How a bridge disaster helped enhance VSD applications

The collapse of the Tacoma Narrows Bridge is a lesson still used in college engineering classes today. Known as “Galloping Gertie,” it’s the story of how in 1940, this suspension bridge self-destructed and fell into the Puget Sound near Seattle. People blamed the wind, but in fact, engineers widely believe it was resonant frequency that caused the disastrous fall.

While bridges are designed to sustain strong winds and even flex with various loads, the Tacoma Narrows Bridge was deemed to have failed due to natural or resonant frequency when strong gales whipped through. The collapse was the dawn of greater focus on natural frequency in engineering practice.

Make sure it’s engineered for the application. Today, we know the benefits of variable frequency control as improving efficiency in manufacturing processes, but were you aware resonance can also impact the compressor’s reliability? Matching the drive to the motor and compressor for the application is important. For VSD applications that are not engineered for the application, a compressor may exhibit rough running or unpleasant sounds as a result of operating at critical speed. A savvy utilities engineer could potentially determine the critical speeds through trial and error, but damage would likely occur before it is caught. This is why I would recommend an engineered solution where the equipment manufacturer has properly paired the VSD to the motor and compressor.

Understanding resonance helps engineers integrate the proper algorithms into the control strategy of a variable speed driven compressor. Simply programming out the possibility for the compressor to operate at critical speed ensures that the compressor will not experience the adverse effects of resonance.

The right drive motor. Other considerations for using VSD in compressor applications include making sure the drive motor is designed for variable speed service.  For traditional induction motors using pulse-width modulated (PWM) drives, the motor would likely need to be rated for “inverter duty,” designed to manage the heat associated with the change in sine wave. Additionally, the motor would often include insulated bearings to protect them from stray currents associated with using variable frequency drives.

Permanent magnet motors with variable speed drives manage heat well with greater air gaps since they do not have one continuous copper winding. Some VSD compressor designs with permanent magnet motors integrate the shaft of the compressor’s compression module with the shaft of the motor. This minimizes frictional losses and also eliminates concerns with motor bearing currents since the motor has no bearings.

Power conditioning. Where electrical power isn’t clean or harmonics are an issue, some additional power conditioning equipment may be required. Early applications of VFD’s saw more frequent issues with harmonics, and proper grounding was often a common and easily rectified issue. Power fluctuations can be rectified with line reactors or active front end technology can be employed; however, AFE is usually not commercially palatable.

Why the focus on VSD? Variable speed drive (VSD) technology has come a long way, and it’s allowing us to make plants more efficient, especially in the case of fluctuating loads. In fact, variable speed drives (VSD) can consume up to 35 percent less energy than fixed-speed compressors for the same applications, since they precisely match the demand for air. But, there are considerations for making sure that the VSD application achieves your efficiency and reliability demands. Investigate the various types of VSD driven compressors and determine the most efficient and reliable with no resonance issues for your application.

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