Calculating gear ratio from the motor's graph for an EV?

[zaraf]

Dear community,

For my electric vehicle project I want to select a gear ratio (single) of my gear box and send this information to the supplier. In order to calculate this ratio I thought of the following steps but am not sure and want some expert to recheck and answer my queries:
1. Calculate wheel torque by using the formula: Torque = Force x Velocity. Here I shall be calculating the Force =drag+rolling resistance+gradient
2. I will be checking the corresponding wheel RPM at the velocity at 1.
3. I will take this RPM and get the motors torque from the Torque vs RPM graph given in the datasheet.
4. Then the Gear ratio would be: Torque on wheels/Torque on engine at the given RPM in 3.

Now my concerns are:
a. For a normal street car, how many scenarios do I have to calculate? For instance if I calculate the resistive force at 50kmh and then determine the GR, would it also be optimal for other speeds such as 10kmh and 50kmh?
b. What about driving uphill as a worst case scenario? Do I calculate GR based on this?
c. In the formula I use in 1. I assume the car is driving at a constant V, what about acceleration cases? Would the GR supply the acceleration requirements I have?

Can someone explain this better :p
Thanks :)

 

[anorlunda]

In engineering, we start with requirements. What is the required speed? acceleration? speed uphill? acceleration uphill? required acceleration from a dead stop? What is the weight of the vehicle?

From those plus some drag and friction estimates, you can calculate the power requirements. That much power, plus losses, is what you need from the electric motor.

How much power can your motor deliver. You need a curve of power versus RPM.

Does the motor need to run at optimum RPM to deliver enough power at any speed? If yes, then you have the gear ratio that relates wheel RPM with motor RPM.

You can do it all with power. You don't really need torque. But you must begin by stating the requirements.

 

[rcgldr]

You don't mention the type of motor or the shape of the torque versus rpm curve. For a typical DC type motor, peak torque occurs at 0 rpm, and decreases linearly to 0 as rpm increases to max rpm (the rpm of no load == 0 torque), with peak power at 1/2 of maximum rpm. For a reluctance type AC motor, depending on the resistance of the rotor and limiting torque to some peak amount due to traction limits, the motor with torque limits has a torque versus rpm curve that is nearly flat (due to torque limiting) until it reaches peak power, after which the torque decreases such that power versus rpm is nearly constant.

 

[CWatters]

1. Calculate wheel torque by using the formula: Torque = Force x Velocity.

Force x velocity = power.