have a fixed KV constant. That means the top speed of the vehicle will be limited by battery voltage at some point. Maybe that limit is high enough not to matter, but it's funny they don't specify it.

Using in-hub motors also means there's a lot of mass on the wheels themselves. I'm not a mechanical engineer, but my understanding is that adding wheel mass makes suspension design a lot more difficult. On top of that, the rest of the car is presumably a couple hundred lb of batteries, and then a bunch of lightweight composites. The batteries will likely be in the floor, which is good for vehicle dynamics. The overall car will be significantly lighter than what people are used to. I think it's possible that the car will have a relatively rough ride, and could possibly have more noise than usual in the passenger compartment.

Crash-safety wise, I wouldn't want to drive that car on a busy highway. In the US, it would likely struggle to pass crash safety testing. I wonder if they are certifying it as a low speed vehicle, which would put it in the same regulatory class as a golf cart. It would have to have a 40kph speed limit on level ground.

It will probably cost more than a Tesla. At the end of the day, slapping a solar roof on a Tesla would probably get a lot of folk to the grocery store and back every few days. Mounting the same panels on the ground and plugging in the Tesla would be even more effective.

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Gravityloss3y ago

Thanks. I would assume you could make the front and rear electric motors optimized for different speeds. That would mean power loss towards top end speeds.

sgtnoodleOP3y ago

Maybe, but I would be surprised if they did that. From a manufacturing point of view, having two different motor types is more expensive.

More importantly, the motor drive electronics would have to be designed specifically to support that. A motor's top speed is limited by the supply voltage because a spinning motor generates its own voltage called back-EMF. When back-EMF matches the supply voltage, the motor drive can't push any current through the motor because there's not enough of a voltage difference. When the motor spins even faster than that, its back-EMF exceeds the supply voltage. This does something funny to the motor drive. The voltage across the FETs in its H-Bridge circuit swaps polarity and the FETs become "reverse biased". A reverse biased FET acts like a diode, and allows current to flow through it. As a result, the motor generates a braking torque. The only way for a motor drive to avoid that would be to have an additional semiconductor in series with the H-Bridge circuit. That would make the drive more expensive, and lower efficiency. It could be done, but I doubt it.

It's also hard to optimize a motor to work well at low speeds, but then have it be efficient at high speeds, even if not using it. One of the ways motors lose efficiency are in what are called "eddy current losses". Just the fact that the motor is spinning means that metal is moving through alternating magnetic fields, and that induces eddy currents in the metal. Those Eddy currents generate a braking torque, and heat up the metal.

This all assumes the motors are permanent magnet brushless motors. I think that's a safe assumption since they are in-hub. There are other types of motors, such as induction, that work differently and don't have the same "speed limit" or Eddy current losses when freewheeling. Most electric vehicles use one or two induction motors. They aren't in-hub, though, because they don't scale down in size well.

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