Power characteristics of air motors are similar to those of series-wound DC motors. With a constant inlet pressure, the brake horsepower of an air motor is zero at zero speed. Power increases with increasing speed until it peaks at around 50% of free speed (maximum speed under no-load conditions), Figure 4.
At the peak point, torque decrease balances speed increase. Power decreases to zero when torque is zero, because all the inlet air power is used to force the volume of air required to maintain this speed through the motor.
Torque output for an air motor of given displacement theoretically is a function of the differential pressure and a constant that depends on the physical parameters of the motor. Therefore, regardless of speed, torque should be constant for a given operating pressure. Actually, this is not the case, because as air flow increases through the motor, pressure losses in the inlet and outlet lines consume a greater portion of the supply. In practice, torque reaches its greatest value shortly beyond zero speed, Figure 4, and falls off rapidly until it reaches zero at free speed.
Starting torque is the maximum torque the motor can produce under load. It is about 75% of stall torque. It takes more torque to start an air motor than to keep it running. Do not confuse stall and starting torques. If the air motor load exceeds its starting torque, the motor will not start.
Stall torque, the maximum torque of an air motor, is about twice the torque at rated horsepower, and can be determined from information on power and speed given in manufacturers' literature. The relationship between torque and rated power is:
T = 5250 P / n
T is torque in lb-ft
P is power in hp, and
n is speed in rpm.
Because stall torque is about twice torque at rated power, if n is 525 rpm, and P is 0.03 hp, then T is 3 ft-lb, and starting torque is 2.25 ft-lb.
Rated power generally refers to maximum horsepower at 90 psi. Although air motors typically can operate at pressures from 20 to 150 psi at the intake, usual practice limits operating pressure to between 30 and 100 psi.
To compare motors rated at different inlet pressures, use this rule of thumb: reduce horsepower 14% for each 10-psi reduction in air pressure. Conversely, a 10-psi reduction in air pressure will cut motor efficiency by 14%. Obviously, this relationship directly affects productivity. Again, this is only a rule of thumb and does not apply exactly to any particular motor model.
Be sure to measure supply pressure at the motor inlet. It is not enough to determine that there is 90-psi supply pressure at the compressor - line losses usually reduce that pressure before it reaches the air motor. There must be 90 psi at the motor inlet for the motor to perform at rated torque and horsepower.
Controlling air pressure supplied to the motor is the simplest and most efficient method of changing the motor's operating characteristics. Conversely, not maintaining the required supply pressure at the motor inlet certainly degrades operating characteristics.
There is no direct relationship between power and speed; that is, the lowest horsepower does not indicate the highest speed or vice versa, Figure 4.
Free speed is the maximum speed of the motor under no-load conditions. For a governed motor, the term free speed actually means free governed speed, or the maximum speed at which the motor will run while the governor is operating.
Design speed is that speed at which rated horsepower is reached. It is about half the free speed of a non-governed motor, and 80% of free governed speed of a governed motor. An air motor operates most efficiently at design speed.
Because air motors are constant-displacement devices, their speed, theoretically, is directly proportional to air flow rate. This is true if there is no leakage, but leakage certainly affects motor speed. Leakage increases with pressure, and is nearly constant at any given pressure. Thus, at fixed speed, air consumption increases as supply pressure increases; at low speeds, a much higher proportion of total flow is lost through leakage.
A typical air motor performance curve, Figure 5, shows that the additional increment of flow per rpm is nearly constant. Notice, though, that total flow per revolution decreases as speed increases. Leakage also decreases slightly as speed increases, because less time is available for leakage.
When the load on an air motor increases, speed decreases until motor torque meets that load requirement. Opening the throttle to the motor to increase inlet air pressure may bring the motor up to rated speed.
For applications involving varying loads, the major consideration is whether the motor can provide enough power for all operating conditions. Motors producing the same maximum horsepower but with different torque characteristics can exhibit substantial differences in speed, depending on load, Figure 6. On the other hand, if you wish to reduce change in speed with varying load, select a motor with a steep torque curve, Figure 7. This is because the steeper the torque curve, the less speed changes with load.
The influence of the load can be reduced by installing speed-reduction gearing between the motor and the load. This decreases output speed while retaining the same power to increase the slope of the torque curve. Remember, maximum power usually occurs at 50% of free speed, so reducing free speed also reduces design speed, Figure 6. Gearing also reduces efficiency.
Another good rule of thumb is to choose an air motor that provides the required horsepower and torque at about 2/3 of available air supply pressure. Full line pressure then can be used for starting and overloads.