Planetary gear system

[ajdelange]

The Models S Plaid has the reverse problem, in that they have to reduce power to the wheels on launch because the electric motors have too much torque, meaning the tyres will break traction up until fairly high speeds. Despite this low ratio gearing they still have to carbon wrap the rotors to stop it from flying apart at high RPM. A higher ratio, which is the primary benefit from a planetary gear would just make the problem worse, not better.

The torque is easily reduced by reducing the magnitude of the d-q vector. It can be switched anywhere within its reachable range to anywhere else in that range (magnitude and phase) within milliseconds. The only gearing that is necessary is that necessary to get the most efficient design motor speed down to range of tyre speeds that typify cruise. There is no need to shift gears (though some designers do have a second gear). Tesla has done the trades and found this un-necessary while outperforming the vehicles that do shift gears.

 

[JBee]

The torque is easily reduced by reducing the magnitude of the d-q vector. It can be switched anywhere within its reachable range to anywhere else in that range (magnitude and phase) within milliseconds. The only gearing that is necessary is that necessary to get the most efficient design motor speed down to range of tyre speeds that typify cruise. There is no need to shift gears (though some designers do have a second gear). Tesla has done the trades and found this un-necessary while outperforming the vehicles that do shift gears.

My argument was that it is unnecessary for Tesla electric motors to use a "planetary gear" because there is a need to increase the deliverable torque to the wheels.

My comparison was that the Model S Plaid actually has to reduce the torque (as you described) because it had too much torque, and that in fact the coefficient of traction of the tyres was the limiting torque factor (otherwise the tyres spin), not the overall motor gear ratio.

As for if more gears is necessary primarily depends on the level of acceleration performance required and the the top speed of the vehicle. The limit there being the rotational velocities of the rotor, which is why we got the CFWIM in the Plaid. It could well be that the Tesla Roadster either has to add some RPM to the CFWIM or that they do actually add a gear to reach top speed. Either way a planetary gear would unlikely help...except if they used is as a CVT transmission like they did in the Prius. using two motors rpm and direction to change the overall ratio.

 

[ajdelange]

We can be more efficient than either the induction motor or the reluctance motor because we can choose what speed the motors are running at. We can run them near their most efficient speed most of the time. You don't get a choice.

Nope because planetary gear systems necessary to give the assortment of ratios that would be necessary to stay at the peak efficiency would required several stages as was done in the automatic transmissions of the days of yore with ICE vehicles where what you are thinking does apply because those engines have narrow torque and efficiency peaks. Electric motors have very broad ones.

We can get a lot more torque at 0 rpm than the motor can put out at 0 rpm because the motors are running at full speed.

ELectric motors can put out full torque at 0 rpm and they do it with nothing more than a garden variety reduction gear. This is how the 2500 kg Teslas are able to get to 60 mph in a few seconds.

Say the field speed needs to run at 5 Hz when the shaft is at 0 rpm, in order to maximize the effect of the induced current in the rotor.

To "maximize the effect of the induced current in the rotor" the flux linkage component of the vector current is increased to whatever it needs to be. Torque is controlled through slip in an induction motor and through field angle relative to the rotor in PM motors..

The linear distance that field can travel is fixed distance based on the radius of the rotor.

That's got nothing to do with it. The torque produced depends on the magnetizing current and the torque current, independently controllable, and either the slip or the angle depending on whether the motor is IM or PM.

If the motor is able to get up to speed and the field can operate at say 400 Hz that is an additional 395 pulses of similar amounts of energy delivered per second. That is why this system is MUCH more powerful than a simple motor.

Again you reveal that you know little about AC machines (disclaimer: I don't know that much either - it is a very complicated subject) or about 3 ø systems. Power is not delivered in "pulses" (these are not DC brushless motors - they are 3 ø AC motors). It is delivered continuously.

Perfect for the Semi (and my truck, I want it in my truck).

Un necessary in either which is why Tesla, who believe it or not know considerably more about these matters than you do, have not elected to use it.

