Calculating Power Requirements for Motors.

During my appreticeship, and final years of high school physics I
studied a little about power, acceleration, work etc...
Since then, I've lost all of my old text books and am trying to
relearn and further my knowledge on this subject. Where I work we
build electric scooters for mobility impaired people. When I asked
the boss, how he worked out the size of the motors that we use, he
said he looked at what was in other scooters on the market, added an
oohaah factor and came up with the final choice. (.250kW @ 1800rpm
The problem seems to have a number of steps.
-Inertia (I remember this well from high school, Mass is 170kg)
-Wind resistance
-Rolling Resistance in the bearings and wheels
-Torque (the wheels are 15" in diameter)
-Friction losses within the gearbox. (Standard NMRV50 from Motavario)
-Motor Efficiency
-Battery Life and Power Consumption (Batteries are 12V 44Amp/Hour with
56Amp peak.)
The drive train consists of two motors driving a wheel each. They are
wired in parralell to the controller providing an electric limited
slip differential. The system is 24Volts, and runs of a standard
forklift controller manufactured by Curtis.
The maximum speed by law for these type of vehicles is 10km/hr,
however we have made some for "private use" rated to 15km/hr by using
different reduction gearboxs. I'm assuming an acceleration to
topspeed in 0.5 sec.
I thought this might be an interesting head scratcher.
View some photos here:
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Thoughts, Comments, Suggestion, (Criticism) Welcome.
Regards Dominic.
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I looked at your website. If those are 15" wheels then Aussies must be giants indeed.
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Well, iffin yer boss didn't bother to do the calcs, I wouldn't worry about it. It actually is not a simple problem. It is an *instructive* problem, if you dig academics, but from a practical pov, most engineers would attack it with simple trial & error.
Also, motors differ in how they deliver their power--high starting torque vs low, etc, which adds to the complexity.
250 watt motors are perty small, tho.
Just an editorial here, not to be taken personally: These scooters are one of the most medically irresponsible devices on the market, quite on par with the unscrupulous and rapacious drug dealing (ie, pill pushing) of MDs. An aging population with mobility problems already has one foot on 1/4" plywood covering a dug-out grave, and the other on the proverbial banana peel. These scooters replace banana peel with ball bearings, and replace the 1/4" plywood with Saranwrap.
The *minute* an aging individual sits down in one of these scooters, the downward spiral of the aging process is accelerated as if the cable of an elevator was clipped, for a litany of long-established physiological reasons.
But, as usual, it takes two to tango, the CorPirateer and the Consumer, for ostensibly shared responsibility in the resulting Mess. But it is by no means a symmetrical tango, as the CorPirateer is able give the Consumer quite the pre-dance Mind-Fuck, so that the ensuing dance is more akin to the Consumer giving CorPirate Merka a free lap dance--with boucou touching allowed.
Looks like I done shot quite the metaphorical wad today.... :)
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Proctologically Violated©®
Ever notice that the fat people are the ones that use the handicapped parking places. The should have DAV parking places in stead.
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"Just an editorial here, not to be taken personally: These scooters are one of the most medically irresponsible devices on the market, quite on par with the unscrupulous and rapacious drug dealing (ie, pill pushing) of MDs. An aging population with mobility problems already has one foot on 1/4" plywood covering a dug-out grave, and the other on the proverbial banana peel. These scooters replace banana peel with ball bearings, and replace the 1/4" plywood with Saranwrap. "
We make two seater scooters that are aimed at couples. Often one partner has some type of mobility issue that restricts thier quality of life. The able bodied person is able to drive the other around. Some of our clients include stroke victims, cerebal paulsy sufferers, motor accident victims, not just ageing pop. (How old are you Proc? You may become a potential customer in the future ;-) )
We are also developing models for golfing, eco tourism, security and robotics. This is where the need for calculating power req. comes in.
Looking up the gearbox manufacturers catalogue, it lists efficiency, but it is not clear how to interpret the data.(It is a multilanguage catalogue, and there is no symbol key or explanation on the efficiency charts) I know that the mounting arrangement is important, because it affects the amount of churn and sling that the oil undergoes. Operating speed is also important, as is temperature because it affects oil viscosity.
I think wind resistance could be discounted, because the speeds are relatively slow. I think after 25km/hr it would become more important.
