Recharging FFE While Being Towed & Applying Brakes

[GREG]

Has anyone tried, in a desperate situation, to add charge to the FFE by being towed and at the same time applying the brakes in order to have regenerative braking?

I guess that this is DC to DC charging or Level 3 charging?

How much energy in Kw per hour could such a scheme add to a battery assuming maximum regenerative braking?

 

[paw160]

Hard to tell if you are just being cheeky or not...

This wouldn't be Level 3 (DC-DC) charging. In fact it would be more like turning you FFE in to a PHEV (with the tow truck being the gas engine!).

If this works at all you would have to tow a very long way to put a significant amount of energy into your battery.

Eliminate the tow truck for a minute and just consider coasting down a long steep hill using the brake to maintain a constant speed. That's a reasonable analogy for the amount of regen you could expect.

 

[davideos]

I wouldn't want to try it, but you could approximate the situation by regenerating going down a long steep hill. Towing with one car while applying the brakes on the other doesn't seem too happy for the car doing the towing. Would probably overheat....I suppose it would depend on how much regen was being applied.

 

[WattsUp]

Has anyone tried, in a desperate situation, to add charge to the FFE by being towed and at the same time applying the brakes in order to have regenerative braking?

You could also just leave it in L, you wouldn't necessarily need to apply the brakes.

 

[michael]

I'd just have the thing put on a flat bed and carried to somewhere with an electrical outlet. That's the safest.

 

[hcsharp]

Has anyone tried, in a desperate situation, to add charge to the FFE by being towed and at the same time applying the brakes in order to have regenerative braking?

I guess that this is DC to DC charging or Level 3 charging?

How much energy in Kw per hour could such a scheme add to a battery assuming maximum regenerative braking?

It's not DC to DC. The motor is AC induction so it would be similar to plugging your car in except probably stronger. I don't know how strong (in terms of kW) the regen is on a FFE but most EVs are capable of much higher charging rates during regen than while plugged in. Regen braking usually doesn't last very long so less overall heat and stress is generated. For this reason you might not want to go very fast.

For the towing idea to work, I would put it in L and not apply the brakes. There's a chance that you might drag the pads against the rotors, creating a safety risk and causing damage. You would probably need all 4 wheels on the ground to avoid problems with traction control. I don't know much about towing methods but I don't think regular tow trucks have the means to do this. Might be easier to push which would scratch up your bumper. I'm still tempted to try it just to see what happens!

 

[jmueller065]

Don't forget about the obvious problem at stoplights: In order to regen the car will have to be on and in L which means that it will try to creep when stopped.
Since there would be no driver to step on the brake the motor may be generating significant torque at that point (with a driver with their foot on the brakes the motor controller knows that the car shouldn't be moving and thus doesn't send any current to the motor. In this towed case there will be no foot on the brake so the motor will attempt to creep...possibly eating up all your regen current in the process!).

 

[v_traveller]

Calll me chicken, but I wouldn't try it. From the manual:

Emergency Towing
In case of a roadside emergency with a disabled vehicle (without access
to wheel dollies, car hauling trailer, or flatbed transport vehicle) your
vehicle (regardless of transmission powertrain configuration) can be flat
towed (all wheels on the ground) under the following conditions:
• Vehicle is facing forward so that it is being towed in a forward direction.
• Place the transmission in N (Neutral). Refer to Brake-shift interlockin the Driving chapter for specific instructions if you cannot move the gear shift lever into N (Neutral).
• Maximum speed is not to exceed 35 mph (56 km/h).
• Maximum distance is 50 miles (80 km).

So when dragging an FFE on all four, it's supposed to be in neutral (Per Ford), in which case regen wouldn't work since the wheels wouldn't be connected to the motor. Anyone ever try braking while in neutral? Maybe I'll try it on the way home...

I suspect that leaving it in drive while flat towing could void the warranty, should any damage occur...

 

[WattsUp]

Anyone ever try braking while in neutral?

Yeah, the mechanical brakes engage, but that's it. There is no regen.

 

[jmueller065]

in which case regen wouldn't work since the wheels wouldn't be connected to the motor

I don't think the FFE has a clutch! I think the wheels are ALWAYS connected to the motor--its just a matter of any current going through the motor or not.

Look at the pictures in these two articles about the FFE's motor: I don't see a clutch anywhere in there.
http://www.electric-vehiclenews.com/2011/01/ford-focus-electric-motor-exposed.html
http://www.engadget.com/2011/01/11/ford-focus-electric-motor-extracted-split-asunder-coppery-guts/

 

[WattsUp]

in which case regen wouldn't work since the wheels wouldn't be connected to the motor

I don't think the FFE has a clutch! I think the wheels are ALWAYS connected to the motor--its just a matter of any current going through the motor or not.

Correct. Regenerative braking only occurs when the motor is electrically connected to the resistive load of charging the battery. The motor is always mechanically connected to the drive wheels (the front wheels, in the case of the FFE).

"Shifting" to neutral is accomplished, basically, by "flipping a switch" somewhere within the car's electrical systems. When the car is "in neutral", there is simply no resistive load on the motor, and therefore very little mechanical resistance against turning the drive wheels via external force (rolling down an incline, etc.).

