Potential drawbacks of VSD’S
-D Sri Mourya, EEE ,B.TECH (III/IV)
INTRODUCTION:
Adjustable speed drive (ASD) or variable-speed drive (VSD) describes equipment used to control the speed of machinery. Many industrial processes such as assembly lines must operate at different speeds for different products. Where process conditions demand adjustment of flow from a pump or fan, varying the speed of the drive may save energy compared with other techniques for flow control. Where speeds may be selected from several different pre-set ranges, usually the drive is said to be adjustable speed. If the output speed can be changed without steps over a range, the drive is usually referred to as variable speed. Adjustable and variable speed drives may be purely mechanical (termed variators), electromechanical, hydraulic, or electronic. They offer many benefits such as energy savings, improved process control, improved system reliability ,yet VSDs have some potential drawbacks, which can be avoided with appropriate design and application.
Potential Drawbacks:
1. Structural Resonance: Resonance conditions can cause excessive vibration levels, which in turn are potentially harmful to equipment and environment. Fixed-speed applications often miss these potential resonance situations because the common excitation harmonics due to running speed, vane passing frequency, plunger frequency, etc., do not coincide with the structural natural frequencies. For VSD applications, the excitation frequencies become variable and the likelihood of encountering a resonance condition within the continuous operating speed range is greatly increased. Pump vibration problems typically occur with bearing housings and the support structure (baseplate for horizontal applications, motor and stool for vertical applications). Pressure pulsations are the common excitation mechanism. These pressure pulsations may be further amplified by acoustic resonance within the pump or the adjacent piping. There are a number of analyses that can be performed to predict and avoid potential resonance situations, including simple hydraulic resonance calculations, Passing frequency analysis, Structural resonance, for example, utilizing Finite Element Analysis, Modal testing of the actual machine. Modal testing can supplement the regular vibration test. Very often, a pump intended for variable speed operation will only be tested at one single speed. 2. ROTOR DYNAMICS: The risk of the rotating element encountering a lateral critical speed increases with the application of a VSD. Lateral critical speeds occur when running speed excitation coincides with one of the rotor’s lateral natural frequencies. The resulting rotor
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