# Motor RPM / MPH Calculations



## KiwiEV (Jul 26, 2007)

Mate, I had the exact same question as you a few weeks back and I was scratching my head trying to use the calculation table that came from EVAmerica.
In the end I found this link was able to answer my max RPM questions.
Also, according to EVAmerica, the max RPM of the FB1-4001A motor is 5600RPM from memory. As long as you know your gearbox's gear ratios then that link will sort you out nicely.


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## mattW (Sep 14, 2007)

Ok i have no idea whether this works or not but could you use the voltage across the motor to work out it's rpm? According to my old physics textbook as the rpm increases so does the back emf, so could you just whack a voltage gauge across the motor and use it like a tacho? (i am assuming it doesn't change with load). If you knew the voltage drop at max rpm then you could just change up then, maybe even work out a gear change light. Kiwi i think i remember you talking about playing with the tacho somewhere but i cant find the thread atm. Anyone know whether this would work?


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## KiwiEV (Jul 26, 2007)

Yep, I'm currently figuring out a way to attach a (very) little generator to the motor's tailshaft to generate current to move the tachometer.

I disconnected the tachometer's tiny needle "motor" input from the built in circuit board in the tach gauge. I ran the input straight past the tach circuit:









Then I re-routed it straight to a low voltage input; this is the result of a 1.5 volt AAA battery:









As you can see, 1.5 volts gives the tachometer just over 4000 RPM. I have a little motor which I connected to my cordless drill and put a meter across. I estimated the drill was spinning at about 2000 RPM and it produced 0.75 volts. I'm expecting that once the little motor is connected to the tail of the drive motor, it will generate up to 2 volts, maybe more.
I have a potentiometer which I will connect into the circuit so I can fine-tune the tachometer's results.

Now this is certainly not going to be accurate throughout the rev range, but as long as I can get it nearly accurate closer to the motor's redline then I'm happy.


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## mattW (Sep 14, 2007)

But what i am saying is, when a voltage, say 120V is applied across a motor, The electrical energy that is converted to kinetic energy has to be accounted for (it isn't 'free'). When the moving electrons in the moving wire pass through the magnetic field, the field tries to slow the electrons down, which reduces the individual energy of the electrons but it also pushes the wire. So a motor with 120V accross it might have a back emf (electromagnetic force) of -50V meaning the total voltage difference across the motor is 70V. So you can skip the small generator step. But i only know this from a text book, you would have to test it. What is the voltage across the motor when it is stationary (i.e when the power has just been turned on) and at full rpm? I don't have a motor i can test it on atm so i'm hoping someone else can.


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## Coley (Jul 26, 2007)

Quote:
So I jacked up 1 tire and spun the motor. I know the figures I got are wrong (5.5 motor rev per 1 tire rev in first). Guess both tires must be moving to be accurate"

You need to spin the raised tire TWO revolutions while counting the motor turns. This is for a non-posi drive train.

If you have a positraction axle, then you turn both of them ONE turn.

My Yugo comes out to 15:0 to 1. in first gear.


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## Hi Torque Electric (Dec 23, 2007)

CPLTECH

I just posted about this on another thread, Please do not run your 9" motor past 5K (5500 max per NetGains specs) as you risk blowing the comm! People might get away with it but I got lots of examples from those who did not, it's just not worth the risk.
Cya
Jim


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