# AC Motor and Speed Calculations.



## Sunking (Aug 10, 2009)

OK have another thread similar to this one that is not gaining any traction, so I though I would ask a different way since I have more info.

OK I am going to take a donor Golf Cart and convert to a LSV. I am going to strip the series wound DC motor and speed controller out and replace them with an AC Induction Motor and Curtis VFD controller. I desire a 30 to 35 mph cruise speed with a little left in the tank for normal graded hills on paved surface.

So what I am asking for is run the numbers or direct me to where I can find a calculator to determine what size motor and controller are required. I think I have all the data, fingers crossed but here goes.

Desired Speed = 35
Vehicle Curb weight = 1000 lbs = 454 Kg
Vehicle full capacity weight = 1500 pounds = 680 Kg mass
Motor RPM @ 35 mph = 4100
Transmission Ratio = Direct 1:1
Differential Ratio = 12.44:1
Tire Diameter = 18 in = 457 mm
Rate of Acceleration = 5 mile/sec or 0 to 35 mph in 7 seconds

I think that is it. 

Thanks!


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## major (Apr 4, 2008)

Sunking said:


> Tire circumference = 18 in = 457 mm


Mean diameter?

What did Brian tell you?


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## major (Apr 4, 2008)

Bob Brant covers it pretty well in chapter 6.


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## kennybobby (Aug 10, 2012)

You will need an estimate of aerodynamic drag coeff and rolling resistance to dial it in closer, also how steep and long and fast do you want to climb those hills. Also an allocation for efficiency.

For a ball park estimate, i take the energy at 1/2 m v^2 divided by the accel time to get the horsepower required (without losses) of 16 Hp.

i haven't checked your work for wheel and gearbox, but since you are a mechanical i assume you did it right, and this would imply needing a motor with 20 ft-lbs of torque at 4100 rpm.

How do these numbers compare with the current setup in the golf cart?


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## Sunking (Aug 10, 2009)

major said:


> Mean diameter?


Yes, my bad



major said:


> What did Brian tell you?


No reply yet.


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## Sunking (Aug 10, 2009)

kennybobby said:


> How do these numbers compare with the current setup in the golf cart?


I wish I could tell you, just bought the donor cart today it's an EZGO 2010 PDS which means a Shunt Wound DC motor, and unfortunately they don't specify the numbers other than 3.3 hp. It goes around 15 to 16 mph so based on that I assume to go 30 will take 12 to 15 hp..

FWIW I am an Electrical Engineer, not mechanical. I know just enough to be dangerous. 

As for the cart itself I posted what specs I know. As for drag I am clueless what rolling resistance for a golf cart. Motors I am looking at are HPEV AC-9 and AC-12.


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## frodus (Apr 12, 2008)

Aero drag isn't much of an issue below like 40mph. That's more for highway worthy vehicles. 

I haven't had time since we talked about the calculations you made.... Major computer issues at home I'm resolving. I will get to it though! Haven't forgotten.


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## PStechPaul (May 1, 2012)

I plugged your numbers into my http://enginuitysystems.com/EVCalculator.htm and got the following:

4.9 MPH/sec, 31 MPH
21 HP, 4100 RPM, 27 lb-ft torque
7:1 gear ratio
504 W-h/mile

These figures are for accelerating at that rate at the top end, or for maintaining that speed on a (rather steep) 23% slope which is equivalent to the 0.22G acceleration desired. You can probably get by with a 10 HP motor or maybe even 5 HP if you can overload it a bit, or back off on the acceleration at the top end. For an AC motor the torque is about proportional to current and you might expect to get 2x to 3x torque for short durations like accelerating to top speed in 7 seconds.

[edit] Those figures were for the unloaded curb weight. For 1500 lb:
31 HP, 40 lb-ft
745 W-h/mile

For just cruising at 31 MPH on average 2% slope:
5.7 HP, 7.3 lb-ft
137 W-h/mile

You must also factor in efficiency, which is probably 70-85% overall.


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## Sunking (Aug 10, 2009)

PStechPaul said:


> I plugged your numbers into my http://enginuitysystems.com/EVCalculator.htm and got the following:
> 
> 4.9 MPH/sec, 31 MPH
> 21 HP, 4100 RPM, 27 lb-ft torque
> ...


No problem for me lowering rate of acceleration, I just pulled a number out of the air. Sounds like a HPEV AC-9 motor with 350 amp controller might work.


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