Because nothing in it is dissipating electrical energy as heat when it stops turning.

When an electric motor stalls, the back-EMF that normally limits current disappears, current rises sharply, and the windings heat rapidly - a stalled electric motor destroys itself within a short time unless protection intervenes.

An air motor stalled simply stops. Compressed air continues to be admitted and it holds torque against the load, but the only energy entering the machine is the air flow, which also carries heat away as it expands. There is no winding to cook and nothing to burn out.

So an air motor can be held at stall indefinitely, restart under full load, and be cycled and reversed continuously without any thermal consideration.

That makes it the natural choice for hoists and winches that must hold and start under load, for mixers that may be started in set material, and for any drive that jams routinely as part of its work.

Torque is highest at stall and falls in a straight line to zero at free speed, with power peaking around the middle.

At zero speed the motor delivers its maximum, or stall, torque. As it speeds up, torque falls steadily, reaching zero at the free speed - the speed it reaches with no load at all.

Because power is torque multiplied by speed, and one rises while the other falls, power is zero at both ends and peaks at approximately half the free speed, where torque is about half the stall torque.

The practical consequences: size the motor so its normal operating point sits near that mid-range for best efficiency; expect the motor to slow down as load increases rather than drawing more current as an electric motor would; and note that it will accelerate towards free speed if the load is lost, which on some drives needs consideration.

Manufacturers publish the curve for each model at a stated supply pressure, and it shifts with pressure.

By throttling the air, which gives stepless control with no electronics at all.

Restrict the flow into the motor and it runs more slowly; open the valve and it speeds up. The adjustment is continuous, immediate and infinitely variable across the range, achieved with a simple flow control valve.

That is a genuine advantage over electric drives, which need a variable frequency drive to do the same thing - the air motor's variable speed capability is inherent and costs nothing.

Pressure regulation is used as well, and the two do different things: throttling the flow limits speed, while reducing the supply pressure lowers the whole torque curve, which limits the torque the motor can develop. That is often used deliberately as a torque limit on assembly tools.

Throttle the exhaust rather than the inlet where smoother low-speed running is wanted - it gives better control at low speed, though it does raise the motor's internal back pressure.

A great deal, and it is the dominant operating cost of any pneumatic drive.

Compressed air is an expensive medium. Producing it consumes electricity at the compressor, and the overall efficiency from electrical input to mechanical output at the air motor is low compared with an electric motor doing the same work - a substantial part of the energy is lost in compression heat, in distribution leakage and in the expansion process itself.

So the honest position is that air motors are chosen for their properties, not their efficiency: stall tolerance, spark-free operation, high power-to-weight ratio, washdown tolerance and simple variable speed.

Where those properties are not required and an electric drive would serve, the electric drive will cost far less to run.

When sizing the air supply, use the motor's consumption at its actual operating point rather than at free speed, and remember it is a continuous demand while running - air motors are often the largest single consumers on a plant air system.

Wherever stall tolerance, spark-free operation or power-to-weight matters more than efficiency.

Hoists and winches are a classic case: they must start under full load, hold a load at stall, and be safe in environments where an electric motor would need expensive protection. Mixers and agitators that may be started in thickened material use them for the same reason.

Hand tools - grinders, drills, screwdrivers, impact wrenches - exploit the very high power-to-weight ratio, since the motor has no windings or magnets and is remarkably light for its output.

Hazardous areas in chemical plant, paint shops, mining and grain handling use them because there is no electrical ignition source at all.

Washdown and wet environments suit them because water ingress is not a problem.

Conveyor drives, tensioners, turntables and packaging machinery round out the list, typically where the drive must stall or reverse frequently.

Most do, though non-lube models exist - and getting this wrong is a common cause of short life.

The vanes slide continuously in their slots and against the housing bore, so most vane motors require an oil mist in the supply air to lubricate that contact. An in-line lubricator delivers it, set to the manufacturer's drop rate.

Run a lubricated motor on dry air and the vanes and bore wear rapidly - it may run for a while and then lose power as clearances open up.

Conversely, oil-free versions are made for food, pharmaceutical and cleanroom use, with vanes in self-lubricating materials. Feeding those a lubricated supply is also wrong, since the oil can degrade the vane material and contaminates the environment the motor was chosen to protect.

Check which type you have, set the lubricator correctly if required, and keep the air dry - water washes lubricant away and corrodes the bore, which is a frequent cause of a motor that has lost power.

Yes, straightforwardly, and this is one of its practical strengths - reversible models change direction simply by switching which port is the inlet.

A reversible vane motor is built symmetrically, with the rotor and vanes arranged so the motor works equally in both directions. A four-way valve switches the supply and exhaust between the two ports, and the motor runs the other way immediately - it can be reversed at full speed under load without damage, since there is no rotating field or stored magnetic energy to fight.

That is why air motors dominate hoisting and winching, where constant reversal is the duty.

Note that unidirectional models exist and are slightly more efficient and produce more torque for a given size, because the porting can be optimised for one direction. Where reversal is not needed, they are the better choice.

On a reversible motor, check whether performance is identical in both directions - some designs are marginally stronger one way.