All of these statements are true
in a general sense. However,
investigation of variable speed drive operation in the context of the process
may yield contradictory results.
It is generally presumed that
variable speed drives operate equipment in a manner whereby the hydraulic
output is matched to the actual process load.
This evokes a sense of efficiency and implies a reduction in electrical
energy consumption. In one application,
a variable speed drive was applied to an air compressor to enable the
substitution of compressed air for a more expensive gas. The electrical energy consumption of the
compressor increased because more compressed air needed to be produced. However, the cost of the expensive gas was
reduced by a greater amount than the increased electrical cost associated with
the compressor. The simple payback for
this project was only 8 months --- despite the increase in electrical energy
consumption. Answer A is wrong in this
application.
It might seem intuitive that reducing the operating
speed of equipment will reduce equipment vibration. This may sound reasonable, but reducing the
speed of cooling tower fans can cause the cooling tower fan to operate at its
resonant frequency. Extended operation
in resonance might shake the tower to the point of failure. Many drives have adjustments that allow the
drive skip over these frequencies to reduce the time that resonance can
occur. Answer C is wrong in this
application.
Additional Complicating Factors
There are many complicating factors that can
come into play when applying variable speed drives to processes. While there may be some relatively standard
ways in which variable speed drives are applied to certain process
configurations, all processes are different and deserve custom
consideration. The benefits of applying
variable speed drives can be substantial and beneficial to the operation, but
you should be careful.
This article originally appeared in Flow Control magazine.