Horsepower Calculations

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electricgreg
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Horsepower Calculations

Hi All!

It looks like visforvoltage.org still exists. I bought an old gas scooter with the intention of converting to electric but ending up buying a new piston and cylinder and rebuilding the carburetor.. as a result I fell off the electric forum for a year or so. Also it looks like I lost my old id. Anyway.....

Does anyone know how to calculate horsepower needed (how many watts should my motor be) given a certain set of conditions? For example, I have a 100 lb scooter, I want to go up a 2% grade, and I would like a top speed of 40 mph. (Regarding range, I would like to reliably travel 30 minutes each way before charging at the end of the day.)

Any weblinks?

electricgreg

chas_stevenson
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Re: Horsepower Calculations

How big are the tires on the scooter?
How much do you weigh?

I am guessing but a scooter that weighs 100 LBs. must have wheels and tires about 12", 40 MPH on 12" wheels is very dangerous. I had a scooter with 12" tires and almost every time I hit a pot hole it did an end-o and it only did about 17 MPH.

Chas S.

andrew
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Re: Horsepower Calculations

Following is from Bidwell's "Secrets of El Ninja":
Force required = Cr x Wt + (Cd x A x V^2)/391

This is assuming a flat surface and steady speed and zero relative wind. Using what you have, and integrating with #s provided by Bidwell...

Cr: Rolling resistance factor = .015
Wt: Weight in pounds = 100 + [your weight] = 250 lbs
Cd: coefficient of drag = .77 (for unfaired recumbent bicycle)
A: Area = 4 square feet (just a guss)
V: Velocity in mph = 40

Force Required = (.015 x 250 lbs) + (.77 x 4 x 1600)/(391) = 16.35

I think the unit for force is pounds.

Pick say an arbitrary wheel size of 1 ft, then the torque required would be 16.35 ft lbs. Given 40 mph, it will need to spin at 560.5 rpm.

Following is from http://en.wikipedia.org/wiki/Torque :
Power (hp) = (torque (lbf x ft) x angular speed (rpm))/5252 = 1.745 hp or 1301.97 watts.

I may have screwed something up, and this doesn't take into account the 2% grade. This is the fist time I've done the above calculations, but they look like they may be close. This doesn't take into account the power required to accelerate, only constant speed.

[url=/forum-topic/motorcycles-and-large-scooters/587-my-kz750-electric-motorcycle-project]KZ750 Motorcycle Conversion[/url]
[url=/forum-topic/motorcycles-and-large-scooters/588-fixing-my-chinese-scooter]900 watt scooter[/url]
Pic from http://www.electri

andrew
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Re: Horsepower Calculations

Given that you can expect about 10 whr/lb of lead-acid in real-world use, 130 lbs of batteries doesn't sound too bad (to provide 1301 watts for an hour), but your power requirements might be a lot more than what I calculated.

[url=/forum-topic/motorcycles-and-large-scooters/587-my-kz750-electric-motorcycle-project]KZ750 Motorcycle Conversion[/url]
[url=/forum-topic/motorcycles-and-large-scooters/588-fixing-my-chinese-scooter]900 watt scooter[/url]
Pic from http://www.electri

Malcolm
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Re: Horsepower Calculations

Around 2,700 Watts, according to this site http://www.kreuzotter.de/english/espeed.htm which a few people have found to match real-life experience well.
This is assuming:
you weigh 157 pounds (default)
roadster-style bike (your machine may have higher rolling resistance)
No wind

If you take this as your AVERAGE power consumption over 1 hour (to allow for acceleration, motor/controller losses and a small reserve) that means you need 2700 Whrs of energy. Of course this is just a rough estimation, but if you play around with the calculator it will give you a good feel for what can be done. It would be a lot easier if you could recharge halfway.

What batteries are you planning to use?

andrew
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Re: Horsepower Calculations

What Malcolm posted is most likely a lot closer. I don't have any confidence in my calculations above, and haven't checked them against real-world data.

[url=/forum-topic/motorcycles-and-large-scooters/587-my-kz750-electric-motorcycle-project]KZ750 Motorcycle Conversion[/url]
[url=/forum-topic/motorcycles-and-large-scooters/588-fixing-my-chinese-scooter]900 watt scooter[/url]
Pic from http://www.electri

Malcolm
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Re: Horsepower Calculations

I think your figure is probably about right for a recumbent Andrew. Air resistance makes a massive difference above 30 mph.

electricgreg
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Re: Horsepower Calculations

Wow! 2700 Watts is a lot!

