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Replacing vintage variable speed drives with VFDs

Publication date: 
April 2004
Jim McKee (deceased), Alabama Electric Motor Service
Type of media: 

The ability to vary the speed of our prime movers has always been desirable. Electric motors are no exception. Many designs and types of motors have been developed over the years to fill this need for variable speed. The most widely used and successful designs include the direct current motor, eddy current coupling/motor combination, three-phase shunt commutator motor (brush shifting or Schrage motor), and various mechanically variable devices such as adjustable pitch pulleys and cone/wheel combinations. EASA service centers often find it desirable or necessary to replace one of these vintage variable speed devices. There are many reasons for replacing older drives; they include improved efficiency, unavailability of repair parts and high maintenance costs. A properly sized Variable Frequency Drive (VFD) /three-phase induction motor combination is often the right choice for the new drive.