The same situation occurs when the car has a fully charged battery, in which case regenerative braking is automatically temporarily disabled (again, by flipping a switch), much like when shifting into neutral. You may have noticed this effect when braking as you drive away right after a full charge... there is no regenerative braking, only the mechanical brakes slow the car. When some of the battery's charge is depleted, and can again provide a resistive load on the motor, regen will be re-enabled.

 

[v_traveller]

Ah, that makes sense. So I wonder why Ford doesn't want us flat-towing except while in neutral.... any guesses?

 

[WattsUp]

Ah, that makes sense. So I wonder why Ford doesn't want us flat-towing except while in neutral.... any guesses?

I think basically so that the car is not in Park. Honestly, saying "shift to Neutral" is simply less ambiguous than saying "shift out of Park" (to what? -- N, D, or L?).

In a normal car with a transmission, Neutral obviously makes mechanical sense for flat-towing. For the FFE, I think (speculatively, to be sure) that N, D, or L really wouldn't make any difference (assuming the car is turned off). But, if I ever did need to tow my FFE like that, it would certainly place it in N, because that's what the manual says.

 

[paw160]

I can't speak for the FFE directly, because I don't know exactly what the motor drive technology is, or how it is configured, but I do have experience with a lot of battery powered motor/drive combinations.

When you mechanically spin a wheel, the motor will begin generating voltage. This is known as "back emf". On most modern motor drives, there are at least four switching devices (igbt's, mosfets, scr's, or similar). As the motor continues to spin (and at higher speeds the back emf is greater), this voltage can grow end up damaging the switching devices or the capacitors on the DC bus.

My guess is that when in neutral, there is some sort of mechanical clutch (or mechanical relay) that disconnects the wheel or the motor.

 

[jmueller065]

My guess is that when in neutral, there is some sort of mechanical clutch (or mechanical relay) that disconnects the wheel or the motor

I don't think so looking at the pictures. (I'm hoping Ford shows that cutaway model at the Detroit auto show in January--most likely not but they may have a display showing all their power options and may throw that up. If they do I plan to take plenty of pictures and inspect it a bit more than the two articles did.)

In addition, the FFE's motor is synchronous meaning that the controller has to create a rotating magnetic field to keep the rotor moving (by energizing different windings in sequence). As such I think the back EMF will cancel itself out as the phase angle of the rotor changes with respect to the windings.

 

[paw160]

The FFE motor is synchronous, but it's a permanent magnet synch motor. This means the rotor is a permanent magnet, so it does not require external excitation.

In normal operation the FFE's motor drive applies a rotating field as you said. But when you spin the wheels manually (as in towing) the rotor actually induces the fields back through the stator. Now the motor is a generator.

If the FFE's controller is on, then the regen system can modulate this induced power back into the battery. However, even then, the regen capacity is probably limited (to prevent over voltage of the DC bus).

It's very possible for a motor's back emf to rapidly rise in voltage to the point where it could damage the battery, bus capacitors, or switching devices.

I'm sure that the FFE's drive system was designed for normal driving conditions, not for having lots of external torque applied.

The question is how much regen power can it handle, and in what mode?

It would be interesting to find out what "Neutral" really means on this car.

 

[WattsUp]

It would be interesting to find out what "Neutral" really means on this car.

It is fairly easy to tell that nothing mechanical happens when shifting to neutral in the FFE. There is nothing jarring, audible, or anything when shifting to neutral, you sense only the instant loss of traction power. It feels purely electronic to me.

 

[hcsharp]

There is no clutch so it has to be electronic or using a contactor. They recommend a maximum 4-wheel towing speed (35 mph) so I wonder if the voltage gets too high if you exceed that. At some point it probably begins to clamp the voltage so you might feel some resistance above a certain speed, whether regenerative or not. That could explain the distance limitation (to avoid overheating the bypass circuit). Or they might just use a contactor. It would be fun to have lunch with the engineers who designed it.

 

[davideos]

I think if you were to tow with all 4 tires on the ground, you'd put the car in Neutral. There would be no generated electricity and no different that shifting into neutral while going down the freeway. With no load, no resistance, so nothing would get hot either. So I don't think that the issue is with that. In fact, I think those instructions are for an focus...gas or electric. I think the reason for the speed limitation might be something related to steering...if that makes sense. I believe when you tow with 4 tires down, you leave the stearing to float...to basically turn itself with the leading car. If you go too fast, you might get some unwanted feedback from that. But honestly, I know nothing about it. Sounds like a google search to me.

 

[davideos]

I think if you were to tow with all 4 tires on the ground, you'd put the car in Neutral. There would be no generated electricity and no different that shifting into neutral while going down the freeway. With no load, no resistance, so nothing would get hot either. So I don't think that the issue is with that. In fact, I think those instructions are for an focus...gas or electric. I think the reason for the speed limitation might be something related to steering...if that makes sense. I believe when you tow with 4 tires down, you leave the stearing to float...to basically turn itself with the leading car. If you go too fast, you might get some unwanted feedback from that. But honestly, I know nothing about it. Sounds like a google search to me.

It seems the issue with flat towing is with the automatic transmission. Since the engine isn't running, then the transmission fluid isn't circulating. I don't know if this would be an issue with the FFE since there would be no difference if the transmission were turning because of the motor or the tires; since they are all connected together and turning either way.