Variable speed electric motor controls
Most electrically driven compressor sets use a constant speed motor, relying on intake throttling for capacity control. There is now coming on to the market an alternative form of capacity control using a variable speed electric motor drive (VSD). This works practically in much the same way as the engine speed control described above. The motive power comes from a variable speed, variable frequency induction motor. This has two major advantages: it is more efficient in part load operation, when compared with a compressor where the capacity control relies on modulation or on a basic load/no load regulation system; and it eliminates the need for a gear train. This form of control includes a frequency convertor in the compressor package and has other features which are possible using advanced electronic monitoring and control. The acceleration and deceleration levels can be controlled, which reduces the mechanical stress on the components of the drive. The concept accurately measures system pressure and monitors the frequency convertor and speed so as to keep the delivery pressure constant within a narrow band.
While the system is more economical in running costs in an application where a substantial part of the time is at part load conditions, this must be offset against the extra capital costs. Figure 9 shows a completed packaged unit.