No tow mode because of permanent magnet motor?

[okashira]

This is because in the event of power loss the parking brake can't be released, so the wheels will not rotate in that case. I'm sure it is not because of BEMF. They don't want a tow trucker driver to discover the brakes are locked after dragging the car so they tell them not to even try it. Most tow trucks carry sufficient equipment that this is not a problem.

Is most certainly because of back emf. That you can't tow evs on the drive wheels, that is.

 

[okashira]

Immediate thought: What happens if there is a fault that causes loss of power while moving? Destroyed motor/inverter????

Just the inverter, yes. The motor won't be damaged.
You are completely right, if you were traveling at 90mph, and you cut 12v power to the inverter , it would quickly go poof.
Exceedingly rare circumstance in practice.

This is why field weaking is so uncommon for hobbiest / diy ev projects.. All it takes is a glitch in your control algorithm and if you lose field weaking for a split second while the motor back emf exceeds the power switch breakdown voltage (650v in the case of the model 3 inverter - about 85mph) the inverter will quickly go poof.

 

[smatthew]

You could probably tow safely to 40mph

In Tow Mode, the brakes will apply after 20 minutes, or if speed goes above 5mph

 

[Unpilot]

If the car is turned on and nothing breaks....isn't towing theoretically the same as driving down a very long hill?

 

[Stellavator]

If you are full SOC, and/or have regen set low, and you coast down a hill, how is this any different from towing?

 

[smatthew]

If the car is turned on and nothing breaks....isn't towing theoretically the same as driving down a very long hill?

The car automatically shifts to park if your butt leaves the seat.
It will stay in neutral without a butt via tow mode, but tow mode turns off at 5mph to prevent accidents.
Do you want to sit in your car for a tow?

I'd rather have my car on a flatbed.

 

[Unpilot]

The car automatically shifts to park if your butt leaves the seat.
It will stay in neutral without a butt via tow mode, but tow mode turns off at 5mph to prevent accidents.
Do you want to sit in your car for a tow?

I'd rather have my car on a flatbed.

Nah....just curios is all...but I would like a Pure electric car I could flat tow behind my pure electric RV.

 

[smatthew]

Nah....just curios is all...but I would like a Pure electric car I could flat tow behind my pure electric RV.

Oh that's simple. Just get a little flatbed trailer.

I think I saw a picture on reddit of a Tesla towing a Tesla

Where are you getting a pure electric RV? Tesla Semi powered?

 

[mongo]

I assure you the model 3 motor will produce a back emf that exceeds the battery voltage by 50mph, not 150mph.

The 3 drive electronics are a buck converter (3 half bridges). In regen, the inductance of the motor windings are used to form a boost converter to charge the pack (from back EMF). There is no such inductor in drive mode.
Where are you getting this information from?

 

[mongo]

Just the inverter, yes. The motor won't be damaged.
You are completely right, if you were traveling at 90mph, and you cut 12v power to the inverter , it would quickly go poof.
Exceedingly rare circumstance in practice.

This is why field weaking is so uncommon for hobbiest / diy ev projects.. All it takes is a glitch in your control algorithm and if you lose field weaking for a split second while the motor back emf exceeds the power switch breakdown voltage (650v in the case of the model 3 inverter - about 85mph) the inverter will quickly go poof.

Edit: I'm less sure on fault modes, but for AC they increase frequency at a constant voltage for field weakening/ speed increase, I suppose the SR part could act the same..
Field weakening is for a separately excited rotor and stator system. The 3's motors are PMSR and AC induction, there is no field weakening option.

Further, the inverter is MOSFET based with reverse body diodes, as long as the pack is still connected, the motor can't overvoltage it.

 

[Unpilot]

Oh that's simple. Just get a little flatbed trailer.

I think I saw a picture on reddit of a Tesla towing a Tesla

Where are you getting a pure electric RV? Tesla Semi powered?

No Electric RV's yet Although Winnebago is working on a small class A electric.
In addition to the electric buses in China and Europe.

Probably will not be mature and available for me in the years I have left.
Trailer is OK but I will probably just go with flat towing a Jeep.

 

[SDRick]

I believe there is a great submarket opportunity for the model 3 if it could be configured to allow a flat tow. All those RV'ers would love it!

 

[mongo]

I believe there is a great submarket opportunity for the model 3 if it could be configured to allow a flat tow. All those RV'ers would love it!

Software seems very doable. In the meantime, could you belt a bag of potatoes to the seat and shift to neutral? Get FSD working and we won't need to tow .

 

[strider]

Trailer is OK but I will probably just go with flat towing a Jeep.

Plug-in hybrid Wrangler in 2020 so they are on the way!

 

[VT_EE]

If you are full SOC, and/or have regen set low, and you coast down a hill, how is this any different from towing?

My question exactly.

 

[okashira]

Edit: I'm less sure on fault modes, but for AC they increase frequency at a constant voltage for field weakening/ speed increase, I suppose the SR part could act the same..
Field weakening is for a separately excited rotor and stator system. The 3's motors are PMSR and AC induction, there is no field weakening option.

Further, the inverter is MOSFET based with reverse body diodes, as long as the pack is still connected, the motor can't overvoltage it.

The body diodes wont do anything. If the back emf exceeds 650v, the mosfet will conduct in the forward direction (with a 650v drop)
The pack can't help either, since it's a fact that the mosfet will conduct at 650V when it is off.
The pack being connected or not wont really make any difference.

Go ahead and apply 700v to two of the phases of the motor and this will happen:

 

[Cdnman0]

I guess if you need a tow, you call Tesla.

 

[mongo]

The body diodes wont do anything. If the back emf exceeds 650v, the mosfet will conduct in the forward direction (with a 650v drop)
The pack can't help either, since it's a fact that the mosfet will conduct at 650V when it is off.
The pack being connected or not wont really make any difference.

Go ahead and apply 700v to two of the phases of the motor and this will happen:

In your diagram the bottom side body diode should be conducting with the current path going through the pack.

Note my disclainer

as long as the pack is still connected

, the pack will not allow 650 V across the FETs due to it's internal resistance (unlike a standard power supply that cannot sink current, been there/done that ). Anything above the current pack voltage will charge it. Beyond that, if Tesla has a voltage clamp in the inverter (or logic to turn on all high (or low) side FETs on loss of pack/ over voltage), it will also suppress the back EMF.

But that still depends on base assumption that the PMSR or AC induction ever produce more Back-EMF than pack voltage (note: this a separate condition from the purposeful signaling to produce regen). Where are you getting the data that they do (at sufficient energy to over voltage the HV rail and its capacitance)?

 

[MP3Mike]

, the pack will not allow 650 V across the FETs due to it's internal resistance (unlike a standard power supply that cannot sink current, been there/done that ). Anything above the current pack voltage will charge it. Beyond that, if Tesla has a voltage clamp in the inverter (or logic to turn on all high (or low) side FETs on loss of pack/ over voltage), it will also suppress the back EMF.

I'm pretty sure that the BMS would open the contactors if the voltage surged above the expected levels. So the pack wouldn't be connected for long.

 

[smatthew]

Theoretically, could Tesla make a "Tow Charge" mode? If you run out of power in the middle of nowhere, just have a tow truck pull you. Regen kicks in, and by the time you're back in town you have a partially charged battery!