[But wait, that's not all.] This system also allows you to get effective and efficient regeneration down to 0 rpm. You get all that for the price of a planetary gear set (and the additional weight of the aluminum case that will need to be beefed up to handle the torque).

It is not necessary to use such a kluge for good regen braking as is demonstrated by the fact that the modern crop of Teslas already have it, with nothing more than simple (relatively) reduction gearing, to the point where one pedal driving to a full stop is available to the driver.

 

[ajdelange]

My argument was that it is unnecessary for Tesla electric motors to use a "planetary gear" because there is a need to increase the deliverable torque to the wheels.

Wasn't disagreeing. Just trying to amplify.

My comparison was that the Model S Plaid actually has to reduce the torque (as you described) because it had too much torque...

Any car does in the sense that the maximum torque is seldom required i.e. when coming out of the blocks or passing. At any other time the motor(s) can supply more torque than is necessary and so we turn the toque knob to a setting less than 11.

 

[Molotov]

Planetary gears can make it even noisier....

 

[Crissa]

Variable gears reduce the available torque for electric motors can produce, because electric motors can produce more torque than transmissions can handle.

-Crissa

 

[JBee]

Variable gears reduce the available torque for electric motors can produce, because electric motors can produce more torque than transmissions can handle.

-Crissa

Not sure who that was directed at, but all Teslas have transmissions that handle the torque just fine. Even the "variable" ones or stepped ones (like Taycan) work too with electric motors and the torque they can deliver.

The Prius CVT was not a rubber band model (like in a moped), it was just a planetary gear with two electric motors driving it.

 

[Crissa]

Not sure who that was directed at, but all Teslas have transmissions that...

...are not variable, which is why I used that word first.

-Crissa

 

[JBee]

...are not variable, which is why I used that word first.

-Crissa

That still confuses me a bit, as all the transmissions I know of in vehicles, typically are used to increase the torque and not reduce it. Be that torque from electric motors or ICE, through variable, stepped or fixed ratio transmissions.

Just like a lever gives you the ability to move something, you couldn't without a lever.

 

[Crissa]

Alas, electric motors can have lots of torque. That's based on their wattage. It's speed they run out of. Even my dinky electric bike with its artificially lowered bottom-end torque can break the tire loose given the least opportunity.

Electric has different torque curve, different tradeoffs from what you're used to, that's all.

-Crissa

 

[JBee]

Alas, electric motors can have lots of torque. That's based on their wattage. It's speed they run out of. Even my dinky electric bike with its artificially lowered bottom-end torque can break the tire loose given the least opportunity.

Electric has different torque curve, different tradeoffs from what you're used to, that's all.

-Crissa

I'm very familiar with electric motor torque curve and those of ICE too. But:

The primary purpose of a transmission in any vehicle is to increase available torque at any given vehicle velocity. Put simply transmissions are designed to increase torque and don't reduce it as you claim. Be they variable, stepped or fixed ratio transmissions.

Further gearbox failure has nothing to do with electric motors or ICE motors, and is just simply because of poor engineering of the transmission. There are many ICE engines with more torque than Plaid that run through decently engineered gearboxes just fine.

Which makes this statement nonsensical:

Variable gears reduce the available torque for electric motors can produce, because electric motors can produce more torque than transmissions can handle.

Maybe a refresh of what torque actually is would help:
https://en.m.wikipedia.org/wiki/Torque

 

[Crissa]

The primary purpose of a transmission in any vehicle is to increase available torque at any given vehicle velocity. Put simply transmissions are designed to increase torque and don't reduce it as you claim. Be they variable, stepped or fixed ratio transmissions.

Sorry, but you're wrong. But just in this one case.

Transmissions in EVs are primarily to increase operational speed, not torque.

And all transmissions - because they have internal resistance - lose some amount of torque. Yes, they may give you more mechanical advantage and increase torque at the wheel, but they're losing it from the parts grinding together all the same.

