[wintogreen74]

>> You still have the mechanical resistance of turning the rotor,

Isn't this exactly what regenerative braking is? There's no mechanical disconnect between the wheels and the motor (i.e. a clutch) so pushing the car essentially turns it into a generator.

[mannykannot]

Your second sentence is correct, but it takes a lot more torque to turn a generator at a given speed when it is driving current through a load, than it does when it is open-circuit.

Inside the motor/generator, what happens is that if there's current flowing in the windings as a result of the EMF induced by turning the rotor, then, loosely speaking, it creates a magnetic field which opposes the rotation - but if it is not connected to some load (or a short-circuit) there's no current, even though there is still an induced EMF.

It's a standard case of the conservation of energy - if it were not the case, you could put an arbitrary number of generators on the same shaft, all supplying power to external loads, and then use the output of a couple of them to power a motor spinning the assembly...

If you are turning a motor/generator slowly, then the difference in torque is not much, but I am responding to the implication that the motor would have to be mechanically disengaged in order to disable the braking effect.

[dinom]

Correct me if I'm wrong but, if I understand things correctly... at this point we're wondering if manual locking hubs are a better (or worse) solution than electrically isolating the motor(s). Right?

And, off the top of my head I'm going to say the manual locking hubs will be better overall. Better from an energy efficiency perspective as well as overall convenience and simplicity of design. They might be worse for Tesla's P&L though since anyone with a tire jack and a standard parts dealer nearby could work on the car.

Then again, besides the fact that I'm a cynic... what the heck do I really know!??!

[mannykannot]

I don't think it would be useful for me to speculate about that. I'm just attempting to clear up some apparent misunderstandings about regenerative braking.

[zh3]

Yes and no. Spinning the motor will generate a voltage but if it's not connected to anything there's no current flow and so the only resistance is friction. For it to act as a brake, something has to use the current - for example, charging the battery or more simply just shorting the motor (try turning an efficient DC motor with the wires open and the wires shorted - even with a stepper motor, you'll find a huge difference).