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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