No tow mode because of permanent magnet motor?

[AntronX]

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!

You can tow charge already by putting the car in drive and getting towed. No different than coasting down a steep hill.

 

[MP3Mike]

You can tow charge already by putting the car in drive and getting towed. No different than coasting down a steep hill.

Only true as long as you have enough power to start the car and put it into drive. (Which we have seen once case where they hit that problem, thought I hope Tesla fixed their particular case via firmware upgrades.)

 

[mongo]

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.

The amount of current needed to overvoltage the pack is huge. If anything, the system would sense the reverse current and shut down the inverter. Regen requires controlled drive signals to generate, so I'm not seeing this situation happening. If it did, the opening time of the contactors is orders of magnitude (multiple millisecond) higher than the switching period (tens of mircoseconds). The electronics would react faster than the physical system.

 

[AntronX]

The amount of current needed to overvoltage the pack is huge. If anything, the system would sense the reverse current and shut down the inverter. Regen requires controlled drive signals to generate, so I'm not seeing this situation happening. If it did, the opening time of the contactors is orders of magnitude (multiple millisecond) higher than the switching period (tens of mircoseconds). The electronics would react faster than the physical system.

The body diodes in mosfets will passively rectify back emf induced current without any intervention from electronics. If battery is left connected, above certain speed this current will begin charging the battery. I assume if towed long enough the battery can be overcharged unless BMS cuts the fuse. Then there is no longer a sink for this rectified current and voltage across mosfets will increase past their blocking capacity and blow them up. All this could have been avoided if Tesla chose to add 2 more contactors between motor and inverter and safe towing would be possible.

 

[mongo]

The body diodes in mosfets will passively rectify back emf induced current without any intervention from electronics. If battery is left connected, above certain speed this current will begin charging the battery. I assume if towed long enough the battery can be overcharged unless BMS cuts the fuse. Then there is no longer a sink for this rectified current and voltage across mosfets will increase past their blocking capacity and blow them up. All this could have been avoided if Tesla chose to add 2 more contactors between motor and inverter and safe towing would be possible.

For that situation to happen, an unexcited motor would need to generate back EMF greater than the pack voltage at towing speed.
An AC induction motor generates no voltage without excitation, so that's out.
The PMSR is primarily SR with PM for smoothing (reportedly). A pure SR motor does not generate back-EMF without excitation, so the only source is from the PM side of things.
Let's say max towing is 70 MPH, top speed is ~140 for the 3. Back-EMF is proportional to speed, so if it can over voltage the pack when towing, it would generate 2x pack voltage at top speed.

For the car to hit this speed would then require some form of boost converter to provide a voltage to the motor higher than the back-EMF, produce a current, and generate torque. It would also require bidirectional switching so that, in the event of a failure while at speed, the motor would not over voltage the pack. (along with needing to block the back EMF to prevent unwanted regen).

However, the simplest drive strategy, and one supported by the tear down by Ingineerix, is that of a 3 phase chopper/ buck converter topology. Thus, the drive electronics cannot increase the voltage to the motor above the pack input voltage, and the back-EMF, even at top speed, must be less than the pack voltage.

Regen is accomplished via the back-EMF, winding inductance, and varying reluctance along with the buck converter operating in reverse as a boost converter.

 

[okashira]

I'm sorry my drawing is not more clear ... But there really is no way having the pack connected will have any effect on protecting the fets.

I think you should sit down and carefully draw a voltage across any of the phase legs and you will see that once you exceed 650v you will just get a dead short (well.. minus 650v)through one of the fets.

All mass market EVs currently available (with permanent magnet motors) will be capable of motor speeds that produce a no load back emf that exceeds, and often far exceeds the nominal battery voltage. I have already mentioned how this is possible and no you don't need a boost converter .


QUOTE="mongo, post: 2947814, member: 60387"]For that situation to happen, an unexcited motor would need to generate back EMF greater than the pack voltage at towing speed.
An AC induction motor generates no voltage without excitation, so that's out.
The PMSR is primarily SR with PM for smoothing (reportedly). A pure SR motor does not generate back-EMF without excitation, so the only source is from the PM side of things.
Let's say max towing is 70 MPH, top speed is ~140 for the 3. Back-EMF is proportional to speed, so if it can over voltage the pack when towing, it would generate 2x pack voltage at top speed.

For the car to hit this speed would then require some form of boost converter to provide a voltage to the motor higher than the back-EMF, produce a current, and generate torque. It would also require bidirectional switching so that, in the event of a failure while at speed, the motor would not over voltage the pack. (along with needing to block the back EMF to prevent unwanted regen).

However, the simplest drive strategy, and one supported by the tear down by Ingineerix, is that of a 3 phase chopper/ buck converter topology. Thus, the drive electronics cannot increase the voltage to the motor above the pack input voltage, and the back-EMF, even at top speed, must be less than the pack voltage.

Regen is accomplished via the back-EMF, winding inductance, and varying reluctance along with the buck converter operating in reverse as a boost converter.[/QUOTE]

 

[mongo]

I'm sorry my drawing is not more clear ... But there really is no way having the pack connected will have any effect on protecting the fets.

I think you should sit down and carefully draw a voltage across any of the phase legs and you will see that once you exceed 650v you will just get a dead short (well.. minus 650v)through one of the fets.

Thst's not how it works. If one terminal of the motor goes positive, the other terminal goes negative causing the bottom FET to conduct. Current flow (if previously driven) dictates that potential plus any inductance will forward bias the bottom diode.

Consider IF the motor developed 410 V and the pack is at 400V. Top FET diode conducts, referencing other end of winding to 400-410 = -10V, that is not going to happen since the bottom FET diodes conducts at 0.7-1.4 or so.

All mass market EVs currently available (with permanent magnet motors) will be capable of motor speeds that produce a no load back emf that exceeds, and often far exceeds the nominal battery voltage. I have already mentioned how this is possible and no you don't need a boost converter .

You have mentioned this (via flux weakening which also drops torque), but not how it is possible on an unexcited motor, unless the windings and PM portion allow for it (which seems like a poor design choice, and not aligned with the magnets being there to reduce ripple, but could be the case).