Bench Talk

When electrical engineers think of motors, they naturally think of electric ones. That makes sense, as electric motors are ubiquitous and used in countless applications. As “The Mystery and Magic of Motor Genealogy” describes, electric motors come in dozens of basic versions—AC, DC, brushed and brushless, synchronous and asynchronous, and many more—and come with a wide range of sizes, power levels, and other attributes. We know how to interface to them, control them, and optimize their performance, thanks to a combination of discrete power-control devices (e.g., MOSFETs, IGBTs), gate drivers, and processor-based algorithms.

Nonetheless, there’s a large class of non-electric motors in widespread though less-visible use: the pneumatic motor. These are not new at all, of course, and have been around in various guises since the mid-1800s, predating electric motors. The steam engine, which was a large part of the Industrial Revolution, was an early type of pneumatic motor, using air pressurized by boiling water in an enclosed vessel to reach the pressure on the order of 1,000psi (about 70bar) and more.

There are, of course, many operating classes of pneumatic-powered motors and engines. Mobile applications include steam-powered locomotives and ships and even the legendary Stanley Steamer car. Tools powered by pneumatic motors—such as drills, jackhammers, and impact drivers—and driven by fixed compressors also exist. In both of these applications, the pressure that drives the motor is generated in real-time as needed. Small, handheld pneumatic motors like those used for dental drills are occasionally encountered as well.

Pneumatic motors are sometimes used to boost power when regular engine power is insufficient—like in heavy-construction machines; this is analogous to using a capacitor to store electrical energy and then draw on it as needed for supporting peak loads or reducing DC ripple. Rather than using a more powerful engine design solely to accommodate its inherently larger size, fuel consumption, and spike loads, machines like those in heavy construction use a compressor, air tank, and pneumatic motor. The compressor keeps the air tank pressurized and the air in the tank drives the motor when the extra power is needed. Pressurized air is used to start internal-combustion engines when electric-motor starters are impractical. If you’ve ever wondered—this is how those 10,000hp diesel engines on ships are started.

Compressed air and pneumatic motors are also used for large-scale power storage, generation, and backup. For example, to generate electricity, a wind turbine or solar-based power system is used, which compresses air for storage in a tank or even an underground cavern. When the primary generating sources are not available or there is a load spike, the compressed air drives a generator. It’s a clean, straightforward solution in many situations.

Why consider using a pneumatic motor instead of an electric one? There are several reasons. First, these motors have very high torque even at low or zero rpm, something which electric motors are not good at providing. They are also very good at high-impact, low-repetition functions such as driving large hammers or even forging presses. Of course, the absence of wiring and associated high voltages and currents make them explosion safe, user safe, and practical in wet environments.

One area where these pneumatic motors have not done well is in using stored energy to power a vehicle without continuous, self-contained replenishment while in motion. Why isn’t compressed air used more often in these applications? The answer is simple: It has relatively low energy density per unit volume. Compressed air at 2,900psi (200bar) has the energy density of a little more than 0.1MJ/L; at 4,300psi (300bar), it goes up to 0.2MJ/L (it’s a function of temperature, too, of course). Compare this to gasoline, which is about 35MJ/L, and Li-ion batteries, which range from about 1 to 2.7MJ/L depending on cell chemistry.

There was a car powered by on-board compressed air tanks, a few years ago, that garnered considerable attention due to the novelty and presumed environmental benefits. However, it literally—and figuratively—did not go anywhere due to a very limited range, inability to power accessories and electronics, and other reasons.

Despite the low energy density, stored-air pneumatic motors are finding innovative new uses. The University of Pittsburgh’s Human Engineering Research Laboratories (HERL) has designed, developed, and built the PneuChair, a waterproof wheelchair for use at the beach, pools, and water parks.

The second iteration of this compressed-air, non-electric wheelchair weighs 36kg and recharges in just a few minutes. With its 1.44L air tank, range is limited to about 4.8km and speed is about 8km/hr, far less than an electric wheelchair but adequate for the targeted application. Despite these limitations, the use of a non-electric motor is a good fit for this small-sized but difficult application.

Although electric motors are more common, pneumatic motors are a good choice when high torque at low (or zero) rpm is required and when driving high-impact, low-repetition functions. Stored-air pneumatic motors are finding new applications, where waterproofing is needed but low energy density will suffice. How are you using pneumatic motors in your designs? Share your experiences in the comments!