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What is neutral for?

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>>Mechanically there is no difference between neutral and tow mode. The motors are always connected to the wheels,...<<

I'm not disagreeing - I know nothing about the mechanics - but I just wonder how in my case (see above) the car just "coasted" when I went into neutral by mistake. The motor/wheels gearing must be very high - I would have thought that if still connected the car would grind to a stop PDQ?
 
I'm not disagreeing - I know nothing about the mechanics - but I just wonder how in my case (see above) the car just "coasted" when I went into neutral by mistake. The motor/wheels gearing must be very high - I would have thought that if still connected the car would grind to a stop PDQ?
The mechanics are exceedingly simple. The motor is connected to the final drive. No clutch like a manual, no torque converter like an automatic.

There's very little friction in the motors; compared to the mass of the car it's inconsequential.

In an ICE; you have compression from the pistons still going up and down. That slows you down unless you use neutral, or you can throw out the clutch if it's a manual. Reverse gear introduces an idler gear to change the rotational direction.

To go backwards in an electric vehicle, the motor goes backwards. The car is capable of the same speeds in reverse, but it's software limited. It's a good thing too - I've read there's an oil pump that only works going forward. That makes sense since the pump would be turning backwards too.
 
I'm not disagreeing - I know nothing about the mechanics - but I just wonder how in my case (see above) the car just "coasted" when I went into neutral by mistake. The motor/wheels gearing must be very high - I would have thought that if still connected the car would grind to a stop PDQ?
You are right on the right track there. There is a 9 to 1 gearing between drive motors and the wheels in constant mesh. One of the motors is a permanent magnet type (the front in your S, rear in my 3). If the car somehow lost all power and control it wont just freewheel with inconsequential friction. The Permanent magnet motor will effectively act like a generator, loading the motor and slow you up pretty quickly. Much like the engine drag on an ICE. However when you bumped the car into neutral, it electronically contrives a neutral, by de-energizing the induction motor & electronically controlling the permanent magnet motor to produce zero torque. Essentially the same effect as feathering the accelerator pedal when driving. This is why the car coasts when you bump it into neutral.
 
You are right on the right track there. There is a 9 to 1 gearing between drive motors and the wheels in constant mesh. One of the motors is a permanent magnet type (the front in your S, rear in my 3). If the car somehow lost all power and control it wont just freewheel with inconsequential friction. The Permanent magnet motor will effectively act like a generator, loading the motor and slow you up pretty quickly. Much like the engine drag on an ICE. However when you bumped the car into neutral, it electronically contrives a neutral, by de-energizing the induction motor & electronically controlling the permanent magnet motor to produce zero torque. Essentially the same effect as feathering the accelerator pedal when driving. This is why the car coasts when you bump it into neutral.

The 9:1 gear ratio means the frictional and eddy current drag of the rotor is scaled up proportionally, and the rotor inertia is scaled up by 81:1. Inertia will be the only item in play on an AC induction, and the PMSR has fairly small eddy losses.

For the AC induction thing you are saying that unpowered is the same as neutral, which I agree with.

For the PMSR type, if unpowered, the drive circuit reverts to diodes feeding the HV bus, not a short. So you can only get as much braking effect from an deenergized inverter as the amount of load on the HV bus (basically zero, if car is dead with contactors open) and then only as much as the speed based back-EMF. The SR part of the motor does not generate back-EMF if the stator is deenergized.
 
Point taken re the Permanent motor, so minimal drag in the failure case & it would not grind to a halt. I think I am still correct though in saying the reason it coasts in neutral, is because of the zero torque request to the Permanent magnet motor. Just highlighting that neutral is created electronically and is not a mechanical disconnect in the drivetrain.
 
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Interesting replies! Just wondering then why the big No-No on towing except for getting it onto a flatbed?
Spinning the PMSR motors does generate voltage which gets fed back onto the HV bus. However, with the car off, this is not controlled which is not good. Especially if the HV system has a safety bleed resistor to drain power when the car is off.
Additionally, none of the coolant or oil pumps (other than the S/X gear driven one) would be active.
 
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>>Spinning the PMSR motors does generate voltage which gets fed back onto the HV bus. However, with the car off, this is not controlled which is not good. Especially if the HV system has a safety bleed resistor to drain power when the car is off.
Additionally, none of the coolant or oil pumps (other than the S/X gear driven one) would be active.<<

Thanks - interesting information!