# Calculating EV Acceleration



## Thomas88 (Apr 27, 2010)

Hi guys,

I'm planning on converting a motorbike to electric. I want something rather sporty, with performance (acceleration at very least) similar to a 600cc sports bike.

I'm trying to calculate the acceleration based off electric motor torque/RPM/power graphs using an iterative method (Jacobi Method for those in the know). At the moment I am simply plugging in the torque and respective rpm values from the graph into excel and letting it work out an equation for torque based on RPM. From there I work out acceleration over a very small time period (dt) and from that i can work out velocity at that instance. I do this many times over and what I'm left with is a velocity vs. time graph.

My issue is, this method doesn't take into account the voltage.

From my understanding of electric motors you control the speed by regulating the voltage. How does the current flow change. Does an electric motor produce the same power throughout the rpm range, so amps decrease as voltage increases. Or does the current stay the same and the power increases as voltage increases. Or neither? Basically, the throttle regulates the voltage, what regulates the current?

Cheers for any help. I realise the first half of what I've said is heavily calculus based but hopefully anyone can answer the second part of my question.


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## DavidDymaxion (Dec 1, 2008)

Simple way: Assume the motor is constant torque up to some rpm, and then constant power after that. It's not too bad an approximation for an AC or sepex motor. Better way: Find the motor's torque curve, and create a lookup table that returns torque as a function of speed.


Thomas88 said:


> Hi guys,
> 
> I'm planning on converting a motorbike to electric. I want something rather sporty, with performance (acceleration at very least) similar to a 600cc sports bike.
> 
> ...


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## Gavin1977 (Sep 2, 2008)

The Throttle controls Current, which is proportional to torque. The controller will supply whatever voltage is required to maintain the current set by the throttle, up to the point where your motor voltage nearly equals your battery voltage. The motor voltage is a function of rpm, and so the motor voltage will increase as rpm increases.


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## maxvtol (Nov 11, 2009)

Thomas88 said:


> Basically, the throttle regulates the voltage, what regulates the current?


I think the controller on the flat part of the torque curve. But basically I think empirical data from the motor charts would be the easiest for the rest. 

The thread in my signature and my spreadsheet may be of some help.


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## BLSTIC (Jun 14, 2010)

You don't need to know the voltage, just the torque that the motor is outputting at that point in time.

In brushed DC motors, voltage input is a speed request. Full voltage = max RPM, zero voltage = zero RPM. Add more voltage (or drop motor rpm through loading it up) and the motor will draw more current in order to make more torque and accellerate. It's kind of like the throttle on your gasoline powered lawnmower, it's hooked to a governor, not the actual carburettor, and half throttle results in half engine speed under most load conditions.

This differs to more complicated motor and speed controller combos that are torque based, and half throttle will give you half torque, regardless of engine speed. Much like the throttle on most road vehicles


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## frodus (Apr 12, 2008)

Gavin1977 said:


> The Throttle controls Current, which is proportional to torque. The controller will supply whatever voltage is required to maintain the current set by the throttle, up to the point where your motor voltage nearly equals your battery voltage. The motor voltage is a function of rpm, and so the motor voltage will increase as rpm increases.


 
not entirely true. It all depends on what type of controller (AC and BLDC are a bit different), what mode its in, and what type of motor.

It also depends on what motor we're talking about and what mode the controller is in (torque mode or speed mode). Torque mode acts as you explained, and with series/sepex, it will give it as much current as the motor wants to match the level of the throttle. 100% throttle = 100% current out of the motor (if the motor "asks" for it). In speed mode, the controller looks at the throttle and tries to match the speed. 100% throttle = 100% speed, and the motor will take all the current it needs to get there, until it reaches current limit.


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## Gavin1977 (Sep 2, 2008)

Frodus. You are quite right.
I made a bit of an assumption about DC, and that the op would want *ice* car/bike feel, and therefore it should be a torque controller.


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## frodus (Apr 12, 2008)

I knew what you meant, just wanted to make sure he knew.... no biggy. Some controllers can be set to run in either mode, so that helps.


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## Franky.EV (Feb 27, 2010)

I've made something that can help you :
http://www.diyelectriccar.com/forums/showthread.php/yes-another-ev-calculator-45278.html
It runs on OpenOffice Basic (V3.2 preferred)


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## Thomas88 (Apr 27, 2010)

Thanks guys for all of your help, it made me understand some things. But this opened the door for more questions and confusion.

How do I work out what amount of current the motor will be drawing at any one instant. For example, I am looking at the performance charts of the Agni 95 series. I can see that for every volt that I add with the throttle, the motor will increase it's speed by 71 revolutions per minute. But the torque graph doesn't change, which makes me think that the current or power is more a subjective thing the the controller decides on rather than a simple calculation. I know I can work out the current from how much power is being used, but how do you know how much power the motor is using whilst accelerating.

Maybe I'm doing this whole thing backwards. I come from an ICE background, do EV's commonly use the torque control method or are they speed based. Do most controllers have to ability to do both?

Sorry if this comes across as rambling, I have trouble getting my thoughts into words, as my mind is usually going a million miles an hour and thinking several things at once


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