Motor Sizing Calculator from Load Torque and Speed

The motor power needed for a load at a speed, and the next standard motor size.

Calculated from standard textbook formulas, with every step shown. The formulas are fixed, so the working does not need checking: what matters is the numbers you type in. Use these calculators only if you are competent to choose the inputs (loads, material strength, dimensions, supports) and check your units and assumptions. For learning and checking your own working: design and sign-off are for a qualified engineer. Drafting Central does not store the numbers you type.

How this calculator works

The power at the driven shaft is the torque times the angular speed. The motor must supply that power plus the losses in the gearbox and couplings, so divide by the efficiency, and multiply by a service factor to allow for overload and starting duty.

The calculator then gives the next standard IEC motor rating. If you give the inertia of the load and the time to reach speed, it also gives the torque and power needed to accelerate it. The motor must also deliver the starting torque and not overheat in repeated starts: check the motor data with the supplier.

Formulas used

  • P = T\,\omega,\quad \omega = \frac{2\pi N}{60}
  • P_{motor} = \frac{P \cdot SF}{\eta}
  • T_a = \frac{J\,\omega}{t_a}

Worked example

With the default numbers in the calculator above:

  1. Power at the driven shaft: 2.513 kW
  2. Motor power needed for the running load: 3.415 kW
  3. Next standard motor size: 4 kW

Change the numbers in the calculator to see the full working, every step, for your own case.

Common questions

What service factor should I use?

About 1.15 to 1.5 depending on the duty. Conveyors, crushers and shock loads are towards the high end.

What are standard motor sizes?

The common IEC ratings: 0.37, 0.55, 0.75, 1.1, 1.5, 2.2, 3, 4, 5.5, 7.5, 11, 15, 18.5, 22, 30, 37, 45, 55, 75, 90 kW and up.

Does it check starting torque?

No. A loaded conveyor or crusher may need several times its running torque to start, so check the motor torque-speed curve.

Related calculators

Further reading: Motor and gearbox manufacturers' selection guides; Shigley, Mechanical Engineering Design.

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