How to calculate electric motor torque

Written by g.k. bayne
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Calculating electric motor torque can be a useful skill for estimating the potential of work a motor can perform. Although not listed on the motor's nameplate data tag, the torque value can be figured out with the information provided. By applying some basic equations you can calculate the electric motor torque for most electrical devices.

Skill level:
Easy

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Things you need

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

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Instructions

  1. 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. 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. 3

    Understand that torque is described as the amount of “force” from any given distance from that force's rotational centre point. In this case the centre point of force will be the exact centre 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. 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.

Tips and warnings

  • Remember that these calculations are theoretical only and do not take into consideration any form of friction or real world applications.

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