"Have you looked at a can of Fosters?
Wes "
Wes, Fosters is Horsepiss!! No self respecting Australian would drink it. Thats why Fosters brewery bought out all the other Aussie breweries.
Guide to drinking Australian beer State by State:
Tasmania: Cascade or Boags Victoria: Carlton Draght, Victoria Bitter, Melbourne Bitter New South Wales: Tooheys Extra Dry Adelaide: See other states Queensland: XXXX (say 4X) "Australians don't give a XXXX for anything else" Western Australia: See other states Northern Territory: Anything!!!! because its so hot (except Fosters).
Favourite boutique beer: Beezneez. Made with Honey, Wheat, and Mountain Sping Water. Its like Angels pissing on your tongue.
hmmm, I'm thirsty now. Maybe I'll go next door to the local brewery.....
Cheers, Dominic.
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Working backwards from performance to get power is a fairly involved calculation; but here's a pretty decent way to approximate what you need.
Think of simple, linear acceleration of the load (scooter and rider) based on Newton's law F=MA. The mass of the load and the desired acceleration give you the (linear) force needed. Now think about that linear force being the tangential force created by a wheel at the point where the wheel's circumference touches the road. The tangential force and the wheel radius can be used to calculate the wheel torque required to accomplish the acceleration. Wheel torque is just motor torque multiplied by gearbox ratio (with efficiency losses thrown in as percentages of ideal, if you want to get fancy.)
So now you know what kind of motor torque you need to get up to speed, which is all that really matters. Cruising after the desired speed has been achieved will always require less torque than accelerating, no matter what the friction losses might be. Look at the torque specs in the motor catalogs, and forget about power ratings. When you find a motor with the needed torque, and the right revs (based on the intended top speed, the gear ratio, and the wheel diameters), then you've got the motor you want.
THEN look at the motor's power numbers, or current draw, and use those to calculate battery size and battery life. This is where friction matters; but it can't be calculated without some good data on the actual friction produced by the scooter itself, the shape and size of the rider, the load on the tires and bearings (which will vary with rider mass, tire inflation, etc.), and other stuff that's only to be found from testing and measuring, like in a wind tunnel or in actual driving conditions.
To do the calculation with your twin motor setup, just start with only half the acceleration, to get the torque needed for a single motor. Two motors will automatically give you twice as much torque, and therefore twice as much get-up-and-go. Then, when you get to battery size and life, double the consumption specified for a single motor.
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Kirk Gordon
Thanks Kirk,
This was the path I'd been going down, and your comments make it seem much clearer. So far I'd calculated that the scooter should be able to climb a 20% grade, with two people giving a total weight of 340kg for 1 minute, without damaging the motors. Something I forgot to mention, the motors are rated to 1.1kw for 1 minute peak power.
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One thing that you can probably do is to find a motor sizing program from one of the motor manufactuors. Sorry, I don't have one for DC motors but one should be available on the Net.
I'm assuming that these are DC permenent magnet motors. Thes motors will probably have a constant torque depending upon the size. Even as the motor speed is increased the torque will be the same but since the speed is increasing the hp is increasing. So, the motor must have enough torque to start rolling under the worst conditions and after that the motor hp will determine the max speed.
Hope this helps.
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Sounds like you're way ahead of me, Dom. But just in case it helps, inclines are easy, too. They're just acceleration, calculated the same way I described earlier. The rate of acceleration when going up a hill is just gravity (9.8 M/Sec/Sec) times the sine of the inclination angle. If you want to add speed while going up a hill, then you just add the gravity factor to the desired rate of increase in speed.
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Kirk Gordon
IF>>>>>> it were to accerlate that fast it would cause whiplash...........and most likely POP A WEELIE I am assuming these are constant torque motors most likely brushed??????
High accerlations require HIGH currents. There are many other factors involved in hwhat your trying to figure out. What you can do is RESEARCH a bit more, learn what OTHERS are utilizing and go from there.
AMP/hrs devided by reserve capacity of the battery along with current draw (higher during accerlation and climbing) ya know drive ways and such.
The controller should have some sort of current limiting control that is adjustable THIS will contol acceration. I am also going to assume these motors are WORM DRIVEN to the wheels. Can this thing be pushed with NO power????? anyways........ reply back to and we can share other information (I am making my own electric drives on other applications of my own.
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