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Old Feb 17, 2007, 06:39 PM
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jstoll's Avatar
United States, KY, Lexington
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50 ft lbs possible?

is there anyone here who could calculate what the dimentions & current draw of a 50 ft lb brushless motor?
could these motors be used on a motorcycle thing?
I am capable of fabricating a stator & have friends with lathes & mills
any aproximate idea ?
any input would be apriciated
JAK
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Old Feb 17, 2007, 07:18 PM
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San Diego, CA, USA
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JAK,

Motors develop horsepower not torque. You could get 50 ft-lbs of torque from a very small motor with a lot of gearing but the RPM would be I suspect lower than lower than what you want.

What kind of motorcycle thing are you talking about. A model or one that you can ride on?

You have not given enough information to get a proper answer.

Chuck
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Old Feb 17, 2007, 07:37 PM
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United States, KY, Lexington
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hey there :-)
I mean 50 ft lbs from the motor only this would start at almost zero RPM & mabey the motor would run at 2500 RPM
I tune cars for a living & run them on dynos often
with engines torque is the only real measureable effect rotational force is torque.
HP is calculated based on the torque curve, so like my cummins TD makes about 500 ft lbs but only like 200BHP because the torque curve is delivered in a verry narrow RPM band
my 2 litre saab makes 440BHP & 430 ft lbs the HP is higher because the torque is delivered over a wide band & much higher in the RPMs
I get your point that you could gear something down but Id be looking to direct drive something
I guess I would be direct driving a wheel (something a person would ride) & the stator would either be in the wheel ? & the tire on the magnet ring?
thanks for taking an intrest & being helpful I know some of you really know your @#$% about these motors & I guess Im asking if they are that much more efficient & long lasting over brushed motors couldent they be used in other things then RC :-)
your input on this is verry much apriciated
take care
JAK
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Old Feb 17, 2007, 08:09 PM
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warner robins ga
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If you know what RPMs you need to run at you could calculate how much HP/Watts out you need. Theres going to be some unit conversions but if i remember right Torque*radians/sec gives you power in HP or Watts depending on what conversions you use. That would be power out. To get power in you need to divide by some effeciency maybe 80%. Then dividing that by your batteries voltage you would get a very very rough estimate of how many amps.
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Old Feb 17, 2007, 08:22 PM
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Toronto (Don Mills), Canada
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I will have to disagree with Chuck. Electric motors generate torque.
When we let them spin we are able to get power from them.

If you read the original LRK outrunner articles here:
part 1
part 2
They explain how to calculate the torque that a given magnet structure is capable of.
You just need to work backwards - start with the torque and arrive at the magnet structure
(Part 1 is the theory, part 2 is more about construction)

Pat MacKenzie
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Old Feb 17, 2007, 08:54 PM
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I "crunched the numbers" and based on the "part 1" formulas here is what I get:

5" diameter ( in the gap) x 2.5" thick should be capable of ~50 foot pounds of torque.

2.5" radius ~ 6.35 cm, so area of the gap is 2*pi*r*h = A = 2*pi*6.35*6.35=253 cm^2
force in gap F = A*4*B^2 = 253*4*1*1= 1012 newtons
torque = F*r = 1012N * .0635m = 64.26 N-M = 47.4 foot pounds. Up the thickness to 2.5*50/47.4 = 2.63 and you will get 50 foot pounds.

Pat MacKenzie
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Old Feb 17, 2007, 09:19 PM
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Yes, I was too quick with my answer. Horsepower is a product of torque and RPM. I with one arm can easily produce 50 foot pounds of torque on a wrench tightening a bolt. But that same one arm will not move a motorcycle very fast.

There are a bunch of motocycles that have been powered by electric motors. Below is a link with one that uses A123 cells for the power source.

http://www.killacycle.com/

Brushless motors can and are used in many applications and are. Including motorcycles, electric cars, electric full size aircraft and boats. And yes they are better than brushed motors.

Chuck
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Old Feb 17, 2007, 09:44 PM
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Oops- missed a factor of 65% for magnet coverage
That makes it about 6" diameter (gap) x 3" long.

As a check I also ran the numbers for a big outrunner - the Plettenberg Predator.
4.74cm gap radius ~ 3.75" diameter
3 cm stator length ~ 1.2" stator length
A= 89.34 cm^2
so F = 2.6*89.34 = 232 N
torque = F*r = 11Nm
Power at 6000 rpm = 100 rps = 628 rad/sec would be
11*628 = 6875 watts.
From the manufacturers data at 6000 rpm it does about 8800 watts output, so the LRK analysis is conservative.
It could be due to the higher magnet coverage- it is more like 70% . This would take it up to 7400 watts.
Closer. Better magnets could take the estimate up another 10% to 8140 watts.

In any case it shows that the simple LRK analysis gets you in the ballpark.


Pat MacKenzie
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Old Feb 17, 2007, 11:37 PM
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United States, KY, Lexington
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thanks guys:-)
it dosent seem like a 50ft lb motor would weigh that much
but the batterys sure would
thanks for crunching the #s
JAK
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Old Feb 18, 2007, 07:57 AM
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United States, KY, Lexington
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so whe you are talking about the diameter of a motor you are measuring from the gap between the magnets & stator on one side to the magnets & stator on the other side?
is that why you keep refering to the gap?

thank you guys verry much for taking the time to get me this information
take care
JAK
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Old Feb 18, 2007, 09:13 AM
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It would be the diameter of the space between the magnets and the stator.
Very close to just the diameter of the stator since the air gap is quite small.
Probably would be clearer to just think about the stator diameter, but the LRK articles makes reference to the air gap.
Pat MacKenzie
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