CVT transmission for an electric motor

  • Thread starter Ayrity
  • Start date
  • #1
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Main Question or Discussion Point

Hey everyone, i am trying to design a small and lightweight cvt (continiously variable transmission) for an electric car that can either be on or off, no variable speeds from the motor. I am having trouble representing the math of the transmission. I have to have the car complete a race of 312 inches, the motor has a top rpm of 4040rpm, and at that rpm torque approaches 0. the graph of the torque vs rpm is piecewise in 2 parts with the following equations when rpm<1248 then torque=-0.0019*rpm=23.3 and also when rpm>1248 then torque=-0.0075*rpm=30.3

anyway, my problem is that i dont know how to represent a smooth change between the infinite gear ratios and relate that to how much time it will take to cross the finish line (312 inches later). obviously i am starting at a velocity of 0, and an rpm of 0. this should give me a torque just as the motor starts to move of 23.3 ounce-inches. i know i want to minimize time, and maximize velocity. i think i have to use an integral to represent the cvt, but im really turning around in circles here. oh and by the way, the diameter of the drive wheels is 3 inches. please can anyone help me out here?
 

Answers and Replies

  • #2
Cliff_J
Science Advisor
789
7
Power = torque * RPM

Graph it out and Pmax ~= 2000RPM

So you bascially have two conditions to figure for:

A) 0 RPM to 2000RPM at the maximum practical gear ratio as the torque falls from 23.3 to 15.3 and the maximum ratio is used to minimize the acceleration needed to the moment of inertia of the engine/CVT plus the smallest acceleration of the car.

B) A steady 2000RPM for Pmax and the gear ratio changes to hold it there over time.

Both should be similar and near linear lines on each side of the inflection point of 2k RPM, its basically a matter of power/mass to find the resultant acceleration.
 
  • #3
hows the electric car going?

I'm also thinking of using a cvt with an electric motor. I'm planning on using two eteks or two perm motors on a motorcycle with the Comet 94c torque converter (that's what they call their cvt). I'm wondering how I can set it up so it will have torque at 0 rpms and also allow me a higher vehicle top speed when the motor reaches top rpms. Any advice? I figure there's some springs and weights or something in the cvt to adjust and also the gearing on the rear wheel. I dont have it all yet to try but want to make sure it's possible first. I've heard of it being done. Youre doing it I imagine.
thanks
john
 
  • #4
12
2
Im wondering why you are using a DC motor and not an AC motor with a VFD/Inverter? I am correct on the use of a DC motor right (the ON/OFF remark)
 
  • #5
1
0
Pease check:

www.warko.it

Electric engines need different rate transmission as IC engines, for this reason Ac motors with inverter is not enough!
If correctly designed it will be big and heavy (and energy expensive) compared with the correspondent motor+CVT.
 

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