Planetary gear system

abebarker

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I think Tesla should start using the planetary gear/[power split] device. It is perfect for induction motors. It allows you to get the power that an induction motor can produce at full speed but deliver it to a shaft at 0 rpm. This would be perfect for the Tesla Semi. It would even be enough torque for a freight train locomotive.

[The power split device allows several advantages;
1) Increased efficiency, because the motor speed is not tied to tire speed, the motors may operate near their most efficient.
2) Much greater low end torque, the motors can deliver full speed power to shaft at 0 RPM's.
3) Effective and efficient regeneration all the way down to 0 RPM.]

I want this technology in my truck.


 
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abebarker

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It is my opinion that the planet carrier/ planetary gears should be going slowly to reduce wear. The faster those gears are flung around the more centrifugal force is going to press the planetary gears into the ring gear, increasing wear. It is logical that the output should be the planetary gears and carrier.

The power curve could be kind of saddle backed, high torque on the low end and high end but a bit of a droop in the mid rpm ranges. This can be minimized through clever selection of motor speeds.

Of course the gear ratio should be that which provide the motors the optimum speed advantage.
 

Crissa

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Why would they need to do that? They usually just throw more motors at something to get more torque. That's where that crazy acceleration comes from.

-Crissa
 
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abebarker

abebarker

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Why would they need to do that? They usually just throw more motors at something to get more torque. That's where that crazy acceleration comes from.

-Crissa
It is really about efficiency. If your motor is most efficient at high speeds but you need to operate at low speeds and high speeds then you can use this planetary gear trick to allow the motors to operate at(or near) their most efficient rpm and the output can be any rpm. This allows the motors to be smaller and, consequently, reduces the weight and mass.
 

Crissa

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https://theconversation.com/heres-why-electric-cars-have-plenty-of-grunt-oomph-and-torque-115356

Electric motors have a very broad curve of power. You can mix and match motors to get a nearly flat torque curve. That's why the Plaid (tri-motor) drive train is so good at what it does, and needs no shifting gear ratios.

My motorcycle goes from 0-95 with nearly no loss in efficiency with only the belt (small gear in front, big in back) acting as a reducer. There's no need to change gears in normal speeds at all. Changing loads are more easily dealt wih more motors because more motors can spread out the amperage without melting.

-Crissa
 
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John K

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Compared to Tesla’s current setup, what benefit in cost, manufacturer, performance and reliability would Tesla achieve converting to a planetary gear system?
 
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abebarker

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John K, I would say that the real benefit is performance. It would cost more, be more difficult to manufacture, and have more moving parts and therefore be less reliable. I believe that all those detriments will be out weighed be the benefit afforded by the increase in performance.

The planetary gear system shifts the efficiency of the motor/s across the rpm range. It moves the efficiency of the motor at high rpm's to low rpm's. This is especially important to Semi truck drivers who really need the performance across the entire rpm range.

Low starting torque has always been the bane of the induction motor. With the planetary gear system, that limitation is removed.
 

Crissa

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I don't see the performance gain, personally. Literally there's programming in my motorcycle that tamps down the available power at low RPM. It doesn't have the logic and sensors for traction control, so it does this rough thing.

Because the moment it breaks loose of friction, it can totally spin up to an amazing rotational speed almost instantly. No ICE powered wheel can do that.

-Crissa
 
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John K

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I am familiar with the process. I am acting as Tesla in this hypothetical conversation.

Quantify performance pulse with a trade off on reduced reliability and greater difficulty manufacturing thus greater cost, is that enough to motivate me as a company to change?

If my intention was to create a vehicle solely on performance, my decision path would be different. If my goal was to create an efficient manufacturing process to create a reliable EV while making it fun to drive, I would have other decision criteria.

Since I am not part of Tesla, the nerve of them never responding to a resume I never sent. I cannot speak for them. I do not see the lure to change current design. Granted, I ,at be missing something.

Regardless of Tesla, nobody can say the gear system is not cool. If they do, they are square.
 
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abebarker

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John K, I get you.