This is why EV torque can't be measured at the motor. Because it's going to be highest there. What matters is what you can get to the tires.

If I and @ajdelange agree on something, it's probably true.

-Crissa

 

[JBee]

Sorry, but you're wrong. But just in this one case.

Transmissions in EVs are primarily to increase operational speed, not torque.

Um no. Many Ev's do not have enough torque for direct drive of the wheels for high acceleration performance, except hub motors and the like, including Teslas. They do have ample speed though.

For example the Model S Plaid, one of the most powerful Ev's out there, with one of the fastest spinning motor rotors (that's why its CF wrapped), has a gear ratio of 8.47:1. That means there is some 8 times more torque at the wheels than what the motor can produce.

And all transmissions - because they have internal resistance - lose some amount of torque. Yes, they may give you more mechanical advantage and increase torque at the wheel, but they're losing it from the parts grinding together all the same.

This is why EV torque can't be measured at the motor. Because it's going to be highest there. What matters is what you can get to the tires.

If I and @ajdelange agree on something, it's probably true.

-Crissa

As before torque is higher at the wheel after the transmission, and not at the motor as you say. This is because "RPM is being traded for torque". Think of a transmission ratio as the length of a simple lever on a fulcrum.


Lets do another calculation to demonstrate:
- Plaid wheel is a 265/35 ZR21 with 719mm diameter and 2.258m circumference
- if it were true that a transmission on the Plaid MS would reduce torque to "increase operational speed" as you say then if they would leave the ratio as 1:1 you would get;
-30,000RPM CRIM motor from Plaid x 2.258m wheel circumference x 60minutes in an hour = 4064kmh or 2540MPH!!

It should be clear that you statement
"Transmissions in EVs are primarily to increase operational speed, not torque"
is in fact exactly opposite to the truth.

As for the torque losses due to "grinding" resistance these are trivial for the torque component, and even for the power transfer are not significant in the grand scheme of things. They're less than the rolling resistance and the deformation of the tyre on the road by far.

 

[Bill906]

The primary purpose of a transmission in any vehicle is to increase available torque at any given vehicle velocity.

In modern ICE transmissions they increase OR decrease available torque depending on what gear. Although the term "overdrive" is somewhat ambiguous, most mechanical discussions use the term to mean a gear ratio where the input speed is slower than the output speed. In that case torque is sacrificed for speed.

My Jeep Grand Cherokee has 8 forward gears. Gears 1-5 are "Underdrive" (input faster than output). Gears 7-8 are "Overdrive" (input slower than output) and 6th gear is 1:1.

First Gear Ratio 4.71:1
Second Gear Ratio 3.14:1
Third Gear Ratio 2.11:1
Fourth Gear Ratio 1.67:1
Fifth Gear Ratio 1.29:1
Sixth Gear Ratio 1.00:1
Seventh Gear Ratio 0.84:1
Eighth Gear Ratio 0.67:1

 

[JBee]

In modern ICE transmissions they increase OR decrease available torque depending on what gear. Although the term "overdrive" is somewhat ambiguous, most mechanical discussions use the term to mean a gear ratio where the input speed is slower than the output speed. In that case torque is sacrificed for speed.

My Jeep Grand Cherokee has 8 forward gears. Gears 1-5 are "Underdrive" (input faster than output). Gears 7-8 are "Overdrive" (input slower than output) and 6th gear is 1:1.

First Gear Ratio 4.71:1
Second Gear Ratio 3.14:1
Third Gear Ratio 2.11:1
Fourth Gear Ratio 1.67:1
Fifth Gear Ratio 1.29:1
Sixth Gear Ratio 1.00:1
Seventh Gear Ratio 0.84:1
Eighth Gear Ratio 0.67:1

Sure I could say "optimise" instead, but the point is that without a high ratio to increase the torque none of them would be able to start to drive.

Otherwise feel free to try to drive your ICE car only in sixth gear and see how you go. ;-)

But seriously you still have a differential gear as well which means you still have a ratio between the motor/ICE and wheels.