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Old May 05, 2012, 03:08 PM
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How to Calculate Electric Motor Torque

Watts = volts * amps
1HP (horsepower) = 746 watts
Torque (T) = ((HP(horsepower) / RPM (rotations per minute))) * 63,025 (constant)

1
Find the wattage from a motor that has the following nameplate data on its metal tag; 120 Volts at 10 amperes with a 3600-RPM. From the formula above we see that watts is equal to volts times amperes. Plugging in the numbers, we have 120 volts times 10 amperes is equal to 1200 watts.

2
Calculate the horsepower that a 1200-watt motor can deliver. 1 HP is equal to 746 watts. We can then divide 1200 watts by 746 watts per HP and the answer is 1.6 HP.

3
Understand that torque is described as the amount of “force” from any given distance from that force's rotational center point. In this case the center point of force will be the exact center of the round motor shaft. Torque values are given in either inch-ounces (in-ozs) or foot-pounds (ft-lb). In the formula described above, the torque value formula will be described in ft-lbs. You can think of 1 ft-lb as the amount of “force” that is placed on an object that has an “arm” 1 foot long and a 1-pound weight hanging from the end of that 1-foot arm.

4
Find the amount of Torque that the 1.6 Hp motor delivers at 3600 RPM. Taking the torque formula one step at a time, divide the HP of 1.6 by the RPM of 3600. The answer will be 0.00044. This is a very small number. Next multiply 0.00044 times the constant of 63,025. The final Torque value of the motor is 28.16 ft-lbs of Torque. This is equivalent to a 28 and 1/8-pound weight hanging on the end of a 1-foot long arm or a 1-pound weight hanging on the end of a 28 foot 2 inch long arm.
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Old May 05, 2012, 03:15 PM
*Miss Geigo*
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thank u bahaa for these important information
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Old May 05, 2012, 03:17 PM
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thx boatmate !
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Old May 06, 2012, 07:05 AM
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I'm a little curious about the formula and where the 63025 constant comes from?

I know for full scale cars the formula is:

HP = (tq * RPM) / 5252

Re-arranging for torque would be:

TQ = (HP * 5252) / RPM

* disclaimer, I know the "proper" formula is: HP = 2pi(RPM * TQ) / 33000, but 2pi/33000 is very close to 5252 (5252.1131220...)

I found a couple sites that reference the same formulas for electric motors as well:

http://www.elec-toolbox.com/Formulas/Motor/mtrform.htm

http://www.wisc-online.com/Objects/V...px?ID=ENG17504

All of this said, none of this helps you pick a motor. Since you need to know the actual current being drawn to make these calculations and not just the theoretical maximum you actually need to have the motor in the boat and be logging current draw. Prop choice, hull type and weight, etc will all effect current draw, so the motor will put out wildly different torque numbers with different setups.

The manufacturers constant wattage output number is much more useful, since they give you a baseline to say how much power it will produce and still be happy. You can easily get double (or more) wattage out of it, but not for long before something goes up in smoke.
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Old May 06, 2012, 12:17 PM
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63025 cts

welcome dude this constant is generated in that way
The constant 63,025 is the truncated value of (33,000 ft•lbf/min) × (12 in/ft)/(2π rad/rev).
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Old May 06, 2012, 03:00 PM
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Not following your math there at all.... why are you converting with 12 in/ft, but not converting between pounds and ounces? No one measures torque in inch/pounds.

Plus above, you mentioned a 1.6hp motor had 28ft/lbs of torque?
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Old May 06, 2012, 04:47 PM
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So torque ghoes down as revs go up.. makes sense
The issue with this equation is that the amount of watts a motor draws is dependant on other factors, not the rated watts on a motor page.
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Old May 06, 2012, 06:39 PM
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The only thing that this equation is good for it calculating hp if you know torque and rpm, or torque if you know hp and rpm.

It's more useful in determining HP on a full scale car engine, since you can't actually measure HP, just torque.
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Old May 06, 2012, 07:22 PM
Thermite + ice = Big boom.
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watts/746=hp
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Old May 06, 2012, 07:38 PM
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Oop wrong thead.
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Old May 06, 2012, 09:06 PM
Never Fast Enough
Hamilton Ontario
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You can't also accurately determine horsepower on an electric motor. It's very difficult to determine the exact efficiency of an electric motor.

The best way to get a ball park value of torque is to use current and the torque constant "kt."

Current can be taken at any point and the torque constant can be found for every motor.
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Old May 07, 2012, 05:25 PM
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!!

ok i will give you all needed details
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Last edited by bahaah; May 07, 2012 at 05:43 PM.
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Old May 07, 2012, 05:27 PM
*Miss Geigo*
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Quote:
Originally Posted by bahaah View Post
chebkon 3amlili halkon abu rabba !!!
sry but what do you mean by abu rabba ? does abu rabba = torque or something !
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Old May 07, 2012, 05:33 PM
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sry
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Last edited by bahaah; May 07, 2012 at 05:42 PM.
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Old May 07, 2012, 05:36 PM
*Miss Geigo*
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can you please correct your information ! we all lost here !
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Last edited by KM.Racing; May 07, 2012 at 05:41 PM.
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