The amount of cost is really limited to a minimum as is the difficulty and reliability problem. The amount of performance gained will be enormous. The torque delivered at a stand still can be increased by 4 times (not sure of the actual difference in starting torque vs full speed torque for Tesla) and the efficiency goes up as well, to near maximum. The increase in weight would be the weight of a planetary gear set, if two motors are already being used. Planetary gear sets are notoriously reliable and long lasting.

It may not be necessary for all situations. A families first car may not need it. Although, it is cheap enough to be a serious consideration.

If Tesla's reputation as a leader is going to hold out into the semi truck market, or even the light truck market, then they may need to adopt this technology. It is ideal for the induction motor. (If you ask me, it almost looks like the drive train was designed to have this planetary gear dropped right in, if you replaced the round inverter with another motor.)
 


Bill906

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An inverter powered induction motor with decent vector control and motor feedback can deliver 100% motor torque at zero speed.

Although I don’t know how Tesla controls their inverters I feel extremely confident in assuming they use vector control and have motor feedback.
 
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ajdelange

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Why would we want a kluge like that? We want the simplest gear train we can have which is, in the case of the tri motor, just a conventional gear or 2 at the right ratio and, in the case of the AWD, a simple differential at the rear. Of course the front motor requires a differential. The reason for simplicity is to eliminate drive train loss thus increasing efficiency.'

No "gear shifting" is required. A fixed ratio suffices because we can get full torque down to 0 speed with induction or synchronous motors using FOC (vector) control which, of course, Teslas (and all other BEV AFAIK, have. FOC permits independent control of flux and torque via separate PI (feedback) controllers. Clearly the art is in the tuning of the controllers.

Now while it is true that we don't need to "shift gears" and that the efficiency curves are very flat with speed and torque they are not completely so thus the motor control scheme may rely more on the front motor in some regimes and more on the rear motor(s) in others and the front and rear motor gearing ratios may be slightly different.
 

JBee

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John K, I get you.

The amount of cost is really limited to a minimum as is the difficulty and reliability problem. The amount of performance gained will be enormous. The torque delivered at a stand still can be increased by 4 times (not sure of the actual difference in starting torque vs full speed torque for Tesla) and the efficiency goes up as well, to near maximum. The increase in weight would be the weight of a planetary gear set, if two motors are already being used. Planetary gear sets are notoriously reliable and long lasting.

It may not be necessary for all situations. A families first car may not need it. Although, it is cheap enough to be a serious consideration.

If Tesla's reputation as a leader is going to hold out into the semi truck market, or even the light truck market, then they may need to adopt this technology. It is ideal for the induction motor. (If you ask me, it almost looks like the drive train was designed to have this planetary gear dropped right in, if you replaced the round inverter with another motor.)
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.

Theres also the problem of gear tooth size to adequately transfer power and torque, a planetary gear would be a fair bit bigger as a result and add a whole bunch of bearings that could fail. Even for semi I don't think it will be necessary as a simple gear on the motor would suffice, have less parts and last longer.

If you're into "cool" gearboxes have a look at the moon rovers harmonic drive.
 
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abebarker

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Why would we want a kluge like that? We want the simplest gear train we can have which is, in the case of the tri motor, just a conventional gear or 2 at the right ratio and, in the case of the AWD, a simple differential at the rear. Of course the front motor requires a differential. The reason for simplicity is to eliminate drive train loss thus increasing efficiency.'

No "gear shifting" is required. A fixed ratio suffices because we can get full torque down to 0 speed with induction or synchronous motors using FOC (vector) control which, of course, Teslas (and all other BEV AFAIK, have. FOC permits independent control of flux and torque via separate PI (feedback) controllers. Clearly the art is in the tuning of the controllers.

Now while it is true that we don't need to "shift gears" and that the efficiency curves are very flat with speed and torque they are not completely so thus the motor control scheme may rely more on the front motor in some regimes and more on the rear motor(s) in others and the front and rear motor gearing ratios may be slightly different.
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.

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. 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. The linear distance that field can travel is fixed distance based on the radius of the rotor. 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. Perfect for the Semi (and my truck, I want it in my truck).

[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).

 

 
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