a. A VSD is a mechanical Variable Speed Device (Drive) which takes many forms, from sliding discs to a variable Vee belt drive which uses a tensioning device to alter the diameter of the Drive pulley, which then increases or reduces the speed.
b. A VFD is an electronic Variable Frequency Device which alters motor speed by altering the Hz frequency of the motor, but is limited to the limits of Ohms law, which does limit the speed differentials by approximately +/- 12.5 %.
c. VSD devices are more commonly used in machines used in a very wide variety of applications for process machinery that may require a powerful positive drive and a wider speed band to control Process throughput management.