Selection of an air motor

For selecting an air motor for any application, the most important parameters to be considered are : Speed and Torque at the desired speed.

Each air motor has a Torque-Speed curve at different air pressures. Although the maximum air pressure required is 6 kg/cm2, it is recommended to consider the air pressure at 4 kg/cm2 and to see whether the curve gives a point where the required torque and speed are available.It is recommend to select the middle position of the curve, and not the end positions of the curve for the selection.

Replacing and electric motor with air motor

A most common mistake done while considering replacing an electric motor with an air motor is that the air motor is selected with the same HP as the electric motor. In case of an electric motor, its speed remains constant throughout and therefore its torque also remains constant. But in an air motor, the speed varies with the air pressure and therefore, the torque also varies. Therefore, an electric motor and an air motor of the same HP will never give the same torque and speed.

As an example, let us consider how to select an air motor equivalent to an electric motor of 1 HP and 1400 rpm.

From the technical specifications of the electric motor, we find that it gives a torque of 0.52 Kg-mt, or approx. 5.2 ntmt at 1400 rpm. Now we should have the air motor to have 5.2 nm approx. at 1400 rpm to give the same performance. Referring to the Torque-Speed curves of different air motors, it will be observed that a 2 HP air motor will give 6 nt-mt torque at approx. 1500 rpm. It is also possible to have a 0.5 HP air motor with a planetary gearbox to give the same torque and speed. Both these air motors will be suitable.