Anti_Climax
Posts: 33
Joined: Fri May 20, 2016 12:22 pm
Location: The Valley of the Sun

Flat towing revisit

Fri Sep 18, 2020 6:12 pm

Been thinking about this and I can only think of one possible problem for flat towing. The parts that don't factor:

On my 2012, there is no steering lock so it would be able to track on a tow bar while "off". When placed in park, the drivetrain is arrested by a "parking pawl" that engages in the transmission. The brakes don't appear to apply unless you pull the handbrake.

You can validate this yourself by parking on level ground and pushing the car forward and back - the wheels will turn slightly, turn the halfshafts, which will be stopped by the pawl *after* the motor and transmission have stopped against their mounts.

When the brakes are engaged - with the pedal if it's on or with the handbrake, the wheels will not shift and all the slop is the drivetrain never factors in. If putting the transmission in park applied the brakes, we'd expect to see the same behavior in both cases.

Speaking of the transmission - it contains zero hydraulics as best I can tell. Components are not moved with fluid pressure, there are no clutches. There's nothing that would rely on the motor being "on" like you see for ICE vehicles. The motor shaft, differential and driveshafts are always connected and moving in concert. There isn't a separate reverse gear, they just energize the motor windings in the opposite order.

There is lubrication happening in the transmission but every component that moves while being driven is moving when the car is coasting in neutral - which is accomplished by "switching off" the regen. If the motor coils are open circuit, no current can flow, so no opposing magnetic field is generated, so nothing "drags" on the motor shaft.

So when in neutral and off we have the steering free, the brakes disengaged and the transmission freewheeling and lubricating itself like normal. Seems totally workable to me. About the only other thing if try to do is physically block the parking pawl arm to prevent any possibility of it engaging.

Here's the biggest obstacle I see, you setup a tow bar and hook up the car... Great, you have a fairly heavy vehicle that requires remote brake control to be legally towed in several states. How do we do that on this car? If an ICE vehicle can have remote braking while not running, you'd think it would be the same for the Focus EV. And I'm pretty sure that aside from the boost vacuum coming from a pump instead of the engine intake, the rest of the braking system is the same.

So, assuming you can get a proper tow bar and remote braking system, you should be able to dingy tow a Focus electric while in neutral and turned off. Handbrake off, parking pawl pinned back if you're paranoid.

What do you all think?
2012 Candy Blue FFE with a 33.5kWh battery pack
*Coming Soon: CHAdeMO*

Heima
Posts: 100
Joined: Thu Feb 21, 2019 11:46 pm

Re: Flat towing revisit

Sun Sep 20, 2020 5:52 pm

Our cars have a permanent magnet motor.
There is no clutch, there is continuous engagement of the motor, to the single speed gearbox, to the differential, to the front wheels.
The wheels turn, the motor turns.
When a magnetic field moves within a coil of wire a current is induced.
So when you pull your FFE behind your RV flat towed, the motor acts like a generator.
That generator is moving current through your inverter.
There are no switches, or disconnects, it is a closed circuit.
The IGBTs within the inverter are unable to adquately shunt off the current, so they burn up.

You might think a possible solution is to power on the FFE, put it into neutral and allow the computer to attempt to compensate.
Well, after a while, that will burn up the IGBTs as well.

So, you need to disconnect the 3-phase cables from either the motor or the inverter. That would open the circuit.
But you would still have to contend with the static electricity generated from the rotor spinning inside of the motor, that would have to be discharged to ground somehow.


The easy answer? Use a dolly, don't flat tow.

Anti_Climax
Posts: 33
Joined: Fri May 20, 2016 12:22 pm
Location: The Valley of the Sun

Re: Flat towing revisit

Mon Sep 21, 2020 6:14 pm

So you're telling me that when the car is on and in neutral, the unpowered windings are not effectively open circuit?

When the motor is being driven, you have IGBTs that power the windings by switching them on a DC bus in the necessary sequence, but each winding is not always "on" and when the IGBTs that connect it to the DC bus are switched off no current flows into or our of the winding. At that point the winding is effectively an open circuit. Unless the motor winding somehow generates such a high voltage as to break down the IGBT, there should be no current flowing in that state.

If the IGBTs were "shunting" anything during coasting, it would generate a drag on the motor just as it does when power is being fed into the regen system - it would just be getting thrown away as heat instead. Based on all that, I can't see any reason the IGBTs would be allowing any current to pass in the windings while coasting in neutral, let alone be fed power by the car to somehow maintain equilibrium.

Edit: I can see the potential for eddy currents in the isolated windings, but that would not be current that gets shunted anywhere and I'd be very surprised if they did more than generate a tiny amount of heat - and I do mean tiny. Certainly open to any data otherwise. Not sure why it would generate static electricity differently than the motor is operating normally or require some active management to avoid.
2012 Candy Blue FFE with a 33.5kWh battery pack
*Coming Soon: CHAdeMO*

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