Here's my thinking.... I was initially planning on a 48V system. Each small battery (motorcyle size) would be rated at 12V, 12 A-h, so 4 in series of these would be 48V, 12 A-h. And then 4 more batteries in parallel would increase the A-h rating to 24 A-h.

Given 24 A-h, I would be able to realistically run 24 amps for only half an hour because of the battery knockdown curves. (Peukert curves and all that stuff...).

So, 24 Ampsx48V=1152 Watts. I could only power a 1152 Watt motor for half an hour. I could discharge the batteries at a higher rate - I heard somewhere that 3x the a-h rating is the max recommended. Going for 2x the A-h rating would give me a 48 Amp discharge rate. 48 Ampsx48V=2304 watts. Now we're getting close, but the batteries could only sustain this for 14 minutes according to Peukert curves.

I think
1) I need higher A-h batteries to sustain the high amps for enough time to get up to an hour or so for commuting time.
2) I gear the scooter to get up to 40 mph instead of relying on brute horsepower. I think the recumbent calculator assumes a 1-1 gear ratio?

Let me know if my calcs or thinking isn't right... which is a very real possibility :-(. Picture below is the scooter i'm bidding on ebay for conversion...f5a2_12.jpg

PJD
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Re: Horsepower Calculations

A while back, I made up a spreadsheet that calculates power and torque (both US and metric) expended at various speeds and hill slopes. It also graphs the power and torque versus speed or wheel RPM.

You ineed to input an assumed coeff. of drag, vehicle (incl. rider) frontal area, vehicle/rider weight, and drive wheel diameter. You then input the steady-state speed for various known percent hill slopes including zero slope. The spreadsheet calculated and plots the power and torque.

Basically it's a poor-man's dynamometer... But you need acces to a variety of hill slopes long enough to obtain a steady state speed (not accleerating of decelerating).

I'd be happy to share the spreadsheet. I can send it as an e-mail attachment, or can it be posted for download on this site?

PJD
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Re: Horsepower Calculations

..of course, you can also use the spreadsheet to calculate power requirements for various speeds and hill slopes...

Malcolm
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Re: Horsepower Calculations

Your figures look good.

The emax scooter uses eight 20Ah batteries in two strings, giving just under 2kWhr nominal capacity. Quoted maximum range is 40 miles, but I think in real life it's more like 20-25. I'm sure there are a few owners here who can tell you their actual range.

You need to gear the scooter so that it will reach your chosen top speed at whatever rpm your motor produces at 48 volts. Then you need to supply the motor with however much power is needed to maintain that top speed.

andrew
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Re: Horsepower Calculations

I think
1) I need higher A-h batteries to sustain the high amps for enough time to get up to an hour or so for commuting time.
2) I gear the scooter to get up to 40 mph instead of relying on brute horsepower. I think the recumbent calculator assumes a 1-1 gear ratio?

In reply to:
1) I wouldn't go into any EV project where you absolutely have to have 30-40 miles without a charge to commute, even a large car/truck. Why? Because it's a real PITA planning your life and your trips around your vehicle, and having your eyes glued to the amp meter. Whats worse, is other factors that effect range like temperature.

2) Gearing doesn't effect how much power it takes to overcome tire and air resistance. However, gearing does effect how efficiently the motor can produce the power required. In general for any given speed, it is most efficient to have the motor run as high rpm as possible while still producing enough power.

Good luck with your project.

[url=/forum-topic/motorcycles-and-large-scooters/587-my-kz750-electric-motorcycle-project]KZ750 Motorcycle Conversion[/url]
[url=/forum-topic/motorcycles-and-large-scooters/588-fixing-my-chinese-scooter]900 watt scooter[/url]
Pic from http://www.electri

electricgreg
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Re: Horsepower Calculations

PJD - Could you send me that calculation spreadsheet or post it? That sounds like a great tool.

PJD
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Re: Horsepower Calculations

The only way I know how to send it is by e-mail attachment. Can you provide an e-mail?

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