Air Compressor Motors And Controls

Electric motors are by far the most popular drivers for industrial air compressors.  Indeed, they are the prime movers for a great many types of industrial rotating equipment.  In their simplest form of operation, rotary motion is induced when current flows through a conductor (the windings) in the presence of a magnetic field (usually by electricity inducing a magnetic field in the rotor.)  In the early days, you’d start one up by flipping a big lever called a knife switch.

Example of a knife switch

These are cumbersome and inherently dangerous…the operators literally have their hand(s) on the conductor.  If the insulation fails, if something mechanical breaks, if they fail to make full contact, electrocution is a very real risk.  Over time, motor starters came in to common use.  Early in their development, they were more popular with higher HP motors, but soon were made for smaller motors as well.

There are several types of modern motor starters:

Full Voltage Starters: The original, and simplest method.  These are similar in theory to the old knife switches, but the operator’s hands aren’t right on the connecting switch.  Full line voltage comes in, and amperage can peak at up to 8 times full load (normal operating) amperage during startup.  This can result in voltage dips…not only in the facility itself, but in the neighborhood.  Remember how the lights always dim in those movies when they throw the switch on the electric chair?  It’s kind of like that.

Reduced Voltage Starters: These are electro-mechanical starters.  Full line voltage is reduced, commonly to 50% initially, and steps up, usually in three increments, back to full.  This keeps the current from jumping so drastically during startup, and reduces the stress on mechanical components…like the motor shaft, bearings, and coupling to the compressor.

Solid State (or “Soft”) Starters: Like the Reduced Voltage types, these reduce the full line voltage coming in as well, but instead of increasing incrementally, they gradually and evenly increase the power to bring the motor to full speed over a set period of time.  They also are beneficial because of the reduced stress on mechanical components.

The Application Engineering team at EXAIR Corporation prides ourselves on our expertise of not only point-of-use compressed air application & products, but a good deal of overall system knowledge as well.  If you have questions about your compressed air system, give me a call.

Russ Bowman
Application Engineer
EXAIR Corporation
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What is an Air Compressor?

Internals of an air compressor

What is an air compressor?  This may seem like a simple question, but it is the heartbeat for most industries.  So, let’s dive into the requirements, myths, and types of air compressors that are commonly used.  Like the name states, air compressors are designed to compress air.  Unlike liquid, air is compressible which means that it can be “squished” into a smaller volume by pressure.  With this stored energy, it can do work for your pneumatic system.

There are two types of air compressors, positive displacement and dynamic.  The core component for most air compressors is an electric motor that spins a shaft.  Positive displacement uses the energy from the motor and the shaft to change volume in an area, like a piston in a reciprocating air compressor or like rotors in a rotary air compressor.  The dynamic types use the energy from the motor and the shaft to create a velocity energy with an impeller.  (You can read more about types of air compressors HERE).

Compressed air is a clean utility that is used in many different ways, and it is much safer than electrical or hydraulic systems.  But most people think that compressed air is free, and it is most certainly not.  Because of the expense, compressed air is considered to be a fourth utility in manufacturing plants.  For an electrical motor to reduce a volume of air by compressing it.  It takes roughly 1 horsepower (746 watts) of power to compress 4 cubic feet (113L) of air every minute to 125 PSI (8.5 bar).  With almost every manufacturing plant in the world utilizing air compressors much larger than 1 horsepower, the amount of energy needed to compress air is extraordinary.

Let’s determine the energy cost to operate an air compressor to make compressed air by Equation 1:

Equation 1:

Cost = hp * 0.746 * hours * rate / (motor efficiency)


Cost – US$

hp – horsepower of motor

0.746 – conversion KW/hp

hours – running time

rate – cost for electricity, US$/KWh

motor efficiency – average for an electric motor is 95%.

As an example, a manufacturing plant operates a 100 HP air compressor in their facility.  The cycle time for the air compressor is roughly 60%.  To calculate the hours of running time per year, I used 250 days/year at 16 hours/day for shifts.  So operating hours equal 250 * 16 * 0.60 = 2,400 hours per year.  The electrical rate at this facility is $0.10/KWh. With these factors, the annual cost to operate the air compressor can be calculated by Equation 1:

Cost = 100hp * 0.746 KW/hp * 2,400hr * $0.10/KWh / 0.95 = $18,846 per year in just electrical costs.

So, what is an air compressor?  The answer is an expensive system to compress air to operate pneumatic systems.  So, efficiency in using compressed air is very important.  EXAIR has been manufacturing Intelligent Compressed Air Products since 1983.  If you need alternative ways to save money when you are using your air compressor, an Application Engineer at EXAIR will be happy to help you.

John Ball
Application Engineer
Twitter: @EXAIR_jb


Compressor internals image courtesy of h080, Creative Commons License.

Cool Motor


One of the best tools we have for Cabinet Cooler calculations is the Cabinet Cooler Sizing Guide (CCSG).  For any given application, this guide (when completed and sent to an EXAIR Application Engineer) provides the necessary details to calculate heat load.

Sometimes, though, we receive these guides for applications better suited to another product.  For example, earlier in the week I received a CCSG for an application needing to keep a motor cool.  The motor is operating outside of the desired temperature range and needs to be cooled down.  A Cabinet Cooler can provide very cold air.  So, it seems like a perfect fit.  But, a Cabinet Cooler has to be mounted and installed on a sealed enclosure.  In an application like this, the Cabinet Cooler will serve little to no purpose.

An Air Amplifier, however, is the perfect fit.  The desired operating temperature of the motor is above ambient, which is a requirement for an Air Amplifier.  It can move enormous amounts of air, but cannot cool the air below ambient temperature.  So, Air Amplifiers have been recommended and are slated for installation.  Another success story on the horizon.

If you have an application which is in need of assistance, don’t hesitate to contact us.

Lee Evans
Application Engineer