# EV Calculations



## xrotaryguy (Jul 26, 2007)

Thot = Tcold + (K x ((Rhot - Rcold) / Rcold)) 

where K = 256.4 for copper 
Thot = hot temperature in deg.C 
Tcold = cold temperature in deg.C 
Rhot = resistance at hot temperature in ohms 
Rcold = resistance at cold temperature in ohms


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## xrotaryguy (Jul 26, 2007)

Hp=(torque*RPM)/5252


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## KiwiEV (Jul 26, 2007)

For those looking to calculate the maximum electric motor RPM in each gear, I recommend clicking on this RPM Calculator
This is essential for those not wanting to over-spin your motors.
The 9 Inch "FB1-4001A" motor from Advanced DC has a maximum RPM of 5600 RPM and without a rev-counter or reliable speed/gear chart memorised, it'll happily overspin and you won't hear it or notice it until it goes pop. That terrifies me a little.


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## xrotaryguy (Jul 26, 2007)

Having a tachometer would be nice too. No sense in spending thousands of $$ on an EV conversion and not wiring up a tach.


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## KiwiEV (Jul 26, 2007)

I hear ya. The Tredia has a tachometer and I think I've figured out a way to make it work on a Curtis controller that doesn't support tachometers.
I've rewired the back of the dash and found that 1.5 volts displays just over 4000 RPM.

This means in theory by using a variable resistor (for tweaking) and a very small generator on the tail shaft of the motor, I can send small voltages through to the tachometer relative to the speed of the motor.
It's crude but it could work with tweaking.
It means I can avoid purchasing and installing an expensive optical sensor to measure the tailshaft RPM and feed it in pulses to the tachometer. That's the professional way apparently. I prefer my idea. 
I wonder if it'll work? 

Here's me experimenting the other night:


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## Mr. Sharkey (Jul 26, 2007)

Hmm, I think we have a lot to discuss concerning tachometers.

First of all, tachometers, as you are probably already aware, requre some kind of frequency-to-voltage conversion to work from a spinning shaft/ignition pulse train/etc.

Your experiment that 1.5 volts DC sends the tach meter movement to about mid scale is a good start, but simply connecting the meter movement to any source of signal derived from a rotational generator is not going to give you what you are after. Even if the match-up was corrected via a variable resistor, you would still be looking at a reading that was non-linear with the speed of the shaft.

A petrol engine tachometer takes a pulses train from the ignition system of the distributor and converts the frequency of the pulses (which is directly proportional to the engine speed) and converts it to a DC voltage/current to run the meter.

The optical sensor tachs do the same thing, except that the pulses are generated by an optical interrupter.

On some diesel engines, a sample of the three phase AC power from inside the alternator is supplied to the V/T converter .There are "black boxes" that can do the conversion and will program for rough calibration of a petrol tach from a variety of frequency generators. See this page for a $60 converter that does this. The generation of the pulse train is up to you.

I can show you how to build a complete tachometer using junk box parts costing $10. All you need to do is couple a disc, gear, or similar serrated interrupter to your motor shaft. A small AC generator of some sort would also work. It's possible that the circuit could be cobbled together to sense motor commutation spikes, but I've never fiddled with that.


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## xrotaryguy (Jul 26, 2007)

Continued Continued Distributor Conversation


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## mattW (Sep 14, 2007)

Ok I dug up a very useful equation from a subject i did at uni about solar cars. Its to work out the power requirement at the wheel at any given speed. The blue side gives you the power needed to overcome rolling resistance and the pink is aerodynamic drag, Crr is Coefficient of rolling resistance Cd is coefficient of drag, A is frontal area (sq m), ρ is the density of air (~1.2kg/cubic m) and v is the speed (m/s). So if you have an intended maximum speed you can use this equation to work out the power you will need to achieve it (+transmission losses). I don't know where you find out your Crr but I think the tyre manufacturer should list it somewhere. Enjoy!


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