The performance of a vane air motor
What to know about a vane air motor's performance
The motor can operate over the complete torque curve
The power curve
Torque is the rotating force that is calculated as force (F) times the length (l) of the lever.
The performance curve for an air motor operating at a constant air pressure
The working point
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
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.