The performance of a vane air motor

Rotary vane air motor efficiency and performance are dependent on the inlet pressure. Air motors exhibit the characteristic linear output torque/speed relationship at a constant inlet pressure. However, by simply regulating the air supply, using the techniques of throttling or pressure regulation, the output of an air motor can easily be modified. One of the features of air motors is that they can operate over the complete torque curve from free speed to standstill without any harm to the motor. The free speed,* or idling speed, is defined as the operating speed where there is no load on the output shaft. *Free Speed = speed at which the outgoing shaft rotates when no load is applied.

The power that an air motor produces is simply the product of torque and speed. Air motors produce a characteristic power curve, or air motor performance curve, with maximum power occurring at around 50% of the free speed. The torque produced at this point is often referred to as “torque at the maximum output.”

Output formula: P = (π x M x n) / 30 M = (30 x P) / (π x n) n = (30 x P) / (π x M) P = power [kW] M = torque [Nm] n = speed [rpm]

When selecting a rotary vane air motor for an application, the first step is to establish the “working point.” This is the combination of the desired operating speed for the motor and the torque required at that point.

Note: The point on the torque/ speed curve where the motor actually operates is called the working point.

 

It should be noted that all rotary vane air motors produce a variable starting torque due to the position of the vanes in the motor. The lowest starting torque value is called the minimum starting torque and can be considered as a guaranteed value at start-up. The variation differs between motor types and must be checked on an individual basis to ensure air motor efficiency. Notably, the torque variation is greater for reversible motors than for non-reversible motors, and therefore, the minimum starting torque is smaller for these motors.

Note: The starting torque is the torque that a motor gives with blocked shaft when you feed full air pressure into it.

The stall torque is the torque that a motor produces when it is braked to a stop from a running condition. The stall torque is not stated among the tabulated data. However, doubling the maximum power torque can approximate the stall torque, e.g., a maximum power torque of 10 Nm equals a stall torque of about 20 Nm.

• Stall torque is the torque provided when the motor is run to standstill. 
• The stall torque varies depending on how fast the motor is braked down to stall. A fast braking down results in higher stall torque than a slow braking down. This depends on the fact that the mass (moment of inertia) from the rotor increases the torque.