How to Calculate Electric Motor Torque
Watts = volts * amps
1HP (horsepower) = 746 watts
Torque (T) = ((HP(horsepower) / RPM (rotations per minute))) * 63,025 (constant)
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.
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.
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.
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.
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:
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.
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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