How to Control Air Motors
FLUID POWER - Design Data Sheet 40
Background Information. The fluid power user
should not overlook the advantages of air motors on certain
applications where they may perform better than either electric
motors or hydraulic motors. They will work in hostile environments
such as extreme heat or cold, in corrosive, flammable, or explosive
atmospheres. They can be controlled with simple valving, can be
instantly reversed, stalled indefinitely without damage, and can be
mounted to run in any position.
Small vane-type and piston-type motors are available up to 3 HP.
Larger piston motors are available up to 25 HP. While most of them
are best suited for high speed, low torque applications, at least
one new design has been introduced for low speed, high torque
They do have certain limitations. They are less efficient than
other motors. They have poor speed regulation; that is, they change
their speed according to the load. If the load should change, their
speed also changes.
Directional Control of Single-Rotation
Figure 1. The simplest start-stop control is any
kind of 2-way valve - ball, gate, globe, plug, spool, placed in the
air supply to the motor inlet.
Figure 2. If the motor must be relieved of an
air block on both ports while stopped, a 3-way valve may be
connected as in this figure.
Figure 3. For a more rapid stop after the air
has been shut off, a 4-way valve, check valve, and needle valve may
be connected as shown. During normal running (solenoid energized),
most of the exhaust air from the motor will go to atmosphere
relatively unrestricted through the check valve and 4-way valve.
While stopping (solenoid de-energized), exhaust air being no longer
able to go through the check valve, is forced through the needle
valve and can be restricted as much as desired.
Directional Control of Reversible Motors
Directional control of air motors is different from that of air
cylinders. Cylinders are allowed to stall against a positive stop,
under pressure, at each end of their stroke. With air motors,
pressure should be removed from both ports while they are stopped
to avoid the excessive waste of air due to their internal leakage.
This calls for a 3-position 4-way valve in which air pressure is
blocked in neutral, either a closed center (Figure
4) or a float center (Figure 5),
according to the needs of the application.
Closed Center Valve.
Figure 5. Float
Torque and Speed Control of Air Motors
Torque of an air motor is directly proportional to the difference
in pressure between inlet and outlet ports.
Running Torque. Output torque on the shaft reaches a maximum just
after the motor starts, then drops off sharply as speed increases
until it becomes zero when the motor reaches its free running
speed. At this speed there is no torque output because all the
torque is consumed internally in overcoming friction and air flow
When the motor is coupled to a load, it rotates the load as fast
as it can according to the torque required by the load. If the load
should decrease, the motor increases its speed until its torque
again matches the load requirement. If the load should increase,
the motor slows down until there is again a match between its
torque output and the load requirement. In this respect an air
motor acts very much like an air cylinder; its speed increases or
decreases as the load respectively decreases or increases.
Stall Torque. While the motor is running, if
the shaft load should be gradually increased, the motor will
continue to decrease its speed until the point of maximum torque is
reached, at perhaps 25 to 100 RPM. Since this is the highest torque
which it can produce, any further increase in torque load will
cause it to suddenly stall.
Starting Torque. Starting torque of an air
motor is quite low, less than 75% of its maximum running torque.
Starting torque can be increased by proper circuit design and
adjustment, by exposing the motor, at the moment of starting, to a
higher pressure than its normal running pressure.
Increasing the Starting Torque. Figure 6. Most
air motor applications will need a speed control valve to set the
speed while the motor is running under load. The key to improve
starting torque is to place the speed control in the motor outlet,
or in a meter-out mode on reversible air motors.
Pressure gauge readings in Figure 6 are shown
just after air pressure is applied but before the motor starts.
Since there is yet no air flow, there will be no pressure drop
across the speed control valve, and full line pressure will appear
across the motor ports. Starting torque is, therefore, a function
of the full line pressure.
Pressure relations at the moment of starting (no air
Figure 7. As the motor comes up to full speed,
pressure across the motor will decrease to its normal running value
as pressure drop appears across the speed control valve due to air
flow. Running torque is, then, a function of line pressure minus
pressure drop-across the speed control valve.
To get full benefit from this method of increasing the starting
torque, the control valve which admits air to start flow to the
motor should be a fast-opening type, like a solenoid valve, and not
a valve which could be gradually opened by an operator.
Pressure relations when motor reaches full speed (full air
Torque and Speed Control of Reversible
Figure 8. A complete control circuit for a
reversible air motor would include a pressure regulator upstream of
the 4-way valve. This controls maximum starting, running, and stall
torque, and prevents excessive pressure from reaching the
Most small air motors are designed to operate at no more than
100 PSI differential pressure across their ports. For example, if
the regulator were set for 120 PSI, this would allow 20 PSI
pressure drop across the flow control valves and still give full
operating pressure across the motor. Starting torque would be
developed at 120 PSI, running torque at 100 PSI.
The circuit should include a pair of
flow control valves in the motor lines connected as meter-out speed
Rather than the closed center 4-way
valve, a float center valve might give more starting torque by
preventing pressure from being trapped between valve and motor when
the motor was stopped.
Torque and speed control of a reversible air motor.
Download a PDF of Fluid
Power Design Data Sheet 40 - How to Control Air
© 1990 by Womack Machine Supply Co. This
company assumes no liability for errors in data nor in safe and/or
satisfactory operation of equipment designed from this