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The performance of a vane air motor

What to know about a vane air motor's performance

Performance air motor

The motor can operate over the complete torque curve

The performance of an air motor is dependent on the inlet pressure. At a constant inlet pressure, air motors exhibit the characteristic linear output torque/ speed relationship. 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 with 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 curve

Torque power illustration

Torque is the rotating force that is calculated as force (F) times the length (l) of the lever.

The power that an air motor produces is simply the product of torque and speed. Air motors produce a characteristic power 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.”

Power curve air motor

The performance curve for an air motor operating at a constant air pressure

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]

The working point

Working point air motor

When selecting an 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.

 

Air consumption

The air consumption for an air motor increases with the motor speed and thus is highest at free speed. Even at standstill condition (with full pressure applied) the motor consumes air. This depends on the internal leakage in the motor.
Note: Air consumption is measured in l/s. This is however not the actual volume that the compressed air occupies in the motor but it is measured as the volume it would occupy if allowed to expand to atmospheric pressure. This is a standard used for all pneumatic equipment.

Starting torque

starting torque performance

At start up the torque varies with the vane position.

It should be noted that all 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. It is notable that 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.

Stall torque

The stall torque is the torque that a motor gives just when it stops after being braked to a stop from a running condition. The stall torque is not stated among the tabulated data. However multiplying the maximum power torque by two can easily approximate the stall torque, i.e., a maximum power torque of 10 Nm equals a stall torque of approximately 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.