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Variable Torque Electric Motor?

I was reading several Threads that discuss driving range of Roadster and can't help but notice that its effective range decreases dramatically at higher speeds due to several factors like wind, tire, drive train loss etc. which is contrary to ICE vehicles where the MPG is the best at highway speeds around 60mph or so. I've also read some discussions where having multi-gear transmission is rather pointless for Electric Cars because it defeats the purpose of having instant torque at any speeds. Not to mention, it is difficult to build a robust and reliable transmission to handle its power output (case in point the original Roadster's 2-speed tranny). My point is, you don't need 100% torque when you are cruising on the freeway thus seems very wasteful use of precious wattage. Is there anyway to reduce the torque output at the electric motor similar to cylinder deactivation system that are used in modern ICE cars to maximize highway driving range? This way the driver can switch between "Econo" and "Sport" model to suit his or her driving needs. Or something more dynamic and sophisticated system that automatically varies the torque by "learning" the driver's behaviors based on steering inputs, acceleration, etc. I think this would be a great feature on Model S since it's designed to be more practical than Roadster.

I don't believe there is full torque all the time in a Tesla,or any car, unless you keep it accelerating to the maximum speed of the car. The reason that an ICE car need some speed to get a good mpg is that it is basically a wasteful technology that consume a lot of fuel even when driving slow or idling. An electric motor, on the other hand, use more of the energy to move the car and less to heat whatever is around it. Air resistance and friction will have a bigger impact on an electric car because it's more energy efficient than an ICE car, especially at low speeds. I think they already have a "torque reduction system" that will limit the power to prevent the system from overheating og breaking. Other than that I can't really see the need for such a system on an electric car.

ICE cars lose mpg too in highway speeds, you just don't realize it because they have very bad efficiency and very small optimal RPM range, and as such you lose a lot more in stop/go traffic and accelerations than any electric vehicle. If you could go steady ~40mph (using optimal RPM) with ICE car you could get much further with single tank of gas than driving that same with 60mph (using same optimal RPM).

I think you (OP) got the physical concept of "torque" mixed with something else.

"full torque" is what you have when you floor the accelerator. Nothing else uses full torque. You don't gain anything by reducing motor max torque, you probably lose some (bigger motors have better efficiency in emotors).

Torque is what causes accelerations/decelerations and steady speed. You have exactly the amount of torque you need to go the speed you want to, no more no less. If you change the torque you change your speed. When (torque at wheels = car air and rolling resistances) you go steady speed. Directly opposite force vectors.

You only get full torque when you mash the go pedal.

The roadster has a range mode which limits power, similar to what you are describing.

Someone with more math skills than I should try to figure out the MPG equivalent for city or highway with the roadster.

And with an electric motor, wouldn't running it at a lower gear ratio put more stress on the engine and create heat that would lower the efficiency of the engine?

IIRC at 50mph MPGe was around 270 depending how you calculate it. It gets better if you do well to wheels vs powerplant to wheels comparison. In city traffic you probably do even better, because EV:s don't use any extra energy in stop/go traffic. Obviously less in highway.

I assume you mean higher gear? Low gear puts less stress on the engine than high gear. Thing is that emotors are so close to perfect that it really doesn't matter much. With higher gear you just accelerate slower. RPM matters, there is a certain cutoff speed for emotors (different for each) once you start to get weaker response, and weaker efficiency, so it is good to have gear ratio that doesn't use those high RPM in usual driving. But even then you still have a very good efficiency (it is just *relatively* less efficient).

Here's the math:
Comparing tank fill up/battery charge for like cars
Gallon of gas contains 1.3*10^8 Joules and the car gets 25 mpg (about 5200 Joule/mile)
Charge the 300 mile battery takes 3.06 *10^8 Joules (about 1000 joules/mile)

It's about 5 times as efficient as a Towncar

If you want to go further in terms of power plant and well to wheel, you have to know the source of the electricity. Based on this number it can go up are down. If you are burning fuel oil as the source (WORST CASE, 50% efficient) or getting it from inland wind (BEST CASE, running at 35% capacity, energy payback approximately 6 months and using the cradle to grave energy cost associated with the windmill as your only energy input)
The range is as follows
2.5 times as efficient (worst case)
310 times as efficient (best case)

I will put some qualifiers on that 310 number. It assumes that only gasoline was the ONLY energy input and ONLY uses the energy used to make the windmill, minimal maintenance, it also assumes that it is a new windmill and there is no grid impact (enough energy in the grid so that another power plant doesn't have to increase capacity to make up for your usage)

"its effective range decreases dramatically at higher speeds due to several factors like wind, tire, drive train loss etc. which is contrary to ICE vehicles where the MPG is the best at highway speeds around 60mph or so."
Not even close to being true. MPG is best at highway speds because the car isn't stopping and starting. And each and every ICE vehicle differs as to at which constant speed its "sweet spot" is located.
There seems to be a confusion here as to "maximum torque."
The torque an electric motor produces is determined by how much juice it's consuming, not by the speed at which its rotor is turning. Those dyno torque and HP numbers are merely indicating how much output is possible at various RPM levels. In other words, if no load is being placed on an electric motor, then the power required to spin the rotor to, say, 5000 RPM, is simply the amount required to keep that rotor and axis spinning at that rate, which is to say, not very much.

Can someone give me an explanation on how the electric motor converts the power from the batter to the wheels. I'm looking at you Timo with your extensive knowledge ;)

Basic three-phase induction motor: http://en.wikipedia.org/wiki/Induction_motor

Basically there is a rotating magnetic field that rotor tries to follow causing torque to the engine. I don't know the exact type of the motor used in Tesla cars, but it is three-phase induction one. Single moving part, very simple basic structure.

Between motor and battery there is PEM (power electronics module) that converts DC from batteries to AC for motor and controls the currents, frequency and voltage. And same in other direction when regenerative braking is used.

Thank you, very good simple explanation.

"Tesla effective range decreases dramatically at higher speeds..."

True of all cars.

Consumer Reports tested ICE cars mileage at speeds. Examples:

55mph 65 mph 75 mph (55-75 difference)
Honda Accord LX-4-cyl 49mpg 42mpg 35mpg (29% drop)
Toyota Rav 4 37 33 27 (27% drop)
Ford Fusion Hybrid 49 41 36 (27% drop)
Fusion Titanium 2.0 4-cyl. 41 34 28 (32% drop)

Perhaps a more readable chart :)

"Tesla effective range decreases dramatically at higher speeds..."

True of all cars.

Consumer Reports tested ICE cars mileage at speeds. Examples:

...........................................55mph 65 mph 75 mph (55-75 difference)
Honda Accord LX-4-cyl.......49mpg 42mpg 35mpg (29% drop)
Toyota Rav 4.......................37........33.........27........27% drop)
Ford Fusion Hybrid..............49........41.........36.......(27% drop)
Fusion Titanium 2.0 4-cyl...41........34..........28.......(32% drop)

JaneW;
The "pre" tag helps with tables:

                      55mph 65 mph 75 mph  
Honda Accord LX-4-cyl 49mpg 42mpg  35mpg (29% drop)
Toyota Rav 4          37    33     27    (27% drop)
Ford Fusion Hybrid    49    41     36    (27% drop)
Fusion Titanium 
   2.0 4-cyl.         41    34     28    (32% drop)

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