The performance of an air motor
The inlet pressure plays a crucial role in determining the performance of an air motor. When the inlet pressure is constant, the air motor will demonstrate a linear relationship between the output torque and speed. Nevertheless, it's possible to adjust the output of an air motor by regulating the air supply through techniques such as throttling or pressure regulation. This allows for easy modification of the air motor's performance.
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 Speed [rpm] output shaft.
Free Speed = speed at which the outgoing shaft rotates when no load is applied.
The power curve
The power output of an air motor is determined by the product of torque and speed. Air motors generate a power curve that exhibits maximum power output at approximately 50% of the free speed. The corresponding torque produced at this point is commonly known as the "torque at maximum output".
- a. Torque
- b. Air consumption
- c. Power
The working point
The working point for an 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.
Torque is the rotating force that is calculated as force (F) times the length (I) of the lever.
The performance curve for an air motor operating at a constant air pressure
Air consumption
The air consumption for an air motor increase 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.
The point at the torque/speed curve where the motor actually operates is called the working point
Starting torque
At start up the torque varies with the vane position.
Starting torque is the maximum torque that can be produced by a motor in order to start rotational movement of the load.
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.
Stall torque
Slow braking & Fast braking
The stall torque is the torque that a motor gives just when it stops after being braked to a stop from a running condition. In other words, when the load exceeds the maximum possible torque of the motor, it stalls.
An easy way to approximate the stall torque is to multiply the maximum power torque by two i.e., a maximum power torque of 10 Nm equals a stall torque of approximately 20 Nm. 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.
