Required Motor Power Calculator

How is motor power calculated?

The first step in motor selection is the power the load needs. Power is force times speed.

• Lifting a load: P = m × g × v, with mass in kilograms and speed in metres per second.
• Pulling or pushing horizontally: P = F × v, where the force is the total resistance at constant speed.
• Rotation: P = T × ω, with angular velocity ω = 2 × π × n / 60.

That power is used at the load itself. The gearbox, belt, chain and bearings between motor and load also absorb power. The power needed at the motor shaft is P = P_load / η, where η is the product of the efficiencies of all drive components. A service margin is added for uncertainty and the result is rounded up to the next standard motor rating.

Example

A 500 kg load is to be lifted at 0.5 m/s. Its weight is 500 × 9.81 = 4903 N and it needs 4903 × 0.5 = 2452 W. With a drive efficiency of 85 percent the motor shaft must deliver 2452 / 0.85 = 2884 W. A margin of 1.2 gives 3461 W. The next standard rating is a 4 kW motor.

Limits

The calculation is for running at constant speed. Starting needs extra torque to accelerate the load, which can govern motor selection for high inertia or frequent starts. Starting torque, duty cycle, ambient temperature and operation on a frequency converter are checked against the manufacturer's